CRC Report No. E-84 

REVIEW OF PRIOR STUDIES OF FUEL 

EFFECTS ON VEHICLE EMISSIONS 

August, 2008 
The Coordinating Research Council, Inc. (CRC) is a non-profit corporation 

supported by the petroleum and automotive equipment industries. CRC 

operates through the committees made up of technical experts from 

industry and government who voluntarily participate. The four main 

areas of research within CRC are: air pollution (atmospheric and 

engineering studies); aviation fuels, lubricants, and equipment 

performance, heavy-duty vehicle fuels, lubricants, and equipment 

performance (e.g., diesel trucks); and light-duty vehicle fuels, lubricants, 

and equipment performance (e.g., passenger cars). CRC’s function is to 

provide the mechanism for joint research conducted by the two industries 

that will help in determining the optimum combination of petroleum 

products and automotive equipment. CRC’s work is limited to research 

that is mutually beneficial to the two industries involved, and all 

information is available to the public. 

CRC makes no warranty expressed or implied on the application of 

information contained in this report. In formulating and approving 

reports, the appropriate committee of the Coordinating Research Council, 

Inc. has not investigated or considered patents which may apply to the 

subject matter. Prospective users of the report are responsible for 

protecting themselves against liability for infringement of patents. 
CRC Project E-84, Final Report 

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Final Report 

Review of Prior Studies of Fuel Effects on Vehicle Emissions

CRC Project E-84 

Prepared for 

Coordinating Research Council 

3650 Mansell Road, Suite 140 

Alpharetta, GA 30022 

August, 2008 

Dr. Albert M. Hochhauser 

12 Celler Rd. 

Edison, NJ 08817 

732-572-5241 

albie.hochhauser@gmail.com
CRC Project E-84, Final Report 

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TABLE OF CONTENTS

1. EXECUTIVE SUMMARY ...................................................................................................................................4

2. INTRODUCTION .............................................................................................................................................6

A. ORGANIZATION OF REPORT..............................................................................................................................7

B. METHODOLOGY................................................................................................................................................7

C. PRESENTATION OF RESULTS..............................................................................................................................8

D. CAVEATS ...........................................................................................................................................................8

3. GASOLINE ......................................................................................................................................................9

A. SULFUR .............................................................................................................................................................9

Reversibility and Aging..................................................................................................................................15

Review Papers ...............................................................................................................................................16

Particulates....................................................................................................................................................17

Sulfur Summary .............................................................................................................................................17

B. AROMATICS AND BENZENE.............................................................................................................................18

Summary of Aromatics and Benzene Effects.................................................................................................23

C. OLEFINS...........................................................................................................................................................23

Summary of Olefin Effects .............................................................................................................................25

D. VAPOR PRESSURE ...........................................................................................................................................25

Summary of RVP Effects ................................................................................................................................27

E. MID‐FILL AND BACK‐END VOLATILITY..............................................................................................................27

Summary of Volatility Effects ........................................................................................................................32

F. OXYGENATES (ETHERS AND ALCOHOLS) .........................................................................................................32

Summary of Oxygenate Effects .....................................................................................................................37

G. GASOLINE SUMMARY AND RESEARCH NEEDS ................................................................................................37

4. DIESEL FUEL .................................................................................................................................................38

A. DENSITY, CETANE AND AROMATICS................................................................................................................40

Light Duty Diesels ..........................................................................................................................................40

Heavy Duty Diesels ........................................................................................................................................45

Summary of Effects of Aromatics, Cetane, Density .......................................................................................49

B. SULFUR............................................................................................................................................................50

Light Duty Diesels ..........................................................................................................................................50

Heavy Duty Diesels ........................................................................................................................................52

Summary of Sulfur Effects .............................................................................................................................54

C. BACK‐END VOLATILITY ....................................................................................................................................54

Light Duty Diesels ..........................................................................................................................................54

Heavy Duty Diesels ........................................................................................................................................56

Summary of Back‐End Effects........................................................................................................................57

D. FATTY ACID ESTERS (FAE)................................................................................................................................57

Light Duty Diesels ..........................................................................................................................................58

Heavy Duty Diesels ........................................................................................................................................60

Summary of FAE Results................................................................................................................................62

E. OTHER OXYGENATES.......................................................................................................................................62

Light Duty Diesels ..........................................................................................................................................62

Heavy Duty Diesels ........................................................................................................................................64
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Summary of Other Oxygenates .....................................................................................................................65

F. FISHER TROPSCH FUELS...................................................................................................................................65

Light Duty Diesels ..........................................................................................................................................66

Heavy Duty Diesels ........................................................................................................................................72

Summary of FT Effects...................................................................................................................................80

G. DIESEL SUMMARY AND RESEARCH NEEDS......................................................................................................80

5. OFF‐ROAD ENGINES AND VEHICLES ..............................................................................................................81

A. SPARK IGNITION ENGINES...............................................................................................................................83

Oxygenates....................................................................................................................................................83

Other Gasoline Properties .............................................................................................................................86

Propane .........................................................................................................................................................88

B. COMPRESSION IGNITION ENGINES .................................................................................................................88

Alcohols .........................................................................................................................................................88

Other Oxygenates..........................................................................................................................................89

Hydrocarbon Fuels.........................................................................................................................................89

C. OFF‐ROAD SUMMARY AND RESEARCH NEEDS................................................................................................91

Spark Ignition Engines ...................................................................................................................................91

Compression Ignition Engines........................................................................................................................91

Research Needs .............................................................................................................................................91

APPENDIX I Statement of Work: CRC E‐84 .................................................................................................................93

APPENDIX II List of Acronyms.....................................................................................................................................94

APPENDIX III Summary of Fuels and Vehicles Database............................................................................................96

APPENDIX IV References.............................................................................................................................................98
CRC Project E-84, Final Report 

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1. EXECUTIVE SUMMARY 

 This report was prepared under contract to the Coordinating Research Council (CRC) in 

fulfillment of Project E-84. It provides a summary of the technical literature describing the effects of fuel 

composition on exhaust emissions of gasoline and diesel vehicles in on-road and off-road applications. 

Extensive literature searches were carried out covering years since 1990 for gasoline vehicles, light-duty 

(LD) diesels and off-road engines/vehicles; and since 1998 for heavy-duty (HD) diesels. Abstracts were 

reviewed carefully and all relevant papers were obtained and read. The search revealed that significant 

research has been carried out in the U.S., Europe and Japan, along with some research in other regions. 

 Directional changes for gasoline vehicles are summarized in the table below. Deciding which 

changes to consider for regulatory purposes requires knowledge of environmental needs, a detailed 

description of the vehicle fleet and a modeling tool that will provide a quantitative estimate. Currently, the 

most complete model is the Predictive Model developed by ARB (California Air Resources Board).[1] 

 To Reduce Gasoline Emissions, Make the Directional Changes Shown Below 

 Aromatics Benzene Olefins Sulfur Oxygenates RVP T50 T90

HC ↓ * ↑ ↓ ↑ ↓ ↓ ↓

CO ↓ * ↑ ↓ ↑ ↓ ↓ ↓

NOx ↑ * ↓ ↓ ↓ 0 ↑ ↑

Toxics ↓ ↓ ↓ ↓ # * ↓ ↓

PM * * * * * * * * 

0 No effect 

* Data are lacking to estimate an effect 

# Data exist, but effect is variable 

 Shown below are areas where existing data is sparse or non-existent and where additional 

research would be helpful in defining fuel effects. If new vehicle technology such as GDI (gasoline direct 

injection) is developed and implemented, additional data would be required with that technology. 

• Effects of fuel parameters on PM emissions. Currently, this is not a particularly important 

area because gasoline vehicle PM emissions are still a small portion of the total 

inventory, but could become more important as diesel PM emissions drop. 

• Effects of RVP on emissions at all temperature levels. 

• Better understanding of the relative impacts of mid-fill volatility, back-end volatility and 

composition on exhaust emissions. 

• Long-term effects of sulfur on aftertreatment components and exhaust emissions. This 

may not be relevant for U.S., Europe and Japan, where sulfur levels are extremely low, 

but could be important for other regions, as they consider future sulfur levels. 

 Significant data exist which relate diesel fuel properties and exhaust emissions, both for HD 

diesels and for HD diesels. In many cases, the conclusions are not consistent and may be contradictory. 

 The following table represents a summary of the current state of knowledge of the impact of fuel 

property changes on diesel exhaust emissions. 
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To Reduce Diesel Emissions, Make the Directional Changes Shown Below 

(Light Duty / Heavy Duty) 

 Density Cetane Aromatics/PAH Sulfur Back-End Fatty Acid 

Esters 

Fischer 

Tropsch 

HC ↓/* ↑/↑ ↑/↓ 0/0 0/↑ ↑/↑ ↑/↑

CO ↓/* ↑/↑ ↑/* 0/0 0/↑ ↑/↑ ↑/↑

NOx */↓ */* ↓/↓ 0/0 #/# ↓/↓ #/↑

PM ↓/* ↓/# ↓/↓ ↓/↓ ↓/↓ ↑/↑ ↑/↑

0 - No effect 

* - Data are lacking to define effect 

# - Data exist, but effect is variable 

 For existing LD and HD diesel technology, a full statistical analysis of existing data would help 

develop an understanding of quantitative effects of fuel parameters on exhaust emissions. This would be 

an update of the work published by EPA in 2001 [2]. An important piece of this effort would be an 

analysis of the ability to determine unambiguously the impacts of density, cetane and aromatics on 

emissions. Much of the published data, with a few notable exceptions, has confounded these 

parameters. If the analysis cannot determine independent effects, then additional experimental data 

would be useful. 

 Additional research on Fatty Acid Esters and Fischer Tropsch liquids would be helpful to 

determine their effects. This research should not use splash blending to prepare fuels, but should blend 

fuels so that the contribution of these blendstocks can be tied to specific properties. A key question, 

especially for Fatty Acid Esters is why they reduce emissions. 

 There is a need for research on diesels with exhaust aftertreatment. Future diesel engines will 

almost certainly contain some form of aftertreatment. When the technology is developed and 

implemented, research should be carried out to determine the impact of fuel variables on engines and 

vehicles containing the new technology. There is some existing data with oxidation catalysts in LD 

diesels and with diesel particulate filters (DPF) , but very little data with Urea-SCR (Selective Catalytic 

Reduction) and NSR (NOx Storage Reduction) systems. 

 Off-road mobile sources are a growing part of the total mobile source inventory, and stringent 

emissions standards are being implemented. Most research to date on this sector has evaluated new 

blendstocks, such as splash blended ethanol in gasoline or splash blended FT in diesel. Prediction of the 

impact of specific fuel blends is, therefore, very difficult. As advanced technology is applied to off-road 

mobile sources, new data should be developed in carefully planned programs to define the relationships 

between fuel properties and exhaust emissions. 

 The following limitations of this analysis should be understood when evaluating the report’s 

findings 

• Many programs did not report the results of a statistical analysis, and did not report the 

statistical significance of the findings. 

• Some programs that reported statistics did not present an evaluation of the Type II errors. 

That is, if no significant results were found, there is no way to tell if the test design was 

powerful enough to have seen a relatively large effect as statistically significant. 

• Some programs tested a limited number of vehicles. Given the known variability in 

response among vehicles, it is difficult to make generalizations from a small sample.
CRC Project E-84, Final Report 

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• In some programs, the fuel properties were not independently varied, so it is not possible 

to assign results to fuel properties unambiguously. Many reports recognize this 

uncertainty, but some do not. 

• Some research programs were designed to demonstrate the benefits of certain fuel 

formulations or fuel blendstocks, and the changes in emissions could not be assigned to 

any one or group of fuel properties. 

2. INTRODUCTION 

 This report provides a summary of the literature describing the effects of fuel properties on 

emissions from on-road and off-road gasoline and diesel-powered vehicles. It is part of a project funded 

by the CRC Project E-84. 

 The Statement of Work issued by CRC is shown as Appendix I. For LD gasoline vehicles, the 

time period covered in this report dates to a similar review by Koehl et al. [3] of research up to 1989. The 

Koehl survey was one of the initial reports of the Auto/Oil Air Quality Improvement Research Program 

(AQIRP) and this report includes all AQIRP research along with other work through the end of 2007. 

 Research on diesel engines has been described separately for LD and HD diesels, for reasons 

discussed below. For HD diesel, the time period covered in this report dates from an extensive review 

published by Lee, Pedley and Hobbs [4] in 1998. Earlier research will be cited only when not covered by 

Lee. Other reviews have been published and these are also cited in this report. For LD diesel, not 

covered by Lee, research going back to 1990 was considered. 

 Off-road mobile sources comprise a wide variety of applications and literature is considered 

separately for spark ignition and compression ignitions engines. Research published since 1990 was 

considered for inclusion in the report. As will be described below, off-road emissions are subject to 

increasingly stringent emission standards and major reductions are either underway or are about to be 

implemented. 

 It should be recognized that the changes in technology in the last two decades is different for 

gasoline and diesel vehicles. For gasoline vehicles, technology changes have been largely evolutionary. 

While extraordinary improvements have been made in catalytic aftertreatment systems, fuel management 

and electronic engine controls, the basic elements of design have stayed constant: 

• Feedback control of air/fuel ratio 

• Port fuel injection 

• Three-way catalysts (TWC) 

 One noteworthy technological addition is the use of on-board diagnostic (OBD) systems to 

monitor the operation of emission control systems and report malfunctions to the driver. 

 For diesel vehicles, changes have been revolutionary and have been driven in part by the 

imposition of stringent emission standards. Direct injection has almost totally replaced indirect injection, 

even for LD diesels. Fuel system and injection system pressures have risen continuously. Injection is 

controlled electronically and can be extremely complex consisting of multiple injection events. Catalytic 

emissions control aftertreatment systems are much more commonplace, and their use will be expanded 

further in the coming years. Overall, engine controls are becoming more sophisticated and consist of 

multiple feedback systems that adjust operating conditions as necessary to maintain low emissions and 

high efficiency. 
CRC Project E-84, Final Report 

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A. ORGANIZATION OF REPORT 

 Gasoline vehicles, diesel vehicles and off-road mobile sources are considered separately; within 

each class of vehicle, the emissions impacts of individual fuel parameters are described. For gasoline 

vehicles, research programs are described chronologically within major geographic regions (U.S., 

Europe, Japan, other). At the end of each section, a summary provides general conclusions and 

highlights areas where the data may be inconsistent. With limited exceptions, only included is research 

that tested fully built vehicles on dynamometers using recognized emission test cycles such as US FTP 

(Federal Test Procedure), European MVEG or Japanese 10.15 mode test. Single cylinder engines tests 

and steady state testing were excluded. 

For diesel vehicles, LD and HD research is considered separately for a number of reasons. 

• Emission standards and test cycles are very different for the two classes 

• Typical duty cycles are very different. HD vehicles operate at high loads a higher fraction 

of the time. 

• Engine technology has been different for LD and HD diesels. For instance, direct 

injection has been introduced much earlier in HD engines. As technology advances, and 

emissions standards tighten, this technology gap is narrowing. 

• Aftertreatment systems have been different for LD and HD diesels. Oxidation catalysts 

are more common on LD diesels, and urea-SCR control of NOx is more common on HD 

diesels. 

 For diesel vehicles, research is described chronologically, generally without consideration of 

where the work was carried out. There is less variability in technology across regions for diesels than for 

gasoline technology, and for LD diesels, more research has been done in Europe and Japan, where 

diesels are more prevalent in the marketplace. Research carried out on fully developed multi-cylinder 

engines using recognized emission cycles was considered. For LD diesels, this generally includes the 

same type of test cycle used for gasoline vehicles. For HD diesels, the test cycle may be a series of 

steady state modes, or a series of transient modes carried out on an engine dynamometer. Exceptions 

were made where the research helped elucidate a particular issue or question. 

 For off-road emissions, spark ignition engines and compression ignition engines are considered 

separately. Smaller engines are generally powered by gasoline spark ignition engines and larger 

applications are generally powered by compression ignition engines using petroleum diesel fuel. It is 

appropriate to consider off-road applications separately from on-road applications for a number of 

reasons: 

• Advanced technology has not usually been applied to off-road sources. 

• Duty cycles are different, and tend to contain more steady state operation. 

• Engine size and power output cover a very broad range. 

 The report also identifies where research might be helpful in furthering the understanding of 

important issues. Research needs are based on an estimate of future technology directions, and the 

state of fuel composition. For instance, studying the effects of sulfur might be helpful, but future sulfur 

levels will be extremely low, and such information might not be particularly relevant for the U.S., Europe 

and Japan.

B. METHODOLOGY 

 The main sources of information for this report were two extensive databases maintained by the 

Society of Automotive Engineers (SAE). The SAE Global Mobility Database contains searchable data on 

publications necessary for this project. The TechSelect Database contains searchable information and 

full text for publications after 1996. Searches were also conducted of EPA, DOE and ARB databases for 

government publications and other information. Most research has been published in the SAE literature, 
CRC Project E-84, Final Report 

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but a significant number of papers have been published elsewhere and these were generally referenced 

in one or more SAE papers or review articles or a number of excellent government agency reviews. 

 The searches were designed to be inclusive. In total, 215 gasoline abstracts, 457 diesel 

abstracts and 106 off-road abstracts were reviewed for relevance. Of these, 109 gasoline papers, 128 

diesel papers and 18 off-road papers were read in full. Appendix III contains tables showing the numbers 

of vehicles and fuels tested in each category, for each major fuel parameter. The numbers presented in 

the tables are simple sums of the totals in each reference. No attempt was made to break down each 

program to determine which vehicles and fuels were used to determine each parameter’s response. 

However, the tables provide the reader with general information concerning the amount of data available 

and how recently it was collected. 

 An Excel spreadsheet was prepared which contains summary information about all the studies 

referenced in this report, as well as the complete reference. The specific information is listed in Appendix 

III, and the spreadsheet itself is available from CRC (www.crcao.org). Using the spreadsheet, it is 

possible to determine the studies that have been carried out to evaluate any given fuel property in broad 

vehicle or engine categories. 

C. PRESENTATION OF RESULTS 

 In general, the impacts of changing fuel properties on emissions are shown as percent change 

because many publications report results only in percentage terms, and stating changes as percentages 

allows a comparison of results across programs. 

 With a few exceptions, results are shown as presented in the original paper. In a few cases, 

minor calculations were carried out to be able to present the results in a meaningful manner. 

 Some difficulties still remain when comparing results among different studies, because: 

• Fuel parameters changes are not common across studies, and reanalyzing the data in 

individual research to provide the results on a common basis is beyond the scope of this 

report. 

• As emission levels drop significantly, small differences in mass emissions represent very 

large percentages, and changes that are not significant from an air quality perspective 

may appear very large. 

• Vehicle response is known to vary greatly from car to car and from model to model. 

Unless a relatively large number of vehicles were tested, different results are to be 

expected. 

 Changes in the concentration of one fuel component necessarily imply equal changes in one or 

more other components. For instance, when the concentration of aromatics is reduced by 10%, the 

concentration of some other component must be increased by 10%. Usually, paraffins are the swing 

component, so that the actual meaning of “reducing aromatics” is “reducing aromatics and replacing them 

by paraffins.” When the change in concentration is in the parts per million range, as is the case for sulfur, 

the changes in concentration of the other fuel components are not measurable and can be ignored. 

D. CAVEATS 

 Advanced engine technology, such as HCCI (Homogeneous Charge Compression Ignition) 

engines, that is not commercially available was generally not included in this report. Design of these 

technologies is far from fixed, and little if any data has been collected on fuel effects. Data that have 

been collected might not be relevant for future commercial designs. 

 When analyzing the literature, comparing results, and trying to draw generalizations, a number of 

problems were encountered. 
CRC Project E-84, Final Report 

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• Many programs did not report the results of a statistical analysis, nor the statistical 

significance of the findings. 

• Some programs that reported statistics did not present an evaluation of the Type II errors. 

If no significant results were found, there is no way to tell if the test design was powerful 

enough to have seen a relatively large effect as statistically significant. 

• Some programs tested a limited number of vehicles. Given the known variability in 

response among vehicles, it is difficult to make generalizations from a small sample. 

• In some programs, the fuel properties are not independently varied, so it is not possible 

to assign results to fuel properties unambiguously. Many reports recognize this 

uncertainty.. 

• Some research programs were designed to demonstrate the benefits of certain fuel 

formulations or fuel blendstocks, and the changes in emissions could not be assigned to 

any one or group of fuel properties. 

 This report includes all data collected in well designed programs and points out any potential 

uncertainties in the design or the analysis. Even programs for which there is limited ability to draw 

unambiguous conclusions can be used in a broader meta analysis of emissions data. Such a meta 

analysis is essentially what was done in developing various emissions models such as the EPA Complex 

Model and the ARB Predictive Model for gasoline. 

3. GASOLINE 

 Emissions data on gasoline vehicles is organized according to fuel properties. Within each fuel 

properties, the literature is summarized first for the U.S. and then for Europe, Japan and other regions of 

the world. Almost all the data covers passenger cars, other LD vehicles, and some motorcycles and 

other two-wheeled vehicles. 

 Various driving cycles have been used to measure exhaust emissions from LD vehicles. In the 

U.S., the FTP75 has been used for many years, and has been supplemented in recent years by the 

US06, which represents more aggressive, high speed driving, and the SC03 which measures emissions 

under conditions when air conditioning is used. In Europe, the UDC (Urban Driving Cycle) and EUDC 

(Extra-Urban Driving Cycle) measure emissions under a variety of driving conditions including cold start. 

In Japan the steady state 10.15 mode test was used for many years and has been supplanted by the 

JC08 test, which has many similarities to the U.S. and European driving cycles. 

 Not much research has been carried out on the differences in fuel effects when measured on the 

different test cycles. It appears that the results are similar, especially where the test cycle includes cold 

start operation. It is beyond the scope of this report to analyze these differences, and for the purposes of 

this analysis, differences in test cycles have generally not been considered. 

A. SULFUR 

 From an exhaust emissions perspective, sulfur concentration is probably the most important 

gasoline property. Present in trace amounts, sulfur does not contribute to octane or other vehicle 

performance attributes, yet it impacts operation of TWC and other components of modern emission 

control systems. Reducing the amount of sulfur in gasoline requires investment in new equipment and 

additional refinery operating expenses. Reducing sulfur concentrations in the refinery consumes energy 

and hydrogen, production of which can reduce the total amount of hydrocarbons available to blend into 

fuel. Reducing sulfur also increases refinery CO2 emissions and can negatively affect fuel properties 

such as octane. 
CRC Project E-84, Final Report 

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 Prior to AQIRP, initiated in 1989, there was little published research on the impact of sulfur. 

Furey and Monroe[5] studied one 1979 car with a TWC and Williamson et al. [6] studied aging in a 

laboratory setting. AQIRP carried out a number of large, well-designed programs to evaluate sulfur 

effects over a wide range of concentrations in different technology vehicles. 

 In the first AQIRP study, Benson et al. [7] tested ten 1989 vehicles with two fuels containing 49 

ppm and 466 ppm sulfur, with results shown below: 

Change in Emissions 

466 ppm S → 49 ppm S 

 HC CO NOx

g/mi -0.38 -0.362 -0.49 

% -16.1 -12.9 -9.0 

Emission changes were highly significant (>>95% confidence level), although there was considerable carto-car variability in response. The effect was virtually instantaneous – occurring within 10 miles – and 

fully reversible. 

 AQIRP tested these same ten vehicles in two subsequent phases, first with 5 fuels between 44 

and 443 ppm sulfur, and then with 3 fuels varying between 11 and 50 ppm sulfur [8]. The objective was 

to test the linearity of the response and to measure toxics in addition to HC, CO and NOx. Overall results 

are summarized below: 

Percent Change in Emissions For Change in Sulfur 

 450 ppm → 50 ppm 50 ppm → 10 ppm 

NMHC - 17.2 - 6.6 

CO - 18.7 - 10.3 

NOx - 8.0 NS 

Benzene - 20.8 - 11.8 

1,3 Butadiene NS NS 

Formaldehyde + 45 NS 

Acetaldehyde - 34 NS 

NS: Not Statistically Significant 

 There was a suggestion that the NMHC response was non-linear, although the significance level 

was only 85%. Specific reactivity using the MIR (Maximum Incremental Reactivity) scale increased by 

8.4% when sulfur was reduced from 450 to 50 ppm. MIR is a calculated estimate of the amount of ozone 

produced by a compound or mixture of compounds under specific atmospheric conditions. It is used by 

ARB to develop comparisons of the relative ozone formation of emissions from different sources or from 

different fuels.[9] 

 Knepper et al. [10] reported on an AQIRP study with seven high emitters that found no impact of 

sulfur between 40 and 440 ppm on exhaust HC and CO, and a 12% reduction in NOx. These results are 

consistent with vehicles that are running rich. Since the oxidation function of the TWC is severely 

inhibited, changes in sulfur have little impact on HC and CO. While running rich, the catalysts still retain 

the ability to reduce NOx and sulfur does have an impact. 

 As part of a larger program, Takei et al. [11] tested one vehicle calibrated to ARB standards of 

0.25 g NMHC/mi. Both fresh and aged catalysts were tested. There was some suggestion that the sulfur 
CRC Project E-84, Final Report 

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effect was not fully reversible, although the test protocol was not discussed in detail. Results are 

summarized below. 

Percent Reduction in Emissions 

300 ppm → 30 ppm Sulfur 

 HC CO NOx

Fresh Catalyst -33% -15% -37% 

Aged Catalyst -15% -19% -20% 

 Takei et al. [12] also tested a 1992 California Camry (0.25 g NMHC/mi standard) on fuels with low 

sulfur (10 ppm or 20 ppm) and higher sulfur (300 ppm) and reported a significant effect on NMOG, and no 

impact on specific reactivity (g O3/g NMOG), but did not report statistics of these conclusions. No results 

were reported for CO and NOx emissions.

 Hirota et al. [13] tested a 1991 Nissan Sentra on four fuels having sulfur concentrations of 10 ppm 

to 300 ppm and reported that TOG (Total Organic Gases), CO, and NOx decreased, although specific 

reactivity was not affected. No statistics were reported or discussed. 

 EPA [14] tested 20 normal emitters (1987-1990) and 16 high emitters (1986-1990) which were 

recruited at a state inspection station. Results comparing two fuels with 371 ppm and 112 ppm sulfur are 

shown below. These values were calculated from data in the paper. 

 Percent Change in Emissions, 371 ppm → 112 ppm Sulfur 

Fleet NMHC CO NOx Benzene Formaldehyde Acetaldehyde 

Normal Emitters - 4.7 - 8.7 - 7.2 - 6.5 + 11.8 + 3.7 

High Emitters - 5.2 + 1.8 - 7.7 + 0.4 + 2.6 - 12.4 

 Considering the smaller sulfur change, these results appear to be consistent with AQIRP normal 

emitters, although there appear to be differences with the AQIRP high emitter results. In the report, 

statistical analysis was presented for a different comparison, so it is not possible to state the statistics for 

the results presented here. 

 In an effort to evaluate the EPA Complex Model, EPA tested ten vehicles (1983-1990 model year) 

on three fuels having sulfur levels of 67, 338 and 685 ppm, and compared the results to the predictions of 

the Complex Model [15]. Significance was evaluated at the 90% confidence level. NMHC reductions 

were substantially larger than predicted by the Complex Model, and NOx reductions were smaller. 

Results are shown below: 

 Percent Change in Emissions for Sulfur Reduction 

 NMHC NOx

 61 → 338 ppm 61 → 685 ppm 61 → 338 ppm 61 → 685 ppm

Testing + 6.7 + 8.9 + 3.6 (NS) + 6.4 

CM Prediction + 5.2 + 12.0 + 10.0 + 14.7 

 Takei et al. [16] tested a 1993 TLEV (Transition Low Emissions Vehicle) and a 1994 TLEV with 

three fuels having sulfur levels of 32, 300 and 500 ppm. The 1994 TLEV was tested with two catalysts 

systems. One catalyst had Pt/Rh noble metal in both underfloor and close-coupled locations. The 

second one had a Pd based catalyst in the close-coupled location. When changing fuels from 300 ppm to 

30 ppm, emissions did not appear to return to baseline. The authors speculated that the 1994 model year 
CRC Project E-84, Final Report 

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vehicle had a leaner calibration than a previously tested 1992 Camry that showed complete reversibility. 

They also accumulated 10,000 miles on two test fuels – 30 ppm and 500 ppm on the 1993 TLEV. FTP 

testing was done with the 30 ppm fuel. No preconditioning was carried out except for the normal LA4s 

used before each FTP. Emissions of NMHC and CO were higher for the catalyst run on 500 ppm, and 

the difference maintained itself over 4 or 5 FTP tests. There was no difference in NOx emissions. 

 In the final AQIRP study, Rutherford et al. [17] reported on testing eight fuels, four of which 

represented two levels of sulfur, nominally 35 ppm and 320 ppm. Ten 1989 LD vehicles and six Federal 

Tier 1 vehicles (0.25 g NMHC/mi) were tested. Results are shown below for regulated and toxics 

emissions. Specific reactivity was calculated and increased as sulfur was reduced, by 2.9% in the 1989 

fleet and by 9.3% in the Tier 1 fleet. The results shown here are generally consistent with earlier AQIRP 

work, and there does not appear to be a major difference between the 1989 fleet and the Tier 1 vehicle 

fleet, when expressed as percentage differences. 

Percent change in Emissions 

320 ppm → 35 ppm Sulfur 

 1989 LDV Fed. Tier 1 LDV 

 

NMHC - 13.3 - 18.5 

CO - 9.8 - 16.4 

NOx - 4.0 (NS) - 8.9 

Benzene - 17.8 - 24.5 

1,3 Butadiene - 8.5 -12.1 

Formaldehyde + 27.2 - 5.2 (NS) 

Acetaldehyde + 2.9 (NS) - 0.9 (NS) 

 Sztenderowicz et al. [18] reported on a program carried out by the Petroleum Environmental 

Research Forum (PERF) on ten 1993/94 vehicles with three fuels having sulfur levels of 25, 300 and 600 

ppm. Nine vehicles were certified to California TLEV standards, and one was certified to Federal Tier 1 

standards. Results were compared to the 300 ppm fuel and are shown below. 

Percent Change in Emissions 

300 ppm → 25 ppm Sulfur 

 HC CO NOx

Ten 1993/94 LDVs - 24% - 21% - 12% (NS) 

 There appeared to be a non-linear response to sulfur, with a greater impact between 25 ppm and 

300 ppm than between 300 ppm and 600 ppm. None of the changes between 300 and 600 was 

statistically significant, while the NMHC and CO changes between 300 and 25 ppm were statistically 

significant. The results were compared to those measured in AQIRP, and it was found that the effects 

measured on a mass basis were somewhat smaller, in part because the overall emissions levels were 

significantly less. Sulfur effects expressed on a percentage basis were similar or somewhat larger, again 

probably reflecting the lower overall emission levels. 

 Takei et al. [19] tested two vehicles with two fuels having sulfur levels of 30 and 300 ppm. One 

was a 1992 model year California vehicle was certified to Tier 1 standards and was tested at 50,000 

miles, and the other was a 1994 model year California vehicle certified to LEV standards and was tested 

at 100,000 miles. NMHC, CO and NOx were reduced by 12%, 14% and 18%, respectively for the Tier 1 

car, and by 21%, 32% and 25% for the LEV car. No statistical significance levels were reported for these 
CRC Project E-84, Final Report 

13

results. The authors also reported that emission levels did not return to baseline levels after operating on 

300 ppm fuel. The effect seemed to be larger for the LEV car, although no statistical analysis was 

presented. 

 Schleyer et al. [20] reported on a program carried out by CRC which measured the impact of 

sulfur in twelve California LD vehicles certified to LEV standards. Four conventional fuels (30 ppm to 630 

ppm) and two California fuels (27 ppm and 148 ppm) were tested. A special preconditioning cycle 

developed in the European Programme for Emissions, Fuels and Engine Technologies (EPEFE) was 

used to remove any issues of fuel carry-over or irreversibility from the results. Results are shown below 

for the fleet averages on the conventional fuels at 100,000 miles: 

Change in Emissions 

630 ppm → 30 ppm Sulfur 

 NMHC CO NOx

grams/mile - 0.03 - 0.70 - 0.23 

Percent - 32 % - 46 % - 61 % 

 There was a wide range in the responses of individual vehicle models, and those with the lowest 

emission levels tended to have the smallest response to sulfur. The response to sulfur concentration for 

the 100,000 mile aged catalysts was non-linear for all three pollutants measured, with a larger response 

at low sulfur levels. The authors also compared results for similar sulfur ranges across four large test 

programs that evaluated technologies ranging from Tier 0 to LEV. When the fleet average impacts were 

expressed in mass terms, the sulfur effect was similar for all fleets, with the possible exception of NOx in 

the LEV fleet. However, when expressed as percentages, the effects were larger as base level emissions 

were reduced with newer technology. 

 Takei et al. [21] tested four stoichiometric gasoline vehicles (LEV, ULEV, SULEV) on fuels 

ranging from near zero to 600 ppm, although the ranges were different for each vehicle. In addition, two 

Japanese direct injection vehicles with NOx Storage Reduction catalysts were tested For the 

stoichiometric vehicles, all the vehicles had a response to sulfur, but the SULEV had a the largest 

response on a relative basis, similar to findings in other programs. The authors found that even at 30 

ppm sulfur, the NSR systems had degraded performance in NOx conversion, NOx storage capacity and 

reversibility. 

 Lyons et al. [22] tested eight 1997/98 model year California vehicles (Tier 1, LEV and ULEV) on 

two fuels with sulfur levels of 40 and 540 ppm. FTP and US06 driving cycles were used, and special 

attention was devoted to reversibility and variations in sulfur response between vehicles. The authors 

concluded that there was a high degree of variability in responses to sulfur, and carried out regression 

analyses to determine what design factors contributed to the differences. The strong correlations 

between design factors did not allow them to draw specific conclusions; however, given appropriate 

resources, experiments could be designed and carried out to develop additional insight into this question. 

 In 2001, the automotive industry in the U.S. reported on a test program carried out on 13 

prototype California TLEVs, LEVs and ULEVs [23]. Three levels of sulfur were tested, 1, 30 and 100 

ppm. No details of the statistical analysis were provided, but the results below appear to be statistically 

significant. Averages quoted in the presentation appear to be geometric averages of the fleet emissions. 

Change in Emissions 

100 ppm → 1 ppm Sulfur 

 NMHC CO NOx

g/mi - 0.017 - 0.642 - 0.059 

Percent - 27% - 42% - 39% 
CRC Project E-84, Final Report 

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Responses are generally linear, although there may be a slight curvature, such that the effect of sulfur is 

less at extremely low values. The authors postulated that this might be due to the effect of sulfur in the 

lubricant. 

 The most recently published study carried out on a relatively large fleet of U.S. vehicles was CRC 

Project E-60 [24], which tested twelve California vehicles certified to LEV and SULEV standards, and two 

European vehicles certified to Euro 3 standards. All vehicles were tested with three fuels (5, 30 and 150 

ppm sulfur) with as-received and aged catalysts. The objective was to measure ammonia emissions, but 

regulated emissions were measured as well. All vehicles were tested using the US FTP and US06 

cycles; the European vehicles were tested over the NEDC (New European Driving Cycle) as well. For the 

California vehicles, there was a small but significant effect of higher sulfur on NOx emissions, and no 

effect on NMHC and CO emissions. Differences were larger on the US06 cycle, and were statistically 

significant for NMHC, CO and NOx. A summary of the results is shown below: 

 Change in Emissions, 150 ppm → 5 ppm Sulfur 

 NMHC CO NOx NH3 N2O 

FTP Test Cycle 

g/mile -0.001 (NS) -0.045 (NS) - 0.022 - 0.003 (NS) - 0.011 

Percent - 2.2 - 6.4 - 31.0 - 15.8 (NS) - 64.7 

US06 Test Cycle 

g/mile - 0.025 - 0.569 - 0.102 - 0.020 - 0.010 

Percent - 64.1 - 10.2 - 70.8 - 21.3 - 90.9 

 In Europe, Morgan et al. [25] carried out a small program on two European cars and sulfur levels 

between 11 ppm and 116 ppm. They used both the European ECE (Economic Commission for Europe) 

and the US FTP test cycles, and concluded that reducing sulfur reduced HC, CO and NOx. No statistical 

analysis was reported and details of the vehicles were minimal. 

 The EPEFE program carried out a large study on the sulfur response in European vehicles [26]. 

Four fuels ranging in sulfur concentration from 18 ppm to 382 ppm were tested in sixteen European 

vehicles, all fitted with TWC, and either single point or multi-point injection (MPI). Results are 

summarized below for the combined ECE/EUDC cycle, known collectively as MVEG 2000. The ECE 

cycle is a cold start urban driving cycle and EUDC is a warmed-up cycle. In the cold start portion, 

emissions are measured starting from key-on, rather than after a warm-up period as in earlier test cycles. 

Sulfur effects are generally larger on percentage basis after the catalysts have reached operating 

temperatures. 

Change in Emissions 

382 ppm → 18 ppm Sulfur 

 HC CO NOx

g/km - 0.015 - 0.113 - 0.019 

Percent - 8.6 - 9.0 - 10.4 

The results were generally linear over the range studied, although there was some suggestion of nonlinearity for HC and CO emissions. Compared to AQIRP, the relative effects seen in this program are 

smaller for HC and CO and similar for NOx. 

 Some measurements of toxics were made but no statistical analysis could be carried out. 

Nevertheless, lower sulfur appeared to lower benzene and had no noticeable effect on 1,3 butadiene, 
CRC Project E-84, Final Report 

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formaldehyde and acetaldehyde. An analysis of vehicle to vehicle differences suggested that palladium 

based catalysts were more sensitive than platinum/rhodium catalysts. Close coupled catalysts appeared 

to be more sensitive to sulfur, especially for NOx, over the EUDC cycle. 

 Kwon et al. [27] tested a direct injection engine calibrated to meet Euro II emission limits. The 

vehicle was equipped with a lean NOx catalyst system. Two sulfur levels (32 and 138 ppm) were tested. 

No significant differences were seen between the two fuels even at 90% confidence level for HC, CO, 

NOx or particulates. 

 Stradling et al. [28] reported on a CONCAWE study of four advanced gasoline vehicles available 

in the European market in 2002. Three were direct injection and one had MPI. Two cars met Euro 3 

standards and two met Euro 4 standards. Four level of sulfur were tested, covering the range 4 ppm to 

148 ppm. The test was designed to see differences of at least 7% for each pollutant measured. All four 

vehicles showed little or no sensitivity to fuel sulfur. No effects were statistically significant for the entire 

NEDC driving cycle, but a few responses were statistically significant on the EUDC portion. The HC 

response was positive for two of four cars the CO response was positive for two cars and negative for 

one car. 

 Akimoto et al. [29] studied three sulfur levels (8, 73, 140 ppm) in one Japanese 1993 model year 

car with MPI and a TWC, using the 10.15 mode emissions test cycle. Reducing sulfur lowered emissions 

of HC, CO, NOx and benzene. The reduction in NOx seemed to be the highest on a relative basis. No 

statistics were reported, so it is difficult to assess the significance of these results on one vehicle. 

 Koseki et al. [30] measured sulfur response in six late model Japanese vehicles – three 

stoichiometric vehicles with TWC, one lean-burn with NOx storage catalyst, and two direct injection 

engines with different NOx reduction catalysts. Each vehicle was tested on fuels varying between 3 ppm 

and 80 ppm or between 4 ppm and 90 ppm. The 10.15 mode Japanese test cycle was used for all the 

tests. The authors concluded that the lean-burn engines exhibited a smaller response to sulfur than the 

stoichiometric engines, although no statistical analysis was presented. 

 In 2000, Hamasaki et al. [31] presented results of the Japanese Clean Air Program (JCAP), which 

compared two fuels (96 ppm and 22 ppm) in eleven late model year vehicles intended to approximate the 

current and future Japanese fleet. Two test cycles were used, the 10.15 mode test and the cold start 11 

mode test. The authors found that reducing sulfur lowered emissions of THC, CO and NOx in both test 

cycles, although the effect was larger in the hot start 10.15 mode test, when the catalysts were operating 

a higher fraction of the time. The relative effects appeared to be higher for the low emitting and lean-burn 

vehicles, because there were fewer pollutants emitted before catalyst light-off. No statistical analyses 

were presented for the results. 

 In Phase II of the JCAP program [32], one MPI vehicle with a TWC and three SIDI (Spark Ignition 

Direct Injection) vehicles with NOx reduction catalysts were tested on sulfur levels of 2, 22 and 86 ppm. 

The MPI vehicle had a smaller response to sulfur and under some test conditions did not respond at all. 

The SIDI vehicles had a stronger response, especially on the 10.15 mode test, for NOx and CO. All 

vehicles showed a flat response of THC to sulfur changes. The SIDI vehicles exceeded the emission 

target at the highest sulfur level, but satisfied the target at the lowest level. 

 Thummadetsak et al. [33] tested six cars representative of the Thai market (model years 1993-

96) with fuels having sulfur levels of 50, 100 and 300 ppm. Four cars had TWC, one had an oxidation 

catalyst and one was a non-catalyst car. For the catalyst equipped vehicles, reducing sulfur from 300 

ppm to 50 ppm lowered THC by 7.6% and acetaldehyde by 28%. Changes in CO, NOx, benzene, 1,3 

butadiene and formaldehyde were not significant. 

REVERSIBILITY AND AGING

 There have been a number of studies, some discussed above, that measured reversibility of the 

sulfur effect. Most of the earlier studies on sulfur effects found that reversibility was complete and 

occurred with a short mileage interval. 
CRC Project E-84, Final Report 

16

 Bjordal et al. [34] studied the effects of catalyst aging on one 1992 European car with two 

different catalyst systems. One was aged on gasoline with 50 ppm sulfur and one on fuel with 450 ppm 

sulfur. Aging was carried out on a test bed using the European EUDC and emissions testing used the 

European Test Cycle which measures emissions from key on. There was no consistent difference in 

NMHC emissions between the catalyst aged on low sulfur fuel and the catalysts aged on high sulfur fuel. 

For CO, aging on high sulfur fuel gave higher emissions at 80,000 km when tested on high sulfur fuel, but 

not at 40,000 km for either high or low sulfur fuel. NOx emissions actually increased more when aged on 

low sulfur fuel, than when aged on high sulfur fuel. Some of the differences were tied to differences in the 

lambda sensors, although it was not known whether these differences were related to the different sulfur 

levels or to subtle manufacturing differences. The results of this test, and the difficulty explaining them, 

show the problems of testing a small number of vehicles and the advantages of larger fleets for this type 

of work. 

 Schleyer et al. [35] reported on a CRC program in which two sulfur levels (30 ppm and 630 ppm) 

were tested over a variety of cycles and the reversibility of the sulfur effect was assessed. Six of the 

twelve vehicles tested in the CRC Sulfur/LEV program [20] were used. Full reversibility was achieved 

when emissions returned to the baseline level achieved with 30 ppm fuel after running on 630 ppm fuel. 

For the entire fleet, NMHC emissions were fully reversible with either the mild LA-4 driving cycle, or the 

more aggressive US06 driving cycle. CO emissions were fully reversible with the US06 cycle, but only 

79% reversible with the LA4 cycle. NOx emissions were 84% reversible with the LA4 cycle and 95% 

reversible with the US06 cycle. Where less than full reversibility is shown, the result is statistically 

significant at the 95% confidence level. In most cases, emissions recovery stabilized within 10 miles of 

driving. 

 Lyons et al. [22] tested eight 1997/98 model year California vehicles (Tier 1, LEV and ULEV) on 

two fuels with sulfur levels of 40 ppm and 540 ppm. FTP and US06 driving cycles were used, and special 

attention was devoted to reversibility and variations in sulfur response between vehicles. With respect to 

reversibility, they concluded, “the effects of sulfur were fully reversible, although in some cases, more 

severe operating conditions such as those encountered in the US06 cycle were required.” 

REVIEW PAPERS

 A number of review papers summarizing and comparing major studies were published in the 

technical literature. Truex [36] evaluated laboratory and vehicle test programs in an effort to understand 

the mechanisms of sulfur’s impact on emissions in gasoline vehicles. Truex reviewed work on noble 

metal loadings, base metal interactions and temperature effects. He also reviewed the then published 

fleet tests. Truex was not particularly sanguine that catalyst systems could be designed without 

sensitivity to sulfur concentrations, but felt that it might be possible to reduce sulfur sensitivity. 

 Hochhauser et al. [37] recently published an analysis of sulfur data in fleets with TWC covering 

seven published studies and one unpublished study. The newest fleets, represented by the CRC E-60 

program, and the CONCAWE program on 2003 vehicles, exhibited relatively low responses to sulfur, less 

sensitive than data from model years 1997 and 1999. The data were analyzed on an absolute basis, and 

a comparison on a percentage basis would not show the same large differences. Shown below are the 

responses for NOx, for the published studies, as presented: 
CRC Project E-84, Final Report 

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Emission 

Control Level Program Sponsor 

NOx Reduction, g/km 

150 ppm → 50 ppm 

US Tier 0 AQIRP 0.004 

US Tier 1 AQIRP 0.006 

Euro 2 EPEFE 0.005 

California LEV CRC 0.040 

California LEV AAM/AIAM 0.050 

Euro 3/4 CONCAWE 0.001 

California LEV CRC 0.013 

 In developing the latest version of the ARB Predictive Model, Uihlein also carried out an analysis 

of previous work, and presented it at a public meeting organized by ARB as part of their effort to update 

the Predictive Model [38]. The table below compares the relative responses at ultra low sulfur levels from 

various large programs on late model vehicles included in the ARB database. His analysis showed that 

at these low levels, results were very similar 

Emission 

Control Level Program Sponsor 

NOx Reduction, % 

20 ppm → 10 ppm 

California LEV CRC - 3% 

California LEV AAM/AIAM - 5% 

California LEV AAMA - 5% 

California LEV CRC - 6% 

PARTICULATES

 Particulate emissions as a function of fuel sulfur were studied in a number of programs carried 

out in Europe. Mohr et al. [39] tested one European gasoline car with two fuels having sulfur levels of 

<10 ppm and 175 ppm and measured particulate emissions using a number of different instruments and 

dilution techniques. Emissions were so low with the gasoline vehicle that it was difficult to draw 

meaningful conclusions. However, the authors measured higher numbers of particles with the higher 

sulfur fuel measured by the CPC (Condensation Particle Counter) under certain test conditions. 

 Ntziachristos et al. [40] tested eleven gasoline vehicles (6 MPI and 5 DI) with three fuels having 

sulfur levels of 6, 45 and 143 ppm. Differences in PM emissions due to fuel sulfur could not be seen 

because of the low overall emission rates. 

 It appears that the impact of sulfur on PM emissions from gasoline vehicles, while theoretically 

possible, is too small to be seen with today’s instruments and test protocols. 

SULFUR SUMMARY

 Reducing sulfur lowers emissions of HC, CO and NOx. Although there is some conflicting data, 

the effects appear to be linear, especially at levels of sulfur below about 150 ppm. Reducing sulfur also 

lowers emissions of the air toxics benzene and 1,3 butadiene, and may increase emissions of 

formaldehyde. The impact on acetaldehyde is less clear. In any case, there is not much recent data on 

toxics effects. 

 In 1989 vehicles, the sulfur effect was shown to be totally reversible under mild driving conditions. 

More recent testing has shown that, in the short term, reversibility is complete only under extreme driving 
CRC Project E-84, Final Report 

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conditions. This is likely the result of tight control of engine conditions and temperatures in newer 

technology, and possibly changes in catalyst formulation. 

 PM levels are generally low in gasoline vehicles; while sulfur probably has an impact, it is 

certainly low relative to diesel vehicles, and probably not of regulatory concern. 

 Research efforts have not been totally successful in developing an understanding of the ways to 

design emission control systems that are insensitive to sulfur. 

 Considering the low sulfur levels present in the U.S., Europe and Japan, and the extensive 

database available on sulfur effects, additional research is not warranted at this time. Changes in 

gasoline technology, or increased emphasis on particulate emissions from gasoline vehicles, might spur 

additional research. 

B. AROMATICS AND BENZENE 

 Aromatics compounds are based on six carbon ring structures, the simplest of which is benzene. 

Higher molecular weight aromatics generally have one or two paraffinic or olefinic sidechains. Aromatics 

composed of multiple rings, are generically described as multi-ring aromatics, polyaromatic hydrocarbons 

(PAH) or polynuclear aromatics (PNA). PAHs have high molecular weight and are present in gasoline at 

low concentrations, if at all. All aromatics have high octane and high energy density. Their boiling points 

are higher than the average boiling point of other gasoline components. 

 When considering aromatics, it is important to recognize that the contribution of individual 

aromatic compounds to emissions performance may differ. Higher molecular weight compounds may 

have a different impact on emissions than those with lower molecular weight. Sometimes carbon number 

is used as a surrogate for molecular weight. Benzene, the simplest and lowest molecular weight 

aromatic, is usually considered separately, because it is a toxic compound and has a strong effect on 

benzene exhaust and evaporative emissions. The benzene concentration of gasoline is also controlled 

by regulation. 

 In the 1980s, average aromatic levels in U.S. gasoline were 32%. There was some evidence that 

aromatics contributed to high emissions and emissions of benzene. AQIRP carried out a large study to 

measure the impact of aromatics on exhaust emissions. Hochhauser et al. [41] reported on AQIRP work 

that measured the effect of aromatic levels between 20% and 45% in twenty 1989 vehicles. (Olefins, 

MTBE and T90 were also studied, and these will be described later.) The effects of this change on fleet 

average emissions are summarized below: 

Percent Change in Emissions 

45% Aromatics → 20% Aromatics 

NMHC CO NOx

- 11.6 - 13.3 + 2.0 

All changes shown above were statistically significant (95% CL), NOx just barely. It was interesting that 

for engine-out emissions, reducing aromatics reduced NOx, the opposite of the effect seen in tailpipe 

emissions. Later studies tried to understand this effect which apparently affects the catalysts. Also noted 

was that the effect on total HC was much smaller that the effect shown above for NMHC. It was shown 

by Hochhauser et al. [42] that reducing aromatics (and consequently increasing paraffins) contributed to 

higher methane emissions and lower NMHC emission rates and that reducing aromatics did not impact 

the specific reactivity of the exhaust (g O3/g HC, MIR scale). 

 There were a number of interactions with other properties. An interaction with MTBE suggested 

that lowering aromatics increased NOx emissions to a greater extent when MTBE was present than when 

it was absent. Also, an interaction with T90 suggested that the heavier molecular weight aromatics were 

more important than the lower molecular weight aromatics in affecting NMHC and CO emissions. 
CRC Project E-84, Final Report 

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 Gorse et al. [43] reported result of toxics measurements from the same experiment and the 

results are shown below: 

Percent Change in Emissions 

45% Aromatics → 20% Aromatics 

Benzene 1,3 Butadiene Formaldehyde Acetaldehyde 

- 41.8 + 11.3 + 23.5 + 19.5 

 The large reduction in exhaust benzene was expected since aromatics can decompose under 

combustion conditions to produce benzene in the exhaust. No attempt was made to explain the 

increases in the other toxic emissions in this experiment. The increase in formaldehyde might be related 

to the increase in exhaust methane seen when aromatics are reduced and are replaced by paraffins, as 

described below. 

 Marshall and Gurney [44] evaluated the impact of individual aromatic compounds on exhaust 

benzene emissions in five 1986 Model Year cars with fifteen fuels and found that fuel benzene had the 

highest impact on exhaust benzene. Other aromatics tested – toluene, ethyl benzene, xylenes and heavy 

reformate (C9+) – had an average effect of about 8% of benzene. 

 API conducted a study on the impact of aromatics as a function of carbon number in ten 1989-90 

model year cars [45]. This effectively allowed them to separate the effects of aromatics and volatility. 

Two sets of fuels were blended. In the first, certain aromatics were removed and replaced by a mix of 

other aromatics of different carbon numbers to keep the total aromatic level constant. In the second set, 

certain aromatics were removed and replaced by equivalent iso-paraffins, effectively controlling distillation 

properties. With the first set of fuels, as the average carbon number of the aromatic compounds became 

heavier, exhaust emissions of NMHC, CO and NOx increased. The effect on NOx was classified as 

borderline significant (0.95>p>0.90). However, when the distillation properties were held constant, shifts 

in the distribution of aromatic carbon content had no effect on exhaust emissions. This implies that what 

had been measured as an aromatic effect was really an effect of distillation properties. 

 Hirota et al. [13] tested two 1991 Nissan Sentras and concluded that reducing aromatics from 

33% to 20% reduced NMOG and aromatics in the exhaust, but did not affect specific reactivity. 

 Takei et al. [16] tested a 1994 Camry TLEV over three aromatics levels, 10%, 29% and 39%. 

Reducing aromatics lowered CO, especially below about 20% aromatics, but had no effect on HC or NOx. 

They did not see the HC effect seen in the AQIRP study, and speculated that lack of effect was because 

they did not vary heavier aromatics, but adjusted aromatic content by adding toluene. Heavier aromatics 

(C9+) are thought to have a larger impact on emissions. 

 Morgan et al. [25] tested 27 cars from Japan, Europe and Canada from model year 1989-91. 

Eight cars had no catalysts, and the others had TWC systems. Six cars were carbureted, three had 

throttle body injection and the others had MPI metering systems. Five fuels were tested on all cars, and 

reducing aromatics seemed to reduce exhaust emissions of HC and CO. However, the authors realized 

that there was a significant correlation between aromatics concentration and T50 levels, so they could not 

state this conclusion with certainty. No statistical analysis was shown to support the conclusions. 

 Jeffrey and Elliott [46] tested five European vehicles with 29 fuels, some of which were from the 

AQIRP study. For the catalysts vehicles tested, they found that aromatics had a significant effect on 

exhaust emissions. As noted below, reducing aromatics lowered HC and CO and increased NOx. 

Percent Change in Emissions 

45% Aromatics → 20% Aromatics 

HC CO NOx

- 10.1 - 12.9 + 10.6 
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These results are very similar to those reported for AQIRP. Similarly, reducing aromatics reduced 

benzene emissions, but increased emissions of the other toxics, 1,3 Butadiene, formaldehyde and 

acetaldehyde. 

 Wedekind et al. [47] studied the effect of aromatics and volatility on emissions in one catalyst 

equipped European car over the European driving cycle (ECE+EUDC). In eleven fuels, aromatics, T50

and T90 were varied independently. As in other programs, reducing aromatics lowered HC and CO and 

increased NOx emissions. Emissions of toxics were also measured. Reducing aromatics lowered 

benzene emissions and raised emissions of 1,3 butadiene. The authors attribute the latter effect to an 

increase in paraffins that occurs when aromatics are lowered. 

 The EPEFE program studied aromatics and mid-fill volatility in sixteen European vehicles with 

nine fuels [48]. The vehicles were prototypes intending to meet the 1996 emissions limits. The year 2000 

driving cycle (ECE+EUDC) was used. A full regression analysis revealed that reducing aromatics 

lowered HC and CO but increased NOx emissions. There were significant interactions between aromatics 

and E100 for HC and NOx. These results are fully consistent with other programs and also showed, as in 

previous studies, that reducing aromatics lowered engine-out NOx. NOx catalyst efficiency dropped for 

the low aromatic fuel and tailpipe NOx emissions increased. Reducing aromatics lowered benzene 

emissions, but increased emissions of 1,3 butadiene, and aldehydes. 

Percent change in Emissions 

50% Aromatics→ 20% Aromatics 

E100 Level 

(Mid-Fill Volatility) HC CO NOx

35 -31.7 - 16.8 - 15.0 

50 - 10.9 - 18.5 - 8.3 

65 - 10.2 - 17.4 - 2.7 

 Bennett et al. [49] tested three Euro 1 and three Euro 2 vehicles with a number of fuels, including 

two matched fuels having aromatics levels of 41.5% and 27.5%. In contrast to results from other 

programs, they did not see an impact on HC or NOx, and CO emissions increased when aromatics were 

reduced. McDonald et al. [50] tested 7 fuels in 12 TWC equipped European vehicles (1990-93 model 

year) and did see the expected results; i.e. reducing aromatics lowered emissions of HC and CO and 

increased NOx emissions. 

 CONCAWE analyzed data from European companies and an Italian industry program on the 

relationship between benzene and aromatics in the fuel and exhaust benzene emissions [51]. Emissions 

data from 21 non-catalyst cars and 34 catalyst cars were used. Data showed that fuel benzene had the 

largest influence on exhaust benzene and that the impact of NBA (non-benzene aromatics) was about 

90% less. A comparison to the EPA Simple and Complex Models was developed as shown below. The 

ratios are similar and the authors concluded that similar mechanisms are at work in all tests. 
CRC Project E-84, Final Report 

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Relative Contribution of Benzene/NBA to 

Exhaust Benzene Emissions 

CONCAWE 

(Catalyst cars) 

EPA Simple 

Model 

EPA Complex 

Model 

Percent 

Benzene 7.8 8.4 

Mass 

Benzene 10.0 8.4 * 

* Based on normal emitters and total aromatics, not NBA 

 The question of the aromatic effect on NOx emissions was investigated by Le Jeune et al. [52] in 

two European vehicles with six full boiling range fuels and 23 fuels blended from individual chemicals 

such as toluene and iso-octane. The vehicles were tested over the European emissions cycle consisting 

of the ECE15 and EUDC driving cycles. Both vehicles exhibited an increase in engine-out NOx emissions 

when aromatics increased. This effect is consistent with the higher energy content and higher flame 

temperatures of aromatic compounds versus paraffinic compounds. Both vehicles also exhibited an 

increase in catalyst efficiency for NOx reduction when aromatics increased, presumably due to the higher 

concentration of aromatics in the exhaust. In one vehicle, which had relatively loose control of A/F 

(Air/Fuel) ratio, the increased efficiency was smaller and did not overcome the higher engine-out 

emissions. The other vehicle had tight control of A/F ratio, a bigger increase in catalyst efficiency, and 

consequently lower tailpipe NOx emissions. These data and conclusions explain the results seen in 

AQIRP and other studies. 

 Van den Brink and McDonald [53] demonstrated in a laboratory reactor that aromatics in the 

exhaust stream contribute to enhanced NOx reduction. Methane and ethane, produced in larger 

quantities with low aromatic/high paraffin fuels, were very unreactive. There were also indications that 

aromatic fuels caused a slight shift in the A/F ratio toward rich mixture. Replacing aromatics with 

paraffins or olefins would increase methane in the exhaust gas which would reduce catalytic NOx activity, 

and would shift the stoichiometry toward the lean side, which would also tend to reduce NOx efficiency. 

 Graupner et al. [54] investigated the NOx/aromatics effect in a bench engine at steady state and 

in a TWC equipped vehicle with a number of carefully blended fuels. Detailed studies showed that 

reducing aromatics in the gasoline increased the amount of hydrogen in the exhaust, and the presence of 

higher levels of hydrogen changed the response of the lambda sensor, causing the engine to shift to a 

slightly leaner stoichiometry. This occurred only when the engine, catalyst and sensors were warmed-up, 

showing that the sensitivity of a vehicle to aromatics levels depends on the calibration of the vehicle. The 

authors also reasoned that using a lambda sensor in a post-catalyst position, where there is little if any 

hydrogen, would mitigate this effect. They activated the feedback control of A/F ratio by using the rear 

oxygen sensor found on OBDII systems, and found that the aromatics/NOx effect was virtually eliminated. 

 The Japanese Clean Air Program (JCAP) studied two levels of aromatics (37% and 22%) in a 

fleet of nineteen vehicles, targeting advanced and future technologies [31]. Among the vehicles were two 

lean-burn GDI vehicles and two lean-burn port injection vehicles. Two test cycles were used, the 10.15 

mode and the 11 mode. It was reported that reducing aromatics lowered HC and CO emissions and 

increased NOx emissions in the 10.15 mode test. In the 11 mode test, reducing aromatics had a different 

impact reducing THC and NOx, and increasing CO. No statistics were provided, and there was no 

discussion about why the responses were different for the two test cycles. 

 Most of the test work on aromatics in the past ten years has focused on direct injection gasoline 

engines. Since these have been more common outside the U.S., most of the published data has come 

from Japan and Europe. Direct injection gasoline engines can operate in a stoichiometric mode or in a 

lean mode. In stoichiometric mode, fuel responses might be similar to port injected engines. In lean 

mode, fuel responses are less predictable, since the combustion regimes are different. Also, in lean 
CRC Project E-84, Final Report 

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mode, control of NOx emissions is similar to diesel engines and if catalytic control of NOx is required, the 

standard TWC would not be appropriate. 

 Kwon et al. [27] studied one lean-burn GDI car, calibrated to meet Euro 2 standards, using the 

Euro 2 test cycle. Two aromatic levels were compared, 40% and 25%, and no statistically significant 

differences were seen in HC, CO, NOx or PM emissions. 

 Stradling et al. [28] tested three GDI cars (two lean-burn and one stoichiometric) complying with 

Euro 3 and Euro 4 emission limits. Emissions control systems for the lean-burn engines consisted of TWC 

and NOx traps. Aromatic levels varied between 26% and 38% in a half-factorial design. Results were 

reported for each of the cars individually, probably because of the significant technology differences. 

Reducing aromatics reduced HC emissions in both lean-burn cars, but not in the stoichiometric GDI car. 

Reducing aromatics also reduced NOx in one of the lean GDI cars and had no effect in the others. The 

test design had good statistical power and would most probably have detected a difference in emissions 

of 7% or greater. 

 In the second phase of JCAP [32], three aromatics levels (8%, 30% and 50%) were tested in 

three lean-burn GDI cars and one port injected stoichiometric car. The impact of reduced aromatics was 

not consistent among the cars, and was not consistent with results reported in the literature for port 

injected vehicles. The authors speculated that the responses were determined by details of the engine 

map that controls how injection timing and duration is impacted by changes in fuel density. 

 Sogawa et al. [55] measured PM emissions at two levels of aromatics (46% and 40%) in a leanburn GDI 1993 model year Japanese car. No differences were reported, possibly because of the 

relatively close aromatic levels, and possibly because the PM levels were so low that the test and 

measurement variances were too high to see differences. 

 Hirose et al. [56] tested one 2005 model year Japanese lean-burn GDI car in 9 different 

gasolines, primarily to test the impact of oxygenates on PM emissions. However, aromatics also varied 

somewhat and a regression analysis showed that aromatics were a large contributor to PM number 

emissions in both the 10.15 mode test cycle and the 11 mode test cycle. 

 Thummadetsak et al. [33] tested six vehicles (1993-96 model year) representative of the fleet in 

Thailand. Four had TWC, one had an oxidation catalyst and one was a non-catalyst carbureted vehicle. 

Three levels of aromatics were tested – 20%, 35% and 45%. Results for the catalyst vehicles are shown 

below: 

Percent change in Emissions, 45% Aromatics → 20 % aromatics 

HC CO NOx Benzene 1,3 Butadiene Formaldehyde Acetaldehyde

-7.5 - 5.96 

(NS) - 14.2 - 36.3 + 1.3 (NS) + 12.6 (NS) + 19.5 (NS) 

Consistent with other programs, reducing aromatics lowered HC emissions. However, NOx emissions 

were reduced also, by an amount more in line with engine-out emissions. It is possible that the: 

• factors described by other researchers for increasing NOx at lower aromatic levels may 

not be present in the Thai marketplace, 

• vehicles are calibrated for a higher aromatics level, 

• lambda sensors are not as sensitive to the aromatic and hydrogen content of the 

exhaust, and 

• control of stoichiometry is not as tight as it is on some of the vehicles tested in other 

programs. 

 A number of studies have examined fuel effects in European 2-wheelers such as motorcycles and 

mopeds. Prati et al. [57] measured emissions from sixteen in-use mopeds and six motorcycles over two 

driving cycles, ECE 47 and ECE 40. In both cycles, emissions are sampled after engine warm-up. All the 
CRC Project E-84, Final Report 

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vehicles had 2-stroke engines and none had catalytic aftertreatment devices. Two fuels were used, 

differing primarily in benzene (1% and 0.7%) and aromatics (28% and 33%), although E100 was also 

different (48% and 58%). The authors concluded that the lower benzene/lower aromatic fuel had lower 

benzene emissions, and that HC, CO and NOx emissions were unaffected by fuel quality. No statistical 

analysis was presented. 

 Jain et al. [58] tested twelve 2-wheelers on three gasolines varying in aromatics (10% to 62%) 

olefins (1% to 26%) and benzene (0.3% to 1.5%). Benzene and aromatics levels were correlated. The 2-

wheelers were 2000-2004 model year Indian vehicles, both new and in-use. Three had 4-stroke engines 

and three were 2-stroke. HC, CO and NOx emissions were not affected by aromatics. Benzene 

emissions were measured and were much higher for the high benzene/high aromatics fuel. 

SUMMARY OF AROMATICS AND BENZENE EFFECTS

 In most studies carried out to date, reducing aromatics lowered emissions of HC, benzene and 

CO. A number of studies suggest that heavier aromatics (C9+) contribute to increased HC/CO emissions 

more than lighter aromatics (C7, C8), although a well designed API study concluded that the effect was 

due to molecular weight and volatility. 

 There is good evidence that reducing aromatics lowers engine-out NOx emissions but also 

lowers the efficiency of TWC systems. The net effect is usually an increase in tailpipe NOx emissions, 

and probably depends on the design of the specific engine control and aftertreatment systems. 

 Benzene and non-benzene aromatics contribute to exhaust benzene emissions. The contribution 

of non-benzene aromatics to exhaust benzene emissions is about 1/10 the contribution of benzene. 

C. OLEFINS 

 Olefins are a class of HC compounds that contain one or more unsaturated or double carboncarbon molecular bonds. The presence of these double bonds makes olefins reactive in the combustion 

process, easy to oxidize over a catalyst, but also highly efficient in forming atmospheric ozone. Although 

light olefins are produced in the combustion process, fuel olefins are present only with molecules having 

five or more carbon atoms. Olefins in the exhaust are major contributors to the total exhaust ozone 

reactivity. 

 AQIRP [41-43] studied the impact of olefin levels on exhaust emissions in 20 1989 model year 

vehicles as part of a larger study evaluating a number of fuel variables. The effect of reducing olefins 

from 20% to 5% on exhaust emissions is shown below: 

Percent Change in Emissions

20% Olefins → 5% Olefins 

NMHC + 7.1 

CO + 1.5 (NS) 

NOx - 6.1 

Specific Reactivity - 8.0 

Benzene - 3.0 (NS) 

1,3 Butadiene - 31.6 

Formaldehyde + 1.3 (NS) 

Acetaldehyde -3.7 (NS) 
CRC Project E-84, Final Report 

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 Olefins are more reactive in the combustion process, and reducing olefins and replacing them 

with paraffins results in a mixture that will not burn as completely or as fast, thereby causing increases in 

HC emissions. Furthermore, the engine-out exhaust, since it contains lower olefins levels, will also be 

less reactive over the catalyst. Engine-out emissions followed the same trend as tailpipe emissions, 

supporting this theory. On the other hand, less complete combustion results in lower heat release and 

therefore lower NOx emissions. CO emissions are generally formed when the mixture is rich, and 

therefore not affected by changes in olefin concentration, which do not affect stoichiometry. 

 Olefins in the fuel were highly correlated with olefins in the exhaust. Since olefins are highly 

reactive in terms of ozone formation, it follows that the specific reactivity of the exhaust is also reduced 

when olefin levels are reduced. 

 The only significant impact of olefins on toxics emissions was to reduce emissions of 1,3 

butadiene. This is not surprising, considering the high correlation between fuel olefins and exhaust 

olefins. 

 In the AQIRP study of high emitters [10], the effects of olefins were almost the same as they were 

on normal emitters. This suggests that the effects are due to the combustion process and not heavily 

dependent on catalyst efficiency or stoichiometry. 

 Jessup et al. [59] purported to study olefins, but found effects that were exactly opposite to the 

AQIRP results. Unfortunately, in the fuel set used by Jessup et al., the olefin and sulfur concentrations 

were hopelessly confounded, making it impossible to separate the two. Interestingly, Jessup et al. also 

claimed that there was no sulfur effect. 

 As part of a larger program, Morgan et al. [25] tested 19 catalyst vehicles from Europe, Japan 

and U.S. on two fuels having olefin levels of 5% and 15%. They concluded that the data suggested that 

lowering olefins would increase HC and reduce NOx emissions. No statistical analysis was presented. 

 Mayotte et al. [14] tested 39 1986-1990 model year LD in-use vehicles with 12 fuels. Twenty-one 

of the vehicles were normal emitters and 12 were high emitters. The authors concluded that the low 

olefins fuel had lower emissions of NOx and CO, with no impact on HC. The authors recognized that the 

variables were not totally separated and the low olefins fuel also had a very low T50 value. 

 In Europe, Jeffrey and Elliott[46] tested five European vehicles with 29 fuels, some of which were 

from the AQIRP study. For the catalysts vehicles tested, results were very similar to those found in 

AQIRP, as shown below: 

Percent Change in Emissions 

20% Olefins → 5% Olefins 

HC CO NOx

- 10.8 Unchanged + 16.5 

 Bennett et al. [49] tested three Euro 1 cars and three Euro 2 cars with seven fuels, two of which 

had different olefin levels (3.9% and 13%). They concluded that reducing olefins reduced emissions of 

1,3 butadiene by about 20%, and in the Euro 1 cars, NOx emissions as well. 

 In a test of one GDI vehicle with a lean NOx catalyst, Kwon et al. [27] found no impact of olefin 

concentration, comparing two fuels having olefin content of 4% and 15%. 

 Stradling et al. [28] tested four Euro3/4 cars, three of which were GDI and one which was MPI. 

Olefin levels varied between 3% and 14% in an eight fuel half-factorial design. An olefin effect was seen 

in only the MPI vehicle, where reducing olefins reduced CO emissions. Differences of more than 7% 

would have been statistically significant. 

 Pentikainen et al. [60] tested four European vehicles, five Japanese vehicles and one US vehicle 

from model years 1999-2003. Two levels of olefins were tested, 4% and 14%. The high olefin levels 

were achieved by replacing alkylate with iso-octene. This evaluated the question of whether olefin effects 
CRC Project E-84, Final Report 

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are caused by heavy olefins. They concluded that increasing the concentration of heavy olefins did not 

affect any of the regulated emissions (HC, CO, NOx), ozone forming potential of the emissions, emissions 

of toxic compounds or PM. The only difference measured was an increase in emissions of iso-butene 

when heavy olefins increased. This was probably a product of iso-octene partial combustion. 

 Thummadetsak et al. [33] studied two olefin levels (5%, 10%) in six 1993-96 model year vehicles 

representative of the vehicle population in Thailand. One vehicle had no catalyst, one had an oxidation 

catalyst and four had TWC systems. In the catalyst vehicles, reducing olefins reduced THC emissions by 

2.5% and benzene emissions by 8.6%. There were no statistically significant olefin effects in the noncatalyst vehicle. 

 Jain et al. [58] tested twelve 2-wheelers on three gasolines varying in aromatics (10% to 62%) 

olefins (1% to 26%) and benzene (0.3% to 1.5%). The fuel variables were not independent because they 

were testing typical refinery fuels. The 2-wheelers were 2000-2004 model year Indian vehicles, both new 

and in-use. Three had 4-stroke engines and three were 2-stroke. CO emissions were higher for the high 

olefin fuel and NOx was not affected. No statistical analysis was provided to support these conclusions. 

SUMMARY OF OLEFIN EFFECTS

 The impact of olefins has been consistent in the literature. Reducing olefins increases emissions 

of HC and 1,3 butadiene, and lowers emissions of NOx. Exhaust specific reactivity is also lowered when 

olefins are reduced. It is fairly certain that the overall reactivity of olefins contributes to these effects. 

D. VAPOR PRESSURE 

 Front-end volatility of gasoline is usually characterized by two measures – vapor pressure and 

T10, the temperature at which 10% of the gasoline evaporated in a laboratory batch distillation (ASTM 

method D 86). Vapor pressure is usually expressed as Reid Vapor Pressure (RVP), which measures 

vapor pressure of the gasoline sample at a fixed vapor-liquid ratio. The original test measured the 

pressure in a bomb, but current versions such as ASTM D5191 use more sophisticated and automated 

equipment that has much better repeatability and reproducibility. 

 RVP and T10 are highly correlated, and only one program [59], discussed below, attempted to 

separate the two variables. In most of the literature, RVP was the controlled variable, and analysis was 

carried out in terms of RVP, not T10. For HC fuels, RVP can be easily varied by changing butane 

concentration in the fuel. 

 The RVP requirement of vehicles varies as a function of temperature. RVP that is too low will 

result in poor starting and cold drivability. RVP that is too high will result in vapor-lock and poor hot 

drivability. ASTM D 439 provides guidance on the appropriate level for RVP as a function of location and 

month. 

 RVP is also controlled to limit evaporative emissions. Even though all U.S. LD vehicles are 

equipped with carbon canisters to trap evaporative HC emissions, RVP can still impact these emissions. 

 Another aspect of RVP is that the addition of ethanol to gasoline increases the RVP by as much 

as 1 psi. This increase is a result of the non-ideal, non-linear blending characteristics of ethanol due to 

hydrogen bonding in the fuel. Other alcohols exhibit the same impact to a greater (methanol) or lesser 

extent (propanol, butanol). For this reason, many of the programs that measured the impact of RVP on 

exhaust emissions, also tested ethanol and other oxygenates. The impact of ethanol and other 

oxygenates will be discussed below. 

 Most [61] reported on a CRC program that tested twelve TWC vehicles with eight fuels at three 

temperatures and two altitudes. Although the program was aimed primarily at evaluating oxygenate 

effects, two different RVP base fuels were tested. The data suggested that reducing RVP from 13 psi to 
CRC Project E-84, Final Report 

26

11.5 psi might reduce CO at 35oF (2oC) and 50oF (10oC), but Most concluded that more work would be 

needed to fully define the effects. 

 AQIRP tested the effects of RVP, ethanol, MTBE and ETBE in twenty 1989 model year vehicles 

with 11 fuels. RVP varied between 8 and 10 psi [62]. A summary of the effects is shown below: 

Percent Change in Emissions, 

Reduce RVP 1 psi in the presence of 

 No Oxygen Ethanol MTBE Average Effect 

of RVP 

NMHC -4.0 (NS) - 11.2 - 1.0 (NS) - 4.3 

CO -10.4 - 14.9 (NS) - 2.4 (NS) - 9.1 

NOx + 1.5 (NS) + 0.4 (NS) - 6.2 (NS) - 0.7 (NS) 

Benzene - 0.4 (NS) + 0.1 (NS) - 8.1 (NS) - 2.4 (NS) 

1,3 Butadiene + 2.8 (NS) + 1.9 (NS) - 0.4 (NS) + 1.8 (NS) 

Formaldehyde - 15.6 (NS) + 61.3 (NS) + 48.4 (NS) + 6.8 (NS) 

Acetaldehyde - 12.8 (NS) + 20.5 (NS) + 537 (NS) - 3.3 (NS) 

 The effects of reducing RVP were statistically similar for the three base fuels, and an average 

was calculated. Reducing RVP was estimated to reduce emissions of NMHC and CO, but did not change 

NOx emissions. The variability of the data for some of the toxics measurements was substantial and even 

large differences were not statistically significant. The effect of RVP on specific reactivity was small and 

not statistically significant. 

 AQIRP also studied RVP changes in seven high emitters. Knepper et al. [10] found there was no 

discernible effect of RVP on any exhaust emissions in the fleet of seven vehicles. 

 Jessup et al. [59] claim to have found an effect of RVP on NOx emissions, such that lowering 

RVP lowers NOx. Unfortunately, in the fuel set, T10 and RVP were hopelessly confounded and the 

regression analysis found that a T10 effect that was exactly opposite the RVP effect, effectively canceling 

it out. This is a common statistical error, and no conclusions about the impact of RVP can be drawn from 

this study. 

 Korotney et al. [63] tested 19 LD vehicles (1986-1990 model year) with eight fuels. A subset of 

the fuels had good separation of RVP. The vehicles were split between eleven normal emitters and eight 

high emitters. Although they did not directly compare the two RVP levels, it appears that there was no 

impact of changing RVP on any of the exhaust emissions. 

 Rutherford et al. [64] reported on a test carried out in Los Angeles and Las Vegas, with 30 in-use 

cars in each location. Two RVP levels were tested (9 psi and 12 psi) under typical wintertime conditions 

[40oF-65oF (4oC-18oC) for Las Vegas and 50oF-70oF (10oC-21oC) for Los Angeles]. The Los Angeles 

base fuel contained 11% MTBE and the Las Vegas base fuel contained 10% ethanol. The authors 

concluded that reducing RVP from 12 psi to 9 psi resulted in 12% reduction in CO emissions under Las 

Vegas test conditions. No significant differences were seen for the Los Angeles tests. Further analysis 

by Hoekman et al. [65] suggested that the lack of effect in Los Angeles might have reflected a poor state 

of repair of some of the vehicles. 

 Bennett et al. [66] tested five 1992-94 model year European cars with six fuels varying in RVP 

and E70, and with two levels of MTBE, 0 and 10%, using the new European test cycle (sampling from key 

on). In non-oxygenated fuels, reducing RVP from 1040 bar to 630 bar (15.1 psi to 9.1 psi) reduced CO by 

13% and NOx by 15% and had no effect on HC. The effects were non-linear, with smaller changes in CO 

and NOx as RVP was lowered. In the presence of MTBE, reducing RVP lowered NOx by 10%, but did not 

affect HC or CO. The effect of RVP on CO was related to the canister purge strategy. Lower RVP fuels 
CRC Project E-84, Final Report 

27

gave less enrichment during canister purge periods, probably because they generate less vapor and do 

not affect stoichiometry as much. 

 Thummadetsak et al. [33] tested three RVP levels (45 kPa, 55 kPa, and 62 kPa) in six cars 

representative of the vehicle population in Thailand. Changes in RVP did not have a statistically 

significant effect on any exhaust emissions. 

SUMMARY OF RVP EFFECTS

 There is limited data on the effect of RVP on exhaust emissions. The most consistent data 

seems to be from the AQIRP study which concluded that reducing RVP would lower exhaust emissions of 

HC and CO. More recent data have been somewhat variable. Since the most important reason to 

reduce RVP is to lower evaporative emissions, and since most gasoline RVP is fairly low, it might be 

worthwhile to collect data on recent model year vehicles to understand the impacts of RVP on exhaust 

emissions at low levels of RVP. There is also limited data available at low temperatures on the RVP 

effects. If this is relevant from a regulatory perspective, then additional data would be useful. 

E. MID-FILL AND BACK-END VOLATILITY 

 Gasoline volatility is usually described by the ASTM D86 distillation test discussed earlier. 

Results of the distillation are plotted on a chart with temperature on the y-axis and percent evaporated on 

the x-axis. Distillation parameters may be described by “T-numbers” or “E-numbers.” Txx refers to the 

temperature at which a certain fraction of the fuel has evaporated in the distillation. Exx refers to the 

percent of the gasoline sample that has evaporated at a given temperature. Both measures are 

equivalent and emissions data may be correlated with either measure. EPA used E-numbers in the 

Complex Model and ARB used T-numbers in the Predictive Model. Gasoline blenders generally prefer to 

use E-numbers since they behave in a linear fashion when mixing different refinery blendstocks. 

 Due to the nature of gasoline blending, there is generally a high degree of correlation between 

distillation values that are close to each other: e.g., T50 and T60 or E70 and E80. When distillation 

measures are far away, say T10 and T90, then there is probably not much correlation between them 

among groups of fuels. 

 Usually, three measures of gasoline volatility are used in order to get good separation of variables 

– front-end, mid-fill and back-end volatility. Front-end volatility, discussed above, is usually expressed as 

T10. Mid-fill volatility is usually expressed as T50, which is basically the median distillation temperature. 

Back-end volatility is usually expressed as T90. Expressed as E-numbers, these parameters are usually 

E100, E200, and E300 (in degrees F). In typically refinery blends, there is a high degree of correlation 

between mid-fill and back-end volatility. When carrying out experimental programs, a great deal of care 

must be spent on choosing blendstocks to be able to determine the independent effects of mid-fill and 

back-end. This was achieved with greater or lesser success in the programs described below. In some 

cases, the authors recognized the lack of orthogonality, while in others it was ignored, calling the 

conclusions into question. 

 Another function stated in terms of volatility is the Drivability Index (DI). First proposed in the 

early 1980s, it has the form: 

 DI = 1.5 x T10 + 3 x T50 + T90

It was found that this equation correlates well with start-up and warm-up driving performance of the 

automotive fleet. The DI equation has been incorporated into gasoline certification standards such as 

ASTM D 4814. Some have suggested that the DI equation should also be used to correlate exhaust 

emissions. As will be shown below, both from a fundamental perspective and from a statistical 

perspective, this is an unrealistic goal. 

 Recently, it has been found that ethanol and other oxygenates do not act like HC in terms of their 

effects on drivability and the driveability equations has been modified to: 
CRC Project E-84, Final Report 

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 DI = 1.5 x T10+ 3 x T50+ T90 + 20 x %oxygen 

 Not much work was done before AQIRP on exhaust emissions and volatility. Clarke [67] had 

done work in the early 1970s, showing the effect of volatility on emissions. Much of the impact was 

believed due to measurable performance debits. Cars that stall or hesitate will certainly have higher 

emissions because of those malfunctions. In the late 1980s, it was realized that emissions might be 

impacted even when performance debits such as driveability demerits could not be measured. 

 Gething [68] correlated emissions with DI and found that HC emissions increased when DI 

increased. However, he did not attempt to determine the best coefficients for the relationship. 

 One of the main objectives of AQIRP was to determine the impact of volatility parameters on 

emissions. In the first published AQIRP study, Hochhauser et al. [41] reported on an experiment which 

varied T90 between two relatively extreme values in 20 1989 model year LD vehicles. Results are shown 

below, along with toxics results reported elsewhere [42, 43]. Lowering T90 reduced NMHC exhaust 

emissions by 23%, and specific reactivity and toxic emissions by similar amounts. This was the largest 

effect measured in the program. There were a number of higher order interactions, some suggesting that 

heavy aromatics were more significant than heavy paraffins or olefins in contributing to NMHC emissions. 

It was speculated that T90 affected emissions, especially during cold start operations, because the heavy 

molecules represented by the back-end of the distillation curve are harder to vaporize and burn before the 

engine has reached full operating temperature. 

Percent Change in Emissions 

T90: 360oF (138oC) → 280oF (182oC) 

NMHC - 22.9 

CO + 1.2 (NS) 

NOx + 4.9 

Specific Reactivity - 18.0 

Benzene - 10.8 

1,3 Butadiene - 36.8 

Formaldehyde - 25.7 

Acetaldehyde - 23.9 

 In AQIRP tests of high emitters [10], reducing T90 had little or no effect on NMHC and CO 

emissions, but lowered NOx emissions significantly. No attempt was made to explain the differences 

between the normal and high emitter results. 

 The AQIRP authors also recognized that T90 and T50 were correlated in the fuel set, and that the 

results could be explained almost as well by considering T50 as the important variable in the regression 

equations, or by using both T50 and T90. The correct interpretation of the results would be that reducing 

both mid-fill and back-end volatility together could help to reduce NMHC emissions. Programs carried 

later in the AQIRP, which studied this effect in more detail and attempted to separate the two variables, 

are discussed below. 

 In a follow-up AQIRP program, Koehl et al. [69] reported on tests carried out in ten 1989 model 

year vehicles. In Matrix A, various refinery streams or portions of the streams were added to a base 

blend. The components varied in carbon number, aromatic and olefin concentrations, and were prepared 

from fluid catalytically cracked and reformate streams. In Matrix B, heavy paraffins and heavy aromatics 

were compared directly, and levels of T50/T90 were varied independently. The latter was accomplished by 

changing isomerate and light alkylate concentrations. The authors concluded that: 

• The response of NMHC to changes in T90 was non-linear, with larger emissions increases 

occurring as heavier components were added. The effects occurred primarily in Bag 1 of the FTP 

test. 
CRC Project E-84, Final Report 

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• The effects can be attributed to distillation characteristics, not chemical composition differences 

between aromatics and paraffins. 

• Increasing T50 levels increased NMHC emissions. The effect did not appear to be non-linear, and 

was smaller than the T90 effect, especially at levels of T90 above 315oF (157oC). 

• Expressing distillation parameters as percent evaporated variables (E-numbers) provided a better 

correlation of NMHC than expressing them as distillation temperature variables (T- numbers). 

• Fuel composition (i.e., aromatics and paraffin levels) were the most useful predictors of toxics 

emissions and ozone reactivity effects. 

 AQIRP tested three vehicles fleets in a more targeted program to measure the independent 

impacts of T50 and T90, reported by Rutherford et al. [17]. Ten 1989 model year vehicles, six 1994 model 

year Tier 1 vehicles, and six advanced technology vehicles were tested. The advanced technology 

vehicles had NMHC emission levels below 0.1 g/mi. Results for the fleets are shown below: 

 Percent Change in Emissions 

 1989 

Model Year 

1994 

Model Year 

Advanced 

Technology 

T50 (215oF → 185oF; 102oC → 85oC) 

 NMHC - 5.8 - 4.0 (NS) - 11.5 

 CO - 5.1 (NS) - 6.9 (NS) - 5.6 

 NOx + 7.8 - 0.6 (NS) + 6.0 (NS) 

 Benzene - 3.9 (NS) - 4.1 (NS) - 10.8 

 1,3 Butadiene - 3.9 (NS) - 4.2 (NS) - 11.5 

 Formaldehyde + 2.6 (NS) + 0.8 (NS) - 5.2 (NS) 

 Acetaldehyde + 1.5 (NS) - 1.5 (NS) - 10.3 (NS) 

 Specific Reactivity - 1.4 - 3.0 (NS) - 0.8 (NS) 

T90 (325oF → 280oF; 163oC → 138oC) 

 NMHC - 7.3 - 8.0 - 10.4 

 CO + 10.7 + 6.6 (NS) + 17.2 

 NOx + 1.2 (NS) + 11.0 + 3.7 (NS) 

 Benzene - 1.8 (NS) - 9.7 + 0.7 (NS) 

 1,3 Butadiene - 19.2 - 22.7 - 17.1 

 Formaldehyde - 11.9 - 15.1 - 17.0 

 Acetaldehyde - 11.3 - 15.1 - 4.6 (NS) 

 Specific Reactivity - 11.5 - 8.8 - 4.9 

 Both T50 and T90 reduced NMHC exhaust emissions by similar amounts for the three fleets. Since 

only two levels were tested, it was not possible to evaluate the non-linearity of the effects. NOx effects 

were generally small and not significant. Lowering T50 seemed to reduce CO, while lowering T90 seemed 

to raise CO; the effects were similar across fleets, although they were not always statistically significant. 

Lowering T50 and T90 generally reduced exhaust emissions of toxics, although the T90 impact was larger 

and more likely to be statistically significant. 

 Kanehara et al [70] tested four 1990 model year vehicles with fuels having three levels of T50

(87oC, 96oC, 110oC) using the US FTP emissions test. They concluded that lowering T50 reduced 

exhaust HC emissions, increased NOx emissions and did not impact CO emissions. No statistical 

analysis was presented and no quantitative analyses were shown. 
CRC Project E-84, Final Report 

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 In a program carried out in vehicles similar to those tested in AQIRP, Jessup et al. [59] found that 

T50 had a large effect on HC emissions, and that both T50 and T90 had large effects on CO emissions. 

 Takei et al. [11] reported on two vehicles certified to emission level of 0.25 g NMHC/mi. 

Reponses to T90 were very similar to those found in AQIRP. They did not find a strong correlation 

between T50 and NMHC emissions. In a subsequent study [16] Takei et al. tested a 1994 model year 

TLEV with fuels that independently varied T50 and T90. Lowering T50 from 105oC to 85oC reduced HC 

emissions, but the effect was not a strong one and CO and NOx emissions were not significantly affected. 

Lowering T90 from 161oC to 135oC also reduced HC emissions, and reducing T90 further achieved no 

further reduction in emissions. Reducing T90 also increased NOx emissions by as much as 40%. These 

results are similar to those found in AQIRP, although the magnitude of the NOx effect is very large. It 

should be recognized that this study only tested 1 car, so the results may not be representative of an 

entire fleet. 

 Takei et al. [19] tested two vehicles - model years 1992 and 1994 – on eight fuels where T50 and 

T90 were varied independently. The non-linear responses of NMHC to T50 and T90 were similar to that 

found in other studies. At low levels of each (T50<90oC, and T90<135oC), NMHC emissions tended to rise. 

The T50 effect was ascribed to the impact of mid-fill volatility on driveability, and the T90 effect was 

ascribed to adsorption and desorption of HC in the oil film present in the combustion chamber. The 

impact of T50 and T90 on CO was similar to its effect on NMHC. T50 had little effect on NOx, and T90 had 

an inconsistent effect for the two cars. 

 Takei et al. [21] tested one LEV and one ULEV on 15 fuels varying in T50, T90, aromatics, olefins, 

RVP, and oxygenate content. Results are shown for the impact of midfill (T50 or E100) and Driveability 

Index only. Above a T50 value of about 100oC, exhaust HC emissions rise sharply, about 40% or 60% in 

20oC. Below this level, emissions are flat. NOx shows the opposite effect. Below a T50 level of 100oC, 

emissions rise by 30% and 50% as T50 is reduced 20oC. 

 The most recent program to study mid-fill and back-end volatility was CRC Project E-67 [71] in 

which 12 California certified vehicles were tested with 12 fuels varying in T50, T90 and ethanol. Three 

levels of each variable were tested in a fractional factorial design that allowed second order interactions 

to be evaluated. The vehicles were 2001-2003 model years and had certification levels of LEV, ULEV 

and SULEV. Fuels properties spanned the 10th and 90th percentiles for T50 and T90 based on U.S. 

summertime surveys. In addition, two levels of T50 were evaluated for speciation and toxics emissions. 

 The effects of T50 and T90 on NMHC emissions were non-linear, similar to the response seen in 

earlier studies. At low levels, the response was lower than at high levels. Fleet average increase in 

emissions of NMHC were 36% and 25% in going from the low T50 (195oF, 91oC) and midpoint T50 (215oF, 

102oC) to the high T50 (235oF, 113oC). The T90 response depended on the ethanol concentration in the 

blend. When no ethanol was present, changing T90 did not affect NMHC emissions. At 5.7% and 10% 

ethanol, the effect was non-linear, with a smaller effect going from low T90 (295oF, 46oC) to mid T90 (330oF 

166oC) than going from mid T90 to high T90 (355oF, 179oC). Effects of T50 and T90 on CO emissions were 

somewhat mixed. The T50 response appeared to be non-linear for fuels that contained ethanol, but was 

essentially flat for the non-ethanol fuels. CO emissions decreased as T90 increased. Neither T50 nor T90

affected NOx emissions. There was a small statistically significant interaction between ethanol and T50. 

NOx emissions tended to decrease as T50 increased when the fuel contained 10% ethanol, but not at 

5.7% or in the absence of ethanol. 

 Reducing T50 reduced formaldehyde, benzene and 1,3 butadiene emissions, but did not affect 

acetaldehyde emissions. 

 Durbin et al. also evaluated the possibility of using Drivability Index to predict NMHC emissions. 

Not surprisingly, the correlation using the DI equation was not as precise as the regression equations in 

predicting NMHC emissions for the fleet of vehicles tested in this program. In the DI equation, the 

coefficients were determined to predict drivability not emissions, while in the emissions regression, the 

coefficients are determined to predict emissions. While the effects may be related, it is simplistic to 

assume that they will be exactly equal. 
CRC Project E-84, Final Report 

31

 A number of programs have been carried out in Europe to measure the emissions impact of midfill and back-end volatility. Jeffrey and Elliott [46] tested three catalyst vehicles (model years 1990 - 1992) 

with a set of 19 fuels in a factorial design. T90 varied between 110oC and 160oC. Lowering T90 from 

160oC to 110oC reduced HC emissions by 17%, very similar to the 22% found for the 1989 model year 

U.S. vehicles in AQIRP. NOx emissions tended to increase (16%), while the CO response was mixed. As 

in other similar programs, the authors recognized that some of the T90 effect could be due to variations in 

T50, since the two were correlated. 

 Wedekind et al. [47] tested one catalyst car (early 90s vintage) with 11 fuels in which three levels 

of T50 and T90 were tested (85oC, 100oC, 115oC for T50 and 122oC, 141oC and 162oC for T90). The 

variables were controlled independently. Volatility was adjusted while maintaining aromatic content 

constant by using lighter and heavier aromatic compounds as needed. As such, the impact of volatility 

cannot be separated from the effects of changing the distribution of aromatic molecular weights. The 

vehicle was tested using the European ECE/EUDC cycle where sampling is started after 40 seconds of 

idling. Reducing T50 from 115oC to 85oC reduced HC emissions less than 20% and CO emissions 

between 20-40%. NOx emissions increased 20-40%. Reducing T90 from 160oC to 120oC lowered HC 

emissions 20-40% and raised NOx emissions by 20-40%. CO emissions were unaffected. 

 CONCAWE conducted a program to determine whether the results seen in AQIRP for U.S. 

vehicles would be similar for European LD vehicles [72, 73]. Seven fuels had T90 levels between 145oC 

and 176oC. RVP and T50 values were held constant. The chemical composition of the back-end was also 

varied in the test fuel design. Aromatics varied in the range 38-50%, olefins in the range 0-9% and 

paraffins in the range 43-54%. Ten European LD vehicles were tested, all having TWC and fuel injection. 

HC and CO emissions increased by 9% and 14%, respectively when T90 increased from 140oC to 160oC, 

but did not change between 160oC and 180oC. NOx emissions behaved in an opposite manner and were 

5% lower when T90 increased versus the base level of 140oC. This behavior is similar in direction to the 

AQIRP results, but smaller in magnitude for HC. Also, the non-linearity seen in AQIRP results is different. 

Here, the lowest level of T90 provides the biggest effect. In AQIRP, changing T90 from the highest level 

provided the biggest benefits. The report recognized that assigning a specific volatility cut-point to an 

emissions effect was not a possibility because the parameters were not independent of each other. 

Within the range of T50 to T90, it was not possible to ascribe the fuel effects to any one distillation point. 

 The EPEFE program [48] tested sixteen prototype vehicles designed to meet 1996 European 

emission limits. Aromatics and E70 were each tested at three levels in a nine-fuel factorial design. E100 

targets were 35%, 50% and 65% corresponding to T50 values of approximately 85oC, 100oC and 120oC. 

The response of HC was non-linear, as seen in other programs. Stated in terms of T50, at high levels of 

T50, reducing T50 caused a large reduction in HC emissions, and at low levels of T50, the impact was very 

small. Mid-fill volatility had little, if any, impact on CO and NOx emissions. 

 Bennett et al. [49] tested three vehicles meeting Euro 1 emission standards (1993), and three 

vehicles meeting Euro 2 standards (1996). E100 varied between 49% and 58%, and E150 varied 

between 80% and 90%. No fuel effects were statistically significant, although the statistical power of the 

test was not clear. 

 McDonald et al. [50] studied aromatics and mid-fill volatility in twelve 1990-93 European catalyst 

cars. Three levels of T50 were tested (75oC, 95oC, 115oC), along with two levels of aromatics. They used 

the European ECE/EUDC test cycle. The authors did not present quantitative results for the impact of 

T50, but showed that the results were very similar to those found in AQIRP studies. 

 Stradling et al. [28] tested three GDI cars and one MPI car with twelve fuels of varying volatility. 

The cars met Euro 3 or Euro 4 emission standards. Mid-fill and back-end were varied together. 

Reducing volatility had no consistent effect on NOx or CO, and increased HC emissions for all the cars. 

When the final boiling point was reduced from 197oC to 176oC, HC emissions were reduced by 9% and 

CO emissions increased 20%. NOx emissions increased in one car and did not change in the others. 

 Akimoto et al. [29] tested one 1993 Japanese vehicle with fuels having three levels of T50 (87oC to 

104oC) and four levels of T90 (130oC to 163oC) that varied independently. They used the Japanese 10.15 
CRC Project E-84, Final Report 

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mode emissions test. T50 had no significant effect on any exhaust emission component. Reducing T90

lowered emissions of HC and CO, but did not affect NOx emissions. No quantitative results were 

provided. 

 Saitoh and Hamasaki reported on the JCAP Program [32] in which three levels of T50 (72oC124oC) and three levels of T90 (119oC – 183oC) were independently varied and tested in one 

stoichiometric MPI and two lean-burn GDI prototype vehicles. One of the GDI vehicles had a NOx

Storage Reduction catalyst. Two test cycles were used – the 10.15 mode (hot) and the 11 mode (cold). 

The results were inconsistent across the vehicles and the test modes; with the small number of vehicles 

tested, it was not possible to draw any general conclusions. 

 Koseki et al. [74] tested one 2004 model year MPI vehicle and one 2005 model year GDI vehicle 

– both stoichiometric - with fuels having two levels of T50 (92oC, 102oC) and T90 (145oC, 168oC). No 

impact of either variable was seen in the vehicles, although the statistical power of the test was not 

discussed. 

 Hirose et al. [56] measured particulate emissions in a 2005 model year lean-burn GDI Japanese 

car with nine fuels. T90 varied between 140oC and 166oC, and T50 varied between 89oC and 100oC in an 

independent manner. There was a statistically significant correlation between T90 and particle numbers in 

both the 10.15 and the 11 mode emission tests, with the coefficients being larger in the case of the 10.15 

mode test. 

 Thummadetsak et al. [33] tested six cars representative of the Thai market (model years 1993-

96) with two fuels having T50 levels of 95oC and 105oC. Four cars had TWC, one had an oxidation 

catalyst and one had no catalyst. The effects of reducing T50 were relatively small, and the only 

statistically significant change was a reduction in benzene exhaust emissions of 8.4%. It is possible that 

the relatively small change in T50 prevented identification of other effects. 

 In 1995 CONCAWE [75] published a thorough literature review and analysis of studies that 

measured the influence of mid-fill and back-end volatility on emissions. They compared studies to date 

from AQIRP, EPEFE, CONCAWE and others, and concluded that the preponderance of evidence 

suggested that mid-fill volatility was more important than back-end volatility. They found differences 

between U.S.-based AQUIRP and European-based CONCAWE programs on the influence of back-end 

composition on emissions (as opposed to back-end volatility). AQIRP [76] found no major impact while 

CONCAWE found some effects. The report was not able to account for these differences, except to point 

out that there were vehicle technology differences between the test fleets. 

SUMMARY OF VOLATILITY EFFECTS

 Most research has shown that reducing T50 and reducing T90 have similar effects – lower 

emissions of HC and CO and higher emissions of NOx. Emissions of toxics and specific reactivity of 

emissions generally follow the HC emissions. The relative size of the effects of T50 and T90 has been 

somewhat variable, possibly reflecting details of the composition of the blendstocks used in the various 

programs. In the most recent CRC program [71], the fuels were blended with great care and the 

variables were almost perfectly orthogonal. While the T50 effect on HC was larger than the T90 effect, the 

results were difficult to interpret and included a number of interactions that were difficult to explain 

technically. 

F. OXYGENATES (ETHERS AND ALCOHOLS) 

 Oxygenates are added to gasoline for a number of reasons, including enhancing octane, 

increasing volume and reducing petroleum use, limiting greenhouse gases, and reducing emissions. 

Oxygenates generally fall into two categories – alcohols and ethers. The lightest alcohol, methanol, once 

seriously considered, is rarely used today. Ethanol, the most common alcohol in use, can be made from 

biomass such as corn or sugar cane, and can reduce the emissions of greenhouse gases, depending on 
CRC Project E-84, Final Report 

33

the manufacturing process. This has been the subject of much scientific debate and will not be discussed 

here. Higher alcohols such as butanol (both normal and iso) have been considered, but are expensive 

and not available in as large quantities as ethanol. Ethanol can be used in all LD vehicles in the U.S. at 

concentrations up to 10%. In some Flexible Fuel Vehicles, it can be used at all concentrations between 

0% and 85%. Ethanol has high octane and affects the volatility of the gasoline when it is splash blended - 

it increases the RVP of the gasoline base blend by up to 1 psi, and also lowers the T50 of the blend by up 

to 20oF (11oC). 

 The most common ether used in gasoline is MTBE (methyl tertiary-butyl ether), usually made by 

reacting methanol and isobutylene. Other ethers commonly considered for use are ETBE (ethyl tertiarybutyl ether) and TAME (tetiary-amyl methyl ether). ETBE is the analog of MTBE made with ethanol, and 

TAME is a reaction product of methanol and iso-pentene. Recently, ethers have been banned from use 

in gasoline in the U.S. because of concerns over groundwater contamination. MTBE is more soluble in 

water than gasoline and less biodegradable than ethanol. Leaks from underground tanks can result in 

MTBE penetrating into groundwater or drinking water supplies. While the health effects of MTBE are not 

fully defined, precaution dictates that ethers no longer be used in gasoline in the U.S. 

 Many of the studies cited below have concluded that the impact of oxygenates on exhaust 

emissions is a function of the oxygen content and the impact of the particular oxygenate on volatility, such 

as RVP or T50. Therefore, in this section, all oxygenates will be considered together. Earlier studies 

tended to focus on ethers, while more recent studies have concentrated on the effects of ethanol, but a 

number of large studies have evaluated both. Hood and Farina [77] published a useful review in 1995 of 

oxygenate impacts, focusing on low ambient temperatures. This report incorporates many of the more 

recent (post-1990) references they cited. 

 AQIRP carried out a number of studies to evaluate oxygenates [41-43]. In the first study, MTBE 

was evaluated along with other variables in 20 1989 model year vehicles. Results comparing nonoxygenated gasoline and gasoline with 15% MTBE are shown below: 

Percent Change in Emissions

MTBE 0% → 15% 

NMHC - 6.4 

CO - 11.1 

NOx + 1.4 (NS) 

Specific Reactivity + 7.3 

Benzene - 5.5 (NS) 

1,3 butadiene - 8.8 

Formaldehyde + 26.8 

Acetaldehyde - 1.9 (NS) 

 The addition of MTBE was accomplished while keeping all other variables constant. It was 

somewhat surprising that vehicles with good control of A/F ratio would still respond to the presence of 

oxygen. The effects were found to be the result of improvements in catalyst efficiency, not engine-out 

emissions, suggesting that stoichiometry over the catalyst was still somewhat variable. There were a 

number of interactions among the fuel variables, most notable between MTBE and aromatics with respect 

to NOx emissions. At low levels of aromatics (20%), addition of MTBE increased NOx emissions by 5%, 

while there was no effect at the higher level of aromatics (45%). This result is relevant because current 

aromatic levels are much closer to the lower end of the range studied than to the higher end, and the 

impact of oxygenates on NOx emissions has been somewhat inconsistent in other studies. 

 Addition of MTBE lowered exhaust emissions of 1,3 butadiene, probably because there were 

lower levels of paraffins, one of the main precursors to olefins in the exhaust. Formaldehyde emissions 
CRC Project E-84, Final Report 

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increased by 27%. Formaldehyde is a known partial combustion product of MTBE so this effect is not 

surprising. Analysis of the speciated exhaust also showed increased levels of isobutylene, another partial 

combustion product of MTBE. The increase of a highly reactive light olefin and formaldehyde probably 

accounted for the increased ozone reactivity seen with MTBE. 

 In a follow-up AQIRP study [62], ethanol (0%, 10%), MTBE (0%, 15%) and ETBE (0%, 17%) 

were tested in ten 1989 vehicles. 

 Percent Change in Emissions from Adding Oxygenate 

Ethanol 

0% → 10% 

MTBE 

0% → 15% 

ETBE 

0% → 17% Average Effect 

NMHC - 5.9 - 7.0 - 6.3 - 6.3 

CO - 13.4 - 9.3 - 14.6 - 12.5 

NOx + 5.1 + 3.6 (NS) + 5.5 (NS) + 4.8 

Reactivity - 1.4 (NS) + 4.4 + 11.4 * 

Benzene - 11.5 - 11.1 - 8.1 (NS) - 10.5 

1,3 Butadiene - 5.8 - 1.7 (NS) - 3.8 (NS) - 4.2 

Formaldehyde + 19.3 (NS) + 15.8 (NS) - 15.7 (NS) + 11.8 (NS) 

Acetaldehyde + 159 - 0.9 (NS) + 254 * 

* Results are statistically different from each other and cannot be 

averaged. 

 The results above are very similar to those measured in the previous AQIRP study and show that, 

with a few exceptions, the effects of the three oxygenates are statistically indistinguishable. The 

formaldehyde results had high variability and no conclusions could be drawn, even though the changes 

were relatively large. Acetaldehyde emissions increased dramatically for ethanol and ETBE because 

acetaldehyde is a known partial combustion product of both compounds. In this experiment, adding 

oxygenates increased NOx emissions by about 5%, while reducing NMHC by 6% and CO by 12%. 

 When AQIRP studied high emitters [10], the benefits of ethanol and MTBE in reducing HC and 

CO emissions were found to be a little larger than in normal emitters, and the increase in NOx was found 

to be about double. These results are consistent with vehicles that are running somewhat rich. 

 AQIRP carried out a direct comparison of MTBE and TAME blended at the same oxygen level of 

2 weight%, in ten 1989 model year vehicles [78]. Reinforcing the conclusion that all oxygenates behave 

in a similar fashion, exhaust emissions for the two fuels were the same, with one exception. The TAME 

fuel had 28% higher emissions of formaldehyde, which the authors attributed to its slightly higher boiling 

point compared to MTBE. 

 Kanehara et al. [70] tested three levels of MTBE (0, 7%, 15%) in three 1990 model year vehicles. 

Interestingly, they reported that HC increased with MTBE, while CO and NOx effects were mixed for the 

three cars. 

 Jessup et al. [59] tested 0% and 15% MTBE in ten LD vehicles ranging from 1984-1990 model 

years, and reported that MTBE did not affect any of the emissions. They did not report the details of the 

statistical analysis so it is not possible to evaluate their findings. 

 Farina [79] carried out a carefully designed experiment comparing a base fuel with two other fuels 

containing either 15% splash blended MTBE or 15% of a splash blended paraffinic HC stream. The HC 

stream was a mixture of hexane isomers with a vapor pressure and boiling range almost equal to MTBE. 

Ten 1989-93 model year LD vehicles were tested and the results are shown below for the fleet averages: 
CRC Project E-84, Final Report 

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Percent Change in Emissions 

Compared to Base Fuel 

 Isomerate Blend MTBE Blend 

NMOG - 10.5 - 11.8 

CO - 5.0 (NS) - 13.0 

NOx - 4.8 (NS) - 4.1 (NS) 

Reactivity - 4.3 - 1.1 (NS) 

Benzene - 24.9 - 22.1 

1,3 Butadiene - 15.2 - 11.6 

Formaldehyde - 1.0 (NS) + 7.1 (NS) 

Acetaldehyde - 4.9 (NS) - 8.2 (NS) 

 Both streams caused similar changes in emissions, except for the possible exception of CO 

emissions, which were lower for MTBE, and specific reactivity, which was lower for the isomerate. Since, 

a number of properties – aromatics and T50 for example – change when the streams were splashed 

blended, it is not possible to say with certainty what caused the emissions effects. However, Farina 

concluded that the only effect that could be caused by oxygen was the reduction in CO, and that some of 

the other effects that have been credited to oxygenates, such as lower HC and higher NOx emissions, are 

probably due to these other changes. 

 As part of the development of the ARB Predictive Model in 2006, ExxonMobil presented data 

from on a study in 1999 with five LEV and ULEV cars [80]. Four fuels were tested, having different levels 

of MTBE (10%) and ethanol (7%, 10%), along with a base fuel. The only statistically significant effect 

was an increase in NOx emissions of up to 30% with the addition of oxygenates. Toyota also presented 

data on nine California vehicles (TLEV, LEV and ULEV) with four fuels containing 10% MTBE or 10% 

ethanol [81]. Compared to the fuel with MTBE, the ethanol containing fuel had 6% lower NMHC and 

5.5% higher NOx emissions. In both programs, presented to ARB, details of the statistics were not 

presented and the main use of the data was to add to the existing database that ARB was using to 

develop their Predictive Model. 

 CRC’s recent Project E-67 measured the effects of volatility and ethanol in twelve 2001-03 model 

year LD vehicles [71]. Three levels of ethanol were tested – 0%, 5.7% and 10% - in a fractional factorial 

statistical design. Emissions with a subset of fuels were speciated. In general ethanol did not affect 

NMHC emissions, the exception being at high levels of T90 (355oF, 179oC). Ethanol had a non-linear 

effect on CO emissions. Adding 5.7% ethanol reduced CO emissions by about 20%, but further increase 

in ethanol concentration up to 10% did not reduce CO emissions any further. The addition of ethanol 

tended to increase NOx emissions, but the effect depended on the T50 value of the fuel. At low levels of 

T50 (195oF, 91oC), the increase was about 10%, while at high levels of T50 (235oF, 113oC), the response 

was flat. With respect to toxics, increasing ethanol concentration from 0 to 10% increased emissions of 

acetaldehyde by 73%. Surprisingly, addition of ethanol also increased emissions of benzene by 18% and 

1,3 Butadiene by 22%. There was a significant ethanol x T50 interaction for NOx emissions. For low 

levels of T50, adding 10% ethanol increased NOx emissions by 12%. For medium levels of T50, they were 

about 5% higher, and for high levels of T50, NOx emissions were unchanged when ethanol was added. 

 A number of studies have also been carried out in European and Japanese vehicles. Jeffrey and 

Elliott [46] tested the impact of MTBE using a number of AQIRP and European fuels with three TWC cars 

(1990-92 model year). Adding 15% MTBE reduced CO in all three cars by 9, 13 and 16%. HC was 

reduced in two cars by 13% and 11%, and NOx was not affected. These results were consistent with 

those from the AQIRP study. 

 Morgan et al. [25] tested 27 cars from Japan, Europe and Canada from model years 1989-91. 

MTBE was varied in a designed matrix of eight fuels. They concluded that 15% MTBE reduces emissions 
CRC Project E-84, Final Report 

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of CO, to a lesser extent HC, and did not affect NOx emissions. No statistical analysis or quantitative 

measures were provided. 

 Bennett et al. [66] tested five European cars (1992-94 model years) with two levels of MTBE (0%, 

10%) at low and high RVP levels, and found that adding 10% MTBE when RVP was high reduced CO 

and NOx emissions by 15% and 7%, respectively. At low RVP levels, adding MTBE did not affect 

emissions at all. In a later program, Bennett et al. [49] tested three Euro 1 cars and three Euro 2 cars 

with fuels having the same two levels of MTBE. They reported that the Euro 2 cars had higher benzene 

emissions when 10% MTBE was present. The benzene effect was unexplained. 

 Graupner et al. [82] carried out a series of tests on European vehicles and fuels with different 

oxygenates – MTBE, ETBE, TAME, TBA (tertiary-butyl alcohol) and ethanol. Fuels were carefully 

blended by replacing isoparaffins with oxygenates, so as to minimize other changes. They concluded 

that for European catalyst vehicles, adding oxygenates reduced CO by a substantial amount (13%), 

reduced HC emissions by a smaller amount (5%), and had little or no effect on NOx emissions. As in the 

AQIRP study, the response of individual vehicles was variable and there were no differences among the 

various oxygenates tested. They were also able to conclude, based on the way in which the fuels were 

blended, that the effects were most likely caused by the presence of oxygen in the molecules, and not by 

other physical or chemical properties. 

 Kwon et al. [27] tested a GDI car meeting the Euro 2 emission standards with two levels of 

MTBE, 0% and 10%, and found no statistically significant fuel effect. 

 Pentikainen et al. [60] compared fuels containing mixed ethers (MTBE, TAME) at 2% oxygen with 

ethanol at 3.5% oxygen. Twelve cars (1999-2002 model years) representative of the Finnish market were 

tested. No differences were measured in THC, CO and NOx emissions. The ethanol fuel had higher 

emissions of 1,3 butadiene, benzene, and acetaldehyde, and lower emissions of formaldehyde. 

 In Japan, Saitoh and Hamasaki [32] tested three fuels with 0%, 6% and 13% MTBE; all other 

properties were held constant. Four cars were tested – three lean-burn GDI and one stoichiometric MPI. 

The only statistically significant effect was a reduction in CO in one of the cars when MTBE was used. 

 As part of the JCAP program, the impact of ETBE on emissions was tested and reported by 

Koseki et al. [83]. Three fuels were tested – 0%, 8% and 17% splash blended ETBE – in six cars and six 

motorcycles. The cars had emission levels at or below the 2005 standards. Five were standard MPI and 

one was GDI. Four motorcycles had 2-stroke engines and two had 4-stroke engines. Emission levels 

ranged from pre-1998 to post-1999, and one had an oxidation catalyst. Two Japanese test cycles were 

used for the cars – the 10.15 mode (hot test) and the 11 mode (cold test). The motorcycles used the 

Japanese motorcycle emission test. For the cars, there was no impact of ETBE at either level on 

emissions of HC, CO or NOx. Acetaldehyde emissions tended to increase with the addition of ETBE but 

the effect was not statistically significant. For the motorcycles, adding 17% ETBE caused a statistically 

significant reduction in CO emissions in four of the six motorcycles. Acetaldehyde emissions increase 

significantly at both levels of ETBE in all six motorcycles. The authors did not present details of the 

analysis or tabulate the quantitative results. 

 In a follow-up study, JCAP studied 8% and 16% ETBE along with 3% ethanol in two low emission 

Japanese cars [74]. One had standard stoichiometric MPI and the other was a stoichiometric GDI 

engine. Both had emission levels that were 50% of the 2005 Japanese standard. All fuels were blended 

to have controlled properties. Almost all comparisons showed that there was not a significant difference 

in emissions when ETBE or ethanol was used in either car. The authors concluded that ETBE or ethanol 

could be used without adverse effects. 

 JCAP [55, 84] also reported on unregulated emissions in one model year 2003 lean-burn GDI car, 

containing a NSR catalyst for NOx reduction. Three gasolines were tested - non-oxygenated, 8% ETBE 

and 3% ethanol. All other properties were the same for the three fuels. Speciated gaseous emissions 

and PM (mass, numbers and particle size distribution) were measured. The only statistically significant 

differences measured was that the ETBE fuel had higher acetaldehyde emissions in the 11 mode cold 

start test, and the ethanol fuel had higher acetaldehyde emissions in the cold start JC08 test. 
CRC Project E-84, Final Report 

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 Thummadetsak [33] compared MTBE (0%, 7.5%, 15%) and ethanol (0%, 7.5%, 15%) blended 

gasolines in six 1993-96 model year cars representing large market share in Thailand, using the 

European driving cycle. Results are shown below for the five catalyst equipped vehicles. These are 

consistent with other published work except for the impact of ethanol on formaldehyde emissions. 

 Percent Change in Emissions 

 MTBE Ethanol 

 0% → 7.5% 0% → 15% 0% → 7.5% 0% → 15% 

THC - 4.6 (NS) - 7.1 - 8.5 - 6.2 

CO - 6.6 (NS) - 16.8 - 23.2 - 31.9 

NOx + 7.0 + 7.2 + 10.7 + 15.2 

Benzene - 4.0 (NS) - 1.6 (NS) - 5.1 (NS) - 1.7 

1,3 Butadiene + 26.7 (NS) + 55.1 (NS) + 4.2 (NS) + 3.4 (NS) 

Formaldehyde + 36.3 (NS) + 20.1 (NS) + 28.9 + 37.2 

Acetaldehyde - 16.4 (NS) + 18.3 (NS) + 90.7 + 231.1 

 A number of studies have been carried out on two wheel vehicles. Subramanian et al. [85] tested 

four 2-stroke motor scooters and a 4-stroke motorcycle commonly sold in India. All met the Euro 2 

emission standards. Non-oxygenated, 5% ethanol and 10% ethanol gasolines were tested. Addition of 

ethanol lowered HC and CO emissions, especially at higher mileages. At low mileage, emissions of CO 

and HC were higher with the ethanol fuels. While the authors attributed this to the effects of aging and 

deposit build-up on the scavenging process in 2-stroke engines, no data was brought to support this 

argument. The ethanol fuels had higher emissions of acetaldehyde on both types of engines. NOx

emissions were not reported because they are generally low with 2-stroke engines. 

 Jaroonjitsathian [86] tested two samples each of three 4-stroke motorcycle models meeting the 

Euro 2 emission standard and common in the Thai market. A non-oxygenated gasoline was compared to 

a splash blended 10% ethanol blend. THC emissions were lower with ethanol on five of the six vehicles. 

For all the vehicles, CO emissions were lower and NOx emissions were higher. 

SUMMARY OF OXYGENATE EFFECTS

 In studies where composition and volatility were controlled, adding oxygenates to gasoline 

resulted in lower HC and CO emissions, even in cars with sophisticated engine control systems. 

Emissions of NOx tended to increase, although this effect was not measured in all programs. There was 

generally no difference between different kinds of oxygenates – alcohols or ethers. The most recent data 

suggest that the CO effect is still relevant, but the HC effect might be smaller or even non-existent in the 

newest technology. 

 While overall toxic emissions may be lower when oxygenates are added to gasoline, oxygenates 

usually increase emissions of aldehydes, the specific aldehyde depends on the chemical structure of the 

oxygenate. MTBE increases formaldehyde emissions and ethanol increase emissions of acetaldehyde. 

G. GASOLINE SUMMARY AND RESEARCH NEEDS 

 A great deal of high quality data exists to define the impact of gasoline composition on exhaust 

emissions from gasoline LD vehicles. Directional changes are summarized in the table below. Many of 

the compositional and volatility changes – except for sulfur and RVP – have both positive and negative 

impacts. Deciding which changes to implement for regulatory purposes requires knowledge of the 
CRC Project E-84, Final Report 

38

environmental needs, a detailed description of the vehicle fleet and a modeling tool that will provide a 

quantitative estimate. The most up-to-date model is the Predictive Model developed by ARB. [1] 

 To Reduce Emissions, Make the Directional Changes Shown Below 

 Aromatics Benzene Olefins Sulfur Oxygenates RVP T50 T90

HC ↓ * ↑ ↓ ↑ ↓ ↓ ↓

CO ↓ * ↑ ↓ ↑ ↓ ↓ ↓

NOx ↑ * ↓ ↓ ↓ 0 ↑ ↑

Toxics ↓ ↓ ↓ ↓ # * ↓ ↓

PM * * * * * * * * 

0 No effect 

* Data are lacking to estimate an effect 

# Data exist, but effect is variable 

 Knowledge gaps which may be closed by additional research are defined below. To a great 

extent, decisions about future research depend on a number of factors, including: 

• Environmental and regulatory requirements 

• Development and implementation of new vehicle technology 

• Advent of new fuels technology and blendstocks 

 Shown below are areas where existing data are sparse or non-existent and where additional data 

would help understand fuel effects. If new vehicle technology such as GDI is developed and 

implemented, then it is important to understand all fuel effects. 

• Effects of fuel parameters on PM emissions. Currently, this is not a particularly important 

area because gasoline vehicle PM emissions are still a small portion of the total 

inventory, but could become more important as diesel PM emissions drop. 

• Effects of RVP on emissions at all temperature levels. 

• Better understanding of the relative impacts of mid-fill volatility, back-end volatility and 

composition on exhaust emissions. 

• Long term effects of sulfur on aftertreatment components and exhaust emissions. This is 

probably not relevant for U.S., Europe and Japan, where sulfur levels are close to zero, 

but could be important for other regions as they consider future sulfur levels. 

 Appendix III contains a compilation of the number of vehicles and fuels tested in programs that 

evaluated various fuel properties. As shown in the table, very little recent data has been collected 

evaluating the effects of RVP and olefins on exhaust emissions. Furthermore, all the recent data on 

aromatics has been measured in GDI equipped vehicles. If these parameters are considered important, 

then additional data may be required before regulatory action is taken. 

4. DIESEL FUEL 

 Emissions data on diesel vehicles is arranged chronologically by fuel property. LD and HD diesel 

engines are described separately for each fuel property. This allows a natural comparison between the 

two engine types. For LD vehicles, research carried out since 1980 is considered. For HD engines and 

vehicles, research since 1998 and not covered in the review article by Lee et al. [4] is considered. 

 When blending diesel fuel for emissions studies, it is especially hard to blend fuel sets in which all 

properties are varied independently. Properties such as aromatics, density and cetane are correlated in 
CRC Project E-84, Final Report 

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the refinery streams typically used to make fully formulated diesel. A few studies have succeeded in 

separating these variables, but in many studies, the variables are not independent and the conclusions 

are therefore somewhat ambiguous. These three variables are considered together in this report. 

 Fluid physical properties such as density and viscosity play a role in diesel emissions, because 

they affect the way that fuel droplets form as they exit the injector nozzle. While density has been studied 

extensively – mostly in conjunction with aromatics and cetane – the effect of viscosity has not received 

much attention. This is likely because it is hard to control viscosity independently of other variables. 

Effects attributed to some of the more common variables, may, in fact, be caused by viscosity changes. 

 In the diesel cycle, combustion is initiated by compressing and heating the gas/liquid mixture in 

the combustion chamber where it ignites spontaneously in the absence of a spark. There are generally 

two combustion regimes. Homogeneous combustion is the burning of that portion of the fuel that has 

evaporated after exiting the fuel injector. Heterogeneous combustion involves burning at the fuel-vapor 

interface of the fuel droplets in the combustion chamber. The relative size of these two regimes is a 

function of engine operation and fuel properties, and can affect combustion chamber temperatures and 

emissions profiles. Some researchers have used this factor to help understand the impact of fuel 

properties on emissions. 

 The operation of diesel engines can be affected by fuel properties to a greater extent than in 

gasoline engines. For example, when fuel density changes, the volume of fuel injected to maintain the 

same power output also changes. Fuel density affects the timing and duration of injection, the onset and 

duration of combustion, and potentially the fraction of exhaust sent back to the combustion chamber 

through the EGR system. This complex relationship has been studied in a few research programs but is 

not fully understood. As diesel engines have become more sophisticated, with more electronic controls 

and feedback mechanisms, the relationships may change and should be studied more extensively. 

 Many studies have been carried out to evaluate specific fuels or fuel blendstocks – biodiesel, 

other oxygenates and Fischer Tropsch (FT) products. In most cases, these materials were splash 

blended into the diesel and no attempt was made to control fuel properties. While this approach may 

provide useful comparison information, it does not provide any understanding of how these materials 

affect emissions, and does not necessarily represent methods of future use. 

 Some papers and reports provided very limited information about the engines or vehicles that 

were tested. Minimum information should include make and model, along with year of production. 

Additional information about design features and emission standards would be helpful as well. 

 From a regulatory and environmental perspective, the important emissions from diesel engines 

are PM and NOx. HC and CO emissions are low in diesel engines, and emission standards for HC and 

CO are rarely limiting on engine design. For this reason, some papers did not report HC and CO 

emissions. PM emissions have been measured in a number of different ways using different instruments 

and sampling conditions. As shown by Chang and Van Gerpen [87], sampling conditions such as dilution 

ratio and temperature profile of the exhaust affect the measured PM emissions. A number of different 

instruments exist to measure and speciate PM emissions, and recent effort has attempted to standardize 

measurement techniques. Some older publications contain data collected with sampling and 

measurement methodology that is different from today’s common usage. 

 A number of excellent reviews have been published in recent years and are referenced in the 

appropriate sections. Among them are reviews by Rickeard et al. [88], Veinblatt et al. [89], Alleman and 

McCormick [90], and EPA [2, 91]. 

 A common problem in evaluating effects and trends is the large amount of seemingly conflicting 

data. Some of the differences seen are real, some are due to poor experimental design involving nonorthogonal fuel sets and limited fleet size, and some are due to incomplete statistical analysis. The best 

way to solve some of these problems is to conduct a meta-analysis and develop a unified statistical 

model. This is a difficult analysis, considering the wide variety of technologies, test cycles and fuel 

parameters found in the literature. In 2001, EPA published such an analysis for HD diesels [2]. 
CRC Project E-84, Final Report 

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A. DENSITY, CETANE AND AROMATICS 

 Density, cetane and aromatics are discussed together because they are very closely associated 

with each other and because many research programs varied them together. In many programs, these 

variables are correlated and it is not possible to distinguish the effects of one over another. As discussed 

below, there is disagreement in the literature about which is the most important variable in terms affecting 

emissions. 

 Cetane is a measure of the combustion property of diesel fuel, somewhat analogous to octane in 

gasoline. Cetane measures the ability of the diesel fuel to auto-ignite under controlled temperature and 

pressure conditions. The major measure is the time delay from fuel injection to auto-ignition in a single 

cylinder engine, compared to a series of reference fuels. Cetane can also be raised by adding cetane 

improvers to diesel. The most common improvers are organic nitrates, such as ethyl hexyl nitrate. One 

important question has been whether cetane achieved though additization has the same emission 

properties as cetane achieved through HC blending. 

 From an emissions perspective, higher cetane numbers mean earlier onset of combustion; 

hence. more complete combustion, but potentially higher emissions of NOx due to higher temperatures in 

the combustion chamber. Some research has questioned this line of reasoning, as discussed below. 

 Different measures of aromatics have been studied in the literature and the consensus is that the 

type of aromatics is at least as important as the amount of aromatics in total. A number of studies have 

examined the contribution of single ring aromatics (mono-aromatics) and multi-ring aromatics (di-, triaromatics, etc.) to emissions. Multi-ring aromatics are generically referred to as Polyaromatic 

Hydrocarbons (PAHs). 

LIGHT DUTY DIESELS

 Weidmann et al. [92] presented data on two Volkswagen Golfs tested with seven diesel fuels 

varying in volatility, density, aromatics and cetane number. Both vehicles were 1983 model year diesels 

with 1.6 liter indirect injection (IDI) engines – one was turbocharged and the other was naturally aspirated. 

The test cycle was the US FTP. The authors concluded that higher density fuels increased emissions of 

HC, CO, NOx and PM. Unfortunately, there was a high degree of correlation between aromatics and 

density, and between cetane and density, so the exact cause of the effects could not be determined with 

any certainty. 

 Betts et al. [93] tested 37 fuels in three IDI cars. One was a production vehicle and the other two 

were prototypes. The production car had EGR but no aftertreatment, while the other two cars had 

oxidation catalysts. The production vehicle met 1993 European emission standards. Fuels were blended 

for three test phases. In the first, parameters varied were cetane, aromatics, back-end distillation (T95), 

and density. In the second, sulfur was varied between 2 ppm and 2200 ppm. In the third, seven fuels 

were blended to reduce the correlation between density and aromatics that was present in the first phase. 

All fuels were tested in the production vehicle, and selected fuels were tested in the two prototypes to 

confirm earlier findings. The focus of the paper was particulates, and only some results were given for 

other emissions. 

 Based on the analysis of the data, fuel density was the most important fuel factor for PM 

emissions, and aromatics had no statistically significant impact. Two operating regions were described. 

In the first, density was within the vehicle’s calibration value and the air-fuel ratio in the engine was high 

enough to assure essentially complete combustion. In this region, there was a linear relationship 

between density and PM emissions with a relatively small slope. 

 In the second operating region, high density caused overfueling and there was incomplete 

combustion because of localized regions of poor mixing in the combustion chamber. PM emissions 

increased rapidly, along with emissions of HC and CO. The response to density was 10 times higher in 

the overfueling region than in the normal operating region. The equations below show the relationships in 

both regions: 
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 Normal operation: PM = - 0.464 + 0.558 x Density + 0.00023 x T95 + 0.022 x Sulfur 

 Overfueling: PM = -4.11 + 6.18 x Density + 0.00068 x T95 – 0.0061 x Cetane 

Ranges over which these equations were developed are shown below. 

Property Range in Tested Fuels 

Property Low High 

Density (g/ml) 0.81 0.87 

Cetane 44 62 

Aromatics (Vol. %) 10 35 

Sulfur (ppm) 2 2200 

 In addition to impacting PM emissions in the overfueling region, increasing cetane also reduced 

HC emissions up to about 48 CN, above which the response was relatively flat. 

 The same authors [94] carried out further tests using fuels from the same set, on a catalyst 

equipped version of the vehicle described above, designed to meet the 1996 European standards, and 

came to the same conclusion – that the most important factor for PM emissions was fuel density. While 

there was no correlation between density and total aromatics, the authors reported a strong correlation 

between density and multi-ring aromatics, and suggested that more research should be done to 

determine the contributions of multi-ring aromatics to PM emissions. 

 Tritthart et al. [95] reported that emissions varied with fuel properties in two diesel prototype cars 

with oxidation catalysts, turbochargers, EGR and electronic controls. One was direct injection and one 

was indirect injection. Emissions were measured over the US FTP the European NEDC and the 

Japanese 10.15 mode cycles. Seven fuels were tested; density varied between 0.826 and 0.845 along 

with aromatics, cetane, sulfur and T90. While there were differences among the fuels tested, density, 

aromatics and cetane were highly correlated and the authors could not ascribe the effects to any one 

property. 

 Lange et al. [96] tested one car, a 1991-1993 model year vehicle with indirect injection, EGR, and 

an oxidation catalyst, meeting Euro 1 emission standards. Although fuel properties were highly correlated 

in the overall fuel set, the authors used pairwise comparisons to conclude that PM emissions were 

affected by density and polyaromatics, but not mono-aromatics. There were no significant NOx effects. 

 Den Ouden et al. [97] published a summary of results on 22 European cars with 40 fuels, 

although not all cars were tested on all fuels. Twenty of the cars were indirect injection, and there was a 

mix of turbocharged/naturally aspirated, EGR and catalyst systems. A number of cars were tested with 

and without oxidation catalysts. Both European and U.S. emission test cycles were used across the fleet. 

Some of the data in this paper were part of a CONCAWE industry program. Only PM effects are 

discussed by Den Ouden et al., but CONCAWE [98] has discussed some of the same data and included 

gaseous emission results as well. 

 For the cars tested without catalysts, the most important fuel variable affecting PM emissions was 

density, and Den Ouden et al. developed the following relationship: 

 Polyaromatics were the second influential parameter, although of lesser importance than density. 

Cetane also seemed to have a small impact, reducing PM as cetane increased. Total aromatics did not 

influence PM emissions at all. 
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When the same analysis was carried out for the catalyst vehicles, the impact of fuel properties 

was much less and no one parameter could be distinguished as most important. This was attributed to 

the fact that the oxidation catalyst reduces PM by oxidizing the fuel-based organic particulates. Other 

components of the total PM such as sulfate, soot and ash become a higher fraction of the total and these 

are not as closely related to the non-sulfur portion of the fuel. 

 In CONCAWE’s analysis of gaseous emissions from six of the vehicles, increasing cetane 

reduced emissions of HC and CO. The effect was larger without oxidation catalysts in place. NOx was 

not affected at all by fuel properties. 

 CONCAWE [99] carried out a series of experiments to answer the question about the influence of 

density on emissions: whether density differences cause changes in engine controls which therefore 

affect emissions, or whether some other mechanism causes the impact on emissions. They tested a 

1992 model year European DI, 2.5 liter engine, with electronic injection control, oxidation catalyst and 

closed loop EGR. Two fuels, differing only in density, were tested. It was determined that fuel density 

affected the fuel pump setting, injection timing and EGR operation. With the advanced technology 

controls not in place, when engine operation was adjusted to compensate for the different fuel densities, 

the effect of changing densities on PM was almost totally removed (93%). When advanced technologies 

were in operation, only partial compensation was achieved (48%). The authors felt that the inability to 

achieve full compensation for density differences stemmed from their inability to fully control EGR 

operation in the advanced engine control unit (ECU). For NOx, they achieved 43% compensation of the 

density effect, but there was some uncertainty about the correction to be used for humidity variations, and 

they felt that the level of compensation might be higher with an accurate correction. 

 Mann et al. [100] carried out a similar program with a 1995 model year DI engine with 

electronically controlled fuel injection, turbocharging and closed loop EGR. The vehicle was mounted in a 

test bed and a sophisticated adjustable controller was used to set engine operating parameters, such as 

injection timing and duration, EGR rate, boost pressure and boost temperature. Fourteen steady state 

modes were chosen to approximate the operating conditions of the European test cycle. The engine was 

operated with four fuels having different densities and PAH levels. (Viscosity varied with density, and can 

be considered a surrogate variable.) The density change increased EGR rate and advanced injection 

timing. Emissions were measured with normal engine operations and when these operating changes 

were reversed. The impact of density on emissions is shown below. Except for CO, the impact of density 

on emissions is reduced by at least half, when operations are adjusted for the different fuel properties. 

The authors argued that some of the EPEFE results described below, where there was a large variation 

in individual vehicles’ response to changes in fuel properties, could be explained by differences in 

calibration strategies for the engine control algorithms. 

Percent Change in Emissions 

Density: 0.86 g/ml → 0.83 g/ml 

 Normal Tune Controlled Tune 

HC - 7% + 3% 

CO - 7% - 8% 

NOx + 8% - 4% 

PM - 12% - 5% 

 This work was further extended by Kwon et al. [101] to other properties including cetane and 

aromatics. In order to collect more data, only four steady state modes were tested. Only results for the 

controlled tune case were presented in the paper, as shown below. In the fuel set, T95 and density were 

highly correlated, so that it is not possible to determine whether the effect shown below is caused by 

changes in density or T95. 
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 Percent Change in Emissions 

Density 

Change 

( - 0.03 g/ml) 

CN Change 

(+ 10 CN) 

Mono 

Aromatics 

(- 7%) 

Polyaromatics 

(- 7%) 

HC - 7% - 7% 0 + 3% 

CO - 15% - 17% 0 + 2% 

NOx - 2% - 3% - 1% - 3% 

PM - 14% + 3% - 4% - 9% 

 Since the authors did not provide the impact of fuel properties in the vehicles in the absence of 

special adjustments, it is not possible to evaluate the importance of compensation in this program. It is 

also difficult to generalize from the results in one vehicle to project how important this effect is in the 

general population. 

 Tsurutani et al. [102] tested one Japanese car - a Toyota Corolla with a naturally aspirated 2 liter 

engine, with EGR but no catalyst – with 45 different fuels. These included fully blended fuels, narrow 

boiling fuels, cetane improved fuels and splash blended fuels. For the splash blended HC fuels, 5% of 

individual compounds were added. Paraffin compounds were n-octadecane (C18H38) and n-eicosane 

(C20H42). The mono-aromatic was dodecylbenzene (C18H30), the di-aromatic was naphthalene (C10H8) 

and the tri-aromatic was phenanthrene (C14H10). The volatility properties are similar and small amounts 

were used, so other fuel properties did not change and it was possible to determine the effects of heavy 

aromatics. European and Japanese test cycles were used. 

 NOx emissions were reduced when total aromatics were lowered. Neither cetane nor total 

aromatics had any effect on PM emissions. Cetane improver lowered PM emissions when starting from a 

relatively low value (46 CN). The fuels in which 5% of individual compounds were added showed that diand tri-aromatics are important contributors to PM emissions, as shown below. 

Added Compound

(5% each) 

Percent Change 

in PM Emissions 

n-octadecane - 1 

n-eicosane + 2 

dodecylbenzene + 3 

naphthalene + 14 

phenanthrene + 22 

 It is clear from these data that paraffins and total aromatics do not have a compositional effect on 

PM emissions, and that polyaromatic compounds increase PM emissions. Regression analysis showed 

that the relative contribution of mono-aromatics/di-aromatics/tri-aromatics was 1/4/5. 

 Although the results were not analyzed in terms of density, fuel density was correlated with 

aromatics, T90 and to a lesser extent cetane, in the fuel set. Therefore, any conclusions about total 

aromatics and density may be somewhat ambiguous. 

 The EPEFE program was a large, well-designed program that tested 19 LD diesel vehicles with 

eleven fuels [103]. The vehicles consisted of 17 passenger cars and 2 LD trucks, and contained a mix of 

DI and IDI, naturally aspirated and turbocharged, electronic and mechanical injection systems. All the 

vehicles had oxidation catalysts, and met the relevant European 1996 emission standards. The fuels 

were blended from refinery streams and consisted of three sets designed to determine the first order 
CRC Project E-84, Final Report 

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(linear) impacts of density, polyaromatics, cetane, and back-end volatility measured as T95. The 

European driving cycle was used. The impacts on fleet average emissions are shown below: 

 Percent Change in Emissions 

Density (g/ml) 

0.855 → 0.820 

PAH 

8% → 1%

Cetane 

51 → 58

HC - 18 + 5.5 - 25 

CO - 16 + 4 - 26 

NOx + 2 - 3.3 0 

PM - 19 - 5 + 5 

 As in other large fleet tests, there were differences, mostly in magnitude and not direction, among 

the vehicles in response to fuel properties. Vehicles with electronic injection systems (mostly DI) were 

generally more sensitive to fuel property changes than those with mechanically controlled injection 

systems (mostly IDI), except for NOx where the reverse was true. One difference was that increasing 

cetane reduced NOx in the DI vehicles and increased NOx in the IDI vehicles. Speciated emissions were 

measured but too few tests were carried out to allow a meaningful statistical analysis. 

 An additional experiment was carried out in which engine tuning was adjusted for different density 

fuels. The response of emissions to density was reduced, but not eliminated, when engine calibrations 

were changed to match densities on three of the more advanced vehicles. The authors were not certain 

whether there is some residual effect that did not respond to calibration adjustment or whether further 

refinement of the calibration would eliminate the density effect totally. 

 As a follow-up to the EPEFE program, CONCAWE [104] tested three diesel cars equipped with 

DI, turbocharged engines. Emissions were controlled with EGR and oxidation catalysts. One car had a 

rotary pump, the second had a common rail injection system, and the third had unit injectors. Although 

built to meet Euro 2 emission standards, the cars had emission levels at or near Euro 3 standards. 

Eleven fuels were carefully blended in two matrices to study cetane (natural and additized) and aromatics 

composition. Results are summarized below for the average responses. Increasing cetane number had 

no measurable effect on NOx and PM, and reduced THC and CO emissions. There was no difference 

between natural and additized cetane. Reducing aromatics only affected CO emissions. No difference 

was seen between the effect of mono-aromatics and poly-aromatics. The differences between these 

results and those obtained in EPEFE were noted, and might be due to the differences in technology or to 

the small number of vehicles tested in this study. 

 

 Percent Change in Emissions 

Cetane Number

49 → 58 

Total Aromatics

30% → 12% 

HC - 15 NS 

CO - 50 - 10 

NOx NS NS 

PM NS NS 

 Tamanouchi et al. [105] tested two 1992 model year cars made for sale in Europe on fuels with 

varying levels of di- and tri-aromatics. Both cars had IDI engines, with turbochargers and oxidation 

catalysts. PAH concentration varied between 0.7% and 12.4%. The authors concluded that reducing 

PAH lowers emissions of THC, CO and PM, and that the effect was smaller when oxidation catalysts 

were in place, although no quantitative results are discussed and no statistics are provided. 
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 Oyama and Kakegawa [106] reported the results of Phase 1 of JCAP in which 13 fuels were 

tested with 14 LD vehicles meeting either 1990-94 or 1997-99 emission regulations. Results were not 

presented in a quantitative fashion, but the study concluded that lower aromatics reduced PM, and NOx

was unchanged. Within the aromatics, tri-aromatics had a larger impact than di-aromatics, which had a 

larger impact than mono-aromatics. 

 In the second Phase of JCAP [107], three diesel cars with advanced aftertreatment were tested. 

One had a NOx Storage Reduction (NSR) catalyst, one had a DPF and one had both. All three had 

electronic injection control and two of them had common rail fuel distribution. Emissions levels were half 

of the 2003 Japanese standards. Fuel effects were generally small. When cetane was increased from 47 

to 54, PM emissions increased by 13% in one of the cars. Other effects were listed as less than 10% and 

not quantified in the paper. 

 JCAP [108, 109] also tested a LD DI diesel engine with common rail injection and equipped with a 

NSR catalyst, DPF and oxidation catalyst aftertreatment system. Reducing total aromatics from 19% to 

5% reduced engine-out PM, THC, CO and NOx emissions. Reducing aromatics reduced tailpipe NOx

emissions, but the effect on tailpipe PM, THC and CO could not be measured because of the extremely 

low levels of emissions. In a second series of tests, the effect of aromatics on NOx did not show a clear 

trend. 

 Nakakita et al. [110] tested ten carefully blended fuels in three single cylinder diesel engines, one 

of which had an optical port to allow spectroscopic measurements. The molecular structure of the fuels 

was carefully controlled to evaluate chemical composition, including ring structure and paraffinic content. 

Emissions were measured at a number of steady state conditions. Nakakita et al. concluded that under 

low and medium load conditions, cycloparaffins (naphthenes) have a higher PM formation tendency than 

paraffins, and that all aromatics have a higher PM formation tendency than paraffins. Two-ring aromatics 

showed a higher PM forming tendencytendency than one-ring aromatics. When cetane values were very 

high, leading to short ignition delay, PM emissions tended to be higher. 

 Bielaczyc et al. [111] tested one LD diesel car meeting Euro 3 emission standards with fuels 

having cetane number between 45 and 63. The engine was 2.0 liter, DI, common rail injection, 

turbocharged, with closed loop EGR and an oxidation catalyst. The European NEDC test cycle was 

used. Increasing cetane number reduced emissions of HC, CO and NOx. The effect seemed to be larger 

when the cetane number was above 50. On the other hand, PM increased up to 55 CN and stayed 

relatively constant above that level. No statistics were provided for this analysis, which is not fully 

consistent with other test results. However, even if the results are statistically significant, tests on a single 

vehicle cannot be used to generalize trends. 

HEAVY DUTY DIESELS 

 In a review of work published prior to 1998, Lee et al. [4] summarized fuel impacts on HD diesel 

engines. The summary is shown below for what were described at the time as “low emission engines.” 

 Change in Emissions 

Reduce 

Density 

Reduce 

Aromatics/PAH 

Increase 

Cetane 

HC ↑ 0/↓ 0 

CO ↑ 0/0 0 

NOx ↓ ↓/↓ ↓

PM 0 0/0 0 
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Lee did not attempt to carry out a full analysis of all the previous work but attempted to identify common 

trends. One of the main research programs relied upon by Lee was the EPEFE program on HD diesels 

[112]. 

 Ryan et al. [113] tested one HD engine with thirteen fuels varying in natural cetane, additized 

cetane, total aromatics, and aromatics type. The engine was a Caterpillar 3176B model, with electronic 

unit injectors. It was tested without EGR to simulate the 1994 emissions standards and with EGR to 

simulate an engine meeting 2004 emission standards. Seven steady state modes were averaged to 

approximate the FTP transient test. PM emissions were measured at only two of the modes. 

 The range of variables is shown below: 

 Min. Max. 

Aromatics, vol. % 9.5 30.9 

Di-aromatics, vol. % 0.5 7 

Density, g/ml 0.8131 0.8636 

Cetane 35.9 56.3 

 A graphical analysis did not show any relationship between fuel properties and emissions for 

either engine configuration. A regression analysis showed that there were some significant variables – 

NOx was correlated with aromatic type, and HC were negatively correlated with cetane. The correlations 

were in general very weak and did not explain much of the variability. In addition, no data were provided 

on the amount of correlation between variables. 

 Tanaka et al. [114] tested one HD, large volume Japanese engine with 28 fuels. It was an 8 liter 

DI, turbocharged engine with no EGR or aftertreatment, meeting 1994 Japanese emission standards. 

The Japanese 13 mode cycle was used for testing emissions. Low sulfur blends and kerosene blends 

were tested, but properties did not vary in a way that would allow any conclusions about effects of 

individual properties. Sixteen fuels were blended to vary concentrations of mono-aromatics, di-aromatics, 

tri-aromatics, and naphthenes. In addition to changes in aromatic concentrations, there were also 

attendant changes in density. Although the conclusions are stated in terms of aromatics, it is possible 

that effects of density are confounding the results. 

 There was a strong correlation between emissions of individual PAH compounds and triaromatics in the fuel, and weaker correlations with other classes of aromatics. The authors suggested 

that the influence of tri-aromatics in the fuel on PAH emissions was promoted by the presence of diaromatics, mono-aromatics and naphthenes. In the absence of tri-aromatics, these other classes would 

not impact PAH emissions. 

 The USEPA Heavy Duty Emissions Work Group [115, 116] (HDEWG) tested a HD engine that 

was representative of 1994-1997 technology and was designed to meet strict NOx emission standards. It 

was a 10.3 liter, DI, turbocharged engine with central electronic injection control and EGR. Eight steady 

state test conditions were chosen to provide good correlation to the FTP. Eighteen fuels varying in 

cetane, density, mono-aromatic content and PAH content were blended in a fractional factorial design 

with excellent orthogonality in the design variables. PM emissions were not reported. Regression 

analysis showed the following significant effects: 
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 Percent Change in Emissions 

Density (g/ml) 

(0.86 → 0.83) 

Mono-Aromatics

(25% → 10%) 

Poly-aromatics

(11% → 3%) 

Cetane 

(45 → 52) 

HC NS - 6.4% - 7.4% - 12.4% 

CO NS NS NS - 13% 

NOx - 4.7% - 3.7% - 2.2% + 1.3% 

 There were no significant differences between natural and additive-based cetane. 

 Mitchell [117] reported on a Canadian program that tested four HD engines with fifteen fuels. 

One engine was the same one as tested in the EPA HDWG program described above. The focus of the 

program was to compare fuels prepared from oil sands with those prepared from crude oil, but three 

levels of aromatics were also evaluated (10%, 20%, and 30%). The program concluded that NOx

emissions were best predicted by aromatics. While cetane number affected NOx emissions in each of the 

engines, the effect was in opposite direction for the engines. Fuel density had an effect on PM in two 

engines, but did not in the others. A further complication in understanding the results was that there was 

a strong correlation between aromatics and density in the fuel set. 

 JCAP [106] tested eight truck engines with sizes ranging from 3.6 to 21 liters. Seven were DI, 

three were turbocharged, one had mechanical injection and three had EGR. They were chosen to 

represent the existing Japanese truck fleet meeting 1990-1994 or 1997-1999 emission standards. The 

Japanese 13 mode test was used. For these engines, five fuels varying in T90 and aromatics were tested. 

 Regression analysis showed that reducing aromatics lowered NOx emissions 

(~0.5%NOx/%aromatics) and had a variable effect on PM emissions. In this fuel set, however, there was 

a strong correlation (0.9) between total aromatics and PAH, so it is not possible to ascribe the aromatics 

effects to total aromatics alone. There was also a correlation between aromatics and density (~0.6) and 

cetane number (- 0.7) that may confound the analysis. 

 In the second part of the program, called the Model Step, two engines representing 1997-1999 

emissions regulations were tested with nine fuels varying in types of aromatics, T90 and sulfur. Density 

and cetane were held within a narrow range and the design variables had low levels of correlation among 

them. Aromatic type had a very marked effect on PM, with tri-aromatics having a larger impact than diaromatics and mono-aromatics (5/2/1). Effects on NOx were mixed, as in the previous experiment. 

 JCAP II evaluated advanced diesel technology [107]. Three truck engines varying in size from 

3.8 liter to 15.7 liter were tested in 14 fuels using the 13 mode emissions test. Three aftertreatment 

systems were tested – DPF, DPF with Urea-SCR, and NSR. Emission levels were targeted to be 1⁄2 the 

2003 Japanese standards. They reported that between values of 47 and 54, cetane number had no 

impact on NOx or PM emissions. 

 JCAP II evaluated NOx emissions with a Urea-SCR aftertreatment system in a 13 liter engine 

with unit injectors and no EGR [108]. Three aromatic levels (5%, 9%, and 19%) were tested using the 

D13 emissions cycle. Density was held constant, but cetane number varied between 57 and 63. No 

impact of aromatics was seen on NOx emissions with the Urea-SCR system in place. 

 Using the same fuel set, JCAP II tested a 9 liter DI turbocharged engine having a common rail 

injection, EGR and a Urea-SCR/DPF aftertreatment system. Two test cycles were used – steady state 

D13 and transient JC05. The only emissions component affected by aromatics content was PM. As 

aromatics were reduced, engine-out PM decreased. Tailpipe emissions were so low with the DPF that 

effects were hard to determine. 

 JCAP [55] measured PM numbers and size distribution in a 9 liter turbocharged engine with EGR 

and Urea-SCR aftertreatment. The engine met 2005 emission standards. No PM emission differences 

were seen between fuels having 10% and 18% aromatics. 
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Khalek et al. [118] tested two 1997/98 model year HD engines with fourteen fuels. One engine was a 6 

liter, turbocharged engine with a mechanical injection system. The other was a 12.7 liter, turbocharged 

engine with unit injectors. Neither engine had aftertreatment systems or EGR. The hot start EPA 

transient test was used. The fuels varied in cetane number, sulfur and aromatics (total and PAH). T90

also varied, but was not reported in the results. The objective of the program was to develop low 

emissions fuels, but some regression analyses were shown. The responses that were similar for both 

engines are shown below. Regression results were not reported for cells that are blank. 

Percent Change in Emissions 

(Average of 2 engines) 

Total Aromatics

(25% → 15%) 

Poly-aromatics 

(11% → 3%) 

Cetane 

(49 → 63) 

HC - 28% 

CO 

NOx - 7% - 6% 

PM NS NS NS 

 As a follow-up to the EPEFE program, CONCAWE [104] tested two HD European DI engines 

equipped with turbochargers. One engine had a high pressure inline pump with no EGR, and the other 

had unit injectors and a cooled EGR system. Neither engine had aftertreatment devices, and both were 

designed to meet the Euro 3 emission standards. Eleven fuels were carefully blended in two matrices to 

study cetane (natural and additized) and aromatics composition. The European Steady State Cycle 

(ESC) and European dynamic Load Response (ELR) test for smoke were used. Results are summarized 

below for the average responses. Increasing cetane number had no measurable effect on any of the 

emissions responses. There was no difference between natural and additized cetane. Reducing 

aromatics only affected HC emissions. No difference was seen between the effect of mono-aromatics 

and poly-aromatics. The differences between these results and those obtained in EPEFE were noted, 

and might be due to the differences in technology or to the small number of vehicles tested in this study. 

 Percent Change in Emissions 

Cetane Number

49 → 58

Total Aromatics

30% → 12%

HC NS - 20% 

CO NS NS 

NOx NS NS 

PM NS NS 

 Kono et al. [119] tested three Japanese DI engines with high pressure injection, two of which 

were turbocharged. All three had EGR, one had an oxidation catalyst, and one had a complex NSR/DPF 

aftertreatment system. The engines were designed for 2 ton cargo trucks; one met the 1998 Japanese 

standards, and the other two met the 2003 standards. All were tested using the JC08 emissions test, 

which involved both cold and hot start operation. Eight fuels were tested, three of which varied in 

aromatic levels (5%, 9%, 16%). Other properties such as density and cetane number varied as well in 

the fuel set, so it is not possible to unambiguously assign effects to individual properties. No detailed 

analysis is presented, but the authors reported that fuel effects on NOx and PM were small (less than 

5%). With the DPF engine, the emissions were too low to be able to discern any fuel effect. They did not 

report any effects of aromatics on HC or CO. 
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 The engine with the oxidation catalyst was tested on the same set of fuels with a number of 

different driving cycles including the JC08, JE05 (HD cycle), USFTP, USHFET, TMG (Japanese city 

driving). Further statistical analysis was also carried out with respect to non-design variables. Kono et al. 

concluded that di-aromatics increased PM emissions and the soot component of these emissions. Higher 

levels of Cetane Index, but not cetane, was found to reduce emissions of NOx and THC. It was not clear 

why cetane index correlates with NOx, and not cetane. Also, the authors acknowledge that there are 

significant correlations among the expanded set of fuel variables. 

 Massa et al. [120] tested two engines commonly used in buses in Brazil. Both were naturally 

aspirated engines, about 4 liter displacement, with no EGR or aftertreatment and contain Euro 3 

technology. One had a common rail injection system and the other had unit injectors. Six fuels were 

tested, varying in cetane between 42 and 48, achieved through blending and with additives. Other 

properties were held constant. Analysis of Variance showed that there were no significant effects of 

cetane on exhaust emissions of HC, CO, NOx or PM. 

SUMMARY OF EFFECTS OF AROMATICS, CETANE, DENSITY

 Taken together, the fuel properties of density, aromatics, especially PAH, and cetane have been 

shown to affect diesel emissions, although contradictory results have been published, as described in 

some detail by Kono et al. [121]. A number of factors combine to make the literature difficult to 

summarize: 

• Lack of orthogonality among variables in many studies. 

• Small number of vehicles/engines tested in many programs combined with high variability 

in vehicle response makes comparisons difficult, because different results are to be 

expected. 

• Different test cycles have been used, some of them representing steady state conditions 

and some representing transient, some containing cold start driving and some containing 

only hot start driving. 

 With these caveats in mind, the following tables are meant to provide a general consensus of 

research for LD and HD diesels. 

 Change in Emissions – Light Duty Diesels 

Reduce 

Density Reduce PAH Increase 

Cetane 

HC ↓ ↑ ↓

CO ↓ ↑ ↓

NOx ↓ 

PM ↓ ↓ ↑

 The table above shows only PAH, not total aromatics. It seems likely that the impact of aromatics 

is caused to a greater extent by the multi-ring aromatics than by the single ring aromatics. 

 The LD effects are for vehicles without aftertreatment. The effects seem to be smaller and more 

difficult to measure with aftertreatment systems in place. They can also be different. Oxidation catalysts 

oxidize the organic fraction of PM, so that PAH, which may contribute to this fraction, would have less of 

an impact with oxidation catalysts in place. All PM effects are small and difficult to measure with DPF in 

place. LD diesels have also been studied to understand the impact of fuel properties on engine 

operation. As technology develops, it is expected that engine operation will be increasingly controlled by 

feedback control mechanisms, and the impacts of fuel properties may change. 
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Finally, development and deployment of new aftertreatment devices to meet strict emissions standards 

may also change the responses. 

 A summary table for HD diesels is shown below: 

 Change in Emissions – Heavy Duty Diesels 

Reduce 

Density Reduce PAH Increase 

Cetane 

HC ↓ ↓

CO ↓

NOx ↓ ↓ 

PM ↓ 

 Reducing density lowers NOx emissions, and reducing PAH content lowers NOx and PM 

emissions, while increasing cetane lowers HC and CO emissions; but there is some evidence that in 

modern engines, PM emissions may increase, as seen more often in LD diesels. As engines become 

more sophisticated in terms of injection control, fuel effects on emissions seem to be lessened. This is 

especially true for DPF systems that have a very high efficiency for PM reduction and are probably not 

much affected by changes in PAH content. 

 More data should be obtained on a wide variety of diesels with aftertreatment systems such as 

Urea-SCR and/or NSR catalysts using the latest test cycles which employ transient and cold start driving 

patterns. 

B. SULFUR 

 Under combustion conditions, sulfur compounds in diesel fuel burn mostly to form the gas sulfur 

dioxide. A relatively small fraction (1%-2%) is oxidized further to sulfur trioxide or sulfuric acid, which may 

associate with water molecules or dissolve in liquid water. The fraction of sulfur that is emitted as sulfate 

also depends on the temperature and oxygen content of the gas stream. Sulfur in diesel fuel can affect 

emissions in two ways. First, sulfates in the exhaust may react with other components in the exhaust to 

form solid sulfate particles. Sulfuric acid droplets may act as nucleation sites for particle formation and 

growth. Sulfur dioxide in the exhaust can be further oxidized over certain catalysts to form additional 

sulfate particles. 

 Diesel sulfur may also affect emissions by degrading the operation of various aftertreatment 

devices. NOx catalysts, especially storage devices are particularly sensitive to sulfur and lose efficiency 

as sulfur rises to 50 ppm and above. Diesel particulate filters are also somewhat sensitive to sulfur and in 

addition to reducing efficiency, high sulfur levels may also cause overall fuel economy to suffer when the 

filter needs to regenerate more often. 

 The effect of sulfur is relatively easy to study, since sulfur levels can be changed without 

impacting other properties. Earlier work focused on the impact of sulfur on particulate mass and 

composition, and more recent work has evaluated the impact of sulfur on aftertreatment devices. When 

typical diesel sulfur levels were 2000 ppm or more, reducing the sulfur content of diesel fuel could have a 

direct impact on PM emissions. However, at levels of 500 ppm and below, the impact of changes in 

sulfur concentration on PM emissions is fairly small. 

LIGHT DUTY DIESELS

 Betts et al. [93] and Fløysand et al. [94] tested a European car with and without an oxidation 

catalyst. The car had an IDI engine with EGR and met the European 1993 emission standards in the 
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non-catalyst configuration. Sulfur had a weak but measurable effect on total PM emissions, and the 

effect was slightly larger with the oxidation catalyst. 

Percent change in PM Emissions

2000 ppm S → 100 ppm S 

Without Catalyst With Catalyst 

- 4.0 - 12.1 

 The National Renewable Energy Laboratory of the Department of Energy (NREL/DOE) 

sponsored DECSE (Diesel Emissions Control – Sulfur Effects Project), which studied sulfur effects in LD 

and HD diesel engines with advanced prototype aftertreatment systems. Results have been published in 

a number of papers and reports [122-124]. One LD vehicle was tested with a 1.9 liter prototype 

DaimlerChrysler engine and a NOx Adsorber Catalyst system. The program measured catalyst efficiency 

and desulfurization effectiveness after aging with sulfur levels of 3, 16, 30 and 78 ppm. In general, with 

higher sulfur levels, the catalyst had to be regenerated more often. Results showed that at 30 ppm and 

below, desulfurization could restore efficiency to acceptable levels. After aging on 78 ppm sulfur, 

exposing the catalyst to repeated desulfurization procedures caused a decline in performance. This 

behavior was not observed with the fuels at or below 30 ppm. It is not known whether this is a feature of 

the specific catalyst tested or whether the behavior is more general. 

 JCAP [106] studied one car with two levels of sulfur, 0 ppm and 400 ppm. The engine was 

naturally aspirated IDI, 2.0 liter with EGR, and was tested with an oxidation catalyst. Relative to the high 

sulfur fuel, the zero sulfur fuel did not affect HC, CO or NOx emissions, but did lower PM emissions by 

45%. With the zero sulfur fuel, sulfate particulate in the exhaust disappeared, the soluble organic fraction 

(SOF) dropped by about 50% and dry soot more than doubled. Reasons for the impacts on SOF and 

soot were not explored. 

 Sluder and West [125, 126] tested a 1999 model year European Mercedes A170 diesel with fuels 

having sulfur levels of 3, 30 and 150 ppm. The engine was a turbocharged, DI design with EGR and two 

oxidation catalysts. Although not certified in the U.S., emissions fell within U.S. Tier 1 standards. In 

addition to the standard configuration, the vehicle was also tested separately with a catalyzed DPF and 

with a NOx adsorber catalyst. Three test cycles were used – FTP, US06 and SC03. 

 In the range of sulfur tested, there was a small (<10%) effect on PM emissions. With the DPF in 

place, there was no degradation in emissions even for the highest sulfur fuel. PM emissions were so low 

(<0.006 g/mi) that background levels were 30%-50% of the measured values. 

 With a NOx adsorber catalyst in place, emissions of NOx were very sensitive to sulfur. After the 

equivalent of 3,000 miles on 30 ppm sulfur, NOx reduction efficiency went from 92% to 80% on the FTP 

and from 89% to 66% on the US06 cycle. Desulfurization cycles were not investigated. 

 Kwon et al. [101] evaluated sulfur effects with a 1995 model year European DI engine with 

electronic controlled fuel injection, turbocharging and closed loop EGR. The vehicle was mounted in a 

test bed and a sophisticated adjustable controller was used to set engine operating parameters, such as 

injection timing and duration, EGR rate, boost pressure and boost temperature. Fourteen steady state 

modes were chosen to approximate the operating conditions of the European test cycle. Reducing sulfur 

levels from 1000 ppm to zero reduced PM emissions by 13%. The same sulfur reduction also increased 

emissions of CO by 6%, although the specific doping compound used – t-butyl disulfide – may have been 

a factor. 

 Bielaczyc et al. [111] tested one LD diesel car meeting Euro 3 emission standards with four fuels 

having sulfur levels of <5, 50, 350 and 2000 ppm. The engine was 2.0 liter, DI, common rail injection, 

turbocharged, with closed loop EGR and an oxidation catalyst. The European NEDC test cycle was 

used. The biggest impact of sulfur was on PM emissions. The 2000 ppm fuel had PM emissions that 

were about 25% higher than any of the other fuels. Sulfur levels did not affect HC or CO emissions for 
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the entire NEDC driving cycle, but there was a small mass effect in the EUDC portion of the cycle where 

catalyst temperatures are higher. Lower sulfur was associated with lower NOx emissions, although the 

effect was smaller for NOx than PM. 

 In the second Phase of JCAP [107] three modern diesel cars with advanced aftertreatment were 

tested. One had a NSR catalyst, one had a Diesel Particulate Filter (DPF) and one had both. All three 

had electronic injection control and two of them had common rail fuel distribution. Emissions levels were 

half of the 2003 Japanese standards. Five fuels were tested having sulfur levels of 10, 46, 94, 298 and 

443 ppm. Both instantaneous emissions and mileage accumulation tests were carried out with different 

sulfur levels. Between 445 ppm and 45 ppm, changes in all emission levels were less than 10%. 

Reducing sulfur also appeared to result in increased levels of the soot component of PM emissions. For 

the vehicle containing a NSR catalyst, catalyst efficiency was significantly higher when using 10 ppm 

sulfur than when using 100 ppm sulfur for 30,000 km. 

 Ntziachristos et al. [40] reported results of the European Particulates project. Fourteen LD 

diesels were tested with four fuels having sulfur levels between 3 ppm and 280 ppm. Three were in the 

same base fuel (8, 38 and 280 ppm) and a fourth was Swedish Class 1 diesel (3 ppm) for reference. 

Most of the cars met or exceeded Euro 3 emission standards, and five were equipped with DPF 

aftertreatment. Over the range of sulfur levels studied, there were no effects of sulfur on PM mass 

emissions for vehicles with or without DPF. When considering particle numbers, the 280 ppm sulfur fuel 

had higher emissions than the other fuels, even with DPF. The higher numbers were due to increases in 

nucleation size particles (< 50 nm diameter). 

 JCAP [108] studied the impact of sulfur on a NSR-DPF catalyst system in a 4 liter LD DI engine, 

with cooled EGR and a common rail injector system. Three sulfur levels were tested: <1, 7 and 46 ppm. 

Two cycles were used for emissions evaluation – the steady state D13 and the transient JE05. Mileage 

was accumulated up to 50,000 km on all three test fuels. Desulfation intervals were geared to each fuel’s 

sulfur concentration. After 50,000 km, there was a significant difference in catalyst efficiency when tested 

on the higher temperature D13 test. NOx conversion efficiency was 85%, 78% and 60% for 1, 7 and 46 

ppm, respectively. There did not appear to be a difference when tested using the JE05 cycle, which had 

conversion efficiencies of about 40%. By varying the desulfation intervals in a second series of tests, the 

authors were able to conclude that the main reason for loss of conversion efficiency is the heat generated 

during desulfation. Since lower sulfur fuel requires less frequent desulfation, conversion efficiency is 

maintained at a higher level when sulfur is low. 

 A passenger car was also tested on the same three fuels over the new Japanese emissions test 

cycle CD34 (JC08) as well as the older 10/15 mode test [109]. After 30,000 km, NOx conversion 

efficiencies were 40-55% for the 46 ppm fuel, and 82-99% for the 1 ppm fuel, depending on test cycle. 

Fuel economy was 5% worse with 46 ppm sulfur than with 7 ppm sulfur fuels. 

HEAVY DUTY DIESELS

 Lee et al. [4] concluded that in the absence of aftertreatment, fuel sulfur only affected the sulfate 

portion of PM emissions. 

 Liang et al. [122] reported the results of DECSE tests with two DPF systems. One was a 

catalyzed DPF system (CDPF) and the other was a continuously regenerating DPF system (CR-DPF). All 

tests were conducted in a Cat 3126 engine, which is usually used in applications that result in low 

exhaust temperatures. This was considered a severe test for the DPF systems. Five sulfur levels were 

tested in a common fuel over the OICA 13 mode test cycle. On an engine-out basis, raising sulfur from 3 

ppm to 350 ppm increased PM emissions by 29%. No changes were observed in the other emissions. 

The impact of sulfur on DPF efficiency is shown below: 
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 PM Efficiency* 

Fuel Sulfur C-DPF CR-DPF 

3 ppm 95% 95 % 

30 ppm 74% 72 % 

150 ppm 0 - 3% 

300 ppm - 122% - 155% 

* Relative to engine-out emissions with 3 

ppm sulfur fuel 

 Efficiencies in the table above can be less than zero because the reference point is the emissions 

with 3 ppm sulfur fuel. Approximately 40-60% of the fuel sulfur was converted to sulfate, thereby 

increasing emissions when sulfur levels are high. 

 The DECSE program also tested lean NOx catalysts in two engines, a Cummins ISM370 and a 

Navistar T444E [123]. The NOx conversion was not affected significantly by changing sulfur levels but 

PM emissions were significantly affected. With 150 ppm and 350 ppm sulfur fuels, engine-out emissions 

of sulfate increased by a factor of 2-5, and tailpipe emissions of sulfate increased by a factor of 10-20. 

These increases are the result of oxidation of SO2 to sulfate over the catalyst system. 

 Khalek et al. [118] tested two 1997/98 model year HD engines with fourteen fuels. One engine 

was a 6 liter, turbocharged engine with a mechanical injection system. The other was a 12.7 liter, 

turbocharged engine with unit injectors. Neither engine had aftertreatment systems or EGR. The hot 

start EPA transient test was conducted. When sulfur was reduced from 500 ppm to 50 ppm, the smaller 

engine had no response to sulfur content, while in the larger engine, PM emissions decreased by 15%. 

Other emissions were not affected. 

 In JCAP II [107] two truck engines with advanced aftertreatment were tested for sulfur effects. 

One engine had electronic injection control with EGR and a DPF. The other had common rail fuel 

distribution with no EGR and DPF+Urea/SCR aftertreatment system. Emission levels were half of the 

2003 Japanese standards. Four fuels were tested having sulfur levels of 46, 94, 298 and 443 ppm. For 

mileage accumulation, fuels having sulfur levels of 9, 44 and 95 ppm were tested. Lowering sulfur 

reduced PM emissions in both engines and did not affect NOx emissions. Mileage accumulation results 

were different for the two engines. For the engine with the DPF aftertreatment, higher sulfur levels 

increased backpressure over the DPF catalyst. After 30,000 km with 100 ppm sulfur, backpressure was 

60 kPa, versus 20 kPa with 10 ppm fuel. The increased backpressure tended to raise the exhaust 

temperature and lower the conversion of NO to NO2. The net effect was that there was no deterioration in 

PM emissions with 100 ppm fuel; however, fuel economy was reduced by 7% because of the higher 

backpressure. The second engine did not show any differences between the fuels over 10,000 km of 

mileage accumulation. 

 Frank et al. [127] tested a Navistar 2001 model year DT466E engine commonly used in urban 

transit buses with a variety of fuels and with a number of different aftertreatment configurations. Engine 

displacement was 7.6 liters, and the engine was turbocharged with electronically controlled injection, and 

no EGR. Although the fuel properties were not well designed to determine effects of individual properties, 

three different sulfur levels were blended with other properties held relatively constant. With an oxidation 

catalyst installed, both engine-out and tailpipe emissions were reported for fuels with sulfur levels 

between 28 ppm and 410 ppm. Engine-out emissions were about 10% lower with 28 ppm sulfur than with 

410 ppm sulfur. For the same fuels, tailpipe PM emissions were reduced about 50%. This reduction is 

probably a result of sulfate production over the oxidation catalyst, although no data were provided for PM 

components. 

 The cooperative European Particulates Project [128] tested ten HD European diesels, ranging 

from Euro 1 technology to prototype Euro 4 and Euro 5 emission levels. All engines were DI with high 

pressure injectors. Three engines had DPFs and two engines had Urea-SCR systems. Four levels of 
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sulfur were tested in one base fuel (8, 38, 280 and 1550 ppm), although not all fuels were tested in all the 

engines. The European Steady State Cycle (ESC) and the European Transient Cycle (ETC) were used. 

Only particulates were measured. 

 Reducing sulfur generally reduced PM emissions. The Euro 4 engine with DPF and the Euro 5 

engine with Urea-SCR gave similar PM mass emissions with 38 ppm and 8 ppm, suggesting that these 

devices might not be sensitive to sulfur over this range. There was also a significant difference between 

the test cycles. The ESC cycle generates much higher exhaust temperatures, and PM emissions were 

much more responsive to sulfur levels. Solid or carbonaceous particles were measured and were not 

sensitive to fuel sulfur levels. The impact of sulfur on particle number was large for the engines equipped 

with DPFs, where sulfur in the fuel contributes to nucleation mode particles in the exhaust downstream of 

the DPF. Evaluation of the particle size distribution confirmed that sulfur impacts particles below 30 nm 

much more than particles above 30 nm. 

 Kono et al. [129] tested a 4.8 liter naturally aspirated DI engine usually used in a 2 ton truck. It 

was equipped with common rail injection, EGR and an oxidation catalyst. Two of the fuels differed only in 

sulfur (7 ppm, 46 ppm). Eight different Japanese and U.S. driving cycles were used. Engine-out and 

tailpipe emissions were measured for all the driving cycles, and sulfur in the range studied had little or no 

impact on emissions. 

SUMMARY OF SULFUR EFFECTS

 The most direct effect of lowering sulfur is to reduce the sulfate portion of PM, without changing 

any of the other engine-out emission components. Lowering sulfur also reduces the number of particles, 

especially in the nanometer range (< 50 nm). 

 DPFs are not particularly sensitive to sulfur, but there is some evidence that higher sulfur levels 

can result in increased backpressure, which may affect fuel economy. PM levels are so low that effects 

are difficult to measure. However, there is some evidence that sulfur has a large measurable effect on 

PM numbers. 

 NSR catalysts have been shown to be sensitive to sulfur above 50 ppm, and possibly require 

sulfur levels much lower than that. Even when the operation of the NSR is not directly affected by sulfur, 

higher sulfur levels require more frequent regeneration and the high temperatures associated with 

regeneration may degrade long-term catalyst activity. Urea-SCR catalyst systems are less sensitive than 

NSR catalysts to sulfur levels. 

C. BACK-END VOLATILITY 

 The amount of high boiling material is a key diesel property. Usually this is defined by T90 or T95

as measured in the ASTM D86 distillation test. Some researchers have used end-point to characterize 

back-end volatility. 

LIGHT DUTY DIESELS

 Betts et al. [93] and Fløysand et al. [94] tested a car with and without an oxidation catalyst. The 

car had a 2.1 liter IDI engine with EGR and met the European 1993 emission standards in the noncatalyst configuration. Sixteen fuels were tested in the non-catalyst configuration and nine with the 

catalyst in place. T95 was varied between 345oC and 370oC, along with aromatics, cetane and density. In 

either configuration, reducing T95 by 20oC reduced PM a statistically significant amount (6% - 7%). No 

results were shown for the influence on other exhaust components. 

 Tritthart et al. [95] tested two cars, one DI and one IDI, both with EGR, turbochargers and 

oxidation catalysts. Seven fuels were tested, with T90 varying between 307oC and 327oC, along with 
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varying cetane, density, aromatics and sulfur. The authors concluded that they did not find a relationship 

between T90 and any of the emission components. 

 Lange et al. [96] tested one car, a 1991-1993 model year (Euro 1 emission standards), with 

indirect injection, EGR, and an oxidation catalyst. Twelve fuels were tested in total. Four of them differed 

in T90 (326oC to 350oC) and polyaromatics and could be used to estimate effects of those parameters. An 

analysis showed that T90 did not affect PM or NOx. No results were shown for HC or CO. 

 Tsurutani et al. [102] tested one Japanese car - a Toyota Corolla with a naturally aspirated 2 liter 

engine, with EGR but no catalyst – with 45 different fuels. These included fully blended fuels, narrow 

boiling fuels, cetane improved fuels, and splash blended fuels. European and Japanese test cycles were 

used. Lowering T90 reduced PM emissions, although it was not possible to separate the physical effects 

(volatility) and chemical effects (polyaromatics). In addition, for some of the fuels, T90 and density were 

highly correlated. 

 The EPEFE program, described in more detail above, tested nineteen LD diesel vehicles with 

eleven fuels [103]. All vehicles had oxidation catalysts, and met the relevant European 1996 emission 

standards. The fuels were blended from refinery streams and consisted of three sets designed to 

determine the first order (linear) impacts of polyaromatics, density, cetane, and back-end volatility 

measured as T95. The European driving cycle was used. The impact of T95 on fleet average emissions 

was calculated from 6 fuels that varied T95 independently of other parameters, and is shown below. 

Percent Change in Emissions

T95: 370oC → 325oC 

HC + 3 (NS) 

CO - 2 (NS) 

NOx + 4.5 

PM - 6.7 

 Trends suggested that reducing T95 increased emissions of benzene and formaldehyde but too 

few tests were carried out to allow a meaningful statistical analysis. 

 Tamanouchi et al. [105] tested two 1992 model year cars made for sale in Europe on fuels with 

levels of T90 varying between 276oC and 336oC. Both cars had IDI engines, with turbochargers and 

oxidation catalysts. Lowering T90 reduced emissions of PM in both cars, reduced THC and CO in one of 

the cars and had no effect on NOx. No quantitative results were discussed and no statistics were 

provided. 

 JCAP carried out a number of studies examining the effect of back-end volatility on emissions 

[106-109]. In the first phase, eight cars were tested on a variety of fuels, and it was determined that 

reducing T90 lowered PM emissions but did not have a consistent impact on HC, CO or NOx. In the 

second phase of JCAP, more advanced vehicles were tested including those with aftertreatment: NOx

reduction catalysts (NSR and Urea-SCR) and PM filters. In most cases, reducing T90 lowered PM 

emissions. The situation with respect to HC, CO and NOx is a bit more complex. In a number of vehicles, 

engine-out and/or tailpipe NOx emissions increased. It was theorized that lower T90 prevented the fuel 

introduced into the combustion chamber during the pilot-injection from igniting. Another possibility is that 

subtle changes in fuels caused changes in the EGR rate, which affect NOx. 

 Kwon et al. [101] adjusted engine operating parameters so that different fuels had the same 

injection and EGR behaviors. In the fuel set, T95 and density were correlated so it is not possible to 

ascribe effects to one property or the other. Results are shown below: 
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Percent Change in Emissions 

Reduce T95 40oC* 

HC - 7% 

CO - 15% 

NOx - 2% 

PM - 14% 

* Highly correlated with density. 

 Nishiumi et al. [130] tested two levels of volatility and cetane in a passenger car and a small 

diesel engine. Both engines were about 2 liter displacement, turbocharged, with EGR and common rail 

injection systems. Four fuels were tested; two had a narrow boiling range and two had a wide boiling 

range. The narrow boiling range fuels had T90 values of 250oC along with T10 values of 220oC. The wide 

boiling range fuels had T90 of 325oC and T10 values of 235oC. Fuel properties were adjusted by distillation 

and changing the normal/iso-paraffin ratios. Results showed that reducing the boiling range lowered PM 

emissions by reducing the SOF portion of the particulates. From the analysis of the fuels and the SOF in 

the particulates, it was determined that the presence of HC heavier than C18 contribute to SOF in the 

particulates. 

HEAVY DUTY DIESELS

 Lee et al. [4] reported that back-end volatility had a small impact on exhaust emissions in HD 

diesels. Reducing T90 or T95 raised emissions of HC and CO and lowered emissions of NOx. PM 

emissions were not affected. They speculated that these small effects might be due to effects of volatility 

and molecular weight on spray patterns. 

 Oyama and Kakegawa [106] reported the results of Phase 1 of JCAP in which five fuels varying in 

T90 and aromatics were tested in eight truck engines. Seven were DI, three were turbocharged, one had 

mechanical injection and three had EGR. Engine sizes ranged from 3.6 to 21 liters. They were chosen to 

represent the existing Japanese truck fleet meeting 1990-1994 or 1997-1999 emission standards. The 

Japanese 13 mode test was used. Regression analysis showed that reducing T90 lowered PM emissions 

(~0.5 %PM/ oC T90) and increased NOx emissions by a smaller amount (~0.1 %NOx/ oC T90). 

 JCAP II [107] evaluated three truck engines varying in size from 3.8 liter to 15.7 liter using the 13 

mode emissions test. Three aftertreatment systems were tested – DPF, DPF + Urea-SCR, and NSR. 

Emission levels were targeted to be one-half the 2003 Japanese standards. When T90 was reduced from 

335oC to 240oC, the engine with NSR catalysts had lower NOx and PM emissions by 25% and 54%, 

respectively. 

 JCAP II evaluated NOx emissions with a Urea-SCR aftertreatment system in a 13 liter engine with 

unit injectors and no EGR [108]. Three fuels with varying levels of T90 (280oC, 300oC, 330oC) were tested 

on the D13 emissions cycle and the JE05 transient test. Cetane was held constant, but density and T10

decreased as T90 was lowered. Results were reported in terms of T90. Reducing T90 increased engineout NOx emissions. It was suggested that for low T90 fuels, the pilot injection did not ignite, therefore 

causing higher heat release, higher temperatures and higher NOx formation during the main combustion 

event. This effect was more noticeable with the JE05 test cycle, which has lower temperatures than the 

D13 test cycle. At the same time, the engine-out effect was counteracted by an increase in catalyst 

efficiency, so that the net effect was a small decrease in NOx emissions. PM emissions decreased, and 

THC increased, both engine-out and tailpipe, when T90 was reduced. 

 Using the same fuel set, JCAP II [55] tested a 9 liter DI turbocharged engine having a common 

rail injection, EGR and a Urea-SCR/DPF aftertreatment system. Two test cycles were used – steady 

state D13 and transient JC05. Lowering T90 increased engine-out emissions of HC and decreased 
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engine-out emissions of PM. Catalyst activity was too high to see an effect in the low tailpipe emissions. 

For NOx, both engine-out and tailpipe emissions increased when T90 was lowered. 

 Kono et al. [119] tested three modern DI engines with high pressure injection, two of which were 

turbocharged. All three had EGR, one had an oxidation catalyst, and one had a complex NSR/DPF 

aftertreatment system. The engines were designed for 2 ton cargo trucks; one met the 1998 Japanese 

standards, and the other two met the 2003 standards. All were tested using the JC08 chassis-based 

emissions test, which involved both cold and hot start operation. Eight fuels were tested, three of which 

varied in T90 (280oC, 300oC, 330oC). Density varied as well in the fuel set, so it is not possible to 

unambiguously assign effects to individual properties. (These fuels appear to be the ones used in JCAP 

II.) No detailed analysis is presented, but the authors reported that fuel effects on NOx were small (less 

than 5%). Lowering T90 reduced PM emissions and increased emissions of HC and CO. 

 Kono et al. [129] tested a Japanese 4.8 liter naturally aspirated DI engine usually used in a 2 ton 

truck. It was equipped with common rail injection, EGR and an oxidation catalyst. The same fuels were 

used as reported previously [119]. Eight different Japanese and U.S. driving cycles were used. Engineout and tailpipe emissions were measured for all the driving cycles. Reducing T90 lowered PM emissions, 

especially engine-out. A 10oC reduction in T90 lowered total PM engine-out emissions between 3% and 

7% in the various test cycles. Tailpipe emissions were reduced by about 3% in four of the eight cycles. 

SOF engine-out emissions rates were also reduced by about 5%; tailpipe SOF emissions were 

unaffected. Soot emissions exhibited a different response. Engine-out soot emissions were unaffected 

by lowering T90, but tailpipe soot emissions decreased about 5%. The authors speculated that the 

oxidation catalyst might convert SOF into soot, so that a reduction in SOF out of the engine would 

translate into a reduction in soot at the tailpipe. 

SUMMARY OF BACK-END EFFECTS

 In LD diesels, reducing back-end volatility (lower T90 or T95) reduced emissions of PM by a 

relatively small amount and did not affect HC or CO. Effects on NOx emissions were mixed, with some 

evidence of an increase. 

 In HD diesels, reducing back-end volatility also reduced PM emissions, and increased emissions 

of HC and CO. For NOx emissions, some studies showed an increase, and some showed a decrease. 

D. FATTY ACID ESTERS (FAE) 

 Esters fit for use in diesel fuel may be made from vegetable oil or animal fat. These esters are 

typically made by reacting oils with an alcohol, usually methanol, in the presence of a base. The resulting 

esters have properties that put them in the diesel boiling range, have high cetane and contain oxygen, 

which may help in the combustion process. On the negative side, they have a lower energy density, and 

may require the use of suitable additives to counter poorer oxidative stability. A wide range of food and 

non-food oils may be used, such as soy, rapeseed, palm, jatropha, jojoba and pongamia. FAE made 

from oils are mixtures of different esters having a range of molecular weights and saturation/unsaturation 

levels. Full description of the chemical composition of various FAEs may be found in the literature [131]. 

 One of the main incentives to use FAE is that they are considered renewable and generally have 

lower greenhouse gas equivalent emissions than petroleum-based diesel fuels. Generically, these 

materials are known as biodiesel. Mixtures of biodiesel and petroleum diesel are usually signified by Bxx, 

where xx refers to the percentage of biodiesel mixed with petroleum diesel. 

 ASTM has written specifications for the pure biodiesel (D 6751), and manufacturers generally 

certify that engines will run on fuel with up to 5% FAE, if ASTM or similar industry specifications are met. 

 A drawback in much research carried out to date is that tests were done with biodiesel that was 

splash blended into a fully formulated diesel and a number of properties changed when the biodiesel was 

added. Changes in emissions are due not just to the change in chemical composition, but also to 
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changes in other properties such as cetane, density and boiling range. If biodiesel is used broadly, it is 

likely that the base fuel will be specially blended so that the entire blend meets, but does not exceed, 

specifications. The emissions benefits might be different in than those reported in the literature. 

 When evaluating the impact of FAE on PM emissions, it is important to measure each component 

of PM emissions. FAE may decrease carbonaceous soot, but probably does not affect sulfate emissions, 

and may increase the soluble organic fraction. 

LIGHT DUTY DIESELS

 Sirman et al. [132] tested a number of fuels in a 2.2 liter Daimler-Benz European DI, 

turbocharged engine with common rail injection system. No information was provided about model year 

or emissions certification. Emissions from a low sulfur diesel were compared to those with a mixture of 

20% soy oil methyl ester (SOME) with low sulfur diesel. Emissions were measured on a 13 mode test on 

an engine dynamometer. With the B20 fuel, NOx emissions increased by 3% and PM emissions were 

lower by 14%. Both changes were statistically significant. 

 Malhotra et al. [133] tested one IDI passenger car from the Indian market, with regular diesel fuel 

and 20% blends of Pongamia Pinnata methyl ester (PPME), Pongamia Pinnata ethyl ester (PPEE) and 

Jatropha Curcas methyl ester (JCME). Results for CO and PM are shown below. No data were 

presented for HC and NOx and no statistical analysis was provided. Results are summarized below. 

Percent Change in Emissions 

Relative to Diesel 

 PPME B20 PPEE B20 JCME B20 

CO - 20 - 6 - 21 

PM - 22 - 9 - 18 

% SOF - 44 - 46 - 33 

 Martini et al. [134] reported on tests of three biodiesels conducted for the European Commission 

in two 2004 model year diesel cars meeting Euro 3 emission standards. Both engines were DI, 

turbocharged, with EGR and oxidation catalysts. One had a common rail injection system and the other 

had unit injectors. Three biodiesel materials were tested: rapeseed methyl ester (RSME), a 50/50 mixture 

of soy methyl ester and sunflower oil methyl ester (SSME), and palm oil methyl ester (POME). RSME 

and SSME were tested neat, and all three were tested in a 30% mixture with two diesel base fuels. 

 Two petroleum diesel base fuels were tested. Diesel fuel 1 was a high quality diesel, and it was 

felt that the benefits of biodiesel might not be apparent. Therefore, a second base diesel, with properties 

closer to average was added to the program and tested in one of the cars. Diesel 2 had higher aromatics 

(30% vs. 20%), higher density (0.839 vs. 0.824) and lower cetane (51.5 vs. 53.5). The test matrix is 

shown below: 
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 Car 1 Car 2 

Pure Biodiesel 

 RSME x x 

 SSME x x 

Diesel Fuels 

 Diesel Fuel1 x x 

 Diesel Fuel 2 x 

30% Blends 

 RSME-Diesel 1 x x 

 SSME-Diesel 1 x x 

 POME-Diesel 1 x x 

 RSME-Diesel 2 x 

 SSME-Diesel 2 x 

 POME-Diesel x 

 Results are shown below for the 30% blends: 

 Percent Change in Emissions versus Base Fuel 

 30% RSME in 30% SSME in 30% POME in 

 Diesel 1 Diesel 2 Diesel 1 Diesel 2 Diesel 1 Diesel 2 

Vehicle 1 

 HC + 29 + 37 + 16 

 CO + 30 + 36 + 3 

 NOx + 1 - 3 - 2 

 PM - 23 - 32 - 27 

Vehicle 2 

 HC + 21 - 2 + 53 + 10 + 6 - 11 

 CO + 36 + 5 + 56 + 18 + 22 - 6 

 NOx + 6 + 1 + 2 + 2 0 0 

 PM + 1 - 9 - 8 - 1 - 10 - 10 

 The three biodiesels had similar directional effects on emissions: HC, CO and NOx emissions 

increased, and PM emissions decreased. POME blend seemed to have smaller impact on HC and CO, 

possibly because of its higher cetane number. The increases in HC and CO occurred mostly during the 

cold start part of the driving cycle (first 300 seconds). The impact of biodiesel on PM was different for the 

two cars, and the authors speculated that it might be the result of different calibration strategies for EGR 

and injection. Analysis of the PM emissions showed that biodiesel changed the make-up of the 

emissions – tending to reduce the soot fraction and to increase the SOF fraction. 

 The negative effects of the biodiesel blends are much smaller when Diesel 2 was used because 

the base fuel itself has higher emissions. 
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HEAVY DUTY DIESELS

 Lee et al. [4] pointed out that most of the research on oxygenates used splash blending and 

therefore could not decouple the changes in the other properties such as density from the addition of the 

oxygenate. They considered all previous conclusions tentative. 

 Clark et al. [135] tested a Navistar HD diesel with biodiesel, petroleum diesel and blends of the 

two. The feedstock oil of the biodiesel material was not disclosed. The engine was a Navistar T444E 

meeting 1994 U.S. emission standards, 7.3 liter displacement, turbocharged, with hydraulically actuated, 

electronically controlled injection system, with no EGR or aftertreatment system. The hot start portion of 

the HD transient FTP was used. Results are shown below for three biodiesels relative to the base 

petroleum diesel. Emissions of HC, CO and PM were reduced, while there was a slight increase in NOx

emissions. Changes in emissions are due to the presence of oxygen and other changes in properties, as 

discussed above. 

Percent Change in Emissions 

Relative to Diesel Fuel 

 B20 B50 B100 

HC - 4.6 - 30 - 61 

CO - 34 - 39 - 49 

NOx + 1.7 0 + 4.3 

PM - 59 * - 32 - 42 

* Questionable result 

Results were compared to earlier data and shown to be consistent when considered on the basis of 

oxygen content. 

 Sharp et al, [136, 137] tested three engines with three fuels – diesel, biodiesel (Soy Oil Methyl 

Ester), and a 20% mixture (B20). One engine was from a highway truck, one from an urban bus, and one 

from a pick-up truck. The truck and bus engines had electronically controlled unit injectors and oxidation 

catalysts. The pickup truck engine had mechanically controlled injection. The HD transient FTP 

emissions test was used. Engine-out emissions of HC were reduced substantially with biodiesel, being 

almost eliminated for the two larger engines. Speciated results showed that the specific ozone reactivity 

was about the same for all fuels. Use of biodiesel substantially reduced (50-75%), but did not eliminate, 

the presence of PAH compounds in the exhaust. It also, as expected, increased emissions of high 

molecular weight oxygenated compounds. Engine-out CO emissions were cut by about 40%, and NOx

emissions increased about 10% with biodiesel. The pickup truck engine had a smaller NOx increase and 

this may be related to the lower pressure injection system and how it responds to other property changes 

(e.g., density). Total engine-out PM emissions decreased by 25-50% due to a combination of an increase 

in fuel-based particulates and a larger decrease in soot. Sulfate emissions also decreased, but these 

were very low and only had a small contribution. B20 demonstrated the same trends as B100, but there 

was some indication that the benefits were somewhat higher than proportional, especially for soot. These 

papers did not present any statistical analysis of the results. 

 Wang et al. [138] tested nine trucks on a chassis dynamometer. Six were equipped with a 

Cummins 855 engine, with model years ranging from 1987 to 1992. This engine’s displacement is 14 

liters. Three trucks were equipped with a 1993/94 DDC series 60 engine, having a displacement of 11.1 

liters. Two fuels were tested, a petroleum diesel and a B30 mixture, using SOME. Two driving cycles 

were used – the WVU (West Virginia University) truck cycle and the WVU 5 mile cycle. Both are similar 

and consist of a series of accelerations, cruises and decelerations. Since, in a chassis based test, the 

vehicle is driven to constant power output, there is no question of adjusting the engine for the lower brake 

specific power levels. PM emissions were 25% lower and CO emissions were 12% lower with B30. HC 

emissions were lower with the Cummins engine and were the same with the DDC engine. NOx emissions 
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results were different for the two engines. With the Cummins engine, B30 increased NOx emissions, and 

with the DDC engine NOx emissions decreased with B30. The authors speculated that the higher NOx

emissions were the result of higher cetane, and reasoned that higher cetane results in an earlier ignition 

and higher temperatures and pressures. Implicit in this argument is that the two engines somehow 

respond to higher cetane in different, undefined ways. 

 McCormick et al. [139] and Graboski et al. [140] tested petroleum diesel, seven FAEs prepared 

from vegetable and animal oils, and 14 pure esters prepared chemically. Some were methyl esters and 

some were ethyl esters. All fuels were tested in a DDC Series 60 engine, with a 1991 emissions 

calibration. The DI turbocharged engine had a displacement of 11.1 liters. The EPA HD transient 

emissions test cycle was used. 

 Almost all esters had higher NOx and lower PM emissions than the petroleum diesel, and the 

methyl esters and ethyl esters had the same emissions. Because of the large numbers of chemically 

similar fuels that were tested, the authors were able to carry out a correlation of bulk properties for the 

ester fuels. They found that NOx emissions were strongly correlated with density and cetane. Over a 

range of density from 0.863 g/ml to 0.895 g/ml, reducing density lowered NOx emissions by about 25%. 

Increasing cetane from 42 to 77 lowered NOx by about 20%. Neither density nor cetane affected PM 

emissions for these fuels. Since the molecular weights of these fuels varied as well, the volatility 

properties changed, although it was not reported. These data suggest that oxygen is the driving variable 

for the effect of biodiesel on PM emissions, but that oxygen alone does not explain the increase in NOx 

emissions relative to petroleum diesel. 

 McCormick et al. [141] tested a number of fuels in order to develop a strategy to mitigate the 

increase in NOx emissions seen with use of biodiesel or biodiesel blends. They tested two biodiesel 

materials, esters of yellow grease and soy oil, along with Fischer Tropsch diesel, cetane improvers and 

low aromatics diesel. The engine was a 1991 calibration DDC Series 60 11.1 liter turbocharged DI 

engine tested over the HD transient FTP test. For B20 fuel, the increase in NOx emissions could be 

mitigated or reversed by adding 46% Fischer Tropsch diesel, reducing aromatics from 30% to 10%, or by 

adding cetane improvers to raise cetane about 0.5 CN. No statistical analysis was presented. 

 Xiaoming et al. [142] tested emissions from two engines with two base diesels and two biodiesel 

blends, B20 and B50. The engines, presumably Chinese models, were 11.9 and 5.8 liter turbocharged DI 

engines, with no information about their emissions control. The Chinese 13 mode steady state test was 

used to measure emissions. B20 was tested in the larger engine and B50 was tested in the smaller 

engine. Responses are shown below. PM and CO responses were similar, while there were differences 

in size for NOx and in direction for HC. No statistical analysis was presented to evaluate the results. 

Percent Change in Emissions

Relative to Diesel Fuel 

 B20/Engine A B50/Engine B 

HC + 7 - 29 

CO - 8 - 11 

NOx + 6.8 + 27 

PM - 13 - 12 

 JCAP [55] measured PM numbers and size distribution in a 9 liter turbocharged engine with EGR 

and Urea-SCR aftertreatment. The engine met 2005 emission standards. B5 (5% RSME) was compared 

to a base diesel, and no obvious effects were seen in particle number or size distribution over the 

transient JE05 test. The focus of this portion of JCAP was not fuel effects, but aftertreatment effects, so 

the conclusions are based on limited testing and may not be generally applicable. 
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SUMMARY OF FAE RESULTS

 In 2002, EPA prepared an analysis of the effect of biodiesel on diesel emissions [91] and 

concluded that for HD diesels, biodiesel reduced emissions of HC, CO and PM and increased emissions 

of NOx. The increase in NOx emissions was much less than the decrease in the other emission 

components. Not enough data were available to draw any conclusions about LD diesels. 

 Based on the data reviewed here, the conclusions presented by EPA are still valid, and would 

seem to apply to LD diesels as well. PM reductions are generally seen in the soot component, while 

emissions of the SOF component may tend to increase. There is good evidence that biodiesel reduces 

PM emissions because of its oxygen content, and that the increase in NOx emissions may be the result of 

biodiesel’s impact on properties such as cetane and density. 

 No data were found that examined FAE impacts in the presence of catalytic aftertreatment 

systems. Neither were data found that tested fully formulated fuels, not splash blended mixtures. 

E. OTHER OXYGENATES 

 A number of other oxygenates have been evaluated in diesel fuel. These include ethers, 

carbonates, and methoxy compounds. One class of methoxy compounds tested is glymes. Mono-glyme 

is 1,2 dimethoxyethane, also known as dimethyl glycol or ethylene glycol dimethyl ether. Di-glyme, triglyme and tetra-glyme are similar compounds with additional dimethyl ether linkages in the backbone of 

the molecule. 

LIGHT DUTY DIESELS

 Tsurutani et al. [102] tested nine oxygenates in a Toyota Corolla with a naturally aspirated 2 liter 

engine, with EGR but no catalyst. The fuels included blends (10%-25%) of mono- di- tri- and tetraglymes, ethers, heavy alcohols and esters. Tests were conducted over the Japanese 10.15 and 

European NEDC cycles. Reductions of PM were measured and there was a strong correlation with 

oxygen content only. Based on tests with 20% di-glyme, the impact on PM emissions was primarily a 

reduction in soot emissions; SOF was not affected. There was little if any impact on NOx emissions. 

 Beatrice et al. [143] tested 10%, 20% and 30% di-glyme in a relatively clean Finnish summer 

diesel. The engine was a 1.9 liter Fiat M724 DI diesel with electronic EGR, common rail injection and an 

oxidation catalyst. Five steady state modes were chosen to approximate the full transient cycle. 

Reductions were observed for HC, CO and soot for all conditions. This was attributed to a higher rate of 

combustion with the oxygenated blends. NOx results were somewhat scattered, with reductions seen in 

2 modes, and increases seen in three modes. The authors argued that higher cetane number of the 

blends may have contributed to the effects measured here. 

 Hess et al. [144] tested a mixture of 20% mono-glyme/80% di-glyme, blended at 5%, 10% and 

15% in a U.S. diesel fuel. A 1.9 liter VW DI, turbocharged engine was tested at six steady state 

conditions. PM emissions decreased with increasing oxygenate content, while HC and NOx emissions 

were generally unaffected. Surprisingly, CO emissions tended to increase with increasing oxygen 

content. The authors argued that oxygenates provide additional oxygen in the rich premix zone and this 

tends to form additional CO. The additional CO reduces the concentration of soot precursors, primarily 

low molecular weight olefins. 

 As part of the Ad Hoc Fuel Group of the PNGV program, Szymkowicz et al. [145] tested 15% 

mono-glyme in a low sulfur diesel fuel, with a prototype 1.3 liter DI turbocharged engine with common rail 

injection and EGR. Six steady state modes were tested and results reported for a weighted average to 

approximate FTP conditions. Mono-glyme reduced PM emissions by about 30%, mostly as dry PM (nonvolatile) and had little or no impact on NOx. HC and CO were not reported. 
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Hilden et al. [146] tested a number of oxygenates, shown below, in a low sulfur diesel fuel. These 

represented a range of volatilities and two different classes (aromatic and aliphatic). 

Compound Acronym Percent Oxygen 

in Molecule 

Percent Oxygen 

in Tested Blend 

Dipropylene glycol 

monomethyl ether DPGME 32% 6% 

Tripropylene glycol 

monomethyl ether TPGME 32% L; 3.5% 

H: 6.5% 

Di-isobutyl adipate DIIBA 25% 6.3% 

Di-isobutyl phthalate DIBP 23% 6% 

 The test engine was a 1.3 liter DI engine with common rail injection. Four steady state test mode 

were used. Only NOx and PM were reported. Results are shown below for the standard EGR 

configuration. 

 Percent Change in Emissions 

Fuel NOx PM 

DPGME + 3.2 - 23 

TPGME-L - 6.3 - 4.5 (NS) 

TPGME-H + 1.5 (NS) - 18.2 

DIIBA - 0.5 (NS) - 27 

DIBP + 8.1 + 16 

 The oxygenates reduced PM emissions, with the exception of the aromatic DIBP. The increase 

with DIBP was the result of higher levels of volatile PM, probably because of the aromatic nature of the 

molecule. There were no differences in particle size distribution among the oxygenates. NOx emission 

effects were small and somewhat variable. The authors suggested that the large reductions in PM could 

be used to increase the EGR rate and thereby optimize both emissions components. 

 Ball et al. [147] tested dimethoxymethane (DMM) blended at 15% into a low sulfur diesel, in a 

DaimlerChrysler 2.2 liter DI engine with common rail injection and closed-loop, variable EGR. Four 

steady state modes were used, and results are provided for the weighted mass emissions, shown below 

Percent Change 

in Emissions 

HC - 14 (NS) 

CO - 4.4 (NS) 

NOx + 18 

PM - 36 

PM-SOF - 23 

 Adding DMM reduced PM, mostly through reductions in SOF, increased emissions of NOx and 

had no statistically significant effect on HC or CO. Emissions of individual compounds such as PAH were 

generally lower but not statistically significant, with the exception of gas phase benzene. 
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 JCAP II [107] tested 10% di-glyme (3.8% oxygen) in two different base fuels in a 2000 model year 

car having a 2.5 liter engine, with EGR, and a catalyzed DPF aftertreatment. NOx emissions were higher 

with the oxygenate for both base fuels. PM results were mixed. In both cases, the effects were less than 

10%, and assumed to be not statistically significant. The small size of the PM effect may be due to the 

presence of the DPF. 

 Kozak et al. [148] tested six oxygenates at 5% blends in a Euro 5 diesel (9 ppm sulfur). Shown 

below, the compounds varied in molecular weight and density. One vehicle was tested – a 1.3 liter DI 

turbocharged engine with common rail injection, closed-loop EGR and an oxidation catalyst. The vehicle 

met Euro 4 emission standards. The European driving cycle (NEDC) was used for the tests. 

Compound Percent Oxygen 

in Tested Blend 

Mono-glyme 1.85 

Di-glyme 2.02 

Tri-glyme 2.12 

Tetra-glyme 2.16 

Diethylene glycol 

dibutyl ether 1.17 

Dipropylene glycol 

dimethyl ether 1.60 

 During the cold start portion of the cycle (UDC), when HC and CO emissions are high, the 

presence of the oxygenates tended to reduce HC and CO emissions by about 20%. Results for NOx and 

PM were about the same for both phases of the test cycle. NOx emissions increase between 2% and 

7%, while PM emissions were reduced between 8% and 13%. There was no obvious correlation between 

oxygen content and emissions effects, possibly because of changes in other properties such as volatility 

and density. 

HEAVY DUTY DIESELS

 Cheng and Dibble [149] tested DMM and diethyl ether (DEE) in concentrations up to 30% in a 

petroleum diesel. A 1993 model year 5.9 liter DI turbocharged Cummins B Series engine was tested at 

nine steady state modes chosen to cover a wide range of the operating map. Results are shown below 

for the 10% and 30% cases. While there were variations across the modes, adding these oxygenates 

generally increased HC and CO emissions, and lowered NOx and PM emissions. The authors speculated 

that high concentrations of these oxygenates may have poor combustibility, may affect the injection spray 

pattern, or may cause the injectors to weep liquid fuel. No statistics were reported for these results. 
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Percent Change in Emissions 

Relative to Diesel Fuel 

 DMM DEE 

10% 

Blend 

30% 

Blend 

10% 

Blend 

30% 

Blend 

HC + 38 + 50 + 30 + 119 

CO + 2 + 31 + 2 + 26 

NOx - 7 - 10 - 1 - 9 

PM - 14 - 35 - 13 - 14 

 Chapman et al. [150] compared diesel fuel with a 5.6% splash blended mixture of CETANERTM 

(2% oxygen) in a HD Navistar 7.3 liter DI turbocharged engine hydraulically activated, electronically 

controlled unit injectors. CETANERTM is a 20/80 mixture of mono-glyme and di-glyme. Eight steady state 

test modes were chosen to approximate the transient FTP emissions test cycle. Using weighted 

averages of the eight mode test, HC and NOx emissions were reduced by 6% and 5.4%, respectively. 

CO emissions were unchanged. PM emissions appear to be reduced, but weighted averages were not 

reported. No statistical analysis was presented. 

 Vertin et al. [151] tested DMM at a number of different splash blended concentrations with a 1993 

model year Cummins B Series 5.9 liter DI turbocharged engine with mechanically controlled fuel injection. 

Nine steady state modes were chosen to cover a variety of operating conditions. Shown below are 

comparisons of 10% and 30% DMM versus the diesel base. These are in surprisingly good agreement 

with the results reported above by Cheng and Dibble. 

Percent Change in Emissions

Diesel Fuel → DMM 

 10% Blend 30% Blend 

HC + 36 + 73 

CO + 1 + 60 

NOx - 7 - 10 

PM - 14 - 35 

SUMMARY OF OTHER OXYGENATES

 Considering the large number of oxygenate compounds that may be used in diesel fuel, there is 

not a lot of data on their emission effects. There is contradictory data and some results do not agree with 

data collected on fatty acid esters. If the impacts on emissions were caused by the presence of oxygen, 

we would expect the results to be similar. While PM emissions are generally lower, HC, CO and NOx

results are mixed. In LD diesels, HC and CO emissions tend to be lower and NOx emissions tend to be 

higher. In HD diesels, HC and CO emissions tend to be higher and NOx emissions tend to be lower. The 

reason for these differences is not clear. 

 If these compounds will be seriously considered for commercial use, more and better data should 

be obtained. 

F. FISHER TROPSCH FUELS 

 The Fischer Tropsch (FT) process is a high temperature catalytic process that reacts CO and H2

to produce a mixture of HC. The specific compounds generated are dependent on reactor design, 
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reaction temperature and pressure, and catalyst composition. Production of the feed stream is usually 

from natural gas, but may also be from petroleum or coal. In some references, FT is referred to as GTL 

(gas-to-liquids). Another blendstock with similar properties is HVO (hydrogenated vegetable oil), and it 

will be discussed in this section. 

 Today, FT products are usually in the diesel boiling range although lighter, naphtha products and 

lubricant basestocks can be made as well. Most FT diesel products require some sort of upgrading to 

improve quality. This includes creating iso-paraffins from normal paraffins and/or removing waxy 

materials to improve cold flow properties. 

 FT fuels usually have extremely high cetane, almost no aromatics and essentially zero sulfur, and 

these properties clearly impact exhaust emissions. FT diesel also has lower volumetric energy density 

than petroleum diesel and, therefore, fuel economy and peak power are lower. Lubricity of FT diesel is 

poorer than petroleum diesel, and, if used by itself, additives to improve lubricity will probably be required. 

 When discussing the literature on FT products, it is important to specify the composition of the 

petroleum diesel against which FT is being compared. Aside from other issues such as model to model 

differences in response, this is probably a major contributor to the different results seen in the literature. 

As with oxygenates, a major limitation in the literature is that most studies were conducted with splash 

blended mixtures of petroleum diesel and FT product, making it difficult to determine the reason for the 

emissions effects. 

 A number of important questions concern the effects of FT diesel, and its ultimate use in the fuel 

pool. 

• Are the effects of FT due to its properties alone? Most studies did not evaluate this 

question. 

• Are the effects linear? 

• How should it be used in practice - alone or in blends? If alone, will this require specials 

engines or special distribution networks? If in blends, will overall fuel quality remain the 

same or will it improve? 

 Unfortunately, much of the research reported in the literature does not help answer these 

questions. 

LIGHT DUTY DIESELS

 Payri et al. [152] tested Sasol FT diesel along with a standard diesel in two samples of a PSA 

diesel car with a 2.0 liter DI turbocharged engine with common rail injection meeting Euro 2 standards. A 

summary of fuel properties is shown below: 

Fuel Property Units Diesel FT 

Density, @15oC g/ml 838 770 

Viscosity mm2/s 2.5 2.1 

Cetane Number 51.4 73 

H/C ratio 1.85 2.15 

Heat of 

Combustion MJ/kg 43.1 43.5 

Results for the two cars are very similar as shown below: 
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Percent Change in Emissions 

Diesel → FT 

 Car 1 Car 2 

HC - 58% - 56% 

CO - 62% - 51% 

NOx - 5% + 3% 

PM - 25% - 41% 

 Payri et al. concluded that an engine could be modified or redesigned to take advantage of the 

properties of FT fuels. An example would be to increase the use of EGR while still maintaining PM 

emissions at an acceptable level. 

 Sirman et al. [132] tested a number of fuels in a 2.2 liter Daimler-Benz European DI, 

turbocharged engine with common rail injection system. No information was provided about model year 

or emissions certification. A sample of Shell SMDS FT diesel was tested along with a 20% blend in low 

sulfur, low aromatics diesel and the low sulfur diesel base fuel. The main property differences were 

cetane (84 vs. 67), density (0.781 vs. 0.811) and T90 (325oC vs. 261oC). Emissions results are 

summarized below. Emissions changes were consistent with the property changes, except for the results 

with FT20 for HC, which increased relatively to the base fuel. 

Percent Change in Emissions

Relative to diesel base fuel 

 FT20 FT 

HC + 28% 0 

CO - 11% - 14% 

NOx - 3% - 3% 

PM - 7% - 19% 

 Szymkowicz et al. [145] tested a FT diesel, an ARB diesel and a low sulfur diesel in a prototype 

1.3 liter DI turbocharged engine with common rail injection and EGR. Two levels of EGR were tested, 

conventional and maximum. Six steady state modes were tested and results reported for a weighted 

average to approximate FTP conditions. Compared to the low sulfur diesel, the FT diesel had higher 

cetane (81 vs. 65), lower specific gravity (0.780 vs. 0.817) and lower aromatics (1.2% vs. 8.9%). Results 

are shown below for the comparison to the low sulfur diesel: 

Percent Change in Emissions

LSD → FT 

Conventional 

EGR 

Maximum 

EGR 

Total PM - 37% - 32% 

NOx - 8% - 2% 

 Johnson et al. [153] tested a pure FT diesel, various cuts of the FT diesel, and blends with 

petroleum diesel fuels in a Peugeot 405 LD diesel car. Fuel property differences are shown below: 
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Fuel Property Units UK Diesel FT 

Density g/ml 0.851 0.775 

Cetane Number 51.1 79 

Aromatics % 29.2 0 

Sulfur ppm 1400 0 

 Over the European emissions cycle, all components of the emissions were reduced: HC by 73%, 

CO by 54%, NOx by 4% and PM by 63%. Different boiling range cuts were prepared and tested, and 

there were no differences in their emissions behavior, suggesting that the heavy paraffinic molecules 

present in FT diesel are not as detrimental to emissions as the heavy molecules present in petroleum 

diesel. The FT diesel was blended with other refinery streams to produce a diesel with similar properties 

to a petroleum diesel. Emissions tests showed that both fuels had similar levels of emissions, suggesting 

that it is the measureable typical properties of the FT material that contribute to its emissions 

performance. Equations developed in the EPEFE program were applied to the emissions data reported 

in this paper and they did a poor job of predicting the performance of the FT fuels. The authors 

suggested that the poor performance was a result of the fuels being outside the range of properties over 

which the equations were developed. However, testing this type of predictive equation on the data from 

one vehicle is not an appropriate test, considering the typically wide range of responses measured in the 

vehicle fleet. 

 Schubert et al. [154] tested a Syntroleum produced FT diesel and a petroleum diesel in a 1999 

model year Volkswagen Golf with a 1.9 liter, DI engine. Major properties of the fuels are summarized 

below. Emissions tests were carried out using the FTP, US06 and HWFET cycles. 

Fuel Property Units Diesel FT 

Specific Gravity 0.8455 0.7716 

Cetane Number 46.7 73.6 

Aromatics wt. % 31 0.7 

Sulfur ppm 300 < 10 

 For the FTP cycle with FT, THC and PM emissions were lower by 25% and NOx emissions were 

12% higher. CO emissions were too low to quantify any differences. Toxics emissions with FT were 34% 

less than with the petroleum diesel. Particle size distribution was similar for both fuels. 

 Schaberg et al. [155] compared a petroleum diesel with a FT diesel produced by Sasol. Fuel 

properties are shown below. 

Fuel Property Units Diesel FT 

Specific Gravity 0.8705 0.7647 

Cetane Number 41.5 74.8 

Aromatics % 38.4 < 1 

Sulfur ppm 404 < 1 

 A 1.9 liter turbocharged VW passenger car engine was tested, and emissions were measured 

over 5 steady state test modes. On average, NOx emissions increased by 6% and PM emissions 

decreased by 50%. Both the soot and volatile fraction of PM were reduced. The smaller numbers of 

nanoparticles were assumed to result from the lack of sulfur available to form nucleation mode particles. 

Changes in HC and CO, while large on a percentage basis, were not statistically significant. 
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 Myburgh et al. [156] tested a FT diesel produced by Sasol and two Japanese petroleum diesels in 

an SUV complying with Japanese 1998 emission standards. The engine was 3.0 liter turbocharged 

diesel with variable EGR, common rail injection and an oxidation catalyst. Properties of the fuels are: 

Fuel Property Units FT Diesel Low S 

Diesel 

Market 

Diesel 

Density g/ml 0.768 0.818 0.832 

Cetane Number 74.1 57.5 58 

Aromatics wt. % 0.1 21.2 24.4 

Sulfur ppm < 1 29 410 

 Emissions of HC and CO were too low to see any differences among the fuels, and there were no 

differences in NOx emissions. PM emissions with the FT diesel were 29% below the low sulfur diesel and 

62% below the market diesel. 

 Nishiumi et al. [130] tested four FT fuels and one petroleum diesel that met Euro 4 standards. 

Two properties were varied by adjusting a full boiling FT diesel – cetane number and boiling range. 

Properties are shown below: 

Fuel Property Units FT A FT B FT C FT C Diesel 

Boiling Range 

(T10-T90) oC 225-328 236-252 221-329 238-254 223-327 

Cetane 

Number 85 85 52 53 52 

Density g/ml 0.7733 0.7671 0.7865 0.7825 0.8295 

Aromatics % 0.3 0.5 0.1 0.0 15.6 

Sulfur ppm < 1 < 1 < 1 < 1 9 

 FT A was the full boiling range FT fuel, and it was adjusted by changing the normal/isoparaffin 

ratio, and/or by cutting out the heavy fractions. The test vehicle was a Toyota Avensis with a 2.0 liter, DI 

turbocharged engine with common rail injection and cooled EGR. Engine calibrations were not adjusted 

to take advantage of fuel properties such as high cetane. The European driving cycle was used for 

emissions tests. Comparisons of the fuels to the petroleum diesel are shown below: 

Percent Change from Diesel 

Fuel FT A FT B FT C FT C 

CN/Boiling Range H/W H/N L/W L/N 

 

PM - 25% - 43% - 40% - 55% 

NOx + 13% + 20% - 5% 0 

THC - 45% - 35% + 35% + 30% 

 In this engine, low cetane helped promote reductions in PM while not increasing NOx and while 

allowing THC to increase. The narrow boiling range seemed to provide somewhat better performance 

with respect to PM emissions. From an analysis of the fuels and the SOF of the particulates, the authors 

concluded that molecules with carbon number greater than 18 are major contributors to PM SOF. 
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 Schaberg et al. [157] carried out tests using the same engine in a car and mounted on a 

dynamometer. The engine was a 2003 model year, Mercedes Benz 2.2 liter turbocharged engine with 

common rail injection, EGR and two oxidation catalysts, meeting Euro 3 emission standards. The 

European emissions test cycle was used. Four fuels were tested – a low sulfur diesel, Sasol FT diesel, a 

50/50 mixture and an 80 Diesel/20 FT mixture. Properties of the fuels are shown below: 

Fuel Property Units Diesel FT 

Density g/ml 0.836 0.768 

Cetane Number 54 74 

Aromatics % 26.8 0.1 

Sulfur ppm 7 < 1 

T95 oC 354 321 

 HC and CO emissions were reduced by more than 90% with FT diesel compared to petroleum 

diesel. This was ascribed to the operation of the oxidation catalyst which absorbs HC and CO during cold 

start and then oxidizes them after operating temperatures are reached. Engine-out emissions are low 

enough that there is no breakthrough during cold start with FT diesel. With petroleum diesel, engine-out 

emissions are greater than the storage capacity of the catalysts and emissions break through to the 

tailpipe. 

 NOx emissions were about the same for all the fuels. PM emissions were reduced about 30% 

with FT diesel. When the 50/50 mixture was tested, the reduction was 22%, slightly more than a linear 

interpolation would predict. A large part of the PM benefit was seen in soot emissions. 

 By adjusting engine operating parameters, particularly EGR, it was estimated that much lower 

NOx emissions could be achieved (35% reduction) while still maintaining the low PM emissions seen in 

the baseline tests. 

 Kitano et al. [158] tested a passenger car with 2.0 liter DI turbocharged engine, with EGR and 

Toyota’s DNPR (Diesel Particulate NOx Reduction) system, an oxidation catalyst, and sensors to control 

A/F ratio and exhaust temperature. Five fuels were tested in the car. Two sets of two fuels each were 

based on two different Shell FT base fuels differing in volatility, and the fifth fuel was a petroleum diesel 

meeting Euro 4 standards. Properties are shown below: 

Fuel Property Units FT-J2 FT-J3 FT-N2 FT-N3 Diesel 

Boiling Range 

(T10-T90) oC 227 -

284 

258 -

344 

173 -

247 

181 -

306 

206 -

328 

Cetane Number 85 85 71.5 71.5 53.4 

Density g/ml 0.773 0.786 0.751 0.764 0.840 

Aromatics % < 0.1 < 0.1 < 0.1 < 0.1 18.9 

Sulfur ppm < 1 < 1 < 1 < 1 33 

 Vehicle emission tests were carried out with and without the DPNR system. In the absence of 

DPNR, HC and CO emissions were reduced by more than 60% and NOx emissions increased about 5%. 

There were no major differences among the FT fuels. PM emissions were reduced between 30% and 

70%, depending on the FT fuel. The reasons for the differences are probably related to the distillation 
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differences, with lower T90 and narrower distillation improving atomization and dispersion, and leading to 

faster evaporation, all of which improves combustion performance and lowers PM emissions. 

 With the DPNR in operation, there were no differences among any of the fuels for NOx and PM 

emissions. It is possible that the effectiveness of the catalyst for PM reduction is so high, that small 

differences cannot be measured. 

 An interesting experiment showed that there seemed to be a sulfur effect on NOx efficiency even 

with the “zero” sulfur FT fuels. The authors attribute this effect to sulfur from the lubricant. 

 Clark et al. [159] tested six cars in two phases to model use of FT material in the short term in 

developing countries, and in the longer term in developed countries. In Phase 1, they tested three cars 

meeting Euro 2/3 emission standards (1996-1999 model years) with GTL blends (15%, 30%, 50%) in two 

regular sulfur base fuels (500 ppm, 1000 ppm). Average results are shown below, along with an 

extrapolated estimate of pure GTL diesel. 

Average Percent Change in Emissions 

Relative to Base Diesel 

15% 

FT 

30% 

FT 

50% 

FT 

100% FT 

(Est.) 

HC - 41 - 57 - 68 - 80 

CO - 26 - 41 - 52 - 67 

NOx + 0.6 - 0.3 - 2.3 - 9.8 

PM - 33 - 41 - 50 - 64 

 In Phase 2, three model year 2003 cars, certified to Euro 4 standards were tested with GTL 

blends (15%, 30%, 50%, 100%) in a low sulfur (40 ppm), higher cetane (54 CN) petroleum diesel. No 

data was provided about aftertreatment systems in these cars. Results in this phase were much more 

variable. HC and CO emissions were consistent across the vehicles and consistent with the Phase 1 

results. For 100% FT, HC benefits averaged 49% and CO benefits averaged 81%. PM differences 

ranged from a benefit of 33% to an increase of 14%. NOx emissions ranged from a benefit of 6% to an 

increase of 18%. The authors speculated that this behavior may be the result of the interaction of the FT 

fuel with engine control systems such as EGR. With lower density fuels such as FT, EGR rates tend to 

decrease. 

 Kitano et al. [160] tested two FT diesels along with a petroleum ultra low sulfur Euro 4 diesel. 

Properties are shown below. An interesting feature of this program is that the base diesel fuel had 

essentially zero sulfur, so it is a little easier to assign reasons for any emissions differences seen. 

Fuel Property Units FT-A FT-B Diesel 

Boiling Range 

(T10-T90) oC 249 - 

339 

174 - 

247 

224 - 

330 

Cetane Number 85.2 71.5 52.7 

Density g/ml 0.7852 0.7510 0.8340

Aromatics Vol. % < 0.1 < 0.1 19.3 

Sulfur ppm < 1 < 1 5 

 The program used a 2.0 liter DI turbocharged engine with common rail injection in a car and on 

an engine dynamometer. The car was tested with and without a DNPR aftertreatment system, using the 

European NEDC test cycle. FT did not affect NOx emissions, with or without aftertreatment. PM 

emissions were lower with FT without aftertreatment. FT-B, which had low T90, had lower PM emissions 
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than FT-A. With the DPNR system in place, PM emissions were so low that differences among the fuels 

could not be discerned. HC and CO emissions were lower with FT in both configurations. 

 One problem noticed with the more volatile FT diesel (FT-B) occurred when it was injected into 

the exhaust to help reduce NOx emissions. It appeared to vaporize rapidly and did not combust properly. 

The authors speculated that engine operations could be optimized for FT properties so that emissions 

could be reduced without the usual degradation in fuel economy. 

HEAVY DUTY DIESELS

 Norton et al. [161] tested emissions from four Class 8 trucks powered with 1996/97 model year 

Caterpillar 3176B 10.3 liter DI turbocharged engines. All trucks were tested on Shell FT diesel and a 

California diesel, shown below: 

Fuel Property Units CA Diesel FT 

Density, @15oC g/ml 0.8337 0.7845 

Cetane Number 54 > 74 

Aromatics wt. % 18.1 0.3 

Sulfur ppm 100 < 5 

 Testing was over a five mile driving course on a chassis dynamometer. Average results for the 

four trucks are shown below. All components of emissions were reduced, although no statistics were 

provided describing the significance of the results. No driving performance difference between the fuels 

was reported. 

Percent Change in Emissions

Diesel → FT 

HC - 40% 

CO - 18% 

NOx - 12% 

PM - 24% 

 Clark et al. [135] tested a Navistar HD diesel with two different FT diesels and with two petroleum 

diesels. Properties of the fuels are shown below. It is interesting to note that FT 1 had a relatively high 

sulfur level. The engine was a Navistar T444E meeting 1994 U.S. emission standards. It was a 7.3 liter 

turbocharged engine with hydraulically actuated, electronically controlled injection system, and no EGR or 

aftertreatment system. The hot start portion of the HD transient FTP was used. 

Fuel Property Units US Diesel CA –Diesel FT 1 FT 2 

Density, @15oC g/ml 0.85 0.8329 0.7905 0.8007 

Cetane Number 44 54 74 51 

Aromatics wt. % < 30 18 0.1 10 

Sulfur ppm 3500 100 < 500 < 10 

 Results are shown below for the fuels. Emissions of all four regulated components HC, CO, NOx 

and PM were reduced. The differences between the two FT samples bracket the reduction seen with the 
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CA diesel, and this is a reflection of the properties of the fuels. The reduction on PM seems small 

compared to other reports of similar fuels. 

Percent Change in Emissions 

Relative to US Diesel 

 CA Diesel FT 1 FT 2 

HC - 21 - 43 - 9 

CO - 31 - 39 - 13 

NOx - 9 - 14 - 6 

PM - 9 - 14 - 15 

 Atkinson et al. [162] tested the same Navistar engine reported by Clark et al. [135] with two fuels, 

a FT diesel and a typical U.S. diesel, shown below. 

Fuel Property Units US Diesel FT 

Specific Gravity 0.838 0.7845 

Cetane Number 48.7 73.7 

Aromatics wt. % 24.7 0.1 

Sulfur ppm < 3500 < 500 

 Emissions were measured at twelve steady state modes to span a wide range of engine 

performance. Considering the average of all test conditions, the FT fuel reduced all emission 

components, HC by 14%, CO by 21%, NOx by 20% and PM by 31%. No statistical analysis was 

presented. 

 The authors present an interesting discussion of the reasons for some of the emission effects. 

The FT fuel, because of its higher cetane, had a shorter ignition delay by about 10 degrees, which 

reduced evaporation before ignition leading to a more consistent heat release shape. The FT fuel also 

had a longer burn duration, even after considering the early ignition. These combine to create a lower 

peak pressure for the FT fuel and therefore lower NOx emissions. While these explanations may be 

accurate for this particular engine technology, other technologies may have different operating maps, and 

newer technologies may adjust operating conditions such as injection timing/duration and EGR operation 

to compensate for differences in fuel properties. Therefore, it is necessary to test a variety of engines 

and technologies. 

 Norton et al. [161] tested a petroleum diesel, a synthetic diesel made by Mossgas, and a 50/50 

mixture. Properties are shown below. The synthetic diesel is made from natural gas through a FT 

reaction that first produces olefins. It differs from typical FT diesel in that it has a significant aromatics 

concentration, and a cetane number that is in the petroleum diesel range. 

Fuel Property Units US Diesel Mossgas FT 

Density g/ml 0.838 0.8042 

Cetane Number 48.7 48.9 

Aromatics wt. % 25 9 

Sulfur ppm 200 < 5 
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 Six urban buses with DDC6V92 2-stroke, DI turbocharged 9 liter engines were tested. Three of 

the buses had rebuilt engines and retrofitted oxidation catalysts. The buses were tested on chassis 

dynamometer using the Central Business District driving cycle (SAE J1376). 

 Average changes from the petroleum diesel are shown below. The synthetic diesel reduced 

emissions of all components. For the 100% synthetic diesel, HC, NOx and PM reductions were similar for 

the two engines groups, and CO effects were different. The 50/50 mixture seemed to give a little more 

than half the benefit of pure synthetic diesel, especially in the rebuilt and retrofitted engines. No statistical 

analysis was provided to assess the significance of the results. 

Percent Change in Emissions 

Relative to US Diesel 

 Synthetic Diesel 50/50 Mixture 

Old 

Engines 

Retrofitted 

Engines 

Old 

Engines 

Retrofitted 

Engines 

HC - 20 - 35 - 7 - 20 

CO - 26 - 49 - 7 - 26 

NOx - 5 - 8 - 5 - 5 

PM - 20 - 31 - 6 - 20 

 Cheng and Dibble [149] tested a petroleum diesel and a FT fuel made at Shell’s Malaysia plant. 

A model year 1993 5.9 liter DI turbocharged Cummins B Series engine was tested at nine steady state 

modes chosen to cover a wide range of the operating map. 

Fuel Property Units US Diesel Shell FT 

Density g/ml 0.8473 0.7837 

Cetane Number 49.1 > 74 

Aromatics wt. % 16 0.3 

Sulfur ppm 98 < 5 

 Results are shown below comparing the FT diesel with the petroleum diesel. There were large 

reductions in all emissions components, similar to reduction seen in other programs. 

Percent Change in Emissions

Diesel → FT 

HC - 45 

CO - 42 

NOx - 9 

PM - 28 

 Schaberg et al. [163] tested two petroleum diesels (U.S. and ARB), a Sasol FT diesel, and two 

mixtures (30 Sasol/70 US and 50 Sasol/50 US). Fuels properties are shown below. 
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Fuel Property Units US LSD ARB Diesel Sasol FT 

Density g/ml 0.8450 0.8432 0.7698 

Cetane Number 46.2 52.6 > 74.8 

Aromatics wt. % 24.5 9.5 0.7 

Sulfur ppm 300 130 < 5 

 The engine used in this program was a 1999 model year DDC Series 60, 12.7 liter DI 

turbocharged engine electronically controlled unit injectors. Both cold start and hot start tests were run 

according to the U.S. transient FTP cycle. 

 Results are shown below for cold and hot cycles. Emissions of all the exhaust components were 

lower for both cycles. Most of the reduction in PM was the result of lower emissions of soot or 

carbonaceous particles. This is probably the result of higher H/C ratio because of the low aromatic levels 

in FT fuels. Sulfate emissions were also lower, tracking the fuel sulfur levels, but these were a relatively 

small fraction of the total PM. Detailed statistics were not reported for these data. 

 Percent Change in Emissions Relative to US Low Sulfur Diesel 

 Hot Start Cold Start 

ARB 

Diesel 

Sasol 

FT S30 S50 ARB 

Diesel 

Sasol 

FT S30 S50 

HC - 23 - 37 - 34 - 33 - 26 - 63 - 46 - 46 

CO - 6 - 37 - 23 - 22 - 12 - 47 - 17 - 31 

NOx - 7 - 17 - 3 - 10 - 15 - 23 - 8 - 18 

PM Total 

Soot 

VOF 

- 7 

- 2 

- 10 

- 37 

- 49 

- 10 

- 18 

- 23 

- 7 

- 9 

- 7 

- 8 

- 15 

- 16 

- 14 

- 46 

- 48 

- 41 

- 19 

- 41 

+ 11 

- 19 

- 14 

- 24 

 Johnson et al. [153] tested three fuels – FT diesel, U.S. low sulfur diesel and ARB diesel in a 

1991 DDC Series 60 11.1liter DI turbocharged engine with electronically controlled injection. Properties 

of the fuels are shown below: 

Fuel Property Units US Diesel ARB Diesel FT 

Density g/ml 0.845 0.842 0.774 

Cetane Number 45.5 50.2 74 

Aromatics wt. % 31.9 8.7 0.26 

Sulfur ppm 300 300 0 

 Emissions are shown below relative to the U.S. low sulfur diesel: 
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Percent Change in 

Emissions 

Relative to US LSD 

 ARB FT 

HC - 22 - 41 

CO - 16 - 45 

NOx - 4 - 9 

PM - 10 - 32 

 Johnson et al. argue that a good use of FT diesel would be as a blendstock, and bring a number 

of examples to show how poorer quality refinery blendstocks could be used in combination with FT diesel 

and still achieve low emissions, while potentially increasing the available volume of diesel fuel. 

 Fanick et al. [164] tested two diesel engines and four diesel fuels. One was a 1999 model year 

5.6 liter Cummins B Series DI turbocharged engine tested on a dynamometer and the other was a similar 

engine tested in a 2000 model year heavy LD pickup truck. The engine was tested using the EPA 

transient HD test cycle, and the pickup was tested using the FTP, HFET and US06 cycles. 

 Properties of the four fuels are shown below: 

Fuel Property Units US Diesel ARB Diesel Swedish Diesel FT 

Density g/ml 0.8455 0.8341 0.8194 0.7716 

Cetane Number 46.7 51.4 52.4 73.6 

Aromatics wt. % 30.7 8.1 3.7 0.7 

Sulfur ppm 300 155 < 10 < 10 

 Results from the HD transient test and the FTP are shown below: 

 Percent Change in Emissions Relative to US Diesel 

 HD Transient FTP 

ARB 

Diesel 

Swedish 

Diesel 

FT 

Diesel 

ARB 

Diesel 

Swedish 

Diesel 

FT 

Diesel 

HC - 16 + 3 - 41 - 11 - 15 - 30 

CO - 15 - 13 - 30 - 11 - 14 - 50 

NOx - 8 - 11 - 20 - 11 - 14 - 23 

PM - 18 - 27 - 40 - 11 - 27 - 33 

Toxics - 25 - 9 - 34 - 8 - 5 - 32 

 Changes in emissions due to fuels were similar for the HD transient and the FTP. For the more 

aggressive driving cycles (HFET and US06), the fuel effects on HC and toxics were smaller and the 

effects on PM were larger. No statistical analysis was reported. 

 McCormick et al. [141] tested a number of fuels in order to develop a strategy to mitigate the 

increase in NOx emissions seen with use of biodiesel or biodiesel blends. Properties of the base diesel 

and FT diesel are shown below. 
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Fuel Property Units US Diesel FT 

Specific Gravity 0.8476 0.7845 

Cetane Number 47.4 > 74.8 

Aromatics vol. % 31.9 * 

Sulfur ppm 430 < 10 

* Aromatics not reported, assumed to be 

extremely low. 

 The engine was a 1991 calibration DDC Series 60 11.1 liter turbocharged DI engine tested with 

the HD transient FTP. Relative to the petroleum diesel, FT diesel reduced emissions of HC by 65%, CO 

by 24%, NOx by 15% and PM by 34%. For B20 fuel, the increase in NOx emissions could be mitigated or 

reversed by adding 46% FT diesel. No statistical analysis was presented. 

 Schaberg et al. [155] compared a petroleum diesel with a FT diesel produced by Sasol. Fuel 

properties are summarized below: 

Fuel Property Units Diesel FT 

Specific Gravity 0.8705 0.7647 

Cetane Number 41.5 74.8 

Aromatics wt % 38.4 < 1 

Sulfur ppm 404 < 1 

 The engine was a Caterpillar 1998 model year C12, a 12 liter DI, turbocharged engine. Four 

steady state modes were tested and results are reported below for the average changes. Note that these 

data are expressed in terms of g emissions/kg fuel, not the usual brake specific basis. While there was 

an average reduction in soot emissions with the FT diesel, in two of the modes, soot emissions increased. 

This may be a consequence of the higher cetane value, although as the authors point out, the reasons for 

this are not clear. Particle size distribution with the FT seemed to be somewhat narrower than with 

petroleum diesel. This may be the result of two trends. One is that the lower sulfur levels lead to fewer 

nucleation particles. The other is that with the overall lower emission level, there is less opportunity for 

agglomeration of large particles. 

Percent Change in Emissions

Diesel → FT 

HC - 15 

CO - 15 

NOx - 17 

PM Total 

PM VOF 

PM Soot 

- 24 

- 43 

- 13 
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 Myburgh et al. [156] tested a FT diesel produced by Sasol and a low sulfur Japanese petroleum 

diesel in a HD vehicle. The engine was a 1998 model year Japanese 6.6 liter DI, naturally aspirated 

diesel with an inline injection pump, EGR, and an oxidation catalyst. Tests were run with and without the 

oxidation catalyst in place. Properties of the fuels are: 

Parameter Units Low S 

Diesel FT 

Density g/ml 0.8185 0.7680 

Cetane Number 57.5 74 

Aromatics wt. % 21.2 0.1 

Sulfur ppm 29 < 1 

 The 13 mode emissions test is designed for an engine dynamometer, and the test was simulated 

on a chassis dynamometer to accommodate the program vehicle. Results are shown below for the noncatalyst configuration comparing the two fuels. In the absence of the catalyst, HC emissions increased 

with FT diesel, but the oxidation catalyst was more efficient with FT diesel, and the post-catalyst 

emissions were lower. 

Percent Change in Emissions 

LS Diesel → FT 

Without 

Catalyst With Catalyst 

HC + 8 - 22 

CO - 22 - 60 

NOx - 15 - 12 

PM - 18 - 21 

 Frank et al. [127] tested a Navistar 2001 model year DT466E engine commonly used in urban 

transit buses with a variety of fuels and with a number of different aftertreatment configurations. Engine 

displacement was 7.6 liters, and the engine was turbocharged with electronically controlled injection, and 

no EGR. A FT diesel and an ultra low sulfur diesel (ULSD) were tested in similar configurations. 

Properties are shown below: 

Fuel Property Units ULSD FT 

Density g/ml 0.8264 0.7833 

Cetane Number 42.9 85.0 

Aromatics wt. % 20.2 0.8 

Sulfur ppm 28 1.2 

 Three aftertreatment configurations were tested – oxidation catalyst (DOC), continuously 

regenerating particulate trap (CRDPF) and DPF with EGR (EGR-DPF). The HD transient FTP test cycle 

was used, and the results are shown for the hot start portion which was run in all configurations. 
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Percent Change in Emissions 

ULSD → FT 

 DOC CRDPF EGR-DPF 

HC - 91 * * 

CO - 89 * + 118#

NOx - 11 - 9 - 11 

PM - 23 * * 

* Values too low to evaluate differences 

# Anomalous result with FT diesel 

 Generally, FT diesel reduced all emissions components. For some of the comparisons, the 

emissions were so low with both fuels that it was not possible to determine if there was a fuel effect. CO 

emissions increased when FT diesel was tested with the EGR-DPF system, but this result is suspect. 

 Yoshitomi et al. [165] tested three FT diesels with different volatilities along with a petroleum 

diesel and some blends in two HD engines. One engine was a 4 liter DI, turbocharged engine with 

common rail injection and DPNR aftertreatment system. The other was a 7.7 liter DI, turbocharged 

engine with a common rail injection system and DPF-NSR aftertreatment. Emissions were measured 

over the Japanese HD transient cycle, JE05. There were no differences in NOx emissions. PM 

emissions were lower by 30-60%. The less volatile FT fuels with lower T90 had lower PM emissions. 

 The authors observed that engine design could be optimized for FT diesels. For instance, 

compression ratio could be reduced while still maintaining low emissions and good stable combustion. 

Efficiency would increase to be equal to petroleum diesel, while still maintaining low emissions. 

 Kuronen et al. [166] tested hydrogenated vegetable oil (HVO) along with two ultra low sulfur 

petroleum diesels. HVO is made in a refinery process that converts vegetable oil to paraffins, and has 

properties similar to FT diesel. Properties of the tested fuels are shown below. 

Fuel Property Units HVO ULSD-A ULSD-B 

Density g/ml 0.779 0.839 0.839 

Cetane Number 82.9 55.5 57.8 

Aromatics wt. % < 0.02 18.4 22.1 

Sulfur ppm < 1 4.5 < 10 

 Two urban buses were tested using the transient Braunschweig City Driving Cycle. One bus (A) 

had a model year 2006 7.3 liter engine with common rail injection and Urea-SCR aftertreatment. The 

other bus (B) had a model year 2006 8.9 liter engine with unit injectors, EGR and an oxidation catalyst. 

 Emission effects are shown below for each bus. Bus A was run with HVO and ULSD-A and Bus 

B was run with HVO and ULSD-B. HC emissions were extremely low, and no differences could be seen. 

CO reductions for Bus B were high with HVO, presumably a reflection of the oxidation catalyst in the 

aftertreatment system. Speciated emissions were measured, and there were no differences between the 

fuels for aldehydes or PAHs. Distribution of PM size was also similar for the fuels. 
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Percent Change in Emissions 

ULSD → HVO 

 Bus A Bus B 

HC - - 

CO - 5 - 50 

NOx - 7 - 9 

PM - 30 - 46 

SUMMARY OF FT EFFECTS

 Almost all studies carried out with FT fuels show a reduction of emissions compared to petroleum 

diesel. There is a difference between LD and HD results with respect to NOx emissions. For HD diesel, 

NOx emissions were always improved with FT fuels, while the results for LD engines were mixed. This is 

the same conclusion reached by Alleman and McCormick [90] in their excellent review published in 2003. 

It is not known what individual properties of FT fuels caused the emissions benefits seen or why there is 

an apparent difference between LD and HD results. 

G. DIESEL SUMMARY AND RESEARCH NEEDS 

 Significant data exist which relate diesel fuel properties and exhaust emissions, both for LD 

diesels and for HD diesels. In many cases, the conclusions are not consistent and may be contradictory. 

Possible reasons for disagreement among the published results include: 

• Non-orthogonal design of fuel parameters. Results are presented in terms of one set of 

variables, but could be also explained in terms of a different set. 

• Small numbers of vehicles or engines were tested and cannot be generalized to the 

entire fleet. 

• Different test cycles were used, which may cause different fuel effects. 

• Special blendstocks were splash blended which does not allow an analysis of the 

reasons for the effects, and which may be dependent on the base fuel used in the 

experiment. 

 The following table represents a summary of the current state of knowledge of the impact of fuel 

property changes on exhaust emissions. 

To Reduce Emissions, Make the Directional Changes Shown Below 

(Light Duty / Heavy Duty) 

 Density Cetane Aromatics/PAH Sulfur Back-End Fatty Acid 

Esters 

Fischer 

Tropsch 

HC ↓/* ↑/↑ ↑/↓ 0/0 0/↑ ↑/↑ ↑/↑

CO ↓/* ↑/↑ ↑/* 0/0 0/↑ ↑/↑ ↑/↑

NOx */↓ */* ↓/↓ 0/0 #/# ↓/↓ #/↑

PM ↓/* ↓/# ↓/↓ ↓/↓ ↓/↓ ↑/↑ ↑/↑

0 - No effect 

* - Data are lacking to define effect 

# - Data exists, but effect is variable 
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 For existing LD and HD diesel technology, a full statistical analysis of existing data would help 

develop an understanding of quantitative effects of fuel parameters on exhaust emissions. This would be 

an update of the work published by EPA in 2001. An important piece of this effort would be an analysis of 

the ability to determine unambiguously the impacts of density, cetane and aromatics on emissions. Much 

of the published data, with a few notable exceptions, has confounded these parameters. If the analysis 

cannot determine independent effects, then additional experimental data would be useful. 

 Additional research on Fatty Acid Esters and Fischer Tropsch liquids would be helpful to 

determine their effects. This research should not use splash blending to prepare fuels, but should blend 

fuels so that the contribution of these blendstocks can be tied to specific properties. A key question, 

especially for Fatty Acid Esters is why they reduce emissions. 

 There is a need for research on diesels with exhaust aftertreatment. Future diesel engines will 

almost certainly contain some form of aftertreatment. When the technology is developed and 

implemented, research should be carried out to determine the impact of fuel variables on engines and 

vehicles containing the new technology. There is some existing data with oxidation catalysts in LD 

diesels and with DPFs, but very little data with Urea-SCR and NSR systems. 

 Appendix III contains a compilation of the number of vehicles and fuels tested in programs that 

evaluated various fuel properties. As shown in the table, there is recent data for most parameters, with 

the possible exception of density (last data published in 2001) and cetane (last data published in 2004). If 

these parameters are considered important from a regulatory perspective, then additional data may be 

required before regulations are considered. 

5. OFF-ROAD ENGINES AND VEHICLES 

 Off-road or nonroad applications consist of many uses and generally refer to internal combustion 

engines used for any application other than automobiles or trucks that travel on roads and highways. 

These include 

• Lawn and garden equipment 

• Small electric generators 

• Construction equipment 

• Mining equipment 

• Recreational vehicles such as snowmobiles 

• Farm vehicles and equipment 

• Marine vessels, including personal watercraft 

• Locomotives 

 For the purposes of this report, locomotives are not included. The list contains a wide variety of 

engine sizes, uses, duty cycles, and operating conditions. Most of the smaller engines are gasoline 

powered, and most of the larger ones are diesel powered. 

 As emissions from on-road vehicles have been reduced, off-road emissions have also been the 

subject of regulatory actions. Typically, emission control technology has advanced faster and been 

implemented sooner for on-road than for off-road applications. The consequence of this fact is that 

emissions from off-road sources are expected to grow much faster than from on-road sources. 

 In May 2004, EPA promulgated Tier 4 emission standards for off-road diesel engines [167]. 

These standards, which were coupled with standards controlling diesel sulfur levels, take effect over the 

years 2007-2015. In their analysis, EPA stated that they expected the controls to be catalyst based and 

enabled by the availability of 15 ppm sulfur diesel fuel. 

 In addition to promulgating new emission standards, EPA also promulgated new test procedures 

based on extensive data collection programs. The new tests involve a more accurate duty cycle and 

incorporate transient testing and cold start operation. 
CRC Project E-84, Final Report 

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 The table below shows the national inventory for controlled pollutants, both with and without the 

Tier 4 off-road diesel emission standards. Without the Tier 4 standards, the relative size of off-road diesel 

emissions will grow as on-road mobile sources are controlled to a greater extent. With controls in place, 

the relative contribution of off-road diesel stays fairly constant. 

National (48 state) Emissions Inventory for Land Based Non-Road Diesel [167] 

 1996 2020 (No New Controls) 2020 (With Tier 4 Controls) 

Short Tons % Mobile 

Sources Short Tons % Mobile 

Sources Short Tons % Mobile 

Sources 

PM2.5 186,507 47.2* 129,058 70.3* 50,065 47.9* 

NOx 1,564,904 12.1 1,119,481 22.2 677,420 14.7 

VOC 220,971 2.7 97,513 2.5 19,372 2.1 

CO 1,004,586 1.3 697,630 0.9 309,593 0.4 

* PM2.5 Percentages do not include gasoline vehicles. 

 The figure below shows the relative sizes of the off-road PM2.5 inventory for 1996 and 2020, with 

and without the Tier 4 standards promulgated in 2004. The size of the circles is proportional to the total 

inventory. With no new controls, off-road will continue to decline as a result of current programs being 

fully implemented. However, the yearly decline in emissions from the current standards diminishes over 

time, and by 2020, off-road emissions account for 70% of the total diesel mobile source PM2.5 inventory. 

With the new Tier 4 standards, reductions in off-road sources keep pace with reductions achieved in onroad sources, and the relative contributions stay constant. 

 Federal emission standards for spark ignition off-road engines were proposed in 2007 and are 

expected to be phased in over the years 2009-2012 [168]. These new regulations are expected to result 

in significant changes in technology, including the introduction of catalytic aftertreatment systems, fuel 

injection, and systems to control evaporative emissions. The table below shows the contribution of spark 

Diesel PM2.5 Mobil Source Inventories

1996 Baseline 2020

No New

Controls

Tier 4 Controls

On‐Road

Diesel

Off‐Road

Diesel
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ignition engines to the total mobile exhaust and evaporative emissions inventory. Emission standards 

limit the relative growth in emissions for this source relative to total mobile sources. 

National (50 state) Emissions Inventory for Non-Road Spark Ignition Engines [168] 

 2001 2020 (No New Controls) 2020 (With Controls) 

 Short Tons % Mobile 

Sources 

Short Tons % Mobile 

Sources 

Short Tons % Mobile 

Sources 

VOC 3,113,628 39.2 2,061,845 41.1 1,567,325 34.7 

CO 23,875,070 27.3 18,579,583 35.6 16,399,358 32.7 

NOx 490,365 4.0 266,903 5.6 194834 4.2 

PM2.5 56,214 12.4 53,466 22.4 48,377 20.7 

 While a great deal of data exists on the relationship between fuel properties and exhaust 

emissions for on-road vehicles, it is not clear that the data are relevant for off-road applications. There 

are major differences in design between on-road and off-road applications. For some applications, size 

and cost considerations make it difficult to adopt advanced technologies that are common on automotive 

engines. For example, virtually all automobiles use fuel injection to meter fuel, while current off-road 

engines use carburetors. 

 On the other hand, the duty cycles for off-road applications tend to be simpler and to include 

more steady state operation. The differences in duty cycles are reflected in the emission test cycles 

adopted by regulators. 

 The literature search conducted for off-road sources included research starting in 1990. A total of 

106 abstracts were reviewed for relevance and 18 papers were purchased and read. Of these, seven 

papers described programs that measured fuel effects in spark ignition engines and eight papers 

described programs that measured fuel effects in compression ignition engines. None of the research 

reported in the literature reported on the independent effect of individual fuel parameters. Where single 

properties such as ethanol concentration were varied, it was generally done in a way that changed many 

properties simultaneously. 

A. SPARK IGNITION ENGINES 

 The published literature on fuel effects on emissions from off-road spark ignition engines and 

vehicles has been limited to small engines used for lawn and farm equipment or for snowmobiles. These 

engines were all manufactured to operate on gasoline, and some were modified to operate on LPG. As 

discussed above, none of the studies evaluated specific chemical or physical fuel characteristics in a 

designed experiment. Either materials were splash blended into fully formulated fuels, or else two or 

more fully formulated fuels were compared. 

OXYGENATES

 Ethanol use was studied in a number of different engines types, and forms the largest body of 

research in the off-road spark ignition area. White et al. [169] tested two snowmobile engines with three 

fuels – base gasoline, splash blended gasohol and an aliphatic gasoline. Both engines were two cylinder 

2-stroke carbureted models having a displacement of around 450 cm3

. Neither one had a catalyst. The 

test cycle consisted of 5 steady state modes on a dynamometer. Lubricants were also tested. Results 

comparing the ethanol fuel and base gasoline with a common lubricant are shown below: 
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Percent Change (based on g/kW-hr) 

Base Gasoline → Gasohol 

 Engine 

 Polaris Arctco 

HC - 16 + 5 

CO - 9 - 6 

NOx + 1 + 6 

PM - 24 - 3 

Benzene - 10 * 

1,3 Butadiene + 7 * 

Formaldehyde + 5 * 

Acetaldehyde + 555 * 

Specific Reactivity 

(g O3/g HC) 

+ 3 * 

* Not tested 

 The results for the Polaris engine are consistent with an engine running rich, where adding an 

oxygenate will lean out the mixture and reduce emissions of unburned and partially burned material. The 

small response of the Arctos engine was not explained by the authors, except to point out that it was a 

water cooled engine. No statistical analysis was presented to evaluate whether the results were 

significant. 

 Bresenham and Reisel [170] studied splash blended ethanol-gasoline mixtures in two models of 

small utility engines. Three engines were tested. Two were Briggs and Stratton 12.5 hp (9.3 kW) slide 

valve engines, and the third was a 12.5 hp (9.3 kW) Kohler overhead valve engine. All three engines 

were carbureted. The test cycle was SAE J1088, which is a series of six steady state conditions. Five 

are loaded at 85% of rated speed and the sixth is a no-load idle mode. Four fuels were tested – a HC 

base and splash blends with 10%, 25% and 50% ethanol. No composition data was provided for the 

base blend. 

 All three engines started out rich and became progressively leaner as the concentration of 

ethanol increased. One of the engines operated just lean of stoichiometric at 50% ethanol, while the 

other two stayed slightly rich. Results are shown below: 

Percent Change (based on g/kW-hr) 

Base Gasoline → Ethanol Blends 

 HC CO NOx

 10% 

EtOH 

20% 

EtOH 

50% 

EtOH 

10% 

EtOH 

20% 

EtOH 

50% 

EtOH 

10% 

EtOH 

20% 

EtOH 

50% 

EtOH 

Engine 1 - 8 - 22 - 72 - 19 - 50 - 92 + 12 + 44 + 85 

Engine 2 - 11 - 37 * - 21 - 62 * + 36 + 152 * 

Engine 3 + 4 - 18 - 40 - 2 - 43 - 74 - 10 + 84 + 211 

* Not tested 

 These results are consistent with the stoichiometry reported for the engine. As more ethanol was 

added, HC and CO emissions decreased and NOx emissions tended to increase. It is interesting to note 
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that the changes in the sum of HC and NOx, the regulated measure of emissions, are very small. No 

statistical analysis of the results was presented. 

 Varde and Clark [171] tested 10% and 85% ethanol blends in a single cylinder carbureted small 

industrial (Class II) engine. Adjustments were made in spark timing, carburetor jets and stoichiometry 

control to accommodate the ethanol blends. The actual stoichiometry was not reported. 

 Emissions were measured at four loads and at two spark timings – standard and maximum brake 

torque. Results were shown graphically. CO and NOx emissions tended to decrease as ethanol 

concentrations increased, while HC emissions tended to increase. No statistics were reported. It was 

suggested that the high heat of vaporization lowered the combustion chamber temperatures and could 

help explain the HC and NOx effects. 

 Martinez and Ganji [172] tested a single cylinder utility engine with a number of ethanol blends. 

The engine was a 2.5 hp Honda, 83 cm3

 displacement with slide valves, a carburetor and fixed timing. It 

is typical of engines used in yard applications and pump and compressor sets. Five fuels were tested, 

comprising pure gasoline, pure ethanol, and 10%, 20% and 40% ethanol blends. Load, stoichiometry and 

spark advance were varied. 

 At equivalent power output, ethanol blends reduced emissions of NOx and CO by up to 60%. HC 

emissions were lower at low output, but were the same at higher outputs. No statistics were reported for 

the results. 

 Merritt and Netemeyer [173] tested a fuel with ETBE in three small engines – a four stroke 

overhead valve engine (160 cm3

), a four stroke side valve engine (188 cm3) and a two stroke engine (60 

cm3

). The four stroke engines complied with EPA 2007 regulations and the two-stroke engine complied 

with EPA 2004 regulations. These were all relatively high volume engines used for lawn and power 

equipment. A fuel with ETBE (2% oxygen) was compared to a non-oxygenated fuel. The ETBE had very 

low sulfur (1 ppm versus 64 ppm) and olefins (1% versus 9%) and no aromatics (versus 29%). The 

CARB SORE (Small Off-Road Emissions) emissions test was used. It consists of six steady state modes. 

Results are shown below: 

Percent Change (based on g/kW-hr) 

Base Gasoline → ETBE Gasoline 

 Engine 

 Four Stroke 

 Side Valve OHV 

Two 

Stroke 

HC - 2.4 - 6.6 - 9.4 

CO - 13.8 - 13.2 - 22.7 

NOx + 2.0 + 3.0 0 

PM * * - 28.9 

Benzene - 88 - 82 - 83 

1,3 Butadiene - 32 - 24 - 42 

Formaldehyde + 32 + 25 + 48 

Acetaldehyde + 2 + 98 + 208 

Total Toxics - 53 - 45 - 43 

Ozone Forming 

Potential 

- 2.1 - 4.4 - 29.7 

* Not tested 
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 Since there were a number of property changes, it is not possible to ascribe emissions effects to 

the use of ETBE. However, some changes may have been due to the addition of the oxygenate. No 

statistical analysis was provided. 

OTHER GASOLINE PROPERTIES

 A number of studies evaluated “clean” gasolines. White et al. [169], in the program described 

above, tested a snowmobile engine with a base gasoline and an aliphatic gasoline. 

 A comparison of the properties of the two fuels is shown below. Properties not shown were 

generally equivalent. 

Fuel Property 

(Units) 

Reference 

Gasoline 

Aliphatic 

Gasoline 

RVP (psi) 8.9 9.4 

Aromatics (Vol. %) 27 1.6 

Benzene (Vol. %) 0.17 0.0 

T90 (oC (oF)) 152 (306) 122 (252) 

 Results comparing the aliphatic and base gasolines are shown below, for a common lubricant: 

Percent Change (based on g/kW-hr) 

Reference Gasoline → Aliphatic Gasoline 

 Polaris Engine 

HC + 33 

CO + 2 

NOx + 12 

PM + 47 

Specific Reactivity 

(g O3/g HC) 

- 25 

 The aliphatic fuel had substantially higher HC and PM emissions. The authors did not attempt to 

explain these effects except to note that the aliphatic fuel had poor combustion quality as evidenced by 

frequent misfire. Fuel consumption increased and maximum power was reduced. It is not clear from the 

fuel properties why these effects were seen. 

 Mayer et al. [174] tested two Briggs and Stratton engines used commonly in yoke mowers in 

Swiss agriculture. One was a two cylinder engine having a displacement of 479 cm3

 and rated at 9.2 kW. 

The other was a single cylinder engine with 399 cm3

 displacement and rated at 8.1 kW. Both complied 

with California’s 1996 emission limits. The test cycle was ISO8178 Type G, which measure emissions at 

six steady state conditions and calculates a weighted average. Both engines were retrofitted with an 

emission control system that included a three way catalyst and adjusted carburetion to produce a leaner 

mixture. 

 Two fuels were tested, a standard Swiss gasoline and a non-aromatic “alkylate” gasoline. The 

authors did not present actual properties of the test fuels, except to say that aromatics, sulfur and olefins 

were typically extremely low for the alkylate fuel. 

 Emission results comparing the two fuels are shown below for tests with and without the catalyst. 
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 Percent Change (based on g/kW-hr) 

Reference Gasoline → Alkylate Gasoline 

 With Catalyst Without Catalyst 

HC 0 - 1 

CO - 22 - 3 

NOx - 29 - 34 

 PM emissions were measured and were not affected by the fuels. No statistics were reported for 

any of the results. The authors did not attempt to explain any of the results, especially the different 

effects for CO when measured with and without a catalyst. 

 In a program described above, Merritt and Netemeyer [173] tested a “Green” fuel in three small 

engines – a four stroke overhead valve engine (160 cm3

), a four stroke side valve engine (188 cm3) and a 

two stroke engine (60 cm3

). The non-oxygenated Green fuel had very low sulfur (1 ppm versus 64 ppm) 

and olefins (1% versus 9%) and no aromatics (versus 29%). Results are shown below: 

Percent Change (based on g/kW-hr) 

Base Gasoline → Green Gasoline 

 Engine Type 

 Four Stroke 

 Side 

Valve 

OHV 

Two 

Stroke 

HC + 9.3 + 2.7 + 1.3 

CO - 10.3 -4.5 + 1.4 

NOx + 0.65 +2.49 - 25.0 

PM * * - 4.8 

Benzene - 80 - 88 - 24 

1,3, Butadiene + 28 0 - 93 

Formaldehyde + 41 + 17 + 42 

Acetaldehyde + 2 - 10 + 1 

Total Toxics - 41 - 57 - 51 

Ozone Forming 

Potential 

+ 21 - 3 - 18 

* Not tested 

 The authors did not discuss possible reasons for the effects seen here, although it’s clear that the 

dramatic reduction in benzene emissions is a result of the lack of any aromatic compounds in the green 

fuel. No statistical analysis was presented. 
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PROPANE

 Sun [175] tested four engines typically used for commercial lawn and garden equipment. All were 

spark ignited, air-cooled, carbureted engines ranging in hp from 5 to 18. They were modified to run on 

LPG with feedback control at the same stoichiometry as with gasoline (lambda = 0.83). The test cycle 

was the six mode SAE J1088 test. Emission results are shown below for the four engines and two fuels. 

 Percent Change (based on g/kW-hr) 

Gasoline → LPG 

 Engine 1 Engine 2 Engine 3 Engine 4 

HC + NOx - 19 - 36 - 69 - 13 

CO - 42 - 39 - 67 - 55 

 The engines had much lower emissions when running on LPG. The authors did not try to explain 

the differences. 

B. COMPRESSION IGNITION ENGINES 

 Compression ignition engines tend to be larger than spark ignition engines, and are common in 

agriculture, industrial and construction applications. In terms of research programs reported in the 

literature, most of the comments made above with respect to spark ignition engines apply to compression 

ignition engines as well. Few, if any, literature reports discuss independent property variations in diesel 

fuel applications. A number of projects measured emissions using methanol and/or ethanol, which are 

unlikely to be used in the near terms in commercial applications. 

ALCOHOLS

 Although alcohols are generally not miscible in diesel fuel, have poor cetane quality, and require 

engine modifications for use in compression ignition engines, a number of studies have been carried out 

on use of alcohols. 

 Hemsley et al. [176] tested methanol in three modified yard-hosteler engines. Cummins CT8.3 

diesel engines were replaced with DDC-71TA methanol engines. The methanol engines had high 

compression ratios (23:1), and catalytic converters. Performance for the two engines was roughly equal. 

Methanol fuel was mixed with 1.5% AvocetTM, a combustion improver. The engine did not require a glow 

plug to initiate combustion because of the high compression ratio and ignition improver. 

 Although the main focus of this project was to demonstrate the potential for methanol fuel, 

emissions were also measured. The methanol fuel reduced all emissions components by large amounts - 

NOx by 28%, PM by 81%, HC by 87% and CO was totally eliminated. It is not clear how much of this 

reduction was due to the presence of a catalyst and how much was due to the use of methanol. 

 Merritt et al. [177] tested three off-road HD John Deere diesel engines with ethanol blends. The 

engines had displacements of 8.1, 6.8 and 12.5 liters, and were all certified to Tier II EPA standards. The 

injection systems were common rail, rotary pump and unit injector respectively. Four fuels were tested – 

base diesel and ethanol blends of 7.7%, 10% and 15% by volume. Three proprietary additives were used 

to keep the ethanol miscible with diesel. The 8-mode EPA test procedure was used for emissions 

measurements. 

 PM emissions decreased by 13-30% with ethanol in the three engines. CO emissions decreased 

up to 15% in the 8.1 and 12.5 liter engines. In the 6.8 liter engine, CO increased as much as 23%. NOx 

emissions were reduced between 5-9% in the 6.8 liter and 12.5 liter engines. NOx emissions increased a 

small amount in the 8.1 liter engine. Emissions of acetaldehyde increased with ethanol in all three 
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engines (27%-139%). Emissions of benzene and 1,3 butadiene were reduced in all engines with the use 

of ethanol. The authors did not discuss the anomalous CO results. Statistics were not presented. 

 Kumar et al. [178] tested ethanol-diesel microemulsions using up to 23% ethanol with 17% ethyl 

acetate as a stabilizer. Ethanol reduced CO emissions at high load, but increased CO emissions at low 

loads. HC emissions also increased over the entire load range. NOx emissions were mixed. 

OTHER OXYGENATES

 Dan et al. [179] tested a 4-stroke single cylinder high speed marine diesel with diesel/DME (dimethyl ether) blends. The engine had a compression ratio of 17.7, a jerk type injection pump, and a 

displacement of 0.638 liter. Three fuels were tested – 100% marine diesel, a 70% diesel/30% DME 

mixture and a 60/40 mixture. NOx emissions were 15-25% lower for the mixtures. The authors 

speculated that the volatile DME increased the pre-mix combustion, causing heat release to be extended 

and peak temperatures to be reduced. Smoke emissions were also reduced with DME mixtures. 

HYDROCARBON FUELS

 Nord and Haupt [180] tested a Valmet diesel engine, commonly used for farm machinery such as 

tractors and harvesters. It was a six cylinder turbocharged engine with 6.6 liter displacement and a rotary 

fuel injection pump. Two fuels were tested. One was a Fischer Tropsch diesel made by Eco-ParTM and 

the second was a Swedish EC-1 diesel. The two fuels had similar compositions except that the EC-1 had 

slightly higher density (0.815 versus 0.799 g/cm3

) and was more volatile (T50 of 230oC versus 253oC). 

Emissions were measured over the ISO 8178 test procedure. Emissions of NOx were reduced 7%, HC 

emissions were reduced 15% and CO was reduced 3%. Emissions of the toxics benzene, 1,3 butadiene, 

formaldehyde and acetaldehyde were also lower with the Fischer Tropsch fuel. 

 Frame et al. [181] tested a military 6.5 liter HD engine using hot start transient dynamometer 

testing with the FTP and a nonroad San Antonio Transit cycle (SAT). Two fuels were tested. One was a 

low sulfur conventional diesel and the other was a synthetic JP-5 fuel made using the Fischer Tropsch 

process by Syntroleum. Fuel properties are summarized below. 

Test Fuel Properties 

Fuel Property (Units) Low Sulfur 

Diesel (LSD)

Synthetic 

JP-5 

Sulfur (ppm) 370 < 1 

Aromatics (Vol. %) 30.9 0.9 

Cetane Number 47.9 69.3 

T50 (oC, (oF)) 260 (500) 220 (428) 

Density (g/cm3) 0.845 0.765 

 A comparison of emissions is shown below for the two test cycles. 
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Percent Change (based on g/kW-hr) 

LSD → FT Diesel 

 FTP Cycle SAT Cycle 

HC - 62 - 72 

CO - 47 - 60 

NOx - 13 - 15 

PM - 55 - 52 

 The changes in emissions are generally in line with the properties of the fuels. PM, for instance, 

would tend to be lowered by reductions in sulfur, density and aromatics (especially PAHs). No statistical 

analysis was reported. 

 Matthews et al. [182] tested a model year 2001 Cummins ISB 190 engine from a Gradall model 

XL3100 grader. No engine details were provided except that the engine did not have any aftertreatment. 

The test cycle was the TxDOT Boom Excavator Cycle. Two fuels were tested. One was a standard low 

sulfur diesel, and the other was a Texas Low Emissions Diesel (TxLED). Properties of the fuels are 

shown below. 

 Test Fuel Properties 

Fuel Property (Units) Texas LSD TxLED 

Sulfur (ppm) 377 2.5 

Aromatics (Vol. %) 33.4 5.9 

Cetane Number 46.8 50.5 

T50 (oC, (oF)) 262 (504) 269 (516) 

Density (g/cm3) 0.8419 0.8454 

 Emissions comparisons are shown below. All differences are statistically significant (95% CL). 

Percent Change (based on g/kW-hr) 

Texas LSD → TxLED 

HC - 38 

CO - 12 

NOx - 6 

PM - 18 

 Birch and Kubesh [183] modified a diesel engine to run on LPG. The diesel engine was a John 

Deere PowerTech 4.5 liter engine used in a model 4510 tractor commonly used for park and right of way 

maintenance. The modifications included adding spark ignition, a new fuel system to accommodate LPG 

and control stoichiometry, and an oxidation catalyst. Although torque and thermal efficiency were 

reduced by about 5% with LPG, emissions were substantially lower on the ISO 8178 test cycle, as shown 

below. The measure of emissions used by the authors was HC+NOx because this is the form of the 

emission standard. 
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Percent Change (based on g/kW-hr) 

Diesel → LPG 

HC + NOx - 44 

CO 0 

PM - 92 

 The LPG engine met 2008 EPA Tier 3 emission standards. The large reduction in HC+NOx was 

driven by a reduction in NOx. This is understandable given that diesel emissions are typically very low in 

HC and CO and high in NOx. A reduction in HC and CO would therefore not be expected to be large if 

there was one at all. 

C. OFF-ROAD SUMMARY AND RESEARCH NEEDS 

 Off-road engines and vehicles are important contributors to mobile source emissions in the United 

States. As emissions from on-road vehicles continues to decline, the relative importance of emissions 

from off-road sources becomes more important and is being subject to control as well. Under current 

programs, off-road sources are predicted to contribute as much as 30-50% of the total in 2020. 

 Not much research has been carried out exploring the impact of fuel quality on exhaust emissions 

from off-road vehicles and equipment. The research that has been published has focused on 

understanding the impact of blendstocks that are splash blended into fully formulated fuels. This fact, 

coupled with the multitude of applications for off-road equipment, makes it difficult to predict the impact of 

fuel properties on exhaust emissions from off-road applications. 

SPARK IGNITION ENGINES

 Most of the research on spark ignition engines has evaluated oxygenates, especially ethanol, in a 

splash blended environment. Results are consistent with engines that were running rich and were leaned 

out by the addition of oxygen. 

 As part of their recent rulemaking proposals for future emission standards, EPA stated that they 

expected to see major technology improvements in off-road gasoline engines over the next 4 years. The 

changes that EPA expects include feedback control of stoichiometry and catalytic aftertreatment. These 

changes suggest that future technology might have different responses than existing off-road technology. 

COMPRESSION IGNITION ENGINES

 Most of the research on compression ignition sources has evaluated fully formulated diesel fuels 

in a way that the effects of individual properties could not be determined. Some studies also evaluated 

alcohols such as ethanol and methanol. The likelihood of these alcohols gaining widespread use in 

commercial diesel fuel is fairly low. 

 Tier 4 emission standards for off-road diesel engines were promulgated by EPA in 2004, and will 

be implemented over the years 2008-2015. EPA expects these standards to be met by adoption of new 

technology, including catalytic aftertreatment. This was one of the main reasons that off-road diesel fuel 

will have a maximum sulfur level of 15 ppm. It is difficult to predict how emissions from these 

technologies will respond to fuel properties. 

RESEARCH NEEDS

 More stringent emission standards have recently been adopted or proposed by EPA for off-road 

sources. It is expected that the new standards will result in development and implementation of new 

technology such as catalytic aftertreatment and feedback control of stoichiometry. Since off-road sources 

are expected to continue to be a major source of emissions in the future, it would be worthwhile to 

develop a substantial database on the relationship between fuel properties and emissions. There is a 
CRC Project E-84, Final Report 

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major benefit to having one fuel distribution system and not require specialize fuels for off-road 

applications. Therefore, development of such a database should center on the current or expected 

properties of gasoline and diesel fuels. 

 Considering that the current database is sparse, considerable thought would have to be given to 

allocation of resources among: 

• spark ignition and compression ignition applications 

• engine types within each major area 

 Appendix III contains a compilation of the number of vehicles and fuels tested in programs that 

evaluated various fuel properties. The table reflects the paucity of data for off-road sources, especially 

considering the large number of different applications. 
CRC Project E-84, Final Report 

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APPENDIX I 

Statement of Work: CRC E-84 

Review of Prior Studies of Fuel Effects on Vehicle Emissions

The Coordinating Research Council (CRC) is seeking a comprehensive review of published 

studies of fuel property effects on vehicle exhaust emissions. The study should cover gasoline 

light-duty vehicles and diesel light and heavy-duty vehicles. CRC will also consider as an option 

a review of fuel effects on non-road emissions. The intent of the study is to update previous 

reviews conducted by Koehl et al. (1) on light-duty gasoline and by Pedley et al. (2) on heavyduty diesel. Highest priority should be given to studies published after these reviews and to 

vehicle technologies that make up the bulk of the current and future US vehicle fleet. 

While the highest priority should be given to studies covering US vehicles, fuels and emission 

testing procedures, relevant studies from Europe, Japan and other regions should also be 

included. Emphasis should be on published studies using established government/industry 

emission measurement procedures simulating in-use vehicle operation and with well designed 

test fuel matrices with independent variation in one or more fuel property that allow individual 

fuel property effects to be evaluated. 

Fuel properties should include regulated properties or those covered in the reviews of Koehl et 

al. and Pedley et al. For gasoline, key fuel properties include RVP, sulfur, distillation properties, 

oxygenate, aromatics, benzene, and olefins and for diesel, cetane - both natural and additized, 

aromatics, density, and distillation. Other properties may be included if experimental information 

indicates that they have a significant impact. The impact of vehicle technology on fuel effects 

including emission control technology should also be considered. The study should include 

mass emissions of criteria pollutants NMHC, NOx, CO and PM, and HC speciation effects 

including air toxics and ozone formation potential. 

References 

1) Koehl W.J., J. D. Benson, V. Burns, R. A. Gorse, A. M. Hochhauser and R.1 Reuter, 

1991, “Effects of Gasoline Composition and Properties on Vehicle Emissions: A Review 

of Prior Studies – Auto/Oil Air Quality Improvement Research Program,” SAE 912321. 

2) Lee R., J. Pedley, and C. Hobbs, 1998, “Fuel Quality Impact on Heavy-Duty Diesel 

Emissions: - A Literature Review,” SAE 982649. 
CRC Project E-84, Final Report 

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APPENDIX II 

List of Acronyms 

A/F – Air Fuel Ratio (mass) 

AQIRP – Auto/Oil Air Quality Improvement Research Program 

ARB – (California) Air Resources Board, sometimes referred to as CARB 

ASTM – ASTM International, voluntary standards development organization; formerly American Society 

for Testing and Materials 

CO – Carbon monoxide 

CRC – Coordinating Research Council 

DECSE – Diesel Emissions Control – Sulfur Effects Project 

DEE – Di-ethyl ether 

DI – Direct Injection 

DMM – Di-methoxy methane 

DPF – Diesel Particulate Filter 

ECE – Economic Commission for Europe (emissions test cycle) 

EGR – Exhaust Gas Recycle 

EPA – United States Environmental Protection Agency 

EPEFE – European Program on Emissions, Fuels and Engine Technologies 

ETBE – Ethyl tertiary-butyl ether 

FAE – Fatty Acid Ester 

FT – Fischer Tropsch 

FTP – Federal Test Procedure (for emissions) 

GDI – Gasoline Direct Injection 

GTL – Gas-to-Liquid 

HC – Hydrocarbons 

HCCI – Homogeneous Charge Compression Ignition 

IDI – Indirect Injection 

JCAP – Japanese Clean Air Program 

JCME – Jatropha curcas methyl ester 

MPI – Multi-Point Injection 

MTBE – Methyl tertiary-butyl ether 

NBA – Non-Benzene Aromatics 

NEDC – New European Driving Cycle 

NMHC – Non-methane hydrocarbons 

NMOG – Non-methane organic gases 

NOx – Oxides of nitrogen 

NREL – National Renewable Energy Laboratory (DOE) 

NS – not statistically significant 

NSR – NOx Storage Reduction 

OBD – On-Board Diagnostics System 

PM – Particulate Matter 

POME – Palm oil methyl ester 

PPEE – Pongamia Pinnata ethyl ester 

PPME – Pongamia Pinnata methyl ester 

RSME – Rapeseed methyl ester 
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SAE – Society of Automotive Engineers 

SIDI – Spark Injection Direct Ignition 

SOF – Soluble Organic Fraction or particulate emissions 

TAME – tertiary-amyl methyl ether 

TLEV – Transitional Low Emissions Vehicle 

TOG – Total Organic Gases 

TWC – Three way catalyst 

Urea-SCR – Urea based Selective Catalytic Reduction 
CRC Project E-84, Final Report 

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APPENDIX III 

Summary of Fuels and Vehicles Database 

 An Excel spreadsheet was prepared, which contains summary information that was used in 

preparing this report. For every data and review reference cited in this report, the following information 

has been tabulated: 

Paper Number (typically SAE number) 

Main author 

Data paper or review paper 

Fuel parameters studied 

Number of fuels tested 

Was PM measured (diesel only) 

Number of vehicle or engines tested 

Country or region represented by fuels/vehicles 

Comments as appropriate 

Full citation 

 The spreadsheet is organized so that it can be sorted easily to determine, for instance, all the 

references that tested one or more fuel parameters. It is available from CRC (www.crcao.org). 

 The three tables shown below contain summary information for the gasoline, diesel and off-road 

areas. They are intended to summarize the general size of the database, and to show when the most 

recent data was collected. In studies that used a factorial design, all fuels were used to determine the 

response of all design parameters. In a number of studies, other statistical designs were used, and not 

all fuels were used to determine the response of all the parameters. Therefore, the tables below might 

contain some values that are somewhat high. .A more detailed analysis of the references would be 

required to determine how many emissions tests were conducted for each parameter. 

Data Summary - Gasoline Studies 

Property # Studies Vehicles Fuels Most Recent Publication Date 

Sulfur 36 360 307 2003 

Aromatics 35 334 346 2007(Most recent work covers GDI) 

Olefins 21 236 215 2007 (Most data >9 years old) 

RVP 14 289 142 1999 

Mid-fill 

Back-end 33 373 430 2007 (19 studies examined both 

back-end and mid-fill) 

Ethers 33 382 345 2007 

Ethanol 18 214 177 2007 
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Data Summary - Diesel Studies 

Property LD/HD # Studies Vehicles Fuels Most Recent 

Publication Date 

Cetane L 15 86 267 2004 

H 13 63 205 2007 

Density L 11 72 194 2001 

H 6 44 132 2000 

Aromatics L 16 109 279 2007 

H 17 90 257 2007 

Sulfur L 13 41 140 2007 

H 12 34 93 2005 

Back-end L 14 81 200 2007 

H 9 51 77 2007 

Oxygenates L 11 19 107 2007 

H 15 33 128 2007 

Fischer-Tropsch L 14 25 77 2007 

H 13 20 77 2007 

Data Summary - Diesel Studies 

Property Gasoline/ 

Diesel # Studies Vehicles Fuels Most Recent 

Publication Date 

Oxygenates G 5 10 18 2006 

D 4 9 13 2007 

Fischer-Tropsch D 2 2 4 2004 

Other G 4 11 10 2005 

D 2 2 4 2005 
CRC Project E-84, Final Report 

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APPENDIX IV 

References 

1. http://www.arb.ca.gov/fuels/gasoline/premodel/PM Users Guide Final_041508.doc, May 1, 2008. 

2. EPA420-P-01-001, "Strategies and Issues in Correlating Diesel Fuel Properties with Emissions", US 

Environmental Protection Agency Office of Transportation and Air Quality. July, 2001. 

3. Koehl, W.J., A. M. Hochhauser, R. M. Reuter, J. D. Benson, V. R. Burns, R. A. Gorse Jr., "Effects of 

Gasoline Composition and Properties on Vehicle Emissions: A Review of Prior Studies", SAE Paper 

912321, 1991. 

4. Lee, R., J. Pedley, C. Hobbs, "Fuel Quality Impact on Heavy Duty Diesel Emissions - A literature 

Review", SAE Paper 982649, 1998. 

5. Furey, R.L., D. R. Monroe, "Fuel Sulfur Effects on the Performance of Automotive Three-Way 

Catalysts During Vehicle Emissions Tests", SAE Paper 811228, 1981. 

6. Williamson, W.B., H. S. Gandhi, M. E. Heyde, G. A. Zawacki, "Deactivation of three-way catalysts 

by fuel contaminants - lead, phosphorus and sulfur", SAE Paper 790942, 1979. 

7. Benson, J.D., V. R. Burns, R. A. Gorse Jr., A. M. Hochhauser, W. J. Koehl, L. J. Painter, R. M. 

Reuter, "Effects of Gasoline Sulfur Level on Mass Exhaust Emissions", SAE Paper 912323, 1991. 

8. Koehl, W.J., J. D. Benson, V. R. Burns, R. A. Gorse Jr., A. M. Hochhauser, J. C. Knepper, W. R. 

Leppard, L. J. Painter, L. A. Rapp, R. M. Reuter, J. A. Rutherford, "Effects of Gasoline Sulfur Level 

on Exhaust Mass and Speciated Emissions: The Question of Linearity", SAE Paper 932727, 1993. 

9. Carter, W.P.L., Development of the SAPRC-07 Chemical Mechanism and Updaed Ozone eactivity 

Scales, Final Report to the Calfornia Air Resources Board. August 31, 2007. 

10. Knepper, J.C., W. J. Koehl, J. D. Benson, V. R. Burns, R. A. Gorse Jr., A. M. Hochhauser, W. R. 

Leppard, L. A. Rapp, R. M. Reuter, "Fuel Effects in High Emitting Vehicles", SAE Paper 930137, 

1993. 

11. Takei, Y., H. Hoshi, M. Kato, M. Okada, K. Abe, "Effects of California Phase 2 Reformulated 

Gasoline Specifications on Exhaust Emission Reduction", SAE Paper 922179, 1992. . 

12. Takei, Y., H. Hoshi, M. Okada, K. Abe, "Effects of Gasoline Components on Exhaust Hydrocarbon 

Components", SAE Paper 932670, 1993. 

13. Hirota, T., K. Kojima, K. Yakushiji, T. Inoue, "Effects of Exhaust Emission Control Devices and Fuel 

Composition on Speciated Emissions of S.I. engines", SAE Paper 922180, 1992. 

14. Mayotte, S.C., C. E. Lindhjem, V. Rao, M. S. Sklar, "Reformulated Gasoline Effects on Exhaust 

Emissions: Phase I: Initial Investigation of Oxygenate, Volatility, Distillation and Sulfur Effects", SAE 

Paper 941973, 1994. 

15. Lindhjem, C.E., "The Effect of Gasoline Reformulation and Sulfur Reduction on Exhaust Emissions 

from Post-1983 but Pre-1990 Vehicles", SAE Paper 950778, 1995. 

16. Takei6, Y., T. Vehara, H. Hoshi, S. Sugiyama, M. Okada, "Effects of California Phase 2 

Reformulated Gasoline Regulations on Exhaust Emissions Reduction: Part 2", SAE Paper 952502, 

1995. 

17. Rutherford, J.A., Koehl, W.J., J. D. Benson, V. R. Burns, A. M. Hochhauser, J. C. Knepper, W. R. 

Leppard, L. J. Painter, L. A. Rapp, B. Rippon, R. M. Reuter, "Effects of Gasoline Properties on 

Emissions of Current and Future Vehicles - T50, T90 and Sulfur Effects", SAE Paper 952510, 1995. 

18. Sztenderowicz, M.L., W. J. Bandy, W. J. Most, S. Jetter, T. L. Sprik, H. Doherty, K. D. Eng, "Effects 

of Fuel Sulfur Level on Emissions From Transitional Low Emission Vehicles", SAE Paper 952561, 

1995. 

19. Takei, Y., T. Uehara, H. Hoshi, S. Sugiyama, M. Okada, "Effect of California Phase 2 Reformulated 

Gasoline Specifications on Exhaust Emission Reduction; Part 3", SAE Paper 972851, 1997. 

20. Schleyer, C.H., R. A. Gorse Jr., R. F. Gunst, G. J. Barnes, J. Eckstrom, K. D. Eng, J. Freel, M. 

Natajaran, A. M. Schlenker, "Effect of Fuel Sulfur on Emissions in California Low Emission 

Vehicles", SAE Paper 982726, 1998. 

21. Takei, Y., Y. Kinugasa, M. Okada, T. Tanaka, Y. Fujimoto, "Fuel Property Requirement for 

Advanced Technology Engines", SAE Paper 2000-01-2019. 

22. Lyons, J.M., D. Lax, S. Welstand, "Investigation of Sulfur Sensitivity and Reversibility in Late-Model 

Vehicles", SAE Paper 1999-01-3676. 

23. http://www.autoalliance.org/archives/archive.php?id=125&cat=Fuel%20Quality. 
CRC Project E-84, Final Report 

99

24. Durbin, T.D., J. W. Miller, J. T. Pisano, T. Younglove, C. G. Sauer, S. H. Rhee, T. Huai, G. I. 

Mackay, "The Effect of Fuel Sulfur on NH3 and Other Emissions from 2000-2001 Model Year 

Vehicle", CRC Project E-60, May, 2003. Also available at www.crcao.com. 

25. Morgan, T.D.B., G. J. den Otter, W. W. Lange, J. Doyon, J. R. Barnes, T. Yamashita, "An Integrated 

Study of the Effects of Gasoline Composition on Exhaust Emissions", SAE Paper 932678, 1993. 

26. Petit, A., J. G. Jeffrey, F. H. Palmer, R. Steinbrink, "European Program on Emissions, Fuels and 

Engine Technologies (EPEFE) "Emissions from gasoline sulphur study", SAE Paper 961071, 1996. 

27. Kwon, Y.K., R. Bazzani, P. J. Bennett, O. Esmilaire, P. Scorletti, T. David, B. Morgan, C. L. 

Goodfellow, M. Lien, W. Broeckx, P. Liiva, "Emissions Response of a European Specification 

Direct- Injection Gasoline Vehicle to a Fuels Matrix Incorporating Independent Variations in Both 

Compositional and Distillation Parameters", SAE Paper 1999-01-3663. 

28. Stradling, R., N. Thompson, R. Bazzani, D. Rickeard, S. D. Bjordal, P. M. Martinez, P. Schmelzle, P. 

Scorletti, G. Wolff, P. J. Zemroch, "Fuel Effects on Regulated Emissions From Modern Gasoline 

Vehicles", SAE Paper 2004-01-1886. See also CONCAWE Report 5/03, "Fuel Effects on Emissions 

from Modern Gasoline Vehicles, Part 1 - Sulfur Effects", July, 2003. 

29. Akimoto, J., T. Kaneko, T. Ichikawa, K. Hamatani, T. Omata, "The Effects of Sulfur on Emissions 

from a S.I. Engine", SAE Paper 961219, 1996. 

30. Koseki, K., T. Uchiyama, M. Kawamura, "A Study on the Effects of Sulfur in Gasoline on Exhaust 

Emissions", SAE Paper 2000-01-1878. 

31. Hamasaki, M., M. Yamaguchi, K. Hirose, "Japan Clean Air Program (JCAP) - Step 1 Study of 

Gasoline Vehicle and Fuel Influence on Emissions", SAE Paper 2000-01-1972. 

32. Saitoh, K., M. Hamasaki, "Effects of Sulfur, Aromatics, T50, T90 and MTBE on Mass Exhaust 

Emission From Vehicles with Advanced Technology - JCAP Gasoline WG STEP II Report", SAE 

Paper 2003-01-1905 or JSAE Technical Paper No. 20030058. 

33. Thummadetsak, T., A. Wuttimongkilchai, S. Tunyapisetsak, T. Kimura, "Effect of Gasoline 

Compositions and Properties on Tailpipe Emissions of Currently Existing Vehicles in Thailand", SAE 

Paper 1999-01-3570. 

34. Bjordal, S.D., C. L. Goodfellow, P. Beckwith, P. J. Bennett, R. J. Brisley, A. J. J. Wilkins, "An 

Evaluation of the Long Term Effects of Gasoline Sulfur Level on Three-Way Catalyst Activity", SAE 

Paper 952421, 1995. 

35. Schleyer, C.H., K. D. Eng, R. A. Gorse, R. F. Gunst, J. Eckstrom, J. Freel, M. Natarajan, A. M. 

Schlenker, "Reversibility of Sulfur Effects on Emissions of California Low-Emission Vehicles", SAE 

Paper 1999-01-1544. 

36. Truex, T.J., "Interaction of Sulfur with Automotive Catalysts and the Impact on Vehicle Emissions - 

A Review", SAE Paper 1999-01-1543. 

37. Hochhauser, A.M., C. H. Schleyer, L. I. Yeh, D. J. Rickeard, "Impact of Fuel Sulfur on Gasoline and 

Diesel Vehicle Emissions", SAE Paper 2006-01-3370. 

38. Uihlein, J.P., Presentation to CARB Workshop, January 26, 2007. Available at 

www.arb.ca.gov/fuels/gasoline/meeting/2007/012607wspaprstn.pdf. 

39. Mohr, M.U.L., G. Margaria, "ACEA Program on the Emissions of Fine Particulates From Passenger 

Cars (2) - Part 2: Effect of Sampling Conditions and Fuel Sulfur Content on the Particle Emission", 

SAE Paper 2003-01-1890 or JSAE Technical Paper No. 20030081. 

40. Ntziachristos L., A.M., Z. Samaras, U. Mathis, M. Mohr, N. Thompson, R. Stradling, L. Forti, C. de 

Serves, "Overview of the European "Particulates" Project on the Characterization of Exhaust 

Particulate Emissions from Road Vehicles: Results for Light-Duty Vehicles", SAE Paper 2004-01-

1985. 

41. Hochhauser, A.M., W. J. Koehl, L. J. Painter, B. H. Rippon, R. M. Reuter, J. A. Rutherford, J. D. 

Benson, V. R. Burns, R. A. Gorse Jr., "The Effect of Aromatics, MTBE, Olefins, and T90 on Mass 

Exhaust Emissions from Current and Older Vehicles", SAE Paper 912322, 1991. 

42. Hochhauser, A.M., J. D. Benson, V. R. Burns, R. A. Gorse Jr., W. J. Koehl, L. J. Painter, R. M. 

Reuter, J. A. Rutherford, "Speciation and Calculated Reactivity of Automotive Emissions and Their 

Relation to Fuel Properties", SAE Paper 920325, 1992. 

43. Gorse Jr., R.A., J. D. Benson, V. R. Burns, A. M. Hochhauser, W. J. Koehl, L. J. Painter, R. M. 

Reuter, B. H. Rippon, "Toxic Air Pollutant Vehicle Exhaust Emissions with Reformulated Gasolines", 

SAE Paper 912324, 1991. 

44. Marshall, W.F., M. D. Gurney, "Effect of Gasoline Composition on Emissions of Aromatic 

Hydrocarbons", SAE Paper 892076, 1989. 

45. API Publication 4557, "Effects of Specific Fuel Aromatic Compounds on Motor Vehicle Exhaust 

Emissions". American Petroleum Institute, Washington, DC, August 1994. 

46. Jeffrey, J.G., N. G. Elliott, "Gasoline Composition Effects in a Range of European Vehicle 

Technologies", SAE Paper 932680, 1993. 
CRC Project E-84, Final Report 

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47. Wedekind, B., P. J. Bennett, C. L. Goodfellow, J. G. Jeffrey, G. F. Marchesi, C. R. McDonald, "The 

Independent Effect of Mid-Range, Back-End Volatility and Aromatics on Emissions from Two 

European Gasoline Engines", SAE Paper 952522, 1995. 

48. Goodfellow, C.L., R. A. Gorse Jr., M. J. Hawkins, J. S. McArragher, "European Program on 

Emissions, Fuels and Engine Technologies (EPEFE)~Gasoline Aromatics/E100 study", SAE Paper 

961072, 1996. 

49. Bennett, P.J., P. Beckwith, S. D. Bjordal, C. L. Goodfellow, "Relative Effects of Vehicle Technology 

and Fuel Formulation on Gasoline Vehicle Exhaust Emissions", SAE Paper 961901, 1996. 

50. McDonald, C.R., T. D. B. Morgan, O. Graupner, E. Wilkinson, "The Independent Effects of Fuel 

Aromatic Content and Mid-Range Volatility on Tailpipe Emissions from Current Technology 

European Vehicle Fleets", SAE Paper 962026, 1996. 

51. CONCAWE Report 96/51, "The Influence of Gasoline Benzene and Aromatics Content on Benzene 

Exhaust Emissions from Non-Catalyst and Catalyst Equipped Cars - A Study of European Data", 

Brussels, January, 1996. Available at www.concawe.be. 

52. LeJeune, A., C. R. McDonald, G. R. Lee, "NOx Aromatics Effects in Catalyst-Equipped Gasoline 

Vehicles", SAE Paper 941869, 1994. 

53. van den Brink, P.J., C. R. McDonald, "The Influence of the Fuel Hydrocarbon Composition on NO 

Conversion in 3-Way Catalysts: The NOx/Aromatics Effect", SAE Paper 952399, 1995. 

54. Graupner, J.O., W. W. Lange, J. Kostka, A. König, "Interaction Between Gasoline Properties and 

Engine Management System and Effects on 3-Way Catalyst Efficiency", SAE Paper 972839, 1997. . 

55. Sogawa, Y., H. Hattori, N. Yanagisawa, M. Hosoya, T. Shoji, Y. Iwakiri, T. Yamashita, T. Ikeda, S. 

Tanaka, K. Takahashi, T. Suzuki, T. Nakajima, Y. Tonegawa, "Nano Particle Emission Evaluation of 

State-of-the-Art Diesel Aftertreatment Technologies (DPF, urea-SCR and DOC), Gasoline 

Combustion Systems (Lean Burn/Stoichiometric DISI and MPI) and Fuel Qualities Effects (EtOH, 

ETBE, FAME, Aromatics and Distillation)", SAE Paper 2007-01-4083. 

56. Hirose, T., H. Komoriya, M. Kojima, T. Tanaka, N. Ohshio, A. Hozumi, S. Tanaka, H. Ohtsuka, 

"Effects of ETBE and EtOH Blending in Gasoline on PM Emission From a Direct Injection Spark 

Ignition Vehicle", SAE Paper 2007-01-4084. 

57. Prati M. V., M.R., N. Violetti, R. Mercoglano, R. Trere, "Regulated and Benzene Emissions of In-Use 

Two-Stroke Mopeds and Motorcycles", SAE Paper 2000-01-0862. 

58. Jain, A.K., S. Pathak, Y. Singh, S. Singh, M. Saxena, M. Subramanian, P. C. Kanal, "Effect of 

Gasoline Composition (Olefins, Aromatics and Benzene) on Exhaust Mass Emissions from TwoWheelers - An Experimental Study", SAE Paper 2007-26-014. 

59. Jessup, P.J., M. C. Croudace, T. Wusz, "An Overview of Unocal's Low Emission Gasoline Research 

Program", SAE Paper 920801, 1992. 

60. Pentikäinen, J., L. Rantanen, P. Aakko, "The Effect of Heavy Olefins and Ethanol on Gasoline 

Emissions", SAE Paper 2004-01-2003. 

61. Most, W.J., "Coordinating Research Council Study of Winter Exhaust Emissions with 

Gasoline/Oxygenate Blends", SAE Paper 892091, 1989. 

62. Reuter, R.M., J. D. Benson, V. R. Burns, R. A. Gorse Jr., A. M. Hochhauser, W. J. Koehl, L. J. 

Painter, B. H. Rippon, J. A. Rutherford, "Effects of Oxygenated Fuels and RVP on Automotive 

Emissions", SAE Paper 920326, 1992. 

63. Korotney, D.J., V. Rao, C. E. Lindhjem, M. S. Sklar, "Reformulated Gasoline Effects on Exhaust 

Emissions: Phase III; Investigation on the Effects of Sulfur, Olefins, Volatility, and Aromatics and the 

Interactions Between Olefins and Volatility or Sulfur", SAE Paper 950782, 1995. 

64. Rutherford, J.A., J. C. Horn, M. Naylor, King Eng, M. Kulakowski, L. A. Rapp, B. Gronowski, V. 

Lawson, "Effects of RVP Reduction on Vehicle CO Emissions During Las Vegas and Los Angeles 

Winter Conditions~Petroleum Environmental Research Forum Project Number 95-06", SAE Paper 

971726, 1997. 

65. Hoekman, S.K., R. S. MacArthur, M. Naylor, J. A. Rutherford, "RVP reduction for control of 

wintertime CO", SAE Paper 981373, 1998. 

66. Bennett, P.J., P. Beckwith, C. L. Goodfellow, K. Skårdalsmo, "The Effect of Gasoline RVP on 

Exhaust Emissions From Current European Vehicles", SAE Paper 952526, 1995. 

67. Clarke, P.J., "Effect of gasoline volatility on exhaust emissions", SAE Paper 720932, 1979. 

68. Gething, J.A., "Distillation Adjustment: An Innovative Step to Gasoline Reformulation", SAE Paper 

910382, 1991. 

69. Koehl, W.J., J. D. Benson, V. R. Burns, R. A. Gorse Jr., A. M. Hochhauser, J. C. Knepper, W. R. 

Leppard, L. J. Painter, L. A. Rapp, R. M. Reuter, J. A. Rutherford, "Effects of Heavy Hydrocarbons 

in Fuel on Exhaust Mass Emissions, Air Toxics and Calculated Reactivity - Auto/Oil Air Quality 

Improvement Research Program", SAE Paper 932723, 1993. 
CRC Project E-84, Final Report 

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70. Kanehara, K., N. Sasajima, M. Nakada, N. Kayanuma, T. Ogawa, "Analyzing the Influence of 

Gasoline Characteristics on Transient Engine Performance", SAE Paper 912392, 1991. 

71. Durbin, T.D., J. W. Miller, T. Younglove, T. Huai, K. Crocker, "Effects of Ethanol and Volatility 

Parameters on Exhaust Emissions", CRC Project E-67, Coordinating Research Council, Inc., 

Alpharetta GA, January, 2006. Available at www.crcao.org. 

72. CONCAWE Report 94/59, "The Influence of Heavy Gasoline Components on the Exhaust 

Emissions of Catalyst Equipped European Vehicles, Part 1 - Regulated Emissions", Brussels, 

December, 1994. Available at www.concawe.be. 

73. CONCAWE Report 95/53, "The Influence of Heavy Gasoline Components on the Exhaust 

Emissions of Catalyst Equipped European Vehicles, Part 2 - Unregulated Emissions", Brussels, 

May, 1995. Available at www.concawe.be. 

74. Koseki, K., A. Hayashi, K. Fukuda, M. Wakita, N. Okabe, M. Watanabe, T. Kikuchi, K. Atsuta, H. 

Aiko, A. Kameoka, M. Kohakura, " Investigations of Compatibility of ETBE Gasoline With Current 

Gasoline Vehicles II", SAE Paper 2007-01-2039 or JSAE Technical Paper No. 20077226. 

75. CONCAWE Report 95/61, "The Influence of Gasoline Mid-Range to Back-End Volatility on Exhaust 

Emissions". Brussels, November, 1995. Available at www.conawe.be. 

76. Koehl, W.J., J. D. Benson, V. R. Burns, R. A. Gorse Jr., A. M. Hochhauser, J. C. Knepper, W. R. 

Leppard, L. J. Painter, L. A. Rapp, R. M. Reuter, J. A. Rutherford, "Effects of Heavy Hydrocarbons 

in Fuel on Exhaust Mass Emissions, Air Toxics and Calculated Reactivity - Auto/Oil Air Quality 

Improvement Research Program", SAE Paper 932723, 1993. 

77. Hood, H., R. Farina, "Emissions From Light Duty Vehicles Operating on Oxygenated Fuels at Low 

Ambient Temperatures: A Review of Published Studies", SAE Paper 952403, 1995. 

78. Koehl, W.J., J. D. Benson, V. R. Burns, R. A. Gorse Jr., A. M. Hochhauser, J. C. Knepper, W. R. 

Leppard, L. J. Painter, L. A. Rapp, R. M. Reuter, J. A. Rutherford, "Comparison of Effects of MTBE 

and TAME on Exhaust and Evaporative Emissions" SAE Paper 932730, 1993. 

79. Farina, R.J., "A Comparison of the Exhaust Emission Effects of MTBE With a Hydrocarbon 

Surrogate of Equivalent Volatility", SAE Paper 952559, 1995. 

80. Exxon Mobil Oxygenate Program, Submission to CARB Predictive Model Database, May, 2006. 

Available at http://www.arb.ca.gov/fuels/gasoline/carfg3/exxonmobil/pstn.pdf. 

81. Crary, B., "Effects of Ethanol on Emissions of Gasoline LDVs", Toyota presentation to CARB, May, 

2000. Available at http://www.arb.ca.gov/fuels/gasoline/carfg3/toyota_prstn.pdf. 

82. Graupner1, J.O., J. S. McArragher, T. D. B. Morgan, "The Effect of MTBE in Gasolines on 

Regulated Exhaust Emissions from Current European Vehicles", SAE Paper 962025, 1996. 

83. Koseki, K., M. Wakita, N. Okabe, T. Kaneko, M. Watanabe, K. Horie, T. Kikuchi, K. Atsuta, H. Aiko, 

A. Kameoka, A. Honma, "Investigations of Compatibility of ETBE Gasoline with Current Gasoline 

Vehicles", SAE Paper 2006-01-3381. 

84. Hattori, H., Y. Sogawa, N. Yanagisawa, M. Hosoya, T. Shoji, Y. Iwakiri, T. Yamashita, T. Ikeda, S. 

Tanaka, K. Takahashi, T. Suzuki, T. Nakajima, Y. Tonegawa, "Unregulated Emissions Evaluation of 

Gasoline Combustion Systems (Lean Burn/Stoichiometric DISI and MPI), State-of-the-Art Diesel 

Aftertreatment Technologies (DPF, urea-SCR and DOC), and Fuel Qualities Effects (EtOH, ETBE, 

Aromatics and FAME)", SAE Paper 2007-01-4082. 

85. Subramanian, M., A. K. Setia, P. C. Kanal, N. K. Pal, S. Nandi, R. K. Malhotra, "Effect of AlcoholBlended Fuels on the Emissions and Field Performance of Two-Stroke- and Four-Stroke-EnginePowered Two Wheelers", SAE Paper 2005-26-034. 

86. Jaroonjitsathian, S., N. Akarapanjavit, S. S. Sa-norh, S. Chanchaona, "Investigation of 2-Wheeler 

Performance, Emissions, Driveability and Durability: Effect of Ethanol-Blended Gasoline", SAE 

Paper 2007-01-2034 or JSAE Technical Paper No. 20077148, 2007. 

87. Chang, D.Y., J. H. Van Gerpen, “Determination of Particulate and Unburned Hydrocarbon 

Emissions from Diesel Engines Fuels with Biodiesel”, SAE Paper 982527, 1998. 

88. Rickeard, D.J., R. Bonetto, M. Signer, "European Program on Emissions, Fuels and Engine 

Technologies (EPEFE) - Comparison of Light and Heavy Duty Diesel Studies", SAE Paper 961075, 

1996. 

89. Veinblatt, M., L. Tartakovsky, M. Gutman, Y. Zvirin, S. Hausberger, "Fuel Effects on Emissions 

From Heavy-Duty Diesel Engines - Results of Recent Research Programs", SAE Paper 2001-24-

0081 or 2001-01-081. 

90. Alleman, T.L., R. L. McCormick, "Fischer-Tropsch Diesel Fuels - Properties and Exhaust Emissions: 

A Literature Review", SAE Paper 2003-01-0763.
CRC Project E-84, Final Report 

102

91. EPA420-P-02-001, “A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions”, US 

Environmental Protection Agency Office of Transportation and Air Quality. October 2002. 

92. Weidmann K., H.M., K. Reders, R. C. Hutcheson, "Diesel Fuel Quality Effects on Exhaust 

Emissions", SAE Paper 881649, 1988. 

93. Betts, W.E., S. A. Fløysand, F. Kvinge, "The Influence of Diesel Fuel Properties on Particulate 

Emissions in European Cars", SAE Paper 922190, 1992. 

94. Fløysand, S.A., F. Kvinge, W. E. Betts, "The Influence of Diesel Fuel Properties on Particulate 

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95. Tritthart, P., R. Cichocki, W. Cartellieri, "Fuel Effects on Emissions in Various Test Cycles in 

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96. Lange, W.W., A. Schäfer, A. Le Jeune, D. Naber, A. A. Reglitzky, M. Gairing, "The Influence of Fuel 

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97. Den Ouden, C.J.J., R. H. Clark, L. T. Cowley, R. J. Stradling, W. W. Lange, C. Maillard, "Fuel 

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100. Mann, N., F. Kvinge, G. Wilson, "Diesel Fuel Effects on Emissions - Towards a Better 

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101. Kwon, Y., N. Mann, D. J. Rickeard, R. Haugland, Kjell Arne, F. Kvinge Ulvund, G. Wilson, "Fuel 

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102. Tsurutani, K., Y. Takei, Y. Fujimoto, J. Matsudaira, M. Kumamoto, "The Effects of Fuel Properties 

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103. Hublin, M., P. G. Gadd, D. E. Hall, K. P. Schindler, "European Programs on Emissions, Fuels and 

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104. CONCAWE Report 4/02, “Evaluation of Diesel Fuel Cetane and Aromatics Effects on Emissions 

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105. Tamanouchi, M., H. Morihisak, H. Arakik, S. Yamada, "Effects of Fuel Properties and Oxidation 

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107. Oyama, K., T. Kakegawa, "Evaluation of Diesel Exhaust Emission of Advanced Emission Control 

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108. Hara, S., T. Kaneko, T. Kakegawa, S. Senbokuya, O. Nakamura, M. Shibuya, H. Itoyama, M. 

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114. Tanaka, S., H. Takizawa, T. Shimizu, K. Sanse, "Effect of Fuel Compositions on PAH in Particulate 

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141. McCormick, R.L., J. R. Alvarez, M. S. Graboski, K. S. Tyson, K. Vertin, "Fuel Additive and Blending 

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143. Beatrice, C., C. Bertoli, N. Del Giacomo, M.na. Migliaccio, “Potentiality of Oxygenated Synthetic 

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144. Hess, H.S., A. L. Boehman, P. J. A. Tijm, F. J. Waller, "Experimental Studies of the Impact of 

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145. Szymkowicz, P.G., D. T. French, C. C. Crellin, “The Effects of Advanced Fuels on the Particulate 

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150. Chapman, E.M., S. V. Bhide, A. L. Boehman, Peter J. A. Tijm, F. J. Waller, “Emission 

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151. Vertin, K.D., J. M. Ohi, D. W. Naegeli, K. H. Childress, G. P. Hagen, C. I. McCarthy, A. S. Chang, R. 

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152. Payri, F., J. Arregle, C. Fenollosa, G. Belot, A. Delage, P. Schaberg, I. Myburgh, “Characterisation 

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153. Johnson1, J.W., P. J. Berlowitz, D. F. Ryan, R. J. Wittenbrink, W. B. Genetti, L. L. Ansell, Y. Kwon, 

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154. Schubert, P.F., B. J. Russell, R. L. Freerks, J. DeVore, E. R. Fanick, "Impact of Ultra-Clean FischerTropsch Diesel Fuel on Emissions in a Light-Duty Passenger Car Diesel Engine", SAE Paper 2002-

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155. Schaberg, P.W., D. D. Zarling, R. W. Waytulonis, D. B. Kittelson, "Exhaust Particle Number and 

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156. Myburgh, I., M. Schnell, K. Oyama, H. Sugano, H. Yokota, S. Tahara, "The Emission Performance 

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162. Atkinson, C.M., G. J. Thompson, M. L. Traver, N. N. Clark, “In-Cylinder Combustion Pressure 

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163. Schaberg, P.W., I. S. Myburgh, J. J. Botha, I. A. Khalek, “Comparative Emissions Performance of 

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164. Fanick, E.R., P. F. Schubert, B. J. Russell, R. L. Freerks, “The Comparison of Emissions 

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166. Kuronen, M., S. Mikkonen, P. Aakko, T. Murtonen, "Hydrotreated Vegetable Oil as Fuel for HeavyDuty Diesel Engines", SAE Paper 2007-01-4031. 

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170. Bresenham, D., J. Reisel, "The Effect of High Ethanol Blends on Emissions From Small Utility 

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171. Varde, K.S., C. P. Clark, “A Comparison of Burn Characteristics and Exhaust Emissions From OffHighway Engines Fueled by E0 and E85”, SAE Paper 2004-28-0045. 

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175. Sun, X., “Low Emission Propane-Fueled Lawn and Garden Engines”, SAE Paper 2000-01-2061. 

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179. Dan, T., M. Hashimoto, I. Isano, A. Kunikawa, “Improvement of Exhaust Gas Emission in Marine 

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182. Matthews, R., M. Hall, J. Anthony, R. Baker, J. Prozzi, R. Machemehl, T. Ullman, D. Lewis, “The 

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