United States Air and Radiation EPA420-D-03-001 

Environmental Protection November 2003 

Agency 

Draft Analysis of

Heavy-Duty Diesel Vehicle

Idle Emission Rates

 Printed on Recycled Paper 
EPA420-D-03-001 

November 2003 

Draft Analysis of Heavy-Duty

Diesel Vehicle Idle Emission Rates

David J. Brzezinski

Edward L. Glover

Assessment and Standards Division

Office of Transportation and Air Quality

U.S. Environmental Protection Agency

NOTICE

This technical report does not necessarily represent final EPA decisions or positions.

It is intended to present technical analysis of issues using data that are currently available.

The purpose in the release of such reports is to facilitate the exchange of

technical information and to inform the public of technical developments which

may form the basis for a final EPA decision, position, or regulatory action.
1.0 INTRODUCTION 

This report describes EPA’s preliminary analysis of heavy-duty diesel vehicle emission 

data under engine idle operating conditions, where vehicle velocity is zero. It presents analysis 

and results for several pollutants and operating parameters. The pollutants include emissions of 

nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbons (HC) and particulate matter (PM) 

emissions. The operating parameters include the effects of both non-discretionary idle and 

discretionary idle, extended idle modes, temperature and parasitic load effects (A/C, heater, 

refrigeration, etc.). 

For purposes of this report, the term "non-discretionary" represents vehicle idle operation 

in which the vehicle typically encounters during normal road operation. It includes the idle 

operation which a vehicle experiences while waiting at a traffic signal or during a relatively short 

stop that lasts less than an hour. It is characterized by an engine speed which is at or near the 

vehicle minimum (typically 600 to 1000 RPM). For future technology vehicles it will 

characterize vehicle operation in which the vehicle’s catalytic converter or other emission 

control system is operating. Throughout this report, the term non-discretionary idle will also be 

known as "curb idle." 

The second term of "discretionary" idle operation is characterized by extending idle 

periods that are frequently several hours in duration, include higher engine speed settings which 

are made by the vehicle operator through the use of a mechanical switch or lever controlled by 

the operator. Discretionary idling most often occurs during long layovers between trips where 

the truck is used as a residence, often referred to as "hoteling." The use of accessories such as 

air conditioning systems or heating systems will also likely occur during discretionary idling and 

affect emissions. For future technology vehicles discretionary idling will include the effect of 

cool-down of the vehicle’s catalytic converter system or other exhaust emission treatments. 

Throughout this report, the term discretionary idle will also be known as "extended idle" or 

"hoteling." 

EPA has done only limited testing to investigate the emission effects of idle emissions 

from heavy-duty diesel vehicles. Instead, this report is primarily the product of a literature 

search of available test results from heavy-duty diesels. As such, it combines the data and 

observations from a variety of recent studies. 

This document is structured into three primary sections and an Appendix section. 

Section 1.0 is this introduction. Section 2.0 briefly describes the databases used in the analysis 

and development of the emission factors. Section 3.0 contains two primary sub-sections. The 

first sub-section describes the analysis of the non-discretionary idle data and presents the 

resulting curb idle emission factors. The second sub-section describes the analysis of the effects 

of accessory load, extended idling and other parameters on idling emissions and uses these 

results to generate extended, discretionary, idle emission rates. 

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2.0 DATA 

This study did not include vehicle emission testing. Instead it is a literature review and 

compilation of existing idle emission data from heavy-duty diesel vehicles. It includes idle 

emission results currently available from eight separate test programs conducted by a variety of 

researchers. 

2.1 High Altitude Study - "Idle Emissions from Heavy-Duty Diesel and Natural Gas 

Vehicles at High Altitude," Robert McCormick, et al, Colorado Institute for Fuels and 

Engine Research, Colorado School of Mines, Journal of the Air and Waste Management 

Association, Revised May 3, 2000. 

Testing was conducted on twelve heavy-duty diesel trucks and twelve transit buses in 

Colorado. Ten of the trucks were Class 8 heavy-duty axle semi-tractors, one was a Class 

7 truck, and one of the vehicles was a school bus. The model year range was from 1990 

through 1998. A typical Denver area wintertime diesel fuel (NFRAQS) was used in all 

tests. Idle measurements were collected during a 20 minute time period. All testing was 

done at 1,609 meters above sea level (high altitude). 

2.2 CCD Study - "Study of Exhaust Emissions from Idling Heavy-duty Diesel Trucks and 

Commercially Available Idle Reducing Devices," Han Lim, US EPA Office of 

Transportation and Air Quality, September 2002. 

Testing was conducted on five trucks in May 2002. The model years ranged from 1985 

through 2001. The vehicles were put through a battery of tests including a variety of 

discretionary and non-discretionary idling conditions. 

2.3 Clean Air Study - "Preliminary Results for Stationary and On-Road Testing of Diesel 

Trucks in Tulare, California," Douglas Lambert, et al, Clean Air Technologies Inc., May 

15, 2002. 

Testing was conducted on 42 diesel trucks in parallel with roadside smoke opacity 

testing. All tests were conducted by the California Air Resources Board (CARB) at a rest 

area near Tulare, California in April 2002. Data collected during this study were 

included in the data provided by IdleAire (below) used in the analysis. 

2.4 IdleAire Study - "NOx Emissions and Fuel Consumption of HDDVs during Extended 

Idle," David K. Irick, University of Tennessee, Bob Wilson, IdleAire Technologies Inc., 

Coordinated Research Council 12th Annual On-Road Vehicle Emission Workshop, San 

Diego, California, April 15-17, 2002. 

A total of 63 trucks (nine in Tennessee, 12 in New York and 42 in California) were tested 

over a battery of idle test conditions including with and without air conditioning. Not all 

trucks were tested under all conditions. Only results from the testing in Tennessee and 

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New York are described in the IdleAire report. The Tulare, California, data are described 

in the Clean Air Study cited above. All analytical equipment for all testing was operated 

by Clean Air Technologies. 

2.5 CRC E-55 Study - "Heavy-duty Vehicle Chassis Dynamometer Testing for Emissions 

Inventory, Air Quality Modeling, Source Apportionment and Air Toxics Emissions 

Inventory," Phase I Interim Report, CRC Project No. E-55/E-59, Mridul Gautam and 

Nigel N. Clark, et al, West Virginia University Research Corporation, July 2002. 

Fourteen trucks were tested with idling times either 900 or 1,800 seconds long. 

2.6 NCHRP Study - "Heavy-duty Vehicle Emissions," National Cooperative Highway 

Research Program Project 25-14, Cambridge Systematics, Inc., with Battelle, Sierra 

Research and West Virginia University. October 2002. 

The idling portion of continuous sampling during transient testing was used to determine 

idling emission rates on two trucks. 

2.7 Metropolitan New York Study - “Internal Report - Idle Emissions from Heavy-Duty 

Diesel Trucks in the New Your Metropolitan Area,” Tang and Munn, New York State 

Dept of Environmental Conservation, November 9, 2001. 

A total of 33 heavy-duty diesel trucks were tested in this study. The model years ranged 

from 1984 through 1999. One hundred seconds of idling were added at the end of the 

WVU five mile transient test driving cycle. 

2.8 UC Davis Study - "Potential Benefits of Utilizing Fuel Cell Auxiliary Power Units in 

Lieu of Heavy-Duty Truck Engine Idling," Broderick, Dwyer, et al. Institute of 

Transportation Studies, University of California - Davis. 

A Class 8 Freightliner Century with a 1999 engine was testing using EPA's on-road 

emissions testing trailer based in Research Triangle Park, North Carolina. Both short (10 

minute) and longer (five hour) measurements were made during idling. Some testing was 

also done on three older trucks. This data was obtained too late to be included in the 

analysis data used for this report. 

2.9 Oak Ridge Study - "Particulate Matter and Aldehyde Emissions from Idling HeavyDuty Diesel Trucks," John M.E. Storey, John F. Thomas, Samuel A. Lewis, Sr., Thang Q. 

Dam, K. Dean Edwards. Oak Ridge National Laboratory. Gerald L. DeVault, Y-12 

National Security Complex. Dominic J. Retrossa, Aberdeen Test Center. Five heavyduty trucks were tested for particulate and NOx emissions under a variety of conditions. 

These are the same trucks used in the CCD study. 

A full set of all of the individual vehicle results used in this analysis with a description of 

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the vehicles tested will be made available electronically along with this report. 

3.0 ANALYSIS 

For purposes of this report, heavy-duty diesel engine idle emissions are modeled by 

determining the curb idle emission factor and adjusting it to account for a range of observed 

idling operational behavior. The development of the curb (or non-discretionary idle mode) idle 

mode emission factors are discussed in Section 3.1. The adjustment factors to account for 

extended idling periods, engine idle speed, and use of accessories such as air conditioning (A/C) 

and heaters are discussed in Section 3.2. All of the adjustments are multiplicative factors that are 

applied to the curb idle emission factors. The general form of the equation is shown in Equation 

3-1. 

Idle EF = Curb_Idle EF * IdleTimeAdj * IdleSpeedAdj * AccessoryAdj Eqn 3-1 

Technically, curb idle and other idle conditions are separate operating modes with unique 

engine operating parameters. However, the limited available samples at all modes suggest that 

statistical characterization of the differences in these modes can provide a more consistent 

estimate of the idle emission rates in each mode rather than independent estimates based on 

small samples. 

3.1 Development of Non-Discretionary Idle Emission Factors 

Test data from the test programs were processed and combined into a comprehensive data 

set. Only those vehicles with results from idle operation similar to which the vehicle typically 

encounters during normal road operation were included in this part of the analysis. Some trucks 

and buses received repeat tests. In these cases, the individual test results were combined into a 

vehicle average emission result. The resulting data set contains 109 trucks and 13 buses. 

The goal of the analysis was to produce average non-discretionary idling emission factors 

in terms of grams per hour for each of four pollutants (NOx, PM, HC, and CO). Once 

determined, these idle emission factors will be used as the basis for the calculation of 

discretionary idle emissions. 

The data set also contained information on vehicle class type, altitude, engine build year 

and odometer readings. From an engineering perspective, these parameters may affect 

emissions. The thinner air of a high altitude region may lead to higher PM and CO emissions and 

possibly lower NOx emissions as a result of less oxygen in the combustion process. Smaller and 

newer vehicles may generally have less emissions than older and larger vehicles. Regression or 

T-Test analysis was performed on the data sample to determine which of these parameters have a 

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significant statistical relationship with the observed emission levels. 

3.1.1 NOx Emission Factors for Non-Discretionary Idle Operation 

The NOx emission analysis explored several factors that include the effect of vehicle 

class, the effect of altitude, the effect of engine model year, the effect of engine size and the 

effect of age as measured by the odometer reading. Only vehicle class (truck versus bus) and 

model year have statistically significant effects. The results suggest that buses have higher nondiscretionary idle NOx emissions than trucks, and that the older (pre-1988 model year) trucks 

have lower rates than the subsequent 1988 through 2006 model year trucks. Other possible 

parameters were also tested using various statistical tests, but were found to be statistically 

insignificant (95% confidence interval). 

Since the results were somewhat surprising, it should be noted that the sample sizes for 

buses and for model years prior to 1988 are rather small. Also, not all of the data points have 

engine size and odometer information to use in the analysis. When additional data becomes 

available, it is recommended that some attention be devoted to further exploring whether transit 

buses have truly different curb idle emission rates than trucks, and further explore potential age 

and mileage effects on curb idle NOx emissions. 

3.1.1.1 NOx Effect of Vehicle Class (Truck versus Bus) 

The first parameter to be analyzed was the effect of vehicle class on NOx emissions. 

This analysis consisted of comparing the mean NOx emission results from the sample of trucks 

and the sample of cars. A standard single sample T-Test was performed, and the results are 

shown in Tables A-1 and A-2 in Appendix A of this document. The sample of 114 trucks shows 

a mean idle NOx emission level of 82.6 g/hr NOx, and the sample of 13 buses shows an emission 

level of 118.9 g/hr. Levene’s test of equal variances shows that equal variances cannot be 

assumed at a 95% confidence level. However, the T-Test result when equal variances are not 

assumed shows a statistically valid relationship with a 2-tailed significant of 0.005 and a T 

statistic of -3.331. These are clearly less than the required 0.05 (95% CI) level, and lead to the 

conclusion that trucks and buses have different mean curb idle NOx emissions. 

The difference between trucks and buses seen in Tables A-1 and A-2 may be the result of 

the confounding effects of high and low altitude on curb idle NOx emissions. To confirm and 

eliminate these altitude effects, the high altitude vehicles were analyzed separately. A one 

sample T-Test was done on eleven trucks and thirteen buses and the results are shown in Table 

A-3 and Table A-4. For high altitude trucks, the mean curb idle NOx emission level was 84.0 

g/hr and the bus mean was 118.9 g/hr. The test was statistically significant at a 95% confidence 

level. This confirms that even among the high altitude vehicles, truck and buses have different 

curb idle NOx emissions. 

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3.1.1.2 NOx Effect of Altitude 

All of the buses in the sample are high altitude tests. A T-Test of using altitude was also 

done on the sample of trucks from both high and low altitude (buses were excluded) to determine 

if altitude is a factor in curb idle emission rates. The statistical results of this test are shown in 

Tables A-5 and A-6 in Appendix A. Here 103 low altitude trucks were tested against eleven 

high altitude ones. The results show statistically similar means of 82.4 g/hr and 84.0 g/hr NOx. 

These two samples cannot be considered different at a 95% confidence level (significance is 

0.769). This suggests that vehicle class is a more important determinant than altitude for NOx 

idle emissions. 

3.1.1.3 NOx Effect of Engine Model Year 

A scatter plot of the curb idle NOx emissions versus engine model year for the trucks in 

the sample is shown in Figure 3-1. We might expect that idle emissions should be decreasing as 

a function of engine model year since emission standards have gradually been made more 

stringent with successive years. However, a regression of the idle NOx emissions versus model 

year (Table A-10 in Appendix A) showed no statistically significant relationship (r2 value is 

0.01 and the significance is 0.260 - these are low values that indicate little correlation). Also, 

with the exception of the pre-1987 model years, the NOx curb idle emissions versus engine 

model year shows no general relationship. 

The idle NOx emissions for the older vehicles in the sample are significantly lower than 

the newer vehicles in the sample. The sample was divided into two strata, using the 1987 model 

year as the dividing point. The pre-1988 model years have a mean idle NOx emission level of 

50.5 g/hr, and the 1988 and later sample has a mean of 85.3 g/hr NOx. A statistical T-Test (see 

Tables A-7 and A-8 in Appendix A) shows a statistically difference between the two sample at a 

greater than 95% confidence level (signification = 0.023). The 1987/1988 model year split 

corresponds to a more stringent NOx emission standards for heavy-duty diesels were put into 

effect in the 1988 model year. Engine improvements, such as higher injection pressures, all tend 

to increase engine efficiency and, correspondingly, increase NOx. Newer vehicles also tend to 

have more accessories, which increase overall engine load and introduce transient loads at idle. 

As a result of this analysis, separate idle NOx emission factors were developed for pre1988 and for 1988 and later model year vehicles. However, since the sample sizes are quite 

small (only nine pre-1987 model years), it is recommended that this be an area for future 

research. 

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Curb Idle NOx (g/hr) 

200 

190 

180 

170 

160 

150 

140 

130 

120 

110 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

CLASS 

Buses 

Trucks 

Figure 3-1 NOx Curb Idle Emissions Versus Engine Model Year 

1970 1980 1990 2000 2010 

Engine Model Year 

3.1.1.4 NOx Effect of Engine Size 

Theoretically, the larger engines should emit more NOx per hour than a smaller one 

given a relatively constant exhaust concentration at idle and no after-treatment. This idea was 

explored in a statistical analysis of curb idle NOx emissions versus engine size in liters of 

displacement. Regression analysis of NOx curb idle emissions and engine size was done and the 

results are shown in Table A-9. The results show a small correlation coefficient ® squared of 

0.028) with no statistical significance (0.114). A graph of average NOx curb idle emissions 

versus the engine size category is shown in Figure 3-2. The wide error bands (95% confidence 

intervals) around the means, the small sample size at the lower engine sizes and general up and 

down nature of the mean values indicate no obvious relationship between engine size and NOx 

emissions in this sample. 

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Figure 3-2 Mean Curb Idle NOx Emissions Versus Engine Size 

191142824625424211536N = 

Curb Idle NOX (g/hr) 

200 

180 

160 

140 

120 

100 

80 

60 

40 

20 

0 

Missing 7.30 8.50 10.00 11.00 12.00 12.70 14.00 14.60 

5.90 7.60 9.00 10.80 11.10 12.10 12.70 14.10 15.00 

Engine SIZE 

3.1.1.5 NOx Effect of Test Program 

The idle NOx emission results were also analyzed versus test program to see if any large 

statistical differences could be observed between the test programs. Large differences between 

the results from different programs might indicate testing or design biases between the programs 

that lead to erroneous conclusions. A list of the test programs is shown in Table A-11a and the 

mean idle NOx emission results for each test program are show in Table A-11b. The CRC E-55 

program (5) is somewhat different with a mean curb idle NOx of 68.0 g/hr versus means from 

the other programs which are generally in the 80 g/hr range. Program 6 (NCHRP) is 

considerably different with a mean of 46.9 g/hr, however this program contains results from only 

two vehicles. A statistical ANOVA analysis (Table A-11c) shows that only program 1 and 5 are 

statistically different from each other at a 95% confidence level (significance = 0.035). Program 

#1 was a high altitude program and that it contained buses (Program #5 was from a low altitude 

area and did not include buses). The general conclusion from this statistical test is the 

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differences that do exist between the samples are reasonable and may be explained by 

differences in sampling. 

3.1.1.6 Pre-2007 Non-Discretionary Idle NOx Emission Factors 

Based on the statistical analysis of the available data, the curb idle NOx emissions for 

model years prior to 2007 are projected to be: 

All Pre-1988 Model Year Trucks and Buses 71.32 g/hr 

All 1988 and Later Trucks 84.69 g/hr 

All 1988 and Later Buses 110.50 g/hr 

3.1.1.7 Estimated 2007 and Later Non-Discretionary Idle NOx Emission Factors 

The 2007 model year heavy duty trucks requires new stringent NOx emission standards. 

These are not directly aimed at idle emissions; however, the new NOx catalysts and particulate 

traps should be effective for non-discretionary idle periods such as waiting at a traffic light for a 

minute or even during a limited idle period up to an hour in length. After this point, it is 

reasonable to assume that the catalyst will be ineffective due to cool down and that idle 

emissions will begin to increase. 

Since no data are available on 2007 and later engines are prototypes the idle NOx 

emission factors will be developed using the ratio of the certification standards in the 1991 

through 1998 time frame (this represents most of the data sample) to those in 2007. The nondiscretionary idle NOx emissions will be reduced accordingly. 

Given certification standards of: 

5.0 g/bhp-hr in 1991 - 1998 time frame for certification 

0.2 g/bhp-hr in 2007+ 

Calculating the ratio of the standards gives: 

0.2 / 5.0 = 0.04 

Applying the ratio to the pre 2007 model year truck and bus emission factors produces the 

2007 and later model year non-discretionary idle NOx emission factors of: 

Truck: 0.04 * 84.69 g/hr = 3.39 g/hr Idle NOx 

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Bus: 0.04 * 110.50 g/hr = 4.42 g/hr Idle NOx 

3.1.2 Particulate Matter (PM) Emission Factors for Non-Discretionary Idle Operation 

3.1.2.1 Pre-2007 Non-Discretionary Idle PM Emission Factor 

The PM emission analysis explored several factors that include the effect of vehicle class, 

the effect of altitude, and the effect of engine model year. These are the same parameters as 

were available for the NOx emission analysis. However, PM emission were not available from 

all of the studies. The High Altitude Study provided 24 vehicle tests. The IdleAire Study 

provided 49 vehicle tests. The CRC E-55 Study provided 14 vehicle tests. The Oak Ridge 

Study provided 5 vehicle tests. 

The conclusion from the analysis suggests that none of the parameters have a statistically 

significant effect on curb idle PM emissions at a confidence level of 95 percent. As a result, this 

paper is proposing that an average curb idle PM emission factor be used for all heavy-duty diesel 

vehicle classes, altitudes and model years prior to 2007. This factor is: 

3.38 g/hr with a standard error of 0.428 g/hr 

Some caution must be used in using this value, since particulate measurements are quite 

variable and easily affected by small differences in the measurement techniques used. The 

available data is not adequate to address these concerns directly. 

Below and in Appendix A the emission statistics are presented that were used to 

determine the average curb idle PM emission factor. Table A-12 shows the mean emission 

factors by altitude type and class type (truck or bus). With the exception of the High Altitude 

Truck results, the curb idle PM emissions were fairly consistent. The eleven high altitude trucks 

show a mean emission factor of 1.41 g/hr and the corresponding low altitude trucks have a mean 

emission factor of 3.78 g/hr. This seems like a sizeable difference. However, an independent 

sample T-Test was done to determine if the results for trucks and buses at high altitude are 

statistically different at a 95% confidence level. The results in Table A-13 show a significance 

of 0.07; this indicates that at high altitude the vehicle classes are not significantly different at 

95% CI, although nearly so. When the combined sample of trucks and buses at high altitude 

(2.34 g/hr) are compared with the low altitude trucks (3.78 g/hr), the difference due to altitude is 

not significant. Table A-14 shows the sample statistics comparing high and low altitude. Table 

A-15 shows the sample statistics comparing trucks and buses. Figure 3-3 graphically shows 

mean curb idle PM emissions versus engine model year. This figure shows considerable 

variance (the 95% confidence intervals are frequently larger than the mean values) of curb idle 

PM versus model year, but no discernable trend in the model year averages. 

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Figure 3-3 Curb Idle PM versus Engine Model Year 

3.1.2.2 2007 and Later Non-Discretionary Idle PM Emission Factor 

151211888105334212121N = 

Curb Idle PM Emissions (g/hr) 

20 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

1978 1986 1989 1991 1993 1995 1997 1999 2001

1985 1987 1990 1992 1994 1996 1998 2000 2002

Engine Model Year 

Since no data are available on 2007 and later model year heavy-duty real engines or

prototype engines, the curb idle PM emission factors were developed by a ratio of the 2007

certification standards to those in the 1991 through 1998 time frame (this represents most of the

data sample). The non-discretionary idle NOx emissions are reduced accordingly.

Given certification standards of: 

0.10 g/bhp-hr in 1991 - 1998 time frame for certification 

0.01 g/bhp-hr in 2007+ 

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Calculating the ratio of the standards gives: 

0.01 / 0.1 = 0.1 

Applying the ratio to the pre 2007 model year truck and bus emission factors produces the 

2007 and later model year non-discretionary idle PM emission factors of: 

Truck: 0.1 * 3.38 g/hr = 0.34 g/hr Idle PM 

3.1.3 Hydrocarbon (HC) Emission Factors for Non-Discretionary Idle Operation 

3.1.3.1 HC Effect of Vehicle Class (Truck versus Bus) and Altitude 

Appendix A contains the HC emission statistics that were used to determine the average curb 

idle HC emission factors. Table A-17 shows the mean emission factors by altitude type and class 

type (truck or bus). In general, the mean curb idle emission factors seem to be similar by vehicle 

class (truck and bus) and by high and low altitude. The accompanying T-Tests in Tables A-18 and 

A-19 confirm no statistically significant differences at a 95% CI exist between classes and altitudes. 

For example, a T-Test of HC and vehicle class shows a significance of more than 0.90 (a very low 

level). Likewise, a T-Test of HC and altitude (See Table A-19) shows a significance of more than 

0.40 (a low level of significance). 

3.1.3.2 HC Effect of Engine Size 

Curb Idle HC Emissions were analyzed versus engine size using least squares regression 

analysis. The results are shown in Tables A-20 in Appendix A. The regression shows poor 

correlation between curb idle HC and engine size. The R Square value is 0.001 and the regression 

significance (Sig.) is 0.736. 

3.1.3.3 HC Effect of Vehicle Odometer 

Curb Idle HC Emissions were analyzed versus odometer using least squares regression 

analysis. The results are shown in Table A-21 in Appendix A. The regression shows poor 

correlation between curb idle HC and odometer. The R Square value is 0.008 and the regression 

significance (Sig.) is 0.468. 

3.1.3.4 HC Effect of Model Year 

Curb Idle HC Emissions were analyzed versus model year using least squares regression 

analysis. The results are shown in Table A-22 in Appendix A. The regression shows modest 

correlation between curb idle HC and model year. The R Square value is 0.132 and the regression 

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significance (Sig.) of 0.000 is statistically significant at more than a 95% Confidence Interval. 

The Curb Idle HC Emissions and the linear regression equation of HC versus model year are 

shown plotted versus engine model year in Figure 3-4. The open green squares are the actual data 

points and the solid blue squares are the predicted values from the regression. The data shows 

considerable scatter and a general lack of data on the older model years. The regression fits the 

expected prediction of lower HC emissions with newer model years. The relatively low value 0.132 

for the r-square correlation coefficient reflects the considerable scatter in the emission data. 

Figure 3-4 Idle HC Emissions Versus Engine Model Year 

Data and Predicted Results 

3.1.3.5 Pre-2007 Non-Discretionary Idle HC Emission Factor 

The HC emission analysis explored several factors that include the effect of vehicle class, 

the effect of altitude, the effect of engine size and the effect of engine model year. These are the 

same parameters as were available for the NOx and PM emission analysis. The HC emission 

data-set contained 122 cases and included members from all of the test programs. The 

1970 1980 1990 2000 2010

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

THC 

YEAR 

Unstandardized Predi 

YEAR 

Engine Model Year 

Idle Hydrocarbon Emissions (g/hr) 
 

 

 

conclusion from the analysis suggests that only the engine model year parameter was a 

statistically significant effect on curb idle HC emissions at a confidence level of 95 percent. 

This was determined through least squares linear regression analysis. The function shown in 

Equation 3-2 and 3-3 would be used to model average curb idle HC emissions for all heavy-duty 

diesel vehicle classes, altitudes and model years between 1978 and 2006. 

Curb Idle HC (g/hr) = 1294.063 - 0.644 * Model Year Eqn 3-2 

Given: 

Curb Idle HC = 20.23 g/hr for Pre-1978 MY Eqn 3-3 

Curb Idle HC = 2.20 g/hr for 2006 MY Eqn 3-4 

Curb Idle HC = 0.22 g/hr for 2007 and later MY Eqn 3-5 

Where Model Year is the engine model year (i.e., 1999). 

The upper curb idle HC emission factor of 20.23 g/hr given in Eqn 3-3 is the value for the 

1978 model year. No earlier engine model year data were available; thus, continuation of 

Equation 3-2 would be extrapolation. Rather than such extrapolation, the emission factor is 

fixed at the 1978 model year level for all previous model years. Additional justification for this 

assumption is that generally the pre-1978 model years were either non-regulated or only lightly 

regulated for PM emissions and should be similar between model years. 

All values past model year 2002 are extrapolation. This extrapolation was done through 

2006 to reflect a gradual possible introduction of diesel catalyst technology. The value for the 

2006 model year is shown as 2.20 g/hr in Equation 3-4. 

The 2007 and later value represents a special problem to model since the new stringent 

PM and NOx standards are applied starting in 2007. These new regulations will likely force the 

complete introduction of catalytic converters and other exhaust after-treatment devices. Such 

devices may drive curb idle HC emissions to near zero levels. This analysis attempts to model 

this future event by reducing the 2006 model year level by 90%. The 90% reduction is 

analogous to the required PM emission reduction in the 2007 diesel rule. It has the effect of 

producing an average curb idle HC emission factor of 0.22 g/hr. This is considerably lower than 

the minimum HC value recorded by a current uncontrolled engine of 1.80 g/hr HC. However, it 

still might be an environmentally conservative estimate given the claims of some future 

technology proponents. 

3.1.4 Carbon Monoxide (CO) Emission Factors for Non-Discretionary Idle Operation 

3.1.4.1 CO Effect of Vehicle Class, Altitude and Model Year 

A single sample T-Test was used to determine the statistical significance of altitude and 

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least squares linear regression was used to determine the effects of engine model year. 

Below and in Appendix A are the emission statistics that were used to determine the 

average curb idle CO emission factors. Table A-23 shows the mean emission factors by altitude 

type and class type (truck or bus). In general, the mean curb idle emission factors seem to be 

similar by vehicle class (truck and bus), but vary considerably by altitude. For example, the 

mean high altitude CO level is 75.3 g/hr while the low altitude mean value is 22.1 g/hr. The 

difference between the two values makes engineering sense since the combustion air at high 

altitude generally has less oxygen per pound of air than low altitude. If not compensated by the 

engine or after-treatment devices, this oxygen deficiency would lead to higher CO emission 

factors at the tailpipe. The accompanying T-Tests (see Table A-24) confirm a statistically 

significant differences at a 95% CI exist between altitudes. For example, a T-Test of HC and 

vehicle class shows a significance of 0.00 (a high level of statistical significance). 

A similar T-Test analysis of CO emissions versus vehicle class done on the high altitude 

sample of 11 trucks and 13 buses (see Table A-25) shows a low level of statistical significance 

(significance = 0.339). As a result, this test eliminated vehicle class as a significant parameter 

in the CO curb idle emission analysis. 

The low altitude curb idle CO emissions were analyzed versus model year using least 

squares regression analysis. The results are shown in Table A-26 in Appendix A. The 

regression shows modest correlation between curb idle CO and model year. The R Square value 

is 0.142, and the regression significance (Sig.) of 0.000 is statistically significant at more than a 

95% Confidence Interval. 

The Curb Idle CO Emissions are shown plotted versus engine model year in Figure 3-5. 

The open green squares are the actual data points and the solid blue squares are the predicted 

values from the linear regression. The data shows considerable scatter and a general lack of 

data on the older model years. Nevertheless, the regression is statistically significant and it fits 

the expected prediction of lower CO emissions with newer model years. 

-15­
Figure 3-5 Idle CO Emissions Versus Engine Model Year 

Data and Predicted Results 

80

70

60

50

40

30

20

1970 1980 1990 2000 2010

Unstandardized Predi 

YEAR

10

CO 

0 YEAR 

Engine Model Year 

3.1.4.2 Pre-2007 Non-Discretionary Idle CO Emission Factor 

The CO emission analysis explored several factors that include the effect of vehicle class, 

the effect of altitude, the effect of engine size and the effect of engine model year. These are the 

same parameters as were available for the NOx and PM emission analysis. The CO emission 

data-set contained 122 cases and included members from all of the test programs. The 

conclusion from the analysis suggests that the altitude parameter and the engine model year 

parameters are statistically significant effects on curb idle CO emissions at a confidence level of 

95 percent. The function shown in Equations 3-6 and 3-7 are proposed for use in modeling 

average curb idle CO emissions for all heavy-duty diesel vehicle classes at LOW altitude and for 

model years between 1978 and 2006. An average CO emission factor of 75.3 g/hr (Eqn 3-10) 

will be used for all Pre-2007 model year high altitude vehicles. 

-16­

Idle Carbon Monoxide Emissions (g/hr) 
 

 

 

 

 

LOW ALTITUDE 

Curb Idle CO (g/hr) = 2415.728 - 1.20 * Model Year Eqn 3-6 

Given: 

Curb Idle CO = 42.13 g/hr for Pre-1978 MY Eqn 3-7 

Curb Idle CO = 8.53g/hr for 2006 MY Eqn 3-8 

Curb Idle CO = 0.85 g/hr for 2007 and later MY Eqn 3-9 

HIGH ALTITUDE 

Curb Idle CO = 75.3 g/hr for Pre-2007 MY Eqn 3-10 

Curb Idle CO = 0.85 g/hr for 2007 and later MY Eqn 3-11 

Where Model Year is the engine model year (i.e., 1999). 

The upper curb idle CO emission factor of 42.13 g/hr is the value for the 1978 model 

year. No earlier engine model year data were available; thus, continuation of Equation 3-6 

would be extrapolation. Rather than such extrapolation, the emission factor is fixed at the 1978 

model year level. 

All values past model year 2002 are extrapolation. This extrapolation was done through 

the 2006 model year. The value for the 2006 model year is shown as 8.53 g/hr in Equation 3-7. 

The 2007 and later value represents a special problem to model since the new stringent 

PM and NOx standards are applied starting in 2007. These new regulations will likely force the 

complete introduction of catalytic converters and other exhaust after-treatment devices. Such 

devices may drive curb idle CO emissions to near zero levels. This analysis attempts to model 

this future event by reducing the 2006 model year level by 90%. The 90% reduction is 

analogous to the required PM emission reduction in the 2007 diesel rule. It has the effect of 

producing an average curb idle CO emission factor of 0.85 g/hr. This is considerably lower than 

the minimum CO value recorded by a current uncontrolled engine of 4.45 g/hr CO. However, it 

still might be an environmentally conservative estimate given the claims of some future 

technology proponents. The same value of 0.85 g/hr was also assumed for high altitude 2007 

and later model years. The assumption is that the high altitude and low altitude vehicles will be 

equally controlled as a result of the stringent 2007 diesel rule requirements. 

-17­
3.2 Development of Discretionary Idle Emission Factors 

Once the non-discretionary or curb idle emission factors are available, they become the 

base emissions to which multiplicative correction factors can be applied to produce load adjusted 

and discretionary or extended idle emission factors for estimating the emission impacts of 

hoteling behavior. The correction factors were determined by analyzing operating parameters 

such as load on the engine (e.g., AC or heat), the speed of the engine or rpm, the amount of time 

spent in idle operation, and impacts of upcoming EPA regulations. 

LOAD 

Engine load during idle is expected to significantly effect idle NOx emission rates. In 

simple terms, the greater the load the greater the emission. However, determining typical load at 

idle requires looking at the average uses of on-board truck and bus systems. Base electrical 

loads, such as lights, battery charger, communications, and computer need about 1-5 kW of 

power. Hoteling loads, such as lighting, simple HVAC, and appliances will need 3-5 kW of 

power. For full truck electrification which includes all of the above, plus water and oil pumps, 

starter, cooling fans, transmission and hydraulic system, brake compressors, and fuel and air 

handling systems requires 5-15 kW of power. According to the American Trucking Association 

(ATA), typical engine load during extended idling operation is in the range of 10 to 30 brake 

horsepower (The Maintenance Council of the American Trucking Associations, The Fleet 

Manager’s Guide to Fuel Economy, 1998). 

RPM 

Truck manufacturers generally recommend higher (than curb idle) engine idle speeds 

when extended idling is expected. Here are a few examples found in operating manuals. 

• Caterpillar: “If extended idle time is required, control the engine RPM to 1000 RPM or 

above 1000 RPM.” (Operation and Maintenance Manual, SEBU7186-04, May 2001, 

page 83. This is the manual for C-10, C-12, 3406E, C-15, and C-16 Truck Engines). 

• Detroit Diesel: “When prolonged idling is necessary, maintain at least 850 RPM 

spring/summer and 1200 RPM fall/winter.” (Operations Guide 6 SE 484 0106, page 8) 

• International: “Maintain a minimum of 1250 idle RPM by use of the Hand Throttle.” 

(Bulletin TSI 96-12-29, “Cold Weather Operation of International Engines.” Guidelines 

for cold weather operation) 

• Cummins: “Operating engines at idle RPM (650 to 1000) in cold ambient temperatures 

wastes fuel, accelerates wear and can result in serious engine damage. . . . If the operator 

insists on prolonged idling, it is recommended that the engine be idled at an RPM which 

-18­
is adequate to maintain coolant temperatures above 140 o

F (60 oC). In 0 oF ambient (-18 o

C), the engine may have to be idled above 1200 RPM.” (Company Bulletin 3379009-03) 

A recent IdleAire survey was done of 100 drivers (through truck stop interviews) and 100 

fleet maintenance managers (by phone) regarding engine speeds used during extended idling. 

(David Irick and Bob Wilson report). The average, weighted by days of operation per year, was 

964 rpm for drivers and 965 rpm for maintenance managers. Nearly 70 percent of these 100 

drivers indicated they operated their trucks at 800 rpm or higher during extended idle. Nearly 

half the drivers indicated that they operate their engines during extended idling at 1000 rpm or 

higher, with about 40 percent reporting the range between 1000 and 1100 rpm. This suggests that 

operating in this rpm range is an established practice by a substantial proportion of drivers. 

As with the driver survey, the fleet survey showed that a high percentage of drivers, 85.5 

percent, use extended idling speeds greater than 800 rpm. Fleets say 76.8 percent of their trucks 

idle at 900 rpm or greater and 56.5 percent use an extended idling speed of 1000 rpm or greater. 

There was little apparent connection between the size of the fleet and the choice of idling 

rpm. On average, fleets of 1000 or fewer trucks chose idling rpm of 965, identical to the overall 

average. The average fleet in the 1000-2999 size range chose a lower engine speed level (with a 

mean 871 rpm), while fleets with 3000 or more trucks reported an average idling engine speed of 

988 rpm. A common theme among fleet owners is that, although most fleets have an official idle 

speed recommendation, it is the driver who chooses idle speed. More often than not, fleet owners 

observe that drivers may choose a speed higher than company recommendations to maximize 

driver comfort levels during extended rest periods. 

In general, the fleet maintenance manager survey may be more reliable. Truck drivers 

are notoriously suspicious of authority and likely to offer socially acceptable answers. The fleet 

maintenance manager data has a more normal, bell shaped, distribution and may have less reason 

to bias their answers either higher or lower. 

IDLE TIME 

Idle duration is an important factor because of engine and future after-treatment device 

cool-down. During long durations (called extended idling) the engine begins from a high 

operating temperature (just off the road) and then cools to a stabilized cooler operating 

temperature as time goes on. Cool down gradually makes engine and after-treatment devices 

less efficient and causes more water condensation in the exhaust. Low exhaust flow rates and 

cool down may also affect emissions by condensing unburned fuel (HC) and collecting 

particulates in the exhaust system. 

NEW REGULATIONS 

-19­
The new emission standards beginning in 2007 do not require control of emissions during 

extended idling. However, it is reasonable to assume that the emission control strategies used to 

reduce driving emissions from these trucks will have an effect on idling emissions, but only as 

long as the devices remain effective once driving has ceased. It is reasonable to assume that 

engine-out idling emission rates will be similar in future model years, but that after-treatment 

devices used to reduce driving emissions will affect the resulting tailpipe idling emissions during 

curb idling. 

Idling emissions during the first minutes of idling will likely be most affected by aftertreatment devices because conversion efficiency of the after-treatment devices remains high. 

Extended idling or “hoteling” idling may last 8 hours per day. Because the effectiveness of 

after-treatment devices will decline as the time spent at idle grows, emission rates of trucks with 

after-treatment devices will increase as idling time increases. The overall fleet average idling 

emission rate from extended idling for trucks certified to the new emission standards will depend 

on the average length of time for extended idling, and the effect of the new standards on idling 

emission rates. 

ELECTRONIC CONTROLS 

The introduction of electronic controls allows engine designers to optimize engine 

performance to the operating mode. At curb idle conditions, the engine designer takes advantage 

of a mode where no useful work is done to adjust engine parameters to minimize emissions. 

However, when idle speed is increased or high load accessories (such as air conditioning) are 

detected, electronic controls allows the engine designer to optimize for power or fuel economy. 

However, the limited available samples at all modes suggest that statistical characterization of 

the differences in these modes can provide a more consistent estimate of the idle emission rates 

in each mode rather than independent estimates based on small samples. 

3.2.1 Discretionary Idle NOx Emission Effects 

3.2.1.1 NOx Effects of Accessory Load 

The CCD study and the IdleAire study include paired test samples showing the effects of 

accessory load on idling NOx emissions from heavy-duty vehicles. The vehicle data from these 

two studies were combined. A truck was tested three times in the CCD study at 600 rpm with no 

load. The value reported in the table, and used in the analysis, is the average of these three 

values. The CCD study results are confounded by the fact that the temperature and humidity 

used with the heater on and the air conditioning on are different than the "no load" case. 

However, since the real-world use of heater and air conditioning accessories are strongly 

correlated to the temperature, the observed effect is appropriate for modeling those cases. A 

sample comparison t-test of the effects observed in the two study samples are not significantly 

different, which suggests that the effects of temperature on idling NOx emissions are not 

-20­
significant. For purposes of this analysis, all the observed effect will be assumed to be the result 

of the accessory loading only and not from any change in temperature. The NOx values in the 

CCD study were adjusted for differences in humidity using the standard Federal Test Procedure 

methods (CFR 86.1342-90). 

A paired sample t-test was used to compare the means of the idle NOx emission tests 

with and without the accessory loads. The t-test indicates that the load effects of air 

conditioning on NOx emissions are significant (0.000) at both low and high idle speeds, while 

the effects of heater load are not significant. The t-test statistics are shown in Appendix B. 

3.2.1.2 NOx Effects of Engine Idle Speed 

The CCD study and IdleAire study results used to evaluate the effects of accessory load 

were also used to investigate the effects of idle speed on NOx emissions. A paired sample t-test 

was used to compare the means of the idle NOx emission tests at low (non-discretionary "curb) 

idle speeds and high (discretionary) idle speeds. The t-test indicates that the effects of 

discretionary idle speeds on NOx emissions are significant (from 0.000 to 0.002) at all loads. 

The t-test statistics are shown in Appendix C. 

3.2.1.3 NOx Effects of Engine Idling Time 

The CCD and the IdleAire study measured the idling NOx emissions from five heavyduty vehicles continuously (second-by-second). All trucks were fully warm when idling began. 

Although data from the CCD study indicated a small (7%) decrease in NOx emissions, 

information from the IdleAire data contradicts this conclusion. Since both samples are 

extremely small, the direct effects of idle time will be assumed to be negligible for this analysis. 

As discussed earlier, the new exhaust after-treatment devices used for 2007 and later 

model years should be effective for non-discretionary idle periods such as waiting at a traffic 

light or during a limited idle period. However, during extended idling periods, the effectiveness 

of these controls will likely fade substantially. This will make the overall average idling 

emission rate for the extended idling period much higher than the expected non-discretionary 

idling emission rate. 

Since no data are available on 2007 and newer engines, the curb idle NOx emission 

factors developed for the estimate of non-discretionary idle NOx emissions will be used to 

represent idling emission rates for the first hour of idling. The idling emission rate after one 

hour of idling will be assumed to return to pre-controlled (pre-2007 model year) rates. Then, 

assuming that all extended idling periods are eight hours long, we can calculate the average 

idling emission rate for extended idling periods for the 2007 and newer engines: 

2007 Extended Idling Rate = (1/8)*2007 Idle Rate + (7/8)*Pre-2007 Idle Rate 

-21­
This method can also be used to determine idle emission rates for idling time periods 

which are less than the eight hours assumed here. The 2007 idle emission rate is a function of 

the pre-2007 idle emission rate, so the extended idle emission rate for 2007 and newer model 

years can be determined directly from the idle emission estimate for pre-2007 model year 

vehicles. 

2007 Extended Idling Rate = [(1/8)* (0.2/5.0) + (7/8)] * Pre-2007 Idle Rate 

These estimates are affected strongly by the assumption of total hours of discretionary 

idling, the effectiveness of the new technologies on idling emissions and the length of time the 

new technologies remain effective during extended idling. As new information is obtained about 

the emission performance of these vehicles, these estimates will need to be updated. 

3.2.1.4 Discretionary NOx Idling Emission Factors 

Table 3-1 summarizes the effect of air conditioning observed in the sample as a 

multiplicative adjustments to the basic idle NOx emissions measured with no load. The 

discretionary idling adjustments are a composite of the effects of higher idling speeds and 

extended idling time. The adjustments assume no interactions. 

Table 3-1 

Discretionary Effects of on Idle NOx Emissions 

Adjustment to Base Idle NOx Emission 

Rate 

Description Speed 

Air 

Conditioning Composite 

Non-Discretionary with A/C On NA 23.7% 23.7% 

Discretionary with No Load 55.5% NA 55.5% 

Discretionary with A/C On 32.6% 26.5% 67.7% 

2007+ Discretionary with No Load 55.5% NA 55.5% 

2007+ Discretionary with A/C On 32.6% 26.5% 67.7% 

Using these adjustments, the non-discretionary idle emission rates described in Section 

3.1 and the travel fractions for Class 8 heavy duty diesel trucks from MOBILE6, we can 

calculate annual average idle emission rates for the fleet for calendar year 2004. This calculation 

assumes that idle times are proportional to vehicle miles traveled and that air conditioning is 

used for five months of the year. The results are shown in Table 3-2. 

-22­
Table 3-2 

Average Heavy Duty Truck Idle NOx Emission Rates 

By Calendar Year 

Average Idle NOx Emission Rate in Grams per Hour 

Travel Fraction Non-Discretionary Discretionary (Hoteling) 

Calendar 

Year 

Pre1988 1988-

2004 No A/C A/C On Annual* No A/C A/C On Annual* 

2004 0.058 0.942 83.91 103.80 92.20 130.49 140.72 134.75 

* Annual assumes five months of A/C use and seven months with no A/C load. 

3.2.2 Particulate Matter (PM) Emission Factors for Discretionary Idle Operation 

The particulate matter measurements made on four of the five heavy duty trucks in the 

Oak Ridge Study were used to estimate the emission rates for PM during modes characteristic of 

discretionary idling, rather than adjusting the non-discretionary, curb idling emission rates for 

particulate matter. The IdleAire study did include 49 paired test samples showing the effects of 

accessory load and idle speed on idling PM emissions from heavy-duty vehicles. These effects 

are discussed below. 

3.2.2.1 PM Effects of Accessory Load 

A paired sample t-test was used to compare the means of the idle PM emission tests with 

and without the accessory loads in the IdleAire study. The t-test indicates that neither the load 

effects on PM emissions of air conditioning nor heater load are significant at either low or high 

idle speeds. All of the t-test statistics are shown in Table B-4 in Appendix B. 

3.2.2.2 PM Effects of Engine Idle Speed 

A paired sample t-test was used to compare the means of the idle PM emission tests at 

low (non-discretionary "curb) idle speeds and high (discretionary) idle speeds in the IdleAire 

study. The t-test indicates that the effects of discretionary idle speeds on PM emissions are 

significant only with the air conditioning on. All of the t-test statistics are shown in Table C-4 in 

Appendix C. 

3.2.2.3 PM Effects of Idling Time 

None of the studies contain the information needed to resolve the effects of extended 

idling time on PM emissions. Pre-2007 model year trucks will be assumed to have the same PM 

emission rate during all hours of operation. This is supported by results in the Oak Ridge study, 

-23­
where PM idling rates appear to be constant over idling time periods exceeding an hour. 

For PM, it is expected that the new after-treatment devices used for 2007 and later model 

years should be effective for both short term non-discretionary idle periods and during extended 

idling periods. No adjustment to the non-discretionary PM idling emission factor is therefore 

needed to account for long-term, discretionary idling. This expectation is quite different than the 

assumptions used for the other pollutants (HC, CO and NOx). 

3.2.2.4 Adjustments to PM Idling Emission Factors 

Table 3-3 summarizes the effect of air conditioning load in combination with high idle 

speeds observed in the sample as a multiplicative adjustments to the basic idle PM emissions 

measured with no load. The idling adjustments are a composite of the effects of higher idling 

speeds and extended idling time, and are applicable to extended idling periods. The time 

adjustment for the 2007 and newer engines is applied to the estimate for pre-2007 model year 

non-discretionary idling PM emission rates and includes the effect of the new standards. The 

adjustments assume no interactions. 

Table 3-3 

Adjustments to Idle PM Emissions 

Adjustment to Base Idle PM Emission Rate 

Description Time 

Speed/Air 

Conditioning Composite 

Extended Idle with A/C On NA 46.9% 46.9% 

2007+ Extended Idle with No Load -11.3% NA -11.3% 

2007+ Extended Idle with A/C On -11.3% 46.9% 30.3% 

3.2.2.5 Discretionary PM Idling Emission Factors 

In the case of particulate matter emissions, it was felt that an adjustment of the base curb 

idle PM emission rate would not be appropriate for use in modeling the effects of extended 

idling (hoteling). Instead, the particulate matter measurements made on four of the five heavy 

duty trucks in the Oak Ridge Study were used to estimate the emission rates for PM during 

modes characteristic of discretionary idling, rather than adjusting the non-discretionary, curb 

idling emission rates for PM. 

Table D-1 in Appendix D shows the PM measurements on the five trucks in the study 

during the different seasonal operating modes with corresponding engine loading. All results 

shown are for the high idling speed case, which is typical of hoteling behavior. The 1992 Ford 

truck has unusually high PM emissions and was dropped from the averages shown in Table D-2. 

Each seasonal average was weighted by the number of months associated with the season (3 

-24­
months for winter, 4 months for spring and fall and 5 months for summer. The resulting 

seasonally weighted average PM emissions are shown in Table 3-4. This number is slightly 

larger than the PM emission factor estimated for non-discretionary idling in Section 3.1.2.1. 

For 2007 model year and later trucks, the emission rate for pre-2007 trucks was adjusted 

using the ratio of the standards, as described in Section 3.1.2.2 and the results are shown in Table 

3-4. This result is slightly lower than the estimate for non-discretionary idle PM emissions 

estimated in Section 3.1.2.2. 

Table 3-4 

Annual Average Discretionary (Hoteling) Idle PM Emissions 

Truck Model Year PM Emission Factor (g/hr) 

2006 and earlier 3.68 

2007 and later 0.33 

3.2.2 Discretionary Idle HC Emission Effects 

3.2.2.1 HC Effects of Accessory Load 

The CCD study has not yet been summarized for HC emissions. However. the IdleAire 

study include paired test samples showing the effects of accessory load on idling HC emissions 

from heavy-duty vehicles. A paired sample t-test was used to compare the means of the idle HC 

emission tests with and without the accessory loads. The t-test indicates that neither the load 

effects on HC emissions of air conditioning nor heater load are significant at either low or high 

idle speeds. All of the t-test statistics are shown in Appendix B. 

3.2.2.2 HC Effects of Engine Idle Speed 

The IdleAire study results used to evaluate the effects of accessory load were also used to 

investigate the effects of idle speed on HC emissions. A paired sample t-test was used to 

compare the means of the idle HC emission tests at low (non-discretionary "curb) idle speeds 

and high (discretionary) idle speeds. The t-test indicates that the effects of discretionary idle 

speeds on HC emissions are significant at all loads. All of the t-test statistics are shown in 

Appendix C. 

3.2.2.3 HC Effects of Engine Idling Time 

The CCD study has not yet been summarized for HC emissions. None of the other 

-25­
studies contains the information needed to resolve the effects of idling time on HC emissions. 

As discussed earlier, the new exhaust after-treatment devices used for 2007 and later 

model years should be effective for non-discretionary idle periods such as waiting at a traffic 

light or during a limited idle period. However, during extended idling periods, the effectiveness 

of these controls will fade substantially. This will make the overall average idling emission rate 

for the extended idling period much higher than the expected non-discretionary idling emission 

rate. 

Since no data are available on 2007 and newer engines, the curb idle HC emission factors 

developed for the estimate of non-discretionary idle HC emissions will be used to represent 

idling emission rates for the first hour of idling. The idling emission rate after one hour of idling 

will be assumed to return to pre-controlled (pre-2007 model year) rates. Then, assuming that all 

extended idling periods are eight hours long, we can calculate the average idling emission rate 

for extended idling periods for the 2007 and newer engines: 

2007 Extended Idling Rate = (1/8)*2007 Idle Rate + (7/8)*Pre-2007 Idle Rate 

This method can also be used to determine idle emission rates for idling time periods 

which are less than the eight hours assumed here. The 2007 idle emission rate is assumed to be 

10% of the pre-2007 idle emission rate, so the extended idle emission rate for 2007 and newer 

model years can be determined directly from the idle emission estimate for pre-2007 model year 

vehicles. 

2007 Extended Idling Rate = [(1/8)* (0.10) + (7/8)] * Pre-2007 Idle Rate 

As new information is obtained about the emission performance of these vehicles, these 

estimates will need to be updated. 

3.2.2.4 Discretionary HC Idling Emission Factors 

Table 3-5 summarizes the effect of discretionary (extended) idling observed in the 

sample as a multiplicative adjustments to the basic idle HC emissions measured with no load. 

The discretionary idling adjustments are a composite of the effects of higher idling speeds and 

extended idling time. The time adjustment for the 2007 and newer engines is applied to the 

estimate for pre-2007 model year non-discretionary idling HC emission rates and includes the 

effect of the new standards. The adjustments assume no interactions. 

Table 3-5

Discretionary Effects of on Idle HC Emissions

-26­
Adjustment to Base Idle HC Emission Rate 

Description Speed Time Composite 

Discretionary 82.6% NA 82.6% 

2007+ Discretionary 82.6% 

3.2.3 Discretionary Idle CO Emission Effects 

3.2.3.1 CO Effects of Accessory Load 

The CCD study has not yet been summarized for CO emissions. However. the IdleAire 

study include paired test samples showing the effects of accessory load on idling CO emissions 

from heavy-duty vehicles. A paired sample t-test was used to compare the means of the idle CO 

emission tests with and without the accessory loads. The t-test indicates that only the load 

effects on CO emissions of air conditioning are significant, and only at low idle speeds. All of 

the t-test statistics are shown in Appendix B. 

3.2.3.2 CO Effects of Engine Idle Speed 

The IdleAire study results used to evaluate the effects of accessory load were also used to 

investigate the effects of idle speed on CO emissions. A paired sample t-test was used to 

compare the means of the idle CO emission tests at low (non-discretionary "curb) idle speeds 

and high (discretionary) idle speeds. The t-test indicates that the effects of discretionary idle 

speeds on CO emissions are significant at all loads. All of the t-test statistics are shown in 

Appendix C. 

3.2.3.3 CO Effects of Engine Idling Time 

The CCD study has not yet been summarized for CO emissions. None of the other 

studies contains the information needed to resolve the effects of idling time on CO emissions. 

As discussed earlier, the new exhaust after-treatment devices used for 2007 and later 

model years should be effective for non-discretionary idle periods such as waiting at a traffic 

light or during a limited idle period. However, during extended idling periods, the effectiveness 

of these controls will fade substantially. This will make the overall average idling emission rate 

for the extended idling period much higher than the expected non-discretionary idling emission 

rate. 

Since no data are available on 2007 and newer engines, the curb idle CO emission factors 

developed for the estimate of non-discretionary idle CO emissions will be used to represent 

idling emission rates for the first hour of idling. The idling emission rate after one hour of idling 

will be assumed to return to pre-controlled (pre-2007 model year) rates. Then, assuming that all 

extended idling periods are eight hours long, we can calculate the average idling emission rate 

for extended idling periods for the 2007 and newer engines: 

-27­
2007 Extended Idling Rate = (1/8)*2007 Idle Rate + (7/8)*Pre-2007 Idle Rate 

This method can also be used to determine idle emission rates for idling time periods 

which are less than the eight hours assumed here. The 2007 idle emission rate is assumed to be 

10% of the pre-2007 idle emission rate, so the extended idle emission rate for 2007 and newer 

model years can be determined directly from the idle emission estimate for pre-2007 model year 

vehicles. 

2007 Extended Idling Rate = [(1/8)* (0.10) + (7/8)] * Pre-2007 Idle Rate 

As new information is obtained about the emission performance of these vehicles, these 

estimates will need to be updated. 

3.2.3.4 Discretionary CO Idling Emission Factors 

Table 3-6 summarizes the effect of discretionary (extended) idling and the effect of air 

conditioning load at low idle speeds observed in the sample as a multiplicative adjustments to 

the basic idle CO emissions measured with no load. The discretionary idling adjustments are a 

composite of the effects of higher idling speeds and extended idling time. The time adjustment 

for the 2007 and newer engines is applied to the estimate for pre-2007 model year nondiscretionary idling CO emission rates and includes the effect of the new standards. The 

adjustments assume no interactions. 

Table 3-6 

Discretionary Effects of on Idle CO Emissions 

Adjustment to Base Idle CO Emission Rate 

Description Speed Time 

Air 

Conditioning Composite 

Non-discretionary with A/C On NA NA 13.4% 13.4% 

Discretionary 217.2% NA NA 217.2% 

2007+ Discretionary 217.2% NA 

-28­
Appendix A

Non-Discretionary Idle Analysis Sample and Statistics

Table A-1 

Average Curb Idle NOx Emissions by Vehicle Class 

Class Mean N Std. Deviation Median 

Truck 114 82.585212 32.298661 3.025047 

Bus 13 118.910769 37.775149 10.476941 

Table A-2 

T-Test Statistics for Curb Idle NOx by Vehicle Class 

Levene's Test for Equality of 

Variances Independent Samples t-test for Equality of Means 

F Sig t df Sig. 

(2-tailed) 

Mean 

Difference 

Std. Error 

Difference 

95% Confidence Interval of 

the Difference 

Lower Upper 

Equal variances 

assumed 

1.059 .305 -3.776 125 .000 -36.325558 9.620520 -55.365761 -17.285355 

Equal variances 

not assumed 

-3.331 14.074 .005 -36.325558 10.904917 -59.702773 -12.948342 

Table A-3 

Average High Altitude Curb Idle NOx Emissions by Vehicle Class 

Class Mean N Std. Deviation Median 

Truck 11 84.038182 24.324821 7.334209 

Bus 13 118.910769 37.775149 10.476941 

Table A-4 

T-Test Statistics for High Altitude Curb Idle NOx by Vehicle Class 

Levene's Test for Equality of 

Variances Independent Samples t-test for Equality of Means 

F Sig t df Sig. 

(2-tailed) 

Mean 

Difference 

Std. Error 

Difference 

95% Confidence Interval of 

the Difference 

Lower Upper 

Equal variances 

assumed 

3.696 .068 -2.630 22 .015 -34.872587 13.257826 -62.367636 -7.377538 

Equal variances 

not assumed 

-2.727 20.683 .013 -34.872587 12.788938 -61.493510 -8.251665 

-29­
Table A-5 

Average Truck Only Curb Idle NOx Emissions by Altitude 

Altitude Mean N Std. Deviation Median 

Low 103 82.430040 33.127697 3.264169 

High 11 84.038182 24.324821 7.334209 

Table A-6 

T-Test Statistics for Truck Only Curb Idle NOx by Altitude 

Levene's Test for Equality of 

Variances Independent Samples t-test for Equality of Means 

F Sig t df Sig. 

(2-tailed) 

Mean 

Difference 

Std. Error 

Difference 

95% Confidence Interval of 

the Difference 

Lower Upper 

Equal variances 

assumed 

2.065 .153 -.156 112 .876 -1.608142 10.289751 -21.995963 18.779680 

Equal variances 

not assumed 

-.200 14.299 .844 -1.608142 8.027791 -18.792332 15.576049 

Table A-7 

Average Truck Only Curb Idle NOx Emissions by Model Year Group 

Model Year Mean N Std. Deviation Median 

Pre-1988 9 50.475000 36.277026 12.092342 

1988 and Newer 105 85.337515 30.583874 2.984680 

Table A-8 

T-Test Statistics for Truck Only Curb Idle NOx by Model Year Group 

Levene's Test for Equality of 

Variances Independent Samples t-test for Equality of Means 

F Sig t df Sig. 

(2-tailed) 

Mean 

Difference 

Std. Error 

Difference 

95% Confidence Interval of 

the Difference 

Lower Upper 

Equal variances 

assumed 

.337 .563 -3.235 112 .002 -34.862515 10.775837 -56.213455 -13.511576 

Equal variances 

not assumed 

-2.799 9.002 .021 -34.862515 12.455242 -63.037336 -6.687695 

-30­
Table A-9 

ANOVA Statistics for Curb Idle NOx by Engine Size 

R R Square Adjusted R Square Std. Error of the Estimate 

.167 .028 .017 33.883323 

Sum of Squares df Mean Square F Significance 

Regression 2926.367 1 2926.367 2.549 .114 

Residual 102179.084 89 1148.080 

Total 105105.451 90

 Unstandardized 

Coefficients

 Standardized 

Coefficients t Sig.

 95% Confidence Interval 

for B 

B Std. Error Beta Lower Bound Upper Bound 

(Constant) 56.531 19.490 2.900 .005 17.804 95.258 

Size 2.541 1.592 .167 1.597 .114 -.621 5.703 

a Predictors: (Constant), Engine Size 

b Dependent Variable: NOx 

Table A-10 

ANOVA Statistics for Curb Idle NOx by Model Year 

R R Square Adjusted R Square Std. Error of the Estimate 

.101 .010 .002 34.5115302 

Sum of Squares df Mean Square F Significance 

Regression 1522.991 1 1522.991 1.279 .260 

Residual 148880.714 125 1191.046 

Total 150403.705 126

 Unstandardized 

Coefficients

 Standardized 

Coefficients t Sig.

 95% Confidence Interval 

for B 

B Std. Error Beta Lower Bound Upper Bound 

(Constant) -1432.999 1343.572 -1.067 .288 -4092.094 1226.097 

Year .762 .673 .101 1.131 .260 -.571 2.094 

a Predictors: (Constant), Model Year 

b Dependent Variable: NOx 

-31­
Table A-11a 

Testing Program Descriptions 

Program 

Number 

Program Description 

1 High Altitude 

Study 

"Idle Emissions from Heavy-Duty Diesel and Natural Gas Vehicles at High 

Altitude," Robert McCormick, et al, Colorado Institute for Fuels and Engine 

Research, Colorado School of Mines, Journal of the Air and Waste 

Management Association, Revised May 3, 2000. 

2 CCD Study "Study of Exhaust Emissions from Idling Heavy-duty Diesel Trucks and 

Commercially Available Idle Reducing Devices," Han Lim, US EPA Office of 

Transportation and Air Quality, September 2002. 

4 Clean Air Tech 

& IdleAire 

Study 

"NOx Emissions and Fuel Consumption of HDDVs during Extended Idle," 

David K. Irick, University of Tennessee, Bob Wilson, IdleAire Technologies 

Inc., Coordinated Research Council 12th Annual On-Road Vehicle Emission 

Workshop, San Diego, California, April 15-17, 2002. Also "Preliminary Results 

for Stationary and On-Road Testing of Diesel Trucks in Tulare, California," 

Douglas Lambert, et al, Clean Air Technologies Inc., May 15, 2002. 

5 CRC E-55 Study "Heavy-duty Vehicle Chassis Dynamometer Testing for Emissions Inventory, 

Air Quality Modeling, Source Apportionment and Air Toxics Emissions 

Inventory," Phase I Interim Report, CRC Project No. E-55/E-59, Mridul 

Gautam and Nigel N. Clark, et al, West Virginia University Research 

Corporation, July 2002. 

6 NCHRP Study "Heavy-duty Vehicle Emissions," National Cooperative Highway Research 

Program Project 25-14, Cambridge Systematics, Inc., with Battelle, Sierra 

Research and West Virginia University. October 2002. 

8 Metropolitan 

New York Study 

“Internal Report - Idle Emissions from Heavy-Duty Diesel Trucks in the New 

Your Metropolitan Area”, Tang and Munn, New York State Dept of 

Environmental Conservation, November 9, 2001. 

Table A-11b 

NOx Emissions Summarized by Testing Program

 Program 

Number 

N Mean Std. Deviation Std. Error 95% Confidence Interval for 

Mean 

Minimum Maximum 

Lower Bound Upper Bound 

1.00 24 102.927500 36.2877542 7.4072068 87.604525 118.250475 25.4400 166.0200 

2.00 5 98.207764 39.8360597 17.8152275 48.744763 147.670765 63.3462 141.6049 

4.00 49 87.088510 31.4434295 4.4919185 78.056902 96.120117 46.8921 166.9950 

5.00 14 68.033929 30.0143504 8.0216726 50.704159 85.363699 15.6950 106.7100 

6.00 2 46.921667 29.1351564 20.6016667 -214.847328 308.690661 26.3200 67.5233 

8.00 33 81.381818 34.3725313 5.9834897 69.193849 93.569788 25.2000 187.2000 

Total 127 86.303576 34.5496773 3.0657881 80.236471 92.370680 15.6950 187.2000 

-32­
Table A-11c 

Multiple Comparisons of Testing Programs 

Dependent Variable: NOx 

Bonferroni 

95% Confidence Interval 

(I) 

Program 

(J) 

Program 

Mean Difference* 

(I-J) 

Std. Error Sig. Lower Bound Upper Bound 

1.00 2.00 4.719736 16.3873777 1.000 -44.353838 53.793311 

4.00 15.838990 8.3053676 .883 -9.032229 40.710210 

5.00 34.893571 11.2104552 .035 1.322788 68.464355 

6.00 56.005833 24.5339313 .363 -17.463377 129.475043 

8.00 21.545682 8.9428496 .262 -5.234539 48.325903 

2.00 1.00 -4.719736 16.3873777 1.000 -53.793311 44.353838 

4.00 11.119254 15.6500211 1.000 -35.746235 57.984743 

5.00 30.173835 17.3671625 1.000 -21.833799 82.181469 

6.00 51.286097 27.8901024 1.000 -32.233489 134.805684 

8.00 16.825946 15.9974557 1.000 -31.079970 64.731861 

4.00 1.00 -15.838990 8.3053676 .883 -40.710210 9.032229 

2.00 -11.119254 15.6500211 1.000 -57.984743 35.746235 

5.00 19.054581 10.1020452 .925 -11.196962 49.306124 

6.00 40.166843 24.0476775 1.000 -31.846234 112.179920 

8.00 5.706691 7.5067682 1.000 -16.773046 28.186429 

5.00 1.00 -34.893571 11.2104552 .035 -68.464355 -1.322788 

2.00 -30.173835 17.3671625 1.000 -82.181469 21.833799 

4.00 -19.054581 10.1020452 .925 -49.306124 11.196962 

6.00 21.112262 25.1989279 1.000 -54.348345 96.572868 

8.00 -13.347890 10.6323443 1.000 -45.187464 18.491685 

6.00 1.00 -56.005833 24.5339313 .363 -129.475043 17.463377 

2.00 -51.286097 27.8901024 1.000 -134.805684 32.233489 

4.00 -40.166843 24.0476775 1.000 -112.179920 31.846234 

5.00 -21.112262 25.1989279 1.000 -96.572868 54.348345 

8.00 -34.460152 24.2752182 1.000 -107.154621 38.234318 

8.00 1.00 -21.545682 8.9428496 .262 -48.325903 5.234539 

2.00 -16.825946 15.9974557 1.000 -64.731861 31.079970 

4.00 -5.706691 7.5067682 1.000 -28.186429 16.773046 

5.00 13.347890 10.6323443 1.000 -18.491685 45.187464 

6.00 34.460152 24.2752182 1.000 -38.234318 107.154621 

* The mean difference is significant at the 0.05 level. 

-33­
Table A-12 

Average Curb Idle PM Emissions by Altitude and Vehicle Class 

Altitude Class Mean N Std. Deviation Median 

Low Truck 3.778967 63 4.428443 2.230947 

All 3.778967 63 4.428443 2.230947 

High Truck 1.418182 11 0.367201 1.380000 

Bus 3.110769 13 2.916523 2.220000 

All 2.335000 24 2.288833 1.410000 

Total Truck 3.428039 74 4.170067 1.996531 

Bus 3.110769 13 2.916523 2.220000 

All 3.380631 87 3.995079 2.017305 

Table A-13 

T-Test Statistics for High Altitude Curb Idle PM by Vehicle Class 

Levene's Test for Equality of 

Variances Independent Samples t-test for Equality of Means 

F Sig t df Sig. 

(2-tailed) 

Mean 

Difference 

Std. Error 

Difference 

95% Confidence Interval of 

the Difference 

Lower Upper 

Equal variances 

assumed 

11.232 .003 -1.906 22 .070 -1.692587 .888243 -3.534692 .149517 

Equal variances 

not assumed 

-2.073 12.449 .060 -1.692587 .816440 -3.464373 7.91985E-02 

-34­
Table A-14 

Curb Idle PM Statistics by Altitude 

ALTITUDE Statistic Std. Error 

Low Mean 3.778967 .557931 

95% Confidence Interval 

for Mean 

Lower Bound 2.663678 

Upper Bound 4.894256 

5% Trimmed Mean 3.206624 

Median 2.230947 

Variance 19.611 

Std. Deviation 4.428443 

Minimum .0800 

Maximum 23.2782 

Range 23.1982 

Interquartile Range 3.431699 

Skewness 2.293 .302 

Kurtosis 6.110 .595 

High Mean 2.335000 .467206 

95% Confidence Interval 

for Mean 

Lower Bound 1.368511 

Upper Bound 3.301489 

5% Trimmed Mean 2.034444 

Median 1.410000 

Variance 5.239 

Std. Deviation 2.288833 

Minimum .3600 

Maximum 10.3800 

Range 10.0200 

Interquartile Range 1.575000 

Skewness 2.425 .472 

Kurtosis 6.279 .918 

-35­
Table A-15 

Curb Idle PM Statistics by Vehicle Class 

CLASS Statistic Std. Error 

Truck Mean 3.428039 .484760 

95% Confidence Interval 

for Mean 

Lower Bound 2.461913 

Upper Bound 4.394165 

5% Trimmed Mean 2.854778 

Median 1.996531 

Variance 17.389 

Std. Deviation 4.170067 

Minimum .0800 

Maximum 23.2782 

Range 23.1982 

Interquartile Range 3.222327 

Skewness 2.552 .279 

Kurtosis 7.612 .552 

Bus Mean 3.110769 .808898 

95% Confidence Interval 

for Mean 

Lower Bound 1.348332 

Upper Bound 4.873207 

5% Trimmed Mean 2.859744 

Median 2.220000 

Variance 8.506 

Std. Deviation 2.916523 

Minimum .3600 

Maximum 10.3800 

Range 10.0200 

Interquartile Range 3.810000 

Skewness 1.514 .616 

Kurtosis 2.078 1.191 

-36­
Table A-16 

Vehicle Sample Size with Available HC Emissions 

Cases Included Excluded Total

 N Percent N Percent N Percent 

THC * Class * Altitude 122 92.4% 10 7.6% 132 100.0% 

Table A-17 

Average Curb Idle HC Emissions by Vehicle Class and Altitude 

Class Altitude Mean N Std. Deviation 

Truck Low 10.495414 98 8.393333

 High 7.690909 11 2.495281

 Total 10.212391 109 8.035518 

Bus High 10.476923 13 9.491434

 Total 10.476923 13 9.491434 

Total Low 10.495414 98 8.393333

 High 9.200000 24 7.191662

 Total 10.240578 122 8.159247 

Table A-18 

T-Test Statistics for Curb Idle THC by Vehicle Class 

Levene's Test for Equality of 

Variances Independent Samples t-test for Equality of Means 

F Sig t df Sig. 

(2-tailed) 

Mean 

Difference 

Std. Error 

Difference 

95% Confidence Interval of 

the Difference 

Lower Upper 

Equal variances 

assumed 

.750 .388 -.110 120 .913 -.264533 2.403949 -5.024184 4.495119 

Equal variances 

not assumed 

-.096 14.128 .925 -.264533 2.742658 -6.141961 5.612896 

-37­
Table A-19 

T-Test Statistics for Curb Idle THC by Altitude 

Levene's Test for Equality of 

Variances Independent Samples t-test for Equality of Means 

F Sig t df Sig. 

(2-tailed) 

Mean 

Difference 

Std. Error 

Difference 

95% Confidence Interval of 

the Difference 

Lower Upper 

Equal variances 

assumed 

.118 .732 .696 120 .488 1.295414 1.862257 -2.391725 4.982553 

Equal variances 

not assumed 

.764 39.852 .449 1.295414 1.695246 -2.131201 4.722029 

Table A-20 

ANOVA Statistics for Curb Idle HC by Engine Size 

R R Square Adjusted R Square Std. Error of the Estimate 

.036 .001 -.010 8.535066 

Sum of Squares df Mean Square F Significance 

Regression 8.329 1 8.329 .114 .736 

Residual 6337.720 87 72.847 

Total 6346.049 88

 Unstandardized 

Coefficients

 Standardized 

Coefficients t Sig.

 95% Confidence Interval 

for B 

B Std. Error Beta Lower Bound Upper Bound 

(Constant) 12.184 4.910 2.481 .015 2.425 21.944 

Size -.136 .401 -.036 -.338 .736 -.933 .662 

a Predictors: (Constant), Engine Size 

b Dependent Variable: THC 

-38­
Table A-21 

ANOVA Statistics for Curb Idle HC by Odometer Mileage 

R R Square Adjusted R Square Std. Error of the Estimate 

.087 .008 -.007 7.146260 

Sum of Squares df Mean Square F Significance 

Regression 27.220 1 27.220 .533 .468 

Residual 3574.833 70 51.069 

Total 3602.053 71

 Unstandardized 

Coefficients

 Standardized 

Coefficients t Sig.

 95% Confidence Interval 

for B 

B Std. Error Beta Lower Bound Upper Bound 

(Constant) 9.270 1.449 6.397 .000 6.380 12.160 

Miles 2.863E-06 .000 .087 .730 .468 .000 .000 

a Predictors: (Constant), Odometer Mileage 

b Dependent Variable: THC 

Table A-22 

ANOVA Statistics for Curb Idle HC by Model Year 

R R Square Adjusted R Square Std. Error of the Estimate 

.363 .132 .125 7.633352 

Sum of Squares df Mean Square F Significance 

Regression 1063.204 1 1063.204 18.247 .000 

Residual 6992.167 120 58.268 

Total 8055.371 121

 Unstandardized 

Coefficients

 Standardized 

Coefficients t Sig.

 95% Confidence Interval 

for B 

B Std. Error Beta Lower Bound Upper Bound 

(Constant) 1294.063 300.548 4.306 .000 699.000 1889.127 

Year -.644 .151 -.363 -4.272 .000 -.942 -.345 

a Predictors: (Constant), Model Year 

b Dependent Variable: THC 

-39­
Table A-23 

Average Curb Idle CO Emissions 

by Vehicle Class and Altitude 

Class Altitude Mean N Std. Deviation 

Truck Low 22.101692 98 15.182583 

High 68.078182 11 31.830396 

Total 26.741521 109 22.235472 

Bus High 81.383077 13 34.330353 

Total 81.383077 13 34.330353 

Total Low 22.101692 98 15.182583 

High 75.285000 24 33.185489 

Total 32.563982 122 29.065045 

Table A-24 

T-Test Statistics for Truck Only Curb Idle CO by Altitude 

Levene's Test for Equality of 

Variances Independent Samples t-test for Equality of Means 

F Sig t df Sig. 

(2-tailed) 

Mean 

Difference 

Std. Error 

Difference 

95% Confidence Interval of 

the Difference 

Lower Upper 

Equal variances 

assumed 

40.902 .000 -11.714 120 .000 53.183308 4.540246 -62.172678 -44.193937 

Equal variances 

not assumed 

-7.657 25.403 .000 53.183308 6.945407 -67.476156 -38.890460 

-40­
Table A-25 

T-Test Statistics for High Altitude Curb Idle CO by Vehicle Class 

Levene's Test for Equality of 

Variances Independent Samples t-test for Equality of Means 

F Sig t df Sig. 

(2-tailed) 

Mean 

Difference 

Std. Error 

Difference 

95% Confidence Interval of 

the Difference 

Lower Upper 

Equal variances 

assumed 

-.978 22 .339 -13.304895 13.608256 -41.526691 14.916900 

Equal variances 

not assumed 

-.984 21.785 .336 -13.304895 13.519105 -41.357821 14.748031 

Table A-26 

ANOVA Statistics for Curb Idle CO by Model Year 

R R Square Adjusted R Square Std. Error of the Estimate 

.377 .142 .133 14.133510 

Sum of Squares df Mean Square F Significance 

Regression 3182.964 1 3182.964 15.934 .000 

Residual 19176.587 96 199.756 

Total 22359.551 97

 Unstandardized 

Coefficients

 Standardized 

Coefficients t Sig.

 95% Confidence Interval 

for B 

B Std. Error Beta Lower Bound Upper Bound 

(Constant) 2415.728 599.642 4.029 .000 1225.449 3606.007 

Year -1.200 .301 -.377 -3.992 .000 -1.796 -0.603 

a Predictors: (Constant), Model Year 

b Dependent Variable: CO 

-41­
Appendix B

Engine Load Analysis Statistics

Table B-1 

Idle HC Emissions Versus Engine Load (g/hr) 

T-Test Paired Samples Statistics 

Pair Description Mean N Std. Deviation Std. Error Mean 

Pair 1 Curb idle with No Load 11.55440 10 4.123644 1.304011 

Pair 1 Curb idle with Heater On 11.26400 10 3.651550 1.154722 

Pair 2 Curb idle with No Load 10.73851 18 3.896791 0.918482 

Pair 2 Curb idle with A/C On 11.22044 18 3.080620 0.726109 

Pair 3 High idle with No Load 17.45338 34 8.815857 1.511907 

Pair 3 High idle with A/C On 16.67390 34 8.191705 1.404866 

Paired Sample Correlations Correlation Significance 

Pair 1 Curb Idle with Heater On 10 0.781 0.008 

Pair 2 Curb Idle with A/C On 18 0.758 0.000 

Pair 3 High Idle with A/C On 34 0.931 0.000 

Paired Differences Mean df Std. Deviation Std. Error Mean 

Pair 1 Curb Idle with Heater On -0.2904 9 2.609318 0.825139 

Pair 2 Curb Idle with A/C On -0.48193 17 2.547206 0.600382 

Pair 3 High Idle with A/C On -0.77948 33 3.219075 0.552067 

Paired Differences 95% Confidence Interval 

of the Difference 

Lower Upper t Sig. (2-tailed) 

Pair 1 Curb Idle with Heater On -2.156994 1.57619 -0.352 0.733 

Pair 2 Curb Idle with A/C On -1.74862 0.784769 -0.803 0.433 

Pair 3 High Idle with A/C On -1.90267 0.343705 -1.412 0.167 

-42­
Table B-2 

Idle CO Emissions Versus Engine Load (g/hr) 

T-Test Paired Samples Statistics 

Pair Description Mean N Std. Deviation Std. Error Mean 

Pair 1 Curb idle with No Load 28.01 10 12.58176 3.978702 

Pair 1 Curb idle with Heater On 30.184 10 14.6011 4.617274 

Pair 2 Curb idle with No Load 26.2485 18 11.27232 2.656911 

Pair 2 Curb idle with A/C On 29.77747 18 13.41937 3.162975 

Pair 3 High idle with No Load 54.63609 34 41.07124 7.043659 

Pair 3 High idle with A/C On 53.24851 34 37.30385 6.397557 

Paired Sample Correlations Correlation Significance 

Pair 1 Curb Idle with Heater On 10 0.984 0.000 

Pair 2 Curb Idle with A/C On 18 0.947 0.000 

Pair 3 High Idle with A/C On 34 0.986 0.000 

Paired Differences Mean df Std. Deviation Std. Error Mean 

Pair 1 Curb Idle with Heater On 2.174 9 3.142922 0.993879 

Pair 2 Curb Idle with A/C On 3.52896 17 4.545774 1.071449 

Pair 3 High Idle with A/C On -1.38758 33 7.518805 1.289464 

Paired Differences 95% Confidence Interval 

of the Difference 

Lower Upper t Sig. (2-tailed) 

Pair 1 Curb Idle with Heater On -0.074311 4.42231 2.187 0.056 

Pair 2 Curb Idle with A/C On 1.2684 5.78952 3.294 0.004 

Pair 3 High Idle with A/C On -4.01102 1.235855 -1.076 0.290 

-43­
Table B-3 

Idle NOx Emissions Versus Engine Load (g/hr) 

T-Test Paired Samples Statistics 

Pair Description Mean N Std. Deviation Std. Error Mean 

Pair 1 Curb idle with No Load 127.754 13 36.296 10.067 

Pair 1 Curb idle with Heater On 122.458 13 36.373 10.088 

Pair 2 Curb idle with No Load 125.430 22 32.433 6.915 

Pair 2 Curb idle with A/C On 155.199 22 38.982 8.311 

Pair 3 High idle with No Load 150.600 5 64.682 28.927 

Pair 3 High idle with Heater On 156.800 5 80.260 35.893 

Pair 4 High idle with No Load 126.468 39 52.893 8.470 

Pair 4 High idle with A/C On 159.960 39 77.232 12.367 

Paired Sample Correlations Correlation Significance 

Pair 1 Curb Idle with Heater On 13 0.566 0.044 

Pair 2 Curb Idle with A/C On 22 0.835 0.000 

Pair 3 High Idle with Heater On 5 0.801 0.104 

Pair 4 High Idle with A/C On 39 0.855 0.000 

Paired Differences Mean df Std. Deviation Std. Error Mean 

Pair 1 Curb Idle with Heater On -5.296 12 33.843 9.386 

Pair 2 Curb Idle with A/C On 29.769 21 21.449 4.573 

Pair 3 High Idle with Heater On 6.200 4 48.080 21.502 

Pair 4 High Idle with A/C On 33.492 38 42.121 6.745 

Paired Differences 95% Confidence Interval 

of the Difference 

Lower Upper t Sig. (2-tailed) 

Pair 1 Curb Idle with Heater On -25.747 15.154 -0.564 0.583 

Pair 2 Curb Idle with A/C On 20.259 39.279 6.510 0.000 

Pair 3 High Idle with Heater On -53.499 65.899 0.288 0.787 

Pair 4 High Idle with A/C On 19.838 47.147 4.966 0.000 

-44­
Table B-4 

Idle PM Emissions Versus Engine Load (g/hr) 

T-Test Paired Samples Statistics 

Pair Description Mean N Std. Deviation Std. Error Mean 

Pair 1 Curb idle with No Load 6.588975 10 5.060315 1.600212 

Pair 1 Curb idle with Heater On 5.661299 10 3.289959 1.040376 

Pair 2 Curb idle with No Load 6.846186 11 4.875843 1.470122 

Pair 2 Curb idle with A/C On 6.333916 11 3.516132 1.060154 

Pair 3 High idle with No Load 4.513723 34 6.044245 1.03658 

Pair 3 High idle with A/C On 4.475141 34 6.336543 1.086708 

Paired Sample Correlations Correlation Significance 

Pair 1 Curb Idle with Heater On 10 0.829 0.003 

Pair 2 Curb Idle with A/C On 11 0.887 0.000 

Pair 3 High Idle with A/C On 34 0.987 0.000 

Paired Differences Mean df Std. Deviation Std. Error Mean 

Pair 1 Curb Idle with Heater On -0.927676 9 2.973875 0.940422 

Pair 2 Curb Idle with A/C On -0.512271 10 2.391163 0.720963 

Pair 3 High Idle with A/C On 0.03858 33 1.049061 0.179912 

Paired Differences 95% Confidence Interval 

of the Difference 

Lower Upper t Sig. (2-tailed) 

Pair 1 Curb Idle with Heater On -3.055058 1.19971 -0.986 0.350 

Pair 2 Curb Idle with A/C On -2.118676 1.09413 -0.711 0.494 

Pair 3 High Idle with A/C On -0.327453 0.40462 0.214 0.832 

-45­
Appendix C

Engine Idle Speed Analysis Statistics

Table C-1 

Idle HC Emissions Versus Idle Speed (g/hr) 

T-Test Paired Samples Statistics 

Pair Description Mean N Std. Deviation Std. Error Mean 

Pair 1 Curb idle with No Load 9.555759 34 4.395221 0.753774 

Pair 1 High idle with No Load 17.45338 34 8.815857 1.511907 

Pair 2 Curb idle with Heater On 11.264 10 3.65155 1.154722 

Pair 2 High idle with Heater On 20.0038 10 7.443559 2.35386 

Pair 3 Curb idle with A/C On 11.22044 18 3.08062 0.726109 

Pair 3 High idle with A/C On 17.44223 18 7.567212 1.783609 

Paired Sample Correlations Correlation Significance 

Pair 1 Speed with No Load 34 0.714 0.000 

Pair 2 Speed with Heater On 10 0.732 0.016 

Pair 3 Speed with A/C On 18 0.687 0.002 

Paired Differences Mean df Std. Deviation Std. Error Mean 

Pair 1 Speed with No Load 7.897625 33 6.460725 1.108005 

Pair 2 Speed with Heater On 8.739800 9 5.379976 1.701298 

Pair 3 Speed with A/C On 6.221792 17 5.893289 1.389061 

Paired Differences 95% Confidence Interval 

of the Difference 

Lower Upper t Sig. (2-tailed) 

Pair 1 Speed with No Load 5.643372 10.15188 7.128 0.000 

Pair 2 Speed with Heater On 4.891197 12.58840 5.137 0.001 

Pair 3 Speed with A/C On 3.291129 9.152456 4.479 0.000 

-46­
Table C-2 

Idle CO Emissions Versus Idle Speed (g/hr) 

T-Test Paired Samples Statistics 

Pair Description Mean N Std. Deviation Std. Error Mean 

Pair 1 Curb idle with No Load 17.22332 34 8.772249 1.504428 

Pair 1 High idle with No Load 54.63609 34 41.07124 7.043659 

Pair 2 Curb idle with Heater On 30.184 10 14.6011 4.617274 

Pair 2 High idle with Heater On 73.742 10 29.07419 9.194065 

Pair 3 Curb idle with A/C On 29.77747 18 13.41937 3.162975 

Pair 3 High idle with A/C On 67.15153 18 38.21423 9.00718 

Paired Sample Correlations Correlation Significance 

Pair 1 Speed with No Load 34 0.630 0.000 

Pair 2 Speed with Heater On 10 0.751 0.012 

Pair 3 Speed with A/C On 18 0.724 0.001 

Paired Differences Mean df Std. Deviation Std. Error Mean 

Pair 1 Speed with No Load 37.41277 33 36.194 6.20722 

Pair 2 Speed with Heater On 43.558 9 20.51483 6.48736 

Pair 3 Speed with A/C On 37.37407 17 29.96987 7.063967 

Paired Differences 95% Confidence Interval 

of the Difference 

Lower Upper t Sig. (2-tailed) 

Pair 1 Speed with No Load 24.78409 50.04146 6.027 0.000 

Pair 2 Speed with Heater On 28.88257 58.23343 6.714 0.000 

Pair 3 Speed with A/C On 22.4704 52.27773 5.291 0.000 

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Table C-3 

Idle NOx Emissions Versus Idle Speed (g/hr) 

T-Test Paired Samples Statistics 

Pair Description Mean N Std. Deviation Std. Error Mean 

Pair 1 Curb idle with No Load 81.12896 38 33.21055 5.387463 

Pair 1 High idle with No Load 126.1909 38 53.57418 8.690879 

Pair 2 Curb idle with Heater On 120.6394 14 35.60214 9.515073 

Pair 2 High idle with Heater On 160.8317 14 48.30586 12.91028 

Pair 3 Curb idle with A/C On 154.5816 23 38.20029 7.965311 

Pair 3 High idle with A/C On 204.9348 23 58.08001 12.11052 

Paired Sample Correlations Correlation Significance 

Pair 1 Speed with No Load 38 0.711 0.000 

Pair 2 Speed with Heater On 14 0.628 0.016 

Pair 3 Speed with A/C On 23 0.492 0.017 

Paired Differences Mean df Std. Deviation Std. Error Mean 

Pair 1 Speed with No Load 45.06191 37 37.98222 6.161529 

Pair 2 Speed with Heater On 40.19236 13 37.94282 10.14065 

Pair 3 Speed with A/C On 50.35326 22 51.47647 10.73359 

Paired Differences 95% Confidence Interval 

of the Difference 

Lower Upper t Sig. (2-tailed) 

Pair 1 Speed with No Load 32.57747 57.54635 7.313 0.000 

Pair 2 Speed with Heater On 18.28482 62.09989 3.963 0.002 

Pair 3 Speed with A/C On 28.09316 72.61335 4.691 0.000 

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Table C-4 

Idle PM Emissions Versus Idle Speed (g/hr) 

T-Test Paired Samples Statistics 

Pair Description Mean N Std. Deviation Std. Error Mean 

Pair 1 Curb idle with No Load 3.536667 34 4.831247 0.828552 

Pair 1 High idle with No Load 4.513723 34 6.044245 1.03658 

Pair 2 Curb idle with Heater On 5.661299 10 3.289959 1.040376 

Pair 2 High idle with Heater On 8.196625 10 5.722588 1.809641 

Pair 3 Curb idle with A/C On 6.333916 11 3.516132 1.060154 

Pair 3 High idle with A/C On 9.301994 11 6.954408 2.096833 

Paired Sample Correlations Correlation Significance 

Pair 1 Speed with No Load 34 0.353 0.040 

Pair 2 Speed with Heater On 10 0.628 0.052 

Pair 3 Speed with A/C On 11 0.876 0.000 

Paired Differences Mean df Std. Deviation Std. Error Mean 

Pair 1 Speed with No Load 0.977056 33 6.264895 1.074421 

Pair 2 Speed with Heater On 2.535326 9 4.463751 1.411562 

Pair 3 Speed with A/C On 2.968078 10 4.229222 1.275159 

Paired Differences 95% Confidence Interval 

of the Difference 

Lower Upper t Sig. (2-tailed) 

Pair 1 Speed with No Load -1.20887 3.162981 0.909 0.370 

Pair 2 Speed with Heater On -0.65785 5.728501 1.796 0.106 

Pair 3 Speed with A/C On 0.126848 5.809309 2.328 0.042 

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Appendix D

Discretionary Idling Particulate Matter Statistics

Table D-1 

Oak Ridge Study Idle PM Emissions Measurements 

Vehicle Description Temperature (F) Load Type PM (g/hr) 

1998 Freightliner with Cummins N14 0 Heater on 5.603 

1998 Freightliner with Cummins N14 65 No load 3.136 

1998 Freightliner with Cummins N14 90 A/C on 2.946 

1992 Ford with Caterpillar 3406 0 Heater on 6.885 

1992 Ford with Caterpillar 3406 65 No load 20.386 

1992 Ford with Caterpillar 3406 90 A/C on 20.574 

1999 Volvo truck, DDC series 60 0 Heater on 4.790 

1999 Volvo truck, DDC series 60 65 No load 3.904 

1999 Volvo truck, DDC series 60 90 A/C on 5.061 

Freightliner, DDC Series 60 0 Heater on 8.208 

Freightliner, DDC Series 60 65 No load 2.480 

Freightliner, DDC Series 60 90 A/C on 3.312 

Exact97 International Caterpillar 0 Heater on 3.347 

Exact97 International Caterpillar 65 No load 2.200 

Exact97 International Caterpillar 90 A/C on 1.439 

Table D-1 

Oak Ridge Study Idle PM Emissions Measurements 

(Excluding the 1992 Ford) 

Temperature (F) N Mean Minimum Maximum Std. Deviation Weighting 

0 4 5.487167 3.34689 8.20828 2.039957 0.250 

65 4 2.930185 2.20022 3.90439 0.758637 0.333 

90 4 3.18931 1.43929 5.06079 1.487654 0.417 

Total 12 3.868887 1.43929 8.20828 1.826509 1.000 

Wgt. Std. Error 

Seasonal Weighting 3.677399 0.42709 

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