Subcontract Report

NREL/SR-540-47819

April 2010

An Experimental and Modeling 

Study of the Flammability of Fuel 

Tank Headspace Vapors from 

Ethanol/Gasoline Fuels

Phase 2: Evaluations of Field Samples and 

Laboratory Blends

D.P. Gardiner, M.F. Bardon, and M. LaViolette

Nexum Research Corporation

Kingston, Ontario, Canada 
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Subcontract Report

NREL/SR-540-47819

April 2010

An Experimental and Modeling 

Study of the Flammability of Fuel 

Tank Headspace Vapors from 

Ethanol/Gasoline Fuels

Phase 2: Evaluations of Field Samples and 

Laboratory Blends

D.P. Gardiner, M.F. Bardon, and M. LaViolette

Nexum Research Corporation 

Kingston, Ontario, Canada

NREL Technical Monitor (Phase 1): M. Melendez

NREL Technical Monitor (Phase 2): Y. Coy

Prepared under Subcontract No. XCI-5-55505-01

Subcontract Period of Performance (Phase 1): August 2005 to December 2008

Subcontract Period of Performance (Phase 2): May 2009 to December 2009
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i

Executive Summary

Background

The flammability of a fuel blend in the head space of a storage tank is controlled by the vapor

pressure of the blend. Conventional gasolines are so volatile that the headspace vapors are 

normally too rich to sustain combustion at most ambient temperatures. Only under very cold 

conditions does the vapor-air mixture in the tank become flammable. 

Because of its different vapor pressure and stoichiometry, pure ethanol produces vapor-air 

mixtures in the tank that are flammable over a wide range of ambient temperatures. Blends of 

ethanol and gasoline form non-ideal mixtures in which the gasoline components render the 

vapor-air mixture in a tank too rich for combustion at warm temperatures. Like gasoline, as 

temperature drops, the vapor-air mixture becomes leaner and eventually falls into the flammable 

region. However, the mixture becomes flammable at higher temperatures than conventional 

gasoline and so presents a greater hazard if a suitable ignition source is present.

Objectives of the Work Reported

This project comprised an experimental study to measure the flammability of fuel vapors at low 

ambient temperatures and further development of a mathematical model to predict the 

temperatures at which flammable vapors were likely to form.

In the current project, two sets of fuels were investigated. The first set consisted of “real world” 

fuel samples obtained from refuelling stations, which are referred to as “field samples” 

throughout this report. These samples included E85 blends, as well as low- and mid-level blends 

containing approximately 10-30% ethanol. Investigations involving these fuels were intended to 

provide information about the possibility of flammable mixtures forming with the fuels that are 

currently available. 

The second set of fuels (provided by Marathon Petroleum Company) was produced in a 

laboratory to systematically investigate the effects of varying ethanol content on the volatility 

and flammability of the resulting blends. These fuels were produced by blending denatured 

ethanol with varying percentages of a single type of gasoline. The gasoline used for the blends 

had properties that were representative of a typical winter grade fuel. Investigations involving 

these fuels were intended to provide information to guide future fuel specifications. Such 

specifications could be developed based upon knowledge of the compromise between avoiding 

flammable mixtures at low ambient temperatures and maximizing the ethanol content of the fuel.

Principal Results

1. Both of the E85 field samples that were tested had vapor pressure values well below the 

minimum Class 3 specifications of ASTM D 5798-09. One of these fuels produced 

flammable vapors over the entire Class 3 temperature range (-5oC and below). The other 

fuel produced flammable vapors at -10oC and below. These results indicate that at least 

some of the ethanol fuels that are currently available when and where Class 3 conditions 

exist are likely to produce flammable vapors within the ambient temperature range where 

they are used.
ii

2. Two mid-level ethanol blends (E20 and E30) were tested. These field samples were 

produced by a blender pump that mixed E10 (the base gasoline available at that location) 

with E85. No significant differences could be measured between vapor flammability limits 

of the E20-E30 samples and the E10 sample they were produced from. These results 

indicate that mid-level blends in this range are unlikely to significantly increase the risk of 

producing flammable vapors over that of the base gasoline used for the blends.

3. Laboratory fuel blends of E55-E83 were produced by blending denatured ethanol with a 

single type of winter volatility gasoline. The flammability limits of these fuels showed a 

nonlinear relationship with both ethanol content and vapor pressure. Consequently, 

reducing the ethanol content of the fuels from typical E85 levels (80% or more) initially 

produced substantial reductions in the temperature at which it was possible for flammable 

vapors to form. Reductions in ethanol content below about 60% had a smaller relative 

impact on the flammability limits.

4. The laboratory fuel blends were produced from a “typical” winter volatility gasoline, with 

a dry vapor pressure equivalent (DVPE) value of 89 kPa. None of the blends produced 

from this gasoline could simultaneously meet the ASTM D5798 requirements for 

minimum vapor pressure (66 kPa) and minimum ethanol content (70%). The vapor 

pressure data for the blends indicated that a reduction in ethanol content to about 65% 

would be needed to achieve the minimum vapor pressure specification. The flammability 

data trends suggest that blends with ethanol content in this region would provide a 

favorable trade-off between avoiding the formation of flammable vapors and maximizing 

the ethanol content of the fuel.

5. Comparisons of the flammability results of the field and laboratory samples indicate that 

vapor pressure (DVPE) alone does not adequately predict the flammability of the fuel tank 

vapors formed at low temperatures. The properties of the hydrocarbon portion of the fuel 

must be considered as well.

6. It is recommended that further testing be carried out with laboratory fuels to examine the 

effects of gasoline volatility on the vapor flammability of the ethanol/gasoline blends. 

Flammability results over a wider range of ethanol content levels (including base gasoline 

without ethanol) are needed to elucidate the interactions between ethanol content, gasoline 

volatility, blend vapor pressure, and vapor flammability. 

7. When some basic properties of the base oxygenate-free gasoline are known, then the 

mathematical model developed in this study can be used to compare the flammability 

characteristics of various gasoline-ethanol blends quite satisfactorily. The model requires 

ASTM D86 distillation data, D4052 specific gravity, and D5191 DVPE of the base 

oxygenate-free gasoline and the percentages of ethanol and gasoline in the blend.

8. The use of conventional flammability data leads to the prediction of higher rich limit 

temperatures than those actually measured in the experimental apparatus used in the study. 

Although an adjustment might be made to better match the predictions to the 
iii

measurements, this is unlikely to be worthwhile, since it would again be apparatus 

dependent and not necessarily applicable to other conditions. Rather, the model is useful 

in comparing blends to each other to assess their relative hazards.

 

9. Since the project had sufficient gasoline data only for the laboratory blends tested for 

flammability, further work is needed to compare ethanol blends containing different 

gasoline base stock. This would allow an assessment to be made of the reliability of the 

model for comparing the relative hazards of different fuel sources.

10. Since most gasolines now contain some oxygenates, the distillation (D86) data for the base 

gasoline alone is often not available. Given the success of the model, it would be desirable 

to attempt to devise a technique for extracting the necessary gasoline data from the D86 

data of a low alcohol blend. That is not trivial, but comparisons such those made of the 

special E0 and E10 fuels studied in this project may allow such a technique to be 

developed, thereby making the model much more accessible for use by others.
iv

Table of Contents

I EXPERIMENTAL STUDY ....................................................................1

1 Introduction.................................................................................................................1

2 Background.................................................................................................................1

2.1 The Flammability Problem .............................................................................1

2.2 Flammability versus Ignitability.....................................................................2

3 Apparatus and Test Procedures...................................................................................3

4 Test Fuels....................................................................................................................4

5 Experimental Results and Discussion.........................................................................5

5.1 Pressure Measurements...................................................................................5

5.2 Upper Flammability Limits.............................................................................8

6 Conclusions from Experimental Study .....................................................................11

II MATHEMATICAL MODELING STUDY..........................................13

7 Introduction...............................................................................................................13

8 Scope of the Present Modeling Study.......................................................................14

9 Validation Using the Specific Tests and Test Fuels .................................................15

10 Modeling the Laboratory Blends..............................................................................18

11 Comparison of Predicted and Measured Flammability of Fuel Tank

Headspace Vapors.....................................................................................................19

12 Conclusions from the Modeling Study .....................................................................21

III OVERALL CONCLUSIONS AND RECOMMENDATIONS............22

13 Conclusions from the Experimental Study ..............................................................22

14 Conclusions from the Modeling Study ....................................................................23

15 References.................................................................................................................24

APPENDIX A: Properties of Field Samples ........................................................26

APPENDIX B: Properties of Laboratory Samples ................................................27

APPENDIX C: Summary of the Models for the Volatility and Flammability

of Ethanol-Gasoline Blends .................................................................28

APPENDIX D: Alberta Research Council−Fuels & Lubricants

Report of Analysis ...........................................................................31
v

List of Figures

Figure 1. Flammability Test Chamber....................................................................................3

Figure 2. Maximum Pressure Rise of Field Fuel Samples......................................................5

Figure 3. Maximum Rate of Pressure Rise of Field Fuel Samples.........................................6

Figure 4. Maximum Pressure Rise of Laboratory Fuel Samples............................................7

Figure 5. Maximum Rate of Pressure Rise of Laboratory Fuel Samples ...............................8

Figure 6. Effect of Ethanol Content on Upper Flammability Limit ......................................9

Figure 7. Effect of Ethanol Content on Vapor Pressure (DVPE) of Test Fuels ...................10

Figure 8. Effect of Vapor Pressure (DVPE) on Upper Flammability Limit of Test Fuels...11

Figure 9. Measured and Predicted Vapor Pressures – Indolene @ 0°C ...............................14

Figure 10. Measured and Predicted Vapor Pressure of ARC Gasoline (V/L=4)..................17

Figure 11. Measured and Predicted Vapor Pressure of ARC E10 Blend (V/L=4)...............17

Figure 12. Measured and Predicted DVPE of Laboratory Samples .....................................19

Figure 13. Measured and Predicted Flammability of Tank Headspace Vapors ...................20

 
1

I EXPERIMENTAL STUDY

1 Introduction

The project described in this report was carried out to examine the potential for flammable 

fuel/air mixtures to form in fuel tanks containing blends of ethanol and gasoline. The current

project followed an earlier project [1] in which an experimental technique and a mathematical 

model were developed to investigate the flammability issue. The ethanol/gasoline blends

evaluated in the first project were experimental fuels originally produced for vehicle drivability 

studies. These blends (referred to as E85 fuels) contained about 70-80% ethanol and a variety of 

hydrocarbon components.

In the current project, two sets of fuels were investigated. The first set consisted of “real world” 

fuel samples obtained from refuelling stations, which are referred to as “field samples” 

throughout this report. These samples included E85 blends, as well as low- and mid-level blends 

containing approximately 10-30% ethanol. Investigations involving these fuels were intended to 

provide information about the possibility of flammable mixtures forming with the fuels that are 

currently available. 

The second set of fuels (provided by Marathon Petroleum Company) was produced in a 

laboratory to systematically investigate the effects of varying ethanol content on the volatility 

and flammability of the resulting blends. These fuels were produced by blending denatured 

ethanol with varying percentages a single type of gasoline. The gasoline used for the blends had 

properties that were representative of a typical winter grade fuel. Investigations involving these 

fuels were intended to provide information to guide future fuel specifications. Such 

specifications could be developed based upon knowledge of the compromise between avoiding 

flammable mixtures at low ambient temperatures and maximizing the ethanol content of the fuel.

2 Background

2.1 The Flammability Problem 

Gasoline is so volatile at most ambient temperatures that the headspace vapors (the gaseous 

fuel/air mixture above the liquid in the fuel tank) are actually too rich to burn, as long as some 

liquid fuel remains. However, as temperature drops, or as the liquid fuel level goes down, the 

volatility of the fuel decreases. As the liquid level drops, there is less fuel vapor mixed with the 

air in the tank.

If the ambient temperature is cold enough and the tank is nearly empty, then the fuel-air mixture 

in the tank becomes flammable and can pose an explosion hazard if ignited. This has always 

been the case with gasoline, but the temperatures and fill levels needed to produce a hazard are 

rarely encountered, so fires and tank explosions are very unlikely, although not impossible. 

Ethanol by itself in a fuel tank produces headspace vapors that are flammable at room 

temperature and over a broad range of commonly encountered ambient temperatures. 

Ethanol/gasoline blends generally have volatility characteristics between those of the two major 
2

constituents. Any given high-alcohol blend tends to produce flammable fuel tank vapors at 

higher (i.e., less cold) temperatures than pure gasoline. The extent of the difference, and hence of 

any increased risk, depends on composition of the gasoline part of the blend and how much 

gasoline is present in the fuel mixture [2-5]. It is therefore prudent to assess the extent of any 

differences in the fire hazards of fuel tank headspace vapors between gasoline and ethanol fuel 

blends.

The standard specifications for fuel ethanol are described in ASTM D5798 [6], which provides 

three different vapor pressure classes for fuel ethanol, depending upon the seasonal and climatic 

conditions that apply. The coldest of these is referred to as Class 3, which encompasses 

geographic areas with 6-h tenth percentile minimum ambient temperature less than or equal to 

-5oC. Since low ambient temperatures present the greatest risk of the formation of flammable 

headspace vapors, Class 3 fuels and climatic conditions are the focus of the current study.

2.2 Flammability versus Ignitability

A fuel-air mixture is considered to be flammable if, when ignited, it produces a flame that can 

propagate throughout the available mixture. A flammable mixture might fail to ignite if the 

ignition source is too weak. However, successful ignition does not guarantee successful flame 

propagation. In some cases, the ignition source is strong enough to initiate a small flame, but the 

flame is extinguished as it moves away from the ignition source. In such cases, only a portion of 

the available mixture is burned, and the mixture is not, therefore, considered flammable.

In the present study, the objective was to determine the flammability of the headspace vapors 

from different fuels. As such, the strategy was to employ an ignition source strong enough to 

ensure that any flammable mixture could be ignited reliably. With this achieved, flammability 

experiments should show evidence of partial combustion when the limits are reached. 

The use of closed combustion chambers and pressure measurements in the experiments provided 

a means of differentiating between ignition failures, successful ignition followed by partial flame 

propagation, and substantially complete flame propagation. Tests in which the mixture fails to 

ignite show no increase in chamber pressure. Partial flame propagation produces a small but 

detectable pressure rise. 

Tests in which most of the mixture in the chamber is burned should exhibit similar peak pressure 

levels. Assuming nearly complete flame propagation, differences in peak pressure levels would 

mainly be due to differing amounts of heat lost to the chamber walls. As the flammability limits 

are approached, greater heat losses (therefore, lower peak pressure levels) are expected due to 

slower combustion. However, even these reduced pressure levels are well above those that occur 

when only a small portion of the mixture is burned (i.e., the partial propagation case).

Another diagnostic tool for examining flame propagation in the chamber is the rate of pressure 

rise. The rate of pressure rise (obtained through differentiation of the pressure signal) reflects the 

overall heat release rate within the chamber. This rate is influenced by the laminar burning 

velocity of the mixture and the surface area of the flame front. For example, a weak flame that 

extinguishes near the ignition source will reach a relatively small maximum surface area and will 

have a relatively low burning velocity prior to extinction. These factors will result in a very low 

rate of pressure rise compared with a case in which combustion is nearly complete. The rate of 
3

pressure rise also provides a sensitive means of comparing cases with similar peak pressure 

levels but different combustion rates. The peak rate of pressure rise falls as the flammability limit 

is approached.

3 Apparatus and Test Procedures

Eight identical test chambers were constructed so that a number of fuel samples could be chilled 

simultaneously. Photographs of one of the test chambers are shown in Figure 1. The interior 

volume of each chamber was 296 mL.

Figure 1. Flammability Test Chamber

The pressure sensor was a Honeywell model 19CP300PA4K absolute pressure sensor with a fullscale range of 0–2,070 kPa (0–300 psia). The sensor had a response time of 0.1 ms, which was 

found to be fast enough to capture the transient features of the pressure rise events during 

combustion. The gas temperature inside the chamber was measured using a Nanmac “right 

angle” K-type thermocouple. This thermocouple was a fast-response design with a low-inertia 

ribbon junction. The thermocouple was not fast enough to detect the actual transient gas 

temperature during combustion, but it could track the dynamic temperature behavior well enough 

to provide relative comparisons between tests.

The temperature of the liquid fuel at the bottom of the chamber was measured using a 

conventional 1.6-mm (1/16 in.) sheathed K-type thermocouple with a grounded junction. This 

thermocouple was positioned so that it would be immersed in the liquid layer (about 5 mm deep). 

The ignition source for the experiments was composed of an automotive spark plug and a 

laboratory programmable ignition system. The ignition system discharged 1 J of stored energy 

during each spark, producing a saw-tooth spark current waveform (50-100 mA) with a duration 

of 1 ms. The spark plug was a Champion model 7034, which had platinum pins in the center and 

ground electrodes. This type of spark plug was selected because the pins ensure that the spark 

location is consistent from spark to spark. The electrode gap was set at 2 mm (0.079 in). This 

particular spark plug model also had a projected tip, which placed the spark gap closer to the 

vertical center of the chamber.

Pressure

Gas Thermocouples Fuel

Compressed Air Valve Fuel Valve
4

The experiments were carried out in an Espec model PUA-3AP temperature chamber. During 

most of the experiments, all eight fuel chambers were chilled together. The fuel chambers were 

placed on racks within the “assured test volume” of the temperature chamber, where (according 

to the manufacturer) the rated temperature uniformity (±0.5oC) should exist. During some tests, 

the same type of fuel was used in each chamber. In other cases, two or three different fuels were 

tested. The choice of fuel for the chambers depended on the test temperature and the results of 

previous tests. 

All the experiments were conducted using a 1/20th (5%) fill level. As shown by Vaivads et al.

[3], the fuel tank headspace vapors of gasoline and alcohol/gasoline blends are flammable at 

higher temperatures if there is less liquid fuel in the tanks. The 5% fill level was selected, in 

consultation with the project sponsor, to represent a worst-case scenario in terms of how low an 

automobile operator might allow the fuel level to become.

The fuel storage containers and the syringes used to transfer the fuel were stored at -12oC. Before 

each test, fuel was extracted with a syringe from one of the fuel storage containers and 

transferred to the appropriate test chamber. The storage container was opened just long enough 

to extract the fuel (to limit vapor losses from the samples), and the test chamber was sealed 

immediately after the fuel was injected through the fuel valve. 

The sealed chambers were placed in the cold chamber, connected to the instrumentation cables, 

and chilled until the gas temperature inside the chamber reached the desired test temperature. 

This initially caused the gas pressure in the chambers to fall below atmospheric pressure. The 

chamber pressure levels were then equalized to atmospheric pressure by quickly opening and 

closing each fuel valve. Following equalization, the chambers were cold soaked until the gas and 

fuel temperatures were within 0.5°C of each other and then maintained at the test temperature 

(±0.5°C) for at least 1 hour before ignition was attempted.

During each ignition attempt, the data acquisition system was activated and the ignition system 

was triggered. The ignition system sparked at a frequency of 2 Hz once triggered. Flammable 

mixtures usually ignited with the first spark.

4 Test Fuels

Two different sets of fuels were tested. The first group of fuels were field samples obtained at 

two different refueling facilities. These facilities were equipped with storage tanks for gasoline 

and E85, and “blender pumps” that could create intermediate ethanol/gasoline blends for the 

“parent” gasoline and E85. The second set of fuels was produced in a laboratory by blending a 

well-characterized gasoline sample (intended to represent typical winter gasoline properties) with 

varying percentages of denatured ethanol. The properties of the test fuels are listed in 

Appendix A. 
5

5 Experimental Results and Discussion

5.1 Pressure Measurements

Rapid City, SD, and Lucerne, MN, blender pump field samples: Figures 2 and 3 show the 

test chamber pressure measurements for the fuel samples obtained from the Rapid City, SD,

blender pump location and an E85 sample from Lucerne, MN. The E20 SD and E30 SD samples 

were created by the blender pump by combining the gasoline (E10 SD) and E85 SD available at 

that location. All of these fuels had relatively low vapor pressure values considering that the 

samples were collected during a season when ASTM Class 3 fuels should have been provided. 

The DVPE values of the E85 SD and E85 MN samples were only 38.4 kPa and 42.4 kPa,

respectively—well below the ASTM D5798 Class 3 minimum value of 66 kPa. The E10, E20, 

and E30 samples had DVPE values of 60.8 kPa, 59.2 kPa, and 58.2 kPa, respectively.

Maximum Pressure Rise

0

100

200

300

400

500

600

700

800

900

-30 -25 -20 -15 -10 -5 0

Temperature (deg C)

Pressure Rise (kPa)

E85 SD (38.4 kPa)

E10 SD (60.8 kPa)

E20 SD (59.2 kPa)

E30 SD (58.2 kPa)

E85 MN (42.4 kPa)

Figure 2. Maximum Pressure Rise of Field Fuel Samples
6

Rate of Pressure Rise

0

2

4

6

8

10

12

14

16

-30 -25 -20 -15 -10 -5 0

Temperature (deg C)

Rate (kPa/ms)

E85 SD (38.4 kPa)

E10 SD (60.8 kPa)

E20 SD (59.2 kPa)

E30 SD (58.2 kPa)

E85 MN (42.4 kPa)

Figure 3. Maximum Rate of Pressure Rise of Field Fuel Samples

As shown in Figure 2, the pressure rise values were either below 100 kPa or above 300 kPa. 

Similar behavior was also seen in the previous study [1]. The tests where only a low pressure rise 

was produced are believed to indicate cases where the mixture could be ignited by the apparatus, 

but only a small portion of the mixture in the chamber was burned. Test results in the higher 

pressure range indicate that a substantial portion of the mixture in the chamber was burned, so 

the mixture was, by definition, flammable. 

Since each fuel tested produced some low pressure results, it is proposed that the spark ignition 

source was strong enough to ensure that the apparatus was capable of identifying true 

flammability limits rather than ignition limits imposed by the ignition source. In other words, the 

spark energy of this apparatus was great enough to ensure that it could ignite any mixture that 

was capable of sustained burning once it was ignited. For all of the fuels tested, only upper 

flammability limits were encountered. That is, the vapor/air mixture in the chamber headspace 

was flammable below a critical temperature and was too rich to burn above that temperature.

The results for the maximum rate of pressure rise shown in Figure 3 provide a relative indication 

of how fast the mixture burned following ignition. This is related to the flame speed of the 

mixture, and affects both the potential destructiveness of combustion within a fuel tank and the 

potential for igniting the mixture with relatively weak sparks. A well-known example of this 

relationship is hydrogen, which is easily ignited and potentially destructive because it has a high 

flame speed.

As shown in Figure 2, the maximum pressure rise results for the E10-E30 samples were almost 

identical. The maximum rate of pressure rise at any given temperature (Figure 3) also varied 
7

little between the E10-E30 samples. Thus, blending E85 with the base E10 gasoline up to E30 

levels had little or no effect on the combustion properties of the fuel.

The E85 field samples were ignitable and flammable at substantially higher temperatures than 

the low-medium level ethanol blends. The E85 SD sample was particularly noteworthy, as it

produced much higher maximum pressure rise rates over the temperature range typical of Class 3 

climatic regions. For example, all of the fuel samples were flammable at -20o C, but the 

maximum rate of pressure rise was nearly 4 times higher with the E85 SD fuel.

An even greater difference would be expected if the E85 fieled samples were compared with 

more volatile winter gasoline, such as the “typical” winter gasoline used to produce the 

laboratory blends for this project (89.2 kPa DVPE). As previously noted, the E10 gasoline 

sample (at 60.8 kPa DVPE) had an unusually low vapor pressure for a winter fuel

Laboratory ethanol/gasoline blends: The pressure rise and rate of pressure rise results for the 

blends of denatured ethanol and winter grade gasoline are shown in Figures 4 and 5. Like the 

field samples discussed earlier, these fuels produced pressure rise results that were bimodal in 

nature (< 100 kPa or > 300 kPa), indicating that the ability to achieve results in the high pressure 

range was not limited by the ignition source. The ranking of the fuel blends was as expected, but 

it can be seen that there were only small differences between E60 and E55 in terms of the 

pressure rise and rate of pressure rise results.

Maximum Pressure Rise

0

100

200

300

400

500

600

700

800

900

-30 -25 -20 -15 -10 -5 0

Temperature (deg C)

Pressure Rise (kPa)

E83 (42.3 kPa)

E75 (53.8 kPa)

E68 (62.1 kPa)

E60 (69.7 kPa)

E55 (72.3 kPa)

Figure 4. Maximum Pressure Rise of Laboratory Fuel Samples
8

Rate of Pressure Rise

0

2

4

6

8

10

12

14

16

-30 -25 -20 -15 -10 -5 0

Temperature (deg C)

Rate (kPa/ms)

E83 (42.3 kPa)

E75 (53.8 kPa)

E68 (62.1 kPa)

E60 (69.7 kPa)

E55 (72.3 kPa)

Figure 5. Maximum Rate of Pressure Rise of Laboratory Fuel Samples

5.2 Upper Flammability Limits 

Based upon the pressure rise data shown in Figures 2 and 4, a statistical analysis was carried out 

to arrive at values for the upper flammability limits of each fuel and their associated 

uncertainties. Statistical analysis of the peak pressure rise data was implemented using a 

PROBIT technique [7]. The use of this digital technique was justified by the binomial nature of 

the data. A threshold pressure of 200 kPa was used to differentiate between flammable mixtures 

that were ignitable but not flammable. 

In the analysis, pressure rise values in the high-pressure range (representing flammable mixtures) 

were assigned a high logic value (1), while pressure rise values in the low-pressure range 

(ignitable but not flammable) and zero pressure rise values (ignition failure) were assigned a low 

logic value (0). The technique generated a most likely curve fit for the temperature region where 

both high-pressure and low-pressure results were recorded in the experiments. From this curve 

fit, the mean temperature value and the upper bound temperature value were determined. The 

criterion chosen for the upper bound temperature was a 50% probability (at 95% confidence) of a 

high-pressure event occurring. 

In the following flammability limit graphs, the mean values have been plotted. The error bars are 

based upon the accuracy of the temperature measurements (conservatively estimated to be within 

±1°C), with difference between the upper bound and the mean value added to the plus side of the 

error bar. Thus, the upper error limits represent the highest temperature at which there would still 

be at least a 50% probability that the mixture was flammable.

In Figure 6, the upper flammability limits of the field samples and the laboratory fuels are plotted 

versus their respective ethanol content (volume % as determined by analysis). Note that since 
9

these are upper (rich) flammability limits, the fuel/air vapors are flammable at or below the 

temperatures show. The E85 SD field sample was flammable at temperatures up to -5oC and was 

therefore flammable over the entire ASTM Class 3 temperature range. The flammability limits of 

the E85 MN field sample and the E83 laboratory blend were somewhat lower, at about 

-10oC and -11oC, respectively.

Flammability Limit Versus Ethanol Vol %

-30

-25

-20

-15

-10

-5

0

0 20 40 60 80 100

Ethanol Content (%)

Temperature (deg C)

E55

E60

E68

E75

E83

E10 SD

E20 SD

E30 SD

E85 SD

E85 MN

Figure 6. Effect of Ethanol Content on Upper Flammability Limit

There was no significant difference in the flammability limits of the E10 SD-E30 SD field 

samples. These results suggest that the E20 and E30 fuels produced by existing blender pumps 

(from E10 and E85 “parent“ fuels) may not pose an increased flammability risk relative to their 

E10 component. As such, they offer a means of increasing the utilization of ethanol without the 

flammability risks inherent in the use of high-level blends such as E85.

The results for the laboratory fuel blends show a nonlinear relationship between the ethanol 

content of the fuel and its upper flammability limit. As such, reducing the ethanol content from 

typical E85 levels initially provides a substantial reduction in the temperature at which 

flammable mixtures are likely to form. Reductions below about 60% ethanol appear to have a 

more modest effect.

Figure 7 shows the measured DVPE values of the test fuels as a function of ethanol content. The 

vapor pressure of the ethanol-free base gasoline used to produce the laboratory blends is also 

shown. The minimum values for ethanol content and vapor pressure for Class 3 fuels (specified 

in ASTM D 5798-09) are depicted for comparison. Based upon the curve fit for the laboratory 

fuels, it appears that the ethanol content of fuels blended with this particular base gasoline would 

have to be reduced to about 65% (below the recommended minimum of 70%) to achieve the 

Class 3 minimum DVPE value of 66 kPa.
10

Vapor Pressure Versus Ethanol Content

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90

Volume % Ethanol

DVPE (kPa)

Laboratory Samples

SD Field Samples

MN Field Sample

Class 3 Minimum Values

 

Figure 7. Effect of Ethanol Content on Vapor Pressure (DVPE) of Test Fuels

It also appears that the E10-E30 field samples had lower DVPE values than similar blends with 

this base gasoline would be expected to have. This suggests that the gasoline portion of the E10-

E30 field samples was less volatile than the “typical” winter gasoline used to produce the 

laboratory blends. On the other hand, the E85 field samples and the E83 laboratory fuel had 

similar vapor pressure values. The E83 and the E85 MN samples had DVPE values of 42.3 kPa 

and 42.4 kPa, respectively, while the E85 SD sample was slightly lower, at 38.4 kPa.

 

Figure 8 shows the upper flammability limits of the fuels as a function of their respective vapor 

pressure values. The data from the laboratory fuels indicates that the relationship between the 

flammability limit and the vapor pressure of the fuels is nonlinear. The impact of increasing 

vapor pressure (achieved by decreasing ethanol content) was diminished when the percentage of 

ethanol was lower. The fact that the E10-E30 field samples had the same flammability limit 

temperature (within the resolution of the experiments), despite significant differences in their 

vapor pressure values, is consistent with this observation for the laboratory samples. Further 

experiments with laboratory samples in the E0-E55 range are needed to confirm this apparent 

trend.
11

Flammability Limit Versus Vapor Pressure

-30

-25

-20

-15

-10

-5

0

30 40 50 60 70 80

DVPE (kPa)

Temperature (deg C)

E55

E60

E68

E75

E83

E10 SD

E20 SD

E30 SD

E85 SD

E85 MN

Figure 8. Effect of Vapor Pressure (DVPE) on Upper Flammability Limit of Test 

Fuels

Note that at similar vapor pressure levels, the flammability limits of the laboratory blends were 

significantly lower than those of the E10-E30 blends. This is consistent with earlier observations 

that the vapor pressure (DVPE value) alone does not adequately predict the flammability of the 

fuel tank vapors formed at low temperatures [1]. Evidently, the properties of the hydrocarbon 

portion of the fuel must be considered as well.

6 Conclusions from Experimental Study

1. Both of the E85 field samples that were tested had vapor pressure values well below the 

minimum Class 3 specifications of ASTM D5798-09. One of these fuels produced 

flammable vapors over the entire Class 3 temperature range (-5oC and below). The other 

fuel produced flammable vapors at -10oC and below. These results indicate that at least 

some of the ethanol fuels that are currently available when and where Class 3 conditions 

exist are likely to produce flammable vapors within the ambient temperature range where 

they are used.

2. Two mid-level ethanol blends (E20 and E30) were tested. These field samples were 

produced by a blender pump that mixed E10 (the base gasoline available at that location) 

with E85. No significant differences could be measured between vapor flammability 

limits of the E20-E30 samples and the E10 sample they were produced from. These 

results indicate that mid-level blends in this range are unlikely to significantly increase 

the risk of producing flammable vapors over that of the base gasoline used for the blends.
12

3. Laboratory fuel blends of E55-E83 were produced by blending denatured ethanol with a 

single type of winter volatility gasoline. The flammability limits of these fuels showed a 

nonlinear relationship with both ethanol content and vapor pressure. Consequently, 

reducing the ethanol content of the fuels from typical E85 levels (80% or more) initially 

produced substantial reductions in the temperature at which it was possible for flammable 

vapors to form. Reductions in ethanol content below about 60% had a smaller relative 

impact on the flammability limits.

4. The laboratory fuel blends were produced from a “typical” winter volatility gasoline, with 

a DVPE value of 89 kPa. None of the blends produced from this gasoline could 

simultaneously meet the ASTM D5798 requirements for minimum vapor pressure (66 

kPa) and minimum ethanol content (70%). The vapor pressure data for the blends 

indicated that a reduction in ethanol content to about 65% would be needed to achieve the 

minimum vapor pressure specification. The flammability data trends suggest that blends 

with ethanol content in this region would provide a favorable trade-off between avoiding 

the formation of flammable vapors and maximizing the ethanol content of the fuel.

5. Comparisons of the flammability results of the field and laboratory samples indicate that 

vapor pressure (DVPE) alone does not adequately predict the flammability of the fuel 

tank vapors formed at low temperatures. The properties of the hydrocarbon portion of the 

fuel must be considered as well.

6. It is recommended that further testing be carried out with laboratory fuels to examine the 

effects of gasoline volatility on the vapor flammability of the ethanol/gasoline blends. 

Flammability results over a wider range of ethanol content levels (including base gasoline 

without ethanol) are needed to elucidate the interactions between ethanol content, 

gasoline volatility, blend vapor pressure, and vapor flammability.
13

II MATHEMATICAL MODELING STUDY

7 Introduction

It is desirable to have a manageable mathematical model to characterize the volatility and 

flammability of ethanol gasoline blends. Such a model would permit the assessment of fuels with 

regard to flammability without resorting to costly and time-consuming combustion tests for 

every candidate fuel. 

In Phase One of this work, it was shown that ethanol-gasoline blends could be represented using 

a modeling technique developed originally for methanol blends. The ethanol is treated using 

conventional methods for pure compounds. Its vapor pressure and vapor phase molecular weight 

are functions of temperature only and do not depend upon how much of the alcohol is 

evaporated. Gasoline, on other hand, is a multi-component mixture; its vapor pressure and vapor

phase molecular weight depend on both the temperature and the extent of evaporation. When 

most of the fuel is liquid, the vapor pressure is high, but as evaporation occurs the vapor pressure 

drops by an order of magnitude, and the molecular weight of the vapor phase increases by a 

factor of 2-3. 

The model being used for this work essentially treats a blend of alcohol and gasoline as a 

pseudo-binary mixture. The gasoline is modeled using a technique that allows both temperature 

and the extent of evaporation to be accounted for in determining vapor pressure and other 

proprieties. The ethanol is treated as a pure compound.

Details of the gasoline model, and the pseudo binary treatment of ethanol-gasoline blends were 

given in the report of Phase 1 [1] and are summarized in Appendix C.

Due to the limited nature of that initial study, and the data available on hydrocarbon components

used in the various blends tested, it was not possible to create models of the hydrocarbon 

components in each blend in accordance with the full modeling technique. Instead, a number of 

existing models for representative gasoline primers were used. Those existing models covered 

representative low, medium and high volatility gasolines, Indolene and light iso-crackate (LIC), 

whose modeling coefficients were known from earlier work.

Two important conclusions were drawn from the modelling results of Phase 1:

1. Volatility at high temperatures such as that used in determining DVPE does not 

necessarily reflect the behaviour of the different ethanol blends at low temperatures. 

A simple comparison of DVPE as an indicator of low temperature flammability is 

insufficient and can be misleading. 

2. Simply using generic gasoline models is not adequate to reliably assess low 

temperature flammability of a particular fuel blend. Details of the actual gasoline 

properties used to make the blend are required.
14

8 Scope of the Present Modeling Study

This present study concentrated on creating software to derive the modeling coefficients from the 

gasoline distillation data and specific gravity. The properties of different gasolines could then be 

easily incorporated into ethanol-gasoline blend models.

This part of the current work created this software in the form of Excel spreadsheets using Visual 

Basic subroutines. The software was then validated using published data and some data derived 

specifically for this project. After validation it was used to model the fuel tank flammability of 

the various blends of interest being tested in the experimental part of the current project.

Reference 8 provided measured vapor pressure for an air-free gasoline with known properties. 

The removal of dissolved air from the gasoline in that work avoided the problem of unknown air 

effects. This issue will be discussed below in connection with the other tests carried out as part of 

this study. 

The experimental data from that study consisted of known masses of the test gasoline contained 

in a container of known volume, held until there was phase equilibrium between the final liquid 

volume and its vapor at the test temperature. To assess the implementation of the gasoline model 

created in this project, the properties of the test gasoline in that 1986 study were inserted into the 

Excel spreadsheet. The spreadsheet provides the coefficients needed to represent the gasoline in 

the basic vapor pressure and molecular weight equations. 

One aspect of the model for this and all the other cases of interest in the present project is that the 

final distribution between liquid and vapor is unknown. The model must be incorporated into a 

larger computer program that will iteratively find the quantity of liquid and vapor pressure in the 

known volume that meets the phase equilibrium requirements. In the present study, a commercial 

equation solving software package (TK Solver) was used for these determinations. Figure 9 

shows the results for a test temperature of 0°C.

Figure 9. Measured and Predicted Vapor Pressures – Indolene @ 0°C
15

In this figure, overall density is simply the mass of fuel injected divided by the internal volume 

of the test chamber. The equilibrium mixtures contain more liquid and less vapor mass towards 

the right, i.e., at higher densities. Points to the left represent mixtures in which there is mostly 

vapor at equilibrium, whereas points towards the right correspond to cases in which most of the 

fuel mass is in the form of liquid. 

As can be seen, the model generally simulates the fuel behavior well. However, points to the 

right, where most fuel is in the liquid phase, seem a bit underpredicted, although most are within 

the uncertainty of the measurements. The model seems to predict slightly lower vapor pressures 

than those measured. This is important for the present study since the cases of particular interest 

are those in which fuel tanks are 5% full or more. Even when only 5% filled with liquid, the 

overwhelming mass of fuel is in the liquid phase. The mass fraction of fuel in the form of vapor

is well below 1% in all cases studied here. 

It is not surprising that the model underpredicts vapor pressure at very low values of mass 

fraction evaporated. This corresponds to the left-most portion of the distillation curve, the region 

of most volatile components—the so-called “light ends.” These components are part of the loss 

in the distillation test and greatly influence the vapor pressures when they are still in the liquid 

phase, before being depleted as evaporation proceeds. This will be discussed later. It is 

noteworthy that despite the fact that less than 1% of the fuel is vaporized, the small differences in 

mass fraction evaporated result in substantial changes in vapor pressure due to the impact of 

small quantities of light ends.

Based on the generally good results such as those shown in the above figure, it was concluded 

that the implementation of the model was successful. A caveat is that the most volatile 

components are expected to be somewhat underpredicted. 

9 Validation Using the Specific Tests and Test Fuels

In this project, the gasoline model is critical since it provides highly volatile components that 

determine the vapor pressure and flammability. To assess the gasoline model implementation 

further, particularly as it affects ethanol blends, tests were carried out in a Grabner apparatus at 

the Alberta Research Council (ARC) using the conditions corresponding to DVPE and Reid 

vapor pressure (RVP) tests, but measuring vapor pressures over a range of temperatures rather 

than merely at 37.8°C (100°F). A winter grade, oxygenate-free gasoline was selected, and a 

blend was then made up using this base gasoline and 10% by volume of a commercial denatured 

ethanol. The properties of the two fuels are included in Appendix D. 

The apparatus is evacuated prior to the tests. As is the case for the standard DVPE and RVP 

tests, a known volume of liquid fuel is inserted into a measuring chamber having a volume five 

times larger than the initial liquid volume. This provides the nominal vapor liquid ratio (V/L) of 

4:1. However, once some liquid fuel evaporates to produce phase equilibrium at the test 

temperature, the actual V/L ratio is less than the nominal 4:1 and must be determined iteratively 

by the software. Once equilibrium is attained at any given temperature, the known volume 

contains not only an unknown quantity of liquid fuel and its vapor in equilibrium at the test 
16

temperature, but also a quantity of air that also contributes to the measured total pressure. This 

air was initially present in the chilled liquid fuel but leaves the liquid phase such that it is in 

equilibrium with the dissolved air remaining in the liquid phase. Therefore, the measurement of 

total pressure in the chamber gives the sum of the vapor pressure of the fuel plus the pressure of 

that part of the air originally dissolved in the liquid fuel that came out of solution to achieve the 

final equilibrium conditions. 

As part of the procedure for the determination of an equivalent of the RVP/DVPE [9], a 

correlation is provided to correct the measured total pressure at the DVPE/RVP temperature of 

37.8°C (100°F) to the DVPE or RVP measured in the conventional apparatus. This is essentially 

the correction for dissolved air in the Grabner apparatus, but that applies only to the DVPE test 

temperature. Due to the conditions in the RVP test, the effect of dissolved air on measured 

pressure is negligibly small. 

To compare the model with the measured values, a dissolved air correction was made; this used 

the empirical data for air solubility in hydrocarbons given in Rose and Cooper [10]. For this 

calculation it was assumed that the original sample (stored at about 0°C) was saturated with air. 

Various amounts of this air present in the original liquid introduced to the fixed volume of the 

apparatus then come out of solution at different temperatures, as described above. The program 

used to determine the final distribution of fuel between liquid and vapor phases also incorporated 

a calculation of the amount of air in the two phases at the known final total volume and 

temperature. 

Figure 10 shows the results. As discussed above, the model tends to underpredict the vapor

pressure. It is derived from the D86 distillation curve, which is heavily influenced by the lightest, 

most volatile components. These constituents are only coarsely captured in the standard 

distillation data. Note that the dissolved air correction corresponds closely to the DVPE/RVP 

correction known to apply at 37.8°C. This gives confidence that the correction to the measured 

pressure to account for dissolved air is valid. 

To allow the model to better represent the volatile components, it was adjusted to give the 

measured DVPE/RVP resulting from the specified ASTM correction. This resulted in the curve 

shown. The DVPE-matched model agrees very well with the measured vapor pressure of this 

gasoline as corrected for dissolved air. 
17

Figure 10. Measured and Predicted Vapor Pressure of ARC Gasoline (V/L=4)

Having matched the model to the ARC gasoline, the model was then set up for the E10 mixture. 

The denaturant was composed of hydrocarbons that make up approximately 5% of the ethanol. 

The denaturant was simply lumped with the gasoline and assumed to have similar properties for 

this purpose. The vapor pressure of this blend (including the adjustment to match the gasoline 

DVPE) was then compared to the measured results in the Grabner temperature sweep for the E10 

blend. Figure 11 shows the results.

Figure 11. Measured and Predicted Vapor Pressure of ARC E10 Blend (V/L=4)
18

As can be seen, the results are reasonably good, but the model is underpredicting the vapor

pressure of the E10. This can only be the result of the non-ideality model being used. In the 

absence of experimental data for other proportions of ethanol and this gasoline, this part of the 

blend model cannot realistically be assessed and adjusted using the two ARC fuels. However the 

laboratory blend data discussed below covers a range of blends from E55-E83, allowing the nonideality model to be assessed and adjusted. 

10 Modeling the Laboratory Blends

As part of the project, the properties of the base gasoline and those of five laboratory blends were 

supplied. Appendix B gives the data received. 

Inserting the gasoline D86 and specific gravity data into the model then allowed the a priori 

prediction of DVPE as a means to assess the accuracy of the model without adjustments. As 

expected, the model again underpredicted the DVPE. Adjusting the model so that the DVPE was 

matched to the Marathon base gasoline then gave results that overpredicted the DVPE of the 

high ethanol blends supplied in this project. The form of the Margules method used for 

determining the activity coefficients was as follows:

T

AX

gasoline

ethanol

e

2

γ =

T

AX

ethanol

gasoline

e

2

γ =

where

γ gasoline = activity coefficient of the gasoline

γ ethanol = activity coefficient of the ethanol

A = the Margules “constant”

X gasoline = mole fraction of gasoline in the blend 

Xethanol = mole fraction of ethanol in the blend

and the Margules constant is itself given by

















− = + ×

ethanol

ethanol

X

X A C C

1 1 2 .

In the original published model [3], the measured vapor pressures of various blends of methanol 

and ethanol were used to derive values of C1 and C2 of 600 and 5.25, respectively.

The DVPE reflects the non-ideality of the blends, so it can used to assess the non-ideality 

coefficients. In the case of the laboratory blends, the measured DVPE values were used to 

determine the values of these coefficients to be used in predicting the flammability of these 

blends. It was found that a good fit to the DVPE values could be obtained using:
19

2.27

600

2

1

= −

=

C

C

With the gasoline model matched to the DVPE of the base gasoline, and the Margules nonideality equations modified as described above, the DVPE predictions were as shown in Figure 

12.

Figure 12. Measured and Predicted DVPE of Laboratory Samples

This matching cannot be considered definitive, even for this particular gasoline, because there 

are no measured data for low alcohol blends. However, it can be seen in this figure that the vapor

pressure is accurately modeled. The model could now be used to predict the flammability of fuel 

tank headspace vapors with reasonable confidence.

11 Comparison of Predicted and Measured Flammability of Fuel Tank

Headspace Vapors

Using the conventional published values of flammability limits for the alcohol and gasoline 

components as described in Appendix C, the model was used to predict the rich flammability 

limits as functions of ambient temperature for the various laboratory blends in the headspace 

vapors of a fuel tank filled to 5% of its capacity. These predicted temperature limits were then 

compared to the measured limits found in the testing part of this project. The results are shown in

Figure 13.
20

Figure 13. Measured and Predicted Flammability of Tank Headspace Vapors

Since the model used the properties of the laboratory blend base gasoline, only the measured 

results for the laboratory blends can be expected to be reflected properly in the predictions. 

Overall, the limit temperatures predicted by the model are somewhat higher than the measured 

values, but the trend of the data is very well represented. The lower measured temperatures 

compared to the predictions are ascribed primarily to the differences in the experimental 

techniques used to derive conventional published flammability compared to the test conditions. 

Published flammability data is normally measured in large-diameter flame tubes, to avoid wall 

effects, under ideal conditions that promote combustion, such as vertically upward flame 

propagation. The reason for such conditions is to provide worst-case scenarios for the safety 

concerns that have been the primary motivator for the tests. In the tests here, or in the real case 

of a vehicle fuel tank, the apparatus and conditions are quite different. Wall effects, ignition 

source location and ignition energy all play a role in making the mixtures harder to ignite in the 

tests than expected based on the standard flammability data. It is to be expected that a given 

mixture will not be ignited at the predicted rich limit but rather at some lower temperature. This 

is indeed what is seen in the figure. 

The trend shown for the laboratory fuels with the range of the measurements is a very good 

match for that of the experimental data. However, there is slight hump on the left of the predicted 

curve. This is due to the non-ideality effects. It may well be a real phenomenon due to the 

characteristics of these blends of polar and nonpolar molecules. On the other hand, the shape 

predicted for the low alcohol blends may be merely an artefact of the particular model used to 

account for non-ideality. To ascertain which is the case, actual combustion tests would need to 
21

be carried out for some lower ethanol blends in that region. However, the difference in the 

predicted rich limit temperatures with or without the hump is fairly small in any case, so this 

anomaly has little practical impact, whether it is real or only mathematically imagined.

It is clear that the model can indeed predict the correct trend and ranking of fuels when the 

gasoline component is correctly represented. The data from the blender pump field sample tests 

also follow the general trend of the model, but too much significance should not be placed on 

this because of the unknown differences in the base gasolines used in those blends. 

12 Conclusions from the Modeling Study

1. When some basic properties of the base oxygenate-free gasoline are known, then the 

mathematical model developed in this study can be used to compare the flammability 

characteristics of various gasoline-ethanol blends quite satisfactorily. The model requires 

ASTM D86 distillation data, D4052 specific gravity, and D5191 DVPE of the base 

oxygenate-free gasoline and the percentages of ethanol and gasoline in the blend.

2. The use of conventional flammability data leads to the prediction of higher rich limit 

temperatures than those actually measured in the experimental apparatus used in the 

study. Although an adjustment might be made to better match the predictions to the 

measurements, this is unlikely to be worthwhile, since it would again be apparatus 

dependent and not necessarily applicable to other conditions. Rather, the model is useful 

in comparing blends to each other to assess their relative hazards.

3. Since the project had sufficient gasoline data only for the laboratory blends tested for 

flammability, further work is needed to compare ethanol blends containing different 

gasoline base stock. This would allow an assessment to be made of the reliability of the 

model for comparing the relative hazards of different fuel sources.

 

4. Since most gasolines now contain some oxygenates, the distillation (D86) data for the 

base gasoline alone is often not available. Given the success of the model, it would be 

desirable to attempt to devise a technique for extracting the necessary gasoline data from 

the D86 data of a low alcohol blend. That is not trivial, but comparisons such as those

made of the special E0 and E10 fuels studied in this project may allow such a technique 

to be developed, thereby making the model much more accessible for use by others.
22

III OVERALL CONCLUSIONS AND RECOMMENDATIONS

13 Conclusions from the Experimental Study

1. Both of the E85 field samples that were tested had vapor pressure values well below the 

minimum Class 3 specifications of ASTM D5798-09. One of these fuels produced 

flammable vapors over the entire Class 3 temperature range (-5oC and below). The other 

fuel produced flammable vapors at -10oC and below. These results indicate that at least 

some of the ethanol fuels that are currently available when and where Class 3 conditions 

exist are likely to produce flammable vapors within the ambient temperature range where 

they are used.

2. Two mid-level ethanol blends (E20 and E30) were tested. These field samples were 

produced by a blender pump that mixed E10 (the base gasoline available at that location) 

with E85. No significant differences could be measured between vapor flammability 

limits of the E20-E30 samples and the E10 sample they were produced from. These 

results indicate that mid-level blends in this range are unlikely to significantly increase 

the risk of producing flammable vapors over that of the base gasoline used for the blends.

3. Laboratory fuel blends of E55-E83 were produced by blending denatured ethanol with a 

single type of winter volatility gasoline. The flammability limits of these fuels showed a 

nonlinear relationship with both ethanol content and vapor pressure. Consequently, 

reducing the ethanol content of the fuels from typical E85 levels (80% or more) initially 

produced substantial reductions in the temperature at which it was possible for flammable 

vapors to form. Reductions in ethanol content below about 60% had a smaller relative 

impact on the flammability limits.

4. The laboratory fuel blends were produced from a “typical” winter volatility gasoline, with 

a DVPE value of 89 kPa. None of the blends produced from this gasoline could 

simultaneously meet the ASTM D5798 requirements for minimum vapor pressure (66 

kPa) and minimum ethanol content (70%). The vapor pressure data for the blends 

indicated that a reduction in ethanol content to about 65% would be needed to achieve the 

minimum vapor pressure specification. The flammability data trends suggest that blends 

with ethanol content in this region would provide a favorable trade-off between avoiding 

the formation of flammable vapors and maximizing the ethanol content of the fuel.

5. Comparisons of the flammability results of the field and laboratory samples indicate that 

vapor pressure (DVPE) alone does not adequately predict the flammability of the fuel 

tank vapors formed at low temperatures. The properties of the hydrocarbon portion of the 

fuel must be considered as well.

6. It is recommended that further testing be carried out with laboratory fuels to examine the 

effects of gasoline volatility on the vapor flammability of the ethanol/gasoline blends. 

Flammability results over a wider range of ethanol content levels (including base gasoline 

without ethanol) are needed to elucidate the interactions between ethanol content, 

gasoline volatility, blend vapor pressure, and vapor flammability. 
23

14 Conclusions from the Modeling Study

1. When some basic properties of the base oxygenate-free gasoline are known, then the 

mathematical model developed in this study can be used to compare the flammability 

characteristics of various gasoline-ethanol blends quite satisfactorily. The model requires 

ASTM D86 distillation data, D4052 specific gravity, and D5191 DVPE of the base 

oxygenate-free gasoline and the percentages of ethanol and gasoline in the blend.

2. The use of conventional flammability data leads to the prediction of higher rich limit 

temperatures than those actually measured in the experimental apparatus used in the 

study. Although an adjustment might be made to better match the predictions to the 

measurements, this is unlikely to be worthwhile, since it would again be apparatus 

dependent and not necessarily applicable to other conditions. Rather, the model is useful 

in comparing blends to each other to assess their relative hazards.

3. Since the project had sufficient gasoline data only for the laboratory blends tested for 

flammability, further work is needed to compare ethanol blends containing different 

gasoline base stock. This would allow an assessment to be made of the reliability of the 

model for comparing the relative hazards of different fuel sources.

 

4. Since most gasolines now contain some oxygenates, the distillation (D86) data for the 

base gasoline alone is often not available. Given the success of the model, it would be 

desirable to attempt to devise a technique for extracting the necessary gasoline data from 

the D86 data of a low alcohol blend. That is not trivial, but comparisons such as those

made of the special E0 and E10 fuels studied in this project may allow such a technique 

to be developed, thereby making the model much more accessible for use by others.
24

15 References

1. Gardiner, D.P., Bardon, M.F., and Pucher, G., An Experimental and Modeling Study of the

Flammability of Fuel Tank Headspace Vapors from High Ethanol Content Fuels,

Subcontract Report No. NREL/SR-540-44040, produced for the DOE National Renewable

Energy Laboratory under Subcontract No. XCI-5-5505-01, September 2008.

2. Persson, H., Bremer, P., Rosell, L., Arrhenius, K., and Lindstrom, K., Fuel Vapor 

Composition and Flammability Properties of E85, SP Report 2007:39, Brandteknik, Boras, 

2007.

3. Vaivads, R.H., Bardon, M.F., Rao, V.K., and Battista, V., “Flammability Tests of 

Alcohol/Gasoline Vapors,” SAE Paper #950401, Society of Automotive Engineers, 

Warrendale, PA, 1995.

4. Vaivads, R., Rao,V.K., Bardon, M.F., and Battista, V., “Volatility and Flammability of 

Variable Fuel Vehicle Tank Contents,” SAE paper # 932776, presented at the Fuels & 

Lubricants Meeting and Exposition, Philadelphia, Oct, 18-21, 1993.

5. Vaivads, R., Bardon, M.F., Rao,V.K., and Battista, V., “Flammability of Alcohol-Gasoline 

Blends in Fuel Tanks,” presented at the 4th International Symposium on Fire Safety Science, 

Ottawa, June 13-17, 1994.

6. “Standard Specification for Fuel Ethanol (Ed75-Ed85)for Automotive Spark-Ignition 

Engines,” ASTM D5798-09, ASTM International, West Conshohocken, PA, February 2009.

7. Lattin, J., Carroll, J.D., and Green, P.E., Analyzing Multivariate Data, Thomson, Brooks and 

Cole, 2003.

8. Bardon, M.F., Rao,V.K., Vaivads, R., and Evans, M.J.B., “Measured and Predicted Effect of 

the Extent of Evaporation on Gasoline Vapour Pressure.” J. Inst. Energy, v59, n441, Dec.

1986.

9. “Standard Test Method for Determination of Vapor Pressure of Crude Oil: VPCRx 

(Expansion Method),” Test D6377-08, ASTM International, West Conshohocken, PA, 2008.

10. Technical Data on Fuel, J.W. Rose and J.R. Cooper, Eds.,7th Edition, British National 

Committee, World Energy Conference, London.

11. Bardon, M.F., and Rao,V.K., “Calculation of Gasoline Volatility”. J.Inst. Energy, v57, 343-

348, Sept 1984

12. Rao, V.K., and Bardon, M.F., “Estimating the Molecular Weight of Petroleum Fractions,” 

Ind. Eng. Chem., Process Des. Dev., v27, n2, April 1985.

13. Wilhoit, R.C., and Zwolinski, B.J., “Physical and Thermodynamic Properties of Aliphatic 

Alcohols, , J. Phys. Chem. Ref. Data, v2 Supp. 1, 1973, pp. 1.40-1.54.
25

14. Bardon, M.F., Nicks, G.W., Rao,V.K., and Vaivads, R., “A Vapour Pressure Model for 

Methanol/Gasoline M85 Blends,” Paper # 870366, Trans SAE, J Fuels and Lubricants, v96, 

Section 5, pp. 38-45, 1987.

15. Zabetakis, M.G., “Flammability Characteristics of Combustible Gases and Vapors,” U.S.

Bureau of Mines, Bulletin 621, Washington, 1965.
26

APPENDIX A

Properties of Field Samples
27

APPENDIX B

Properties of Laboratory Samples

Product E83 E75 E68 E60 E55 EtOH 93RBOB

(13 psi)

Date 10/14/2009 10/14/2009 10/14/2009 10/14/2009 10/14/2009 10/12/2009 10/12/2009

Sample ID 3057180 3057179 3057178 3057177 3057168 3055540 3055538

D4052 Specific Gravity at 60°F 0.7821 0.7748 0.7680 0.7610 0.7584 0.7933 0.7128

D5191 ASTM DVPE, psi 6.14 7.81 9.02 10.12 10.49 -- 12.94

D5501 EtOH, Vol % 83.81 75.49 68.38 59.57 56.49 96.98 N.D.

D5501 MeOH, Vol % 0.04 0.03 0.03 0.02 0.02 0.04 N.D.

E1064 H2O, Wt % 0.68 0.61 0.58 0.51 0.49 0.74 --

D86 IBP, °F 118.5 107.4 96.8 93.2 90.7 -- 78.4

D86 T 10 Evaporated, °F 165.6 161.3 154.1 146.6 144.2 -- 110.5

D86 T 20 Evaporated, °F 169.0 167.5 164.1 161.5 159.9 -- 140

D86 T 30 Evaporated, °F 170.4 169.6 167.1 165.4 164.6 -- 179.2

D86 T 50 Evaporated, °F 171.6 171.9 170.3 169.0 168.4 -- 223.9

D86 T 70 Evaporated, °F 172.2 173.1 172.2 171.8 171.3 -- 245.9

D86 T 90 Evaporated, °F 173.1 174.7 174.6 175.2 175.2 -- 313.9

D86 End Pt, °F 252.1 343.8 353.5 358.5 361.1 -- 386.2

D86 Recovery, Vol % 98.4 97.8 97.8 97.3 97.5 -- 97.1

D86 Residue, Vol % 0.8 0.9 1.0 1.0 1.1 -- 0.9

D86 Loss, % 0.7 1.2 1.1 1.5 1.2 -- 2.0

D86 E 200, % 98.9 98.0 96.7 95.3 94.4 -- 36.1

D86 E 300, % Not appl. 98.1 96.7 95.4 94.5 -- 87.7
28

Appendix C

Summary of the Models for the Volatility and Flammability of EthanolGasoline Blends

Volatility Characteristics of Gasolines

For a pure compound such as a single-component hydrocarbon, the composition and hence 

molecular weight of the vapor and liquid phases are constant and identical regardless of how 

much has evaporated, and the vapor pressure and enthalpy of evaporation are functions only of 

temperature. 

The vapour pressure of pure compounds can be adequately described for most practical purposes 

by a simple equation such as the classical Clausius-Clapeyron equation, as follows:

exp( / ) Psat = C1 −C2 T (1)

where 

Psat = equilibrium saturation pressure

T = absolute temperature 

C1 and C2 = constants for any given pure substance

On the other hand, commercial gasolines and other refinery products are mixtures of hundreds of 

individual pure hydrocarbon compounds. As a result, the compositions of the liquid and vapour 

phases vary continuously as the fuel evaporates. Light volatile fractions evaporate first, followed 

progressively by the heavier molecular weight compounds in the fuel. 

Modeling real fuels can in principle be done by expressing the vapour pressure of each of the 

hundreds of components using Equation 1 with the constants applicable for each component, and 

then combining all components using Raoult’s Law for ideal mixtures, namely:

i sat

i N

i

Pfuel sat ∑XiP

=

=

= 1

(2)

where 

Pfuel sat = total vapour pressure of the hydrocarbon blend 

Xi = mole fraction of component i in the liquid phase of the blend at equilibrium

Pi sat = equilibrium saturation pressure of component i alone 

In practice, this requires the detailed composition of the fuel blend to be known, and the 

constants C1 and C2 for each component to be available. Equation 2 can then be used to find the 

vapour pressure of the blend at some given mass fraction evaporated (say, 20%). However, 

iteration is required because the mole fractions of each compound in the two phases are different 

from each other and are no longer the same as that of the initial mixture before evaporation 

occurred. A computer model can be developed on this basis, but the code for such a model tends 

to be large and relatively slow, and the exact composition of each blend to be evaluated must be 

known. Non-ideal mixtures, such as blends of gasoline and ethanol, further complicate the 

process. 
29

For many practical analyses, a simple and fast model is needed to allow blend volatility to be 

calculated. It has been shown [11, 12] that hydrocarbon blends such as gasoline can be modeled

quite satisfactorily for many purposes by describing their vapor pressure and other properties 

using the form of the Clausius-Clapeyron equation but using appropriate polynomial functions 

instead of the two constants C1 and C2 as follows: 

exp( / ) Psat = f1 − f 2 T (3)

where 

Psat = equilibrium saturation pressure

T = absolute temperature 

f1 and f2 = functions of the extent of evaporation defined by VF where 

VF = mass fraction of the mixture in the vapour phase 

Equation 3 essentially separates the effects of temperature, expressed directly in the exponential 

term of this Clausius-Clapeyron format, from the extent of evaporation, contained exclusively 

within the functions f1 and f2. These latter two functions can be expressed as polynomials in the 

vapour fraction, VF. References [11] and [12] showed how the functions can be derived using 

only the ASTM D86 distillation data for the mixture and its specific gravity. 

Despite the drastic simplification of using Equation 3 to represent the complex evaporative 

behaviour of a hydrocarbon mixture, experimental measurements [9] showed that this method 

gave quite satisfactory predictions of hydrocarbon vapour pressure over a range of temperatures 

from 0oC to 40oC. Subsequent measurements to temperatures as low as -40oC showed that the 

method worked well over the entire range (-40oC to +40oC) of interest for ambient conditions in

North America. 

Volatility Characteristics of Ethanol

The ethanol vapour pressure is modeled using the Antoine Equation:

log /( ) 10 Psat ethanol = A − B C + T

The values of the coefficients used for ethanol were taken from Wilhoit and Zwolinski [13].

Volatility Characteristics of Gasoline/Alcohol Blends

Based on the success of this simplified approach to modeling hydrocarbon blends, the method 

was then extended to encompass blends of methanol or ethanol with gasoline. The approach [14] 

was to treat any blend of an alcohol with a hydrocarbon as if it were a pseudo-binary mixture, 

that is, a mixture of a single hydrocarbon component represented by the model described above 

(Equation 3), and the alcohol, represented by the Clausius-Clapeyron or Antoine equations as 

usual for pure compounds. 

Such blends are somewhat more complicated to model than those involving only hydrocarbons, 

because mixtures of strongly polar compounds such as alcohols do not form ideal mixtures with 

nonpolar compounds such as hydrocarbons. Thus Raoult’s Law (Equation 2) does not apply as 
30

written and a modification to account for the non-ideality of the resulting alcohol/hydrocarbon 

mixture was used. For a blend such as 85% ethanol and gasoline (E85), this has the form:

PE g X gPgsat e XePesat = γ + γ 85

where 

PE85 = equilibrium saturation pressure of the E85 mixture

g γ = activity coefficient for gasoline in the blend 

e γ = activity coefficient for methanol in the blend

Xg = mole fraction of gasoline in the liquid phase 

Xe = mole fraction of ethanol in the liquid phase

Pgsat = saturated vapour pressure of the gasoline 

Pesat = saturated vapour pressure of the methanol 

Note that Xg and Xe as well as Pgsat vary continuously as the fuel evaporates. Furthermore, at any 

point in the evaporation process, when the overall vapour fraction VFE85 is some fixed value 

(between 0 and 1), the vapour fraction of the gasoline component VFg is different from that of 

the ethanol VFe and both are different from VFE85 . Representative values for the two activity 

coefficients were determined for use with gasoline alcohol blends as part of that and subsequent 

work. Since the mole fractions of each of the two components in the liquid phase are not known 

a priori for some overall value of vapour fraction for the blend, iteration is required to determine 

the vapour pressure and other volatility characteristics needed, such as the molecular weight of 

the vapour phase. This makes the model more complex to use but still much simpler than the 

more rigorous methods. Experimental measurements over the temperature range -40oC to +40oC 

again showed that the model works quite satisfactorily for predicting the volatility properties of 

such pseudo-binary alcohol/gasoline blends [14].

Modeling the Flammability Characteristics of Gasoline/Alcohol Blends

The volatility model allows the composition of the vapour phase to be determined at any given 

temperature and fuel fill level in a fuel tank. Using published flammability for the gasoline and 

ethanol [15], combined using the LeChatelier mixing rule, then allows the resulting flammability 

of the vapour phase to be determined. Further details and other references are given in the report 

submitted for the first phase of this project [1]. 
31

APPENDIX D

Alberta Research Council − Fuels & Lubricants

Report of Analysis
32
F1146-E(10/2008)

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

April 2010

2. REPORT TYPE

Subcontract Report

3. DATES COVERED (From - To)

August 2005-December 2009

4. TITLE AND SUBTITLE

Experimental and Modeling Study of the Flammability of Fuel Tank 

Headspace Vapors from Ethanol/Gasoline Fuels, Phase 2: 

Evaluations of Field Samples and Laboratory Blends

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

D. Gardiner, M. Bardon, and M. LaViolette

5d. PROJECT NUMBER

NREL/SR-540-47819

5e. TASK NUMBER

FC080071

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Nexum Research Corporation

Kingston, Ontario, Canada

8. PERFORMING ORGANIZATION

REPORT NUMBER

Report No. 100101,

Subcontract No. XCI-5-55505-01

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

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NREL

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NREL/SR-540-47819

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13. SUPPLEMENTARY NOTES

NREL Technical Monitor: W. Clark 

14. ABSTRACT (Maximum 200 Words)

An experimental study measured the flammability of fuel vapors at low ambient temperatures and developed a 

mathematical model to predict the temperatures at which flammable vapors were likely to form. Results indicate that 

some currently available ethanol fuels are likely to produce flammable vapors within the ambient temperature range 

under Class 3 conditions. Results also indicate that mid-level ethanol blends (E20 and E30) are unlikely to 

significantly increase the risk of producing flammable vapors over that of the base gasoline used for the blends.

15. SUBJECT TERMS

ethanol; ethanol blends; fuel flammability studies; ethanol emissions; fuel tank vapors; gasoline vapors 

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