United States

Environmental Protection

Agency

Air and Radiation EPA420-P-98-015

May 1998

Update Heavy-Duty

Engine Emission

Conversion Factors for

MOBILE6:

Analysis of BSFCs and

Calculation of HeavyDuty Engine Emission

Conversion Factors

 Printed on Recycled Paper
05 May 1998

P R E P A R E D F O R

U.S. Environmental Protection Agency

Motor Vehicle Emissions Laboratory

2565 Plymouth Road

Ann Arbor, Michigan 48105

UPDATE HEAVY-DUTY

ENGINE EMISSION

CONVERSION FACTORS

FOR MOBILE6

Analysis of BSFCs and Calculation of

Heavy-Duty Engine Emission

Conversion Factors
Prepared for:

U.S. Environmental Protection Agency

Motor Vehicle Emissions Laboratory

2565 Plymouth Road

Ann Arbor, Michigan 48105

Prepared by:

Louis Browning

ARCADIS Geraghty & Miller, Inc.

555 Clyde Avenue

P.O. Box 7044

Mountain View

California 94039

Tel 650 961 5700

Fax 650 254 2496

Our Ref.:

SJ007258

Date:

05 May 1998

Update Heavy-Duty Engine

Emission Conversion

Factors for MOBILE6

Analysis of BSFCs and

Calculation of Heavy-Duty

Engine Emission Conversion

Factors
This report and the information and data described herein have been funded by the USEPA under

Contract 68-C6-0068, Work Assignments #0-03 and 1-02. It is being released for information purposes

only. It may not reflect the views and positions of the USEPA on the topics and issues discussed, and

no official endorsement by USEPA of the report or its conclusions should be inferred.

This report has not been peer or administratively reviewed.
 Numbers in brackets refer to references in Section VII of this report. 1

1

I. INTRODUCTION

The USEPA highway emission factor model, MOBILE5a, calculates average in-use emission

factors for hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NO ) for eight x

categories of vehicles including heavy-duty gasoline (HDGV) and heavy-duty diesel (HDDV) vehicles

(all vehicles with a gross vehicle weight of 8501 pounds or more). These emission factors are

expressed in units of grams per mile (g/mi) and are used in combination with data on vehicle miles

traveled (VMT) to estimate highway vehicle contributions to mobile source emission inventories.

However, since emission standards for both gasoline and diesel heavy-duty vehicles are expressed in

terms of grams per brake-horsepower-hour (g/bhp-hr), conversion factors in terms of brakehorsepower-hour per mile (bhp-hr/mi) must be used to convert the emission certification data from

engine testing to in-use grams per mile. These conversion factors have been calculated several times

over the last 15 years with the last update completed by EPA in 1988 for all heavy-duty vehicles [1] .1

The conversion factors used in MOBILE5a were calculated from the following expression:

Fuel Density (lb/gal) Conversion Factor (bhp-hr/mi) = --------------------------------------------------------- (I.1)

 BSFC (lb/bhp-hr) x Fuel Economy (mi/gal)

where BSFC is brake-specific fuel economy.

There are two approaches for determining inputs to the above equation. One is to use brake

specific fuel consumption (BSFC) and fuel economy for the in-use driving cycle to calculate the

conversion factor. This would assume that the emissions factors in terms of grams of pollutant per unit

work (g/bhp-hr) is only a function of the work required to move a truck or bus one mile, no matter how

that mile is driven. This is clearly not the case for any of the pollutants as shown by Kitchen and

Damico [2] and Brown et al. [3]. Kitchen and Damico studied several bus engines on both an engine

dynamometer and in a bus on a chassis dynamometer over several different driving cycles. They found

for all emissions that the conversion factors increased with increasingly heavier duty cycles. Brown

et al. computed conversion factors for in-use class 8 heavy-duty trucks and found similar results.

The second approach is to use BSFC for the certification test cycle in which the emissions

factors were generated and fuel economy for the in-use duty cycle. Historically, modelers have used

BSFC from the certification test cycle because it was readily available from certification records. 

Dividing the emission rates by BSFC give emissions in terms of grams of pollutant per pound of fuel.

As shown by Dreher and Harley [4], emissions generally vary less with duty cycle when expressed in

these terms. Dividing this factor by fuel economy from the in-use driving cycle and multiplying by

fuel density (as is done in the above equation) gives a more accurate conversion factor for different

heavy-duty engine duty cycles. This is the approach that has been utilized in this study.

There are also some issues with the available fuel economy data for in-use trucks, namely the

1992 Truck Inventory and Use Survey (TIUS) [5]. If the fuel economy data represents a different
2

driving cycle than is actually used on the road, there can be an error in the conversion factor calculated.

This effect has been documented by Sierra Research [6].

The current TIUS data represents model year 1992 and earlier engines. However, the trend

since the 1994 model year has been a large movement in diesel engines to sophisticated electronic

control. These engines generally have a different NO -fuel economy trade-off than previous x

mechanically injected engines. Almost all on-highway diesel engines will be electronically controlled

by 1998. Projecting conversion factors using older fuel economy data can present errors.

The conversion factors previously calculated for use in MOBILE5 and the ones calculated in

this report are most likely only reasonable estimates of in-use NO emissions. Emissions of CO and x

particulates (PM) are less a function of the force required to drive a truck or bus (bhp-hr) than the

frequency and severity of the transients in the duty cycle. While emissions of HC are not directly

related to transients, they also are not a direct function of engine load. Having a different in-use duty

cycle from the one used to generate emissions profiles on the engine dynamometer could result in very

different conversion factors for each pollutant. This point was demonstrated for transit buses by

Kitchen and Damico [2].

The best approach for determining conversion factors would be to develop in-use driving cycles

and then test a statistically significant number of trucks and buses over those cycles to determine

conversion factors for each pollutant and driving cycle. However, due to the significant resources

required in terms of both time and money, and the limited availability of chassis dynamometer testing

capability for heavy-duty trucks and buses, use of the present methodology employed in MOBILE5 as

updated with newer data should, at least, provide reasonable estimates of in-use truck and bus NOx

emissions [6].

With the above caveats in mind, this report updates conversion factors used in MOBILE5 for

all weight classes listed in Table 1. Since the most recent previous analysis and calculation of

conversion factors [1] was based on actual data only through the 1986 model year, it is the purpose of

this work to calculate conversion factors for model years 1987 through 1996 and project conversion

factors from 1997 through 2050.

This report discusses the calculation of average engine brake-specific fuel consumption(BSFC)

for model years 1987 through 1996 and calculates conversion factors for all weight classes listed in

Table 1. In addition, it projects conversion factors for years 1997 through 2050. Calculation of fuel

economy, non-engine fuel economy improvements and fuel density was detailed in a separate report

[7]. 

II. CALCULATION OF CLASS SPECIFIC BSFCs by MODEL YEAR

To calculate average BSFCs for each category listed in Table 1, data on engine family specific

BSFC for model years 1987 through 1996 were requested from eight engine manufacturers (three

gasoline and five diesel). Six manufacturers supplied data for analysis. BSFCs for other

manufacturers’ engines were estimated using the data obtained from the six manufacturers for similar

engines based upon the engine horsepower, engine specifications (determined from the engine family

codes) and engineering knowledge of the various engine families. Engine family sales data for 1988
3

through 1995 was obtained from USEPA and used to weight the BSFCs. Sales data were first

categorized into weight classes using manufacturer suggestions, engine horsepower and actual vehicle

populations for each model year [8]. Engine family BSFCs were then weighted by sales fractions in

each category listed in Table 1. BSFC for the certification cycle was used for all weight classes. Since

the individual engine BSFCs and sales data was proprietary, it is not reproduced in this report.

Sales-weighted BSFC for all diesel truck weight classes, calculated as discussed above, are

shown in Table 2. Sales-weighted BSFC for all gasoline truck weight classes are shown in Table 3.

TIUS provided no data for class 8B gasoline trucks and therefore no BSFC or conversion factor for that

class are calculated.

Table 1. Vehicle weight classes

Designation Description Weight (lb)

Gross Vehicle

HDGV (class 2B) Light heavy-duty gasoline vehicles 8501-10,000

HDGV (class 3) Light heavy-duty gasoline vehicles 10,001-14,000

HDGV (class 4) Heavy heavy-duty gasoline vehicles 14,001-16,000

HDGV (class 5) Heavy heavy-duty gasoline vehicles 16,001-19,500

HDGV (class 6) Heavy heavy-duty gasoline vehicles 19,501-26,000

HDGV (class 7) Heavy heavy-duty gasoline vehicles 26,001-33,000

HDGV (class 8A) Heavy heavy-duty gasoline vehicles 33,001-60,000

HDGV (class 8B) Heavy heavy-duty gasoline vehicles >60,000

HDGTB Gasoline transit buses all

HDGSB Gasoline school buses all

HDGCB Gasoline intercity buses all

HDDV (class 2B) Light heavy-duty diesel trucks 8501-10,000

HDDV (class 3) Light heavy-duty diesel trucks 10,001-14,000

HDDV (class 4) Light heavy-duty diesel trucks 14,001-16,000

HDDV (class 5) Light heavy-duty diesel trucks 16,001-19,500

HDDV (class 6) Medium heavy-duty diesel trucks 19,501-26,000

HDDV (class 7) Medium heavy-duty diesel trucks 26,001-33,000

HDDV (class 8A) Heavy heavy-duty diesel trucks 33,001-60,000

HDDV (class 8B) Heavy heavy-duty diesel trucks >60,000

HDDTB Diesel transit buses all

HDDSB Diesel school buses all

HDDCB Diesel intercity buses all

In order to weight the BSFC data for bus engines, population data was taken from a number of

sources. Transit bus engine populations for model years 1987 through 1995 were taken from the
4

APTA 1995 Transit Passenger Vehicle Fleet Inventory [9] and are shown in Table 4 for diesel buses

and Table 5 for gasoline buses. School bus counts of vehicle sizes for model years 1990 through 1996

were taken from School Bus Fleet 1997 Fact Book [10] and are shown in Table 6. Intercity bus diesel

engine assumptions by model year based upon conversations with bus manufacturers are shown in

Table 7. Gasoline intercity bus engines were assumed to be equally split among the “big three”

gasoline engine manufacturers, Chrysler, Ford and General Motors. Population-weighted BSFCs for

the three bus classes are shown in Table 8.

Table 2. Sales-weighted BSFC for diesel trucks

(lb/bhp-hr)

Model Weight Class

Year 2B 3 4 5 6 7 8A 8B

1988 0.553 0.544 0.478 0.465 0.444 0.414 0.403 0.395 

1989 0.536 0.528 0.508 0.460 0.432 0.410 0.397 0.385 

1990 0.545 0.535 0.504 0.453 0.432 0.397 0.397 0.375 

1991 0.504 0.491 0.485 0.460 0.416 0.397 0.388 0.385 

1992 0.494 0.491 0.450 0.450 0.450 0.395 0.400 0.407 

1993 0.527 0.521 0.498 0.469 0.418 0.430 0.429 0.387 

1994 0.516 0.500 0.490 0.444 0.431 0.415 0.392 0.371 

1995 0.511 0.504 0.502 0.467 0.427 0.426 0.392 0.373 

Table 3. Sales-weighted BSFC for gasoline trucks

(lb/bhp-hr)

Model Weight Class

Year 2B 3 4 5 6 7 8A

1988 0.611 0.626 0.642 0.640 0.642 0.640 0.638 

1989 0.614 0.613 0.627 0.627 0.641 0.644 0.616 

1990 0.607 0.610 0.611 0.607 0.638 0.639 0.621 

1991 0.602 0.602 0.602 0.601 0.600 0.599 0.598 

1992 0.588 0.595 0.604 0.606 0.602 0.600 0.600 

1993 0.570 0.577 0.589 0.596 0.597 0.600 0.600 

1994 0.570 0.587 0.608 0.607 0.604 0.602 0.600 

1995 0.565 0.585 0.591 0.587 0.589 0.578 0.557 

Table 4. Diesel transit bus inventory by engine type

(U.S. in-service population)
5

Model DDC Cummins Other

Year Series 50 6V-92 8V-92 L-10 Engines

1987 2189 33 355 238

1988 1826 5 683 142

1989 2983 102 239 96

1990 2910 34 1087 204

1991 1979 1 189 180

1992 1394 50 365 78

1993 257 1473 12 361 148

1994 1604 243 11 603 28

1995 1370 200 333 21

Table 5. Gasoline transit bus inventory by engine manufacturer

(U.S. in-service population)

Model Manufacturer

Year Chrysler Ford GM

87 3 1

88 2

89 1 12

90 19 7

91 24

92 7

93 4

94 36 3
 Types A & B are generally smaller school buses with the engine in the front. Types C 2

and D are generally larger school buses, Type C has a front engine and Type D has an engine in

the rear or midship.

6

Table 6. School bus inventory by bus type2

(U.S. in-service population)

Diesel Gasoline

MY A&B C D A&B

90 2225 23670 6286 3575

91 3756 21370 6864 3554

92 3820 16444 5444 2856

93 3535 18928 6734 3244

94 3215 21005 7321 3504

95 2216 20861 9671 3638

96 2225 22016 9270 3723

Table 7. Intercity diesel bus engine assumptions by model year

(% of U.S. in-service population)

Model DDC Cummins

Year Series 60 6V-92TA 8V-92TA L-10

1987 60% 30% 10%

1988 60% 30% 10%

1989 60% 30% 10%

1990 60% 30% 10%

1991 60% 30% 10%

1992 60% 30% 10%

1993 15% 60% 15% 10%

1994 50% 30% 10% 10%

1995 75% 15% 10%
 Sales data was only available for model years 1988 through 1995. 3

7

Table 8. Sales- weighted bus BSFC

(lb/bhp-hr)

Model Diesel Gasoline

Year Transit Intercity School Transit Intercity School

1988 0.427 0.427 0.550 0.610 

1989 0.451 0.451 0.600 0.601 

1990 0.432 0.432 0.421 0.615 0.604 0.600

1991 0.438 0.438 0.420 0.598 0.595 0.595

1992 0.447 0.447 0.410 0.598 0.585 0.590

1993 0.440 0.440 0.407 0.541 0.569 0.585

1994 0.399 0.399 0.396 0.544 0.568 0.580

1995 0.402 0.407 0.391 0.569 0.575

A regression analysis was performed for BSFCs by model year for each weight class and a

logarithmic curve (y = a + b*ln(x)) was used to extrapolate values prior to 1988 and after 1995 . These 3

curves are shown in Table 9. Curve fit BSFCs for diesel trucks are shown in Table 10 and BSFCs used

for determining conversion factors for 1987 in MOBILE5 [1] shown in Table 11. As shown by this

comparison, the curve fits produced reasonable values when compared to MOBILE5 estimates.

Table 9. Curve fit equations for BSFCs by weight class and fuel

Class Gasoline Diesel

2B y = -0.7211*ln(x) + 3.8473 y = -0.4806*ln(x) + 2.6959

3 y = -0.5656*ln(x) + 3.1535 y = -0.5183*ln(x) + 2.8529

4 y = -0.5583*ln(x) + 3.1319 y = -0.1780*ln(x) + 1.2897

5 y = -0.5435*ln(x) + 3.0630 y = -0.0349*ln(x) + 0.6162

6 y = -0.7339*ln(x) + 3.9284 y = -0.1706*ln(x) + 1.1985

7 y = -0.8224*ln(x) + 4.3266 y = -0.0863*ln(x) + 0.7854

8A y = -0.7681*ln(x) + 4.0725 y = -0.1141*ln(x) + 0.9107

8B N/A y = -0.2003*ln(x) + 1.2858

Transit y = -0.8652*ln(x) + 4.4842 y = -0.5058*ln(x) + 2.7092

Intercity y = -0.4951*ln(x) + 2.8221 y = - 0.3648*ln(x) + 2.0764

School y = -0.4648*ln(x) + 2.6918 y = -0.5311*ln(x) + 2.8123

y = BSFC (lb/bhp-hr)

x = MY - 1900
8

Table 10. Curve fit diesel truck BSFC

(lb/bhp-hr)

Model Weight Class

Year 2B 3 4 5 6 7 8A 8B

1987 0.550 0.538 0.495 0.460 0.437 0.400 0.401 0.391 

1988 0.544 0.532 0.493 0.460 0.435 0.399 0.400 0.389 

1989 0.539 0.526 0.491 0.460 0.433 0.398 0.399 0.387 

1990 0.533 0.521 0.489 0.459 0.431 0.397 0.397 0.384 

1991 0.528 0.515 0.487 0.459 0.429 0.396 0.396 0.382 

1992 0.523 0.509 0.485 0.458 0.427 0.395 0.395 0.380 

1993 0.518 0.504 0.483 0.458 0.425 0.394 0.394 0.378 

1994 0.512 0.498 0.481 0.458 0.423 0.393 0.392 0.376 

1995 0.507 0.493 0.479 0.457 0.422 0.392 0.391 0.374 

1996 0.502 0.487 0.477 0.457 0.420 0.391 0.390 0.372 

Table 11. MOBILE5 1987 diesel truck BSFC

(lb/bhp-hr)

Weight Class

2B 3-5 6 7 8A 8B

0.54 0.51 0.45 0.44 0.41 0.39

Curve fit BSFCs for gasoline trucks are shown in Table 12 and estimated BSFCs from

MOBILE5 for 1987 model year [1] are shown in Table 13. As shown by this comparison, these curve

fits also produced reasonable values when compared to MOBILE5 estimates.

Curve fit bus BSFCs are shown in Table 14 for both diesel and gasoline buses for model years

1987 to 1996. BSFCs used for MOBILE5 conversion factors [1] for 1987 buses are shown in Table

15.
9

Table 12. Curve fit gasoline truck BSFCs

(lb/bhp-hr)

Model Weight Class

Year 2B 3 4 5 6 7 8A

1987 0.627 0.628 0.638 0.636 0.651 0.654 0.642 

1988 0.619 0.621 0.631 0.630 0.642 0.644 0.633 

1989 0.611 0.615 0.625 0.624 0.634 0.635 0.625 

1990 0.602 0.608 0.618 0.618 0.626 0.626 0.616 

1991 0.595 0.602 0.612 0.612 0.618 0.617 0.608 

1992 0.587 0.596 0.606 0.606 0.610 0.608 0.599 

1993 0.579 0.590 0.600 0.600 0.602 0.599 0.591 

1994 0.571 0.584 0.594 0.594 0.594 0.590 0.583 

1995 0.563 0.578 0.588 0.588 0.586 0.581 0.575 

1996 0.556 0.572 0.582 0.582 0.579 0.573 0.567 

Table 13. MOBILE5 1987 gasoline truck BSFC

(lb/bhp-hr)

Weight Class

2B 3-5 6 7 8A

0.62 0.62 0.66 0.65 0.63

Table 14. Curve fit bus BSFCs

(lb/bhp-hr)

Model Diesel Gasoline

Year Transit Intercity School Transit Intercity School

1987 0.450 0.447 0.424 0.620 0.611 0.616

1988 0.445 0.443 0.423 0.610 0.605 0.611

1989 0.439 0.439 0.422 0.601 0.600 0.604

1990 0.433 0.435 0.421 0.591 0.594 0.600

1991 0.428 0.431 0.420 0.581 0.589 0.595

1992 0.422 0.427 0.411 0.572 0.583 0.590

1993 0.417 0.423 0.404 0.563 0.578 0.585

1994 0.411 0.419 0.398 0.553 0.573 0.580

1995 0.406 0.415 0.391 0.544 0.567 0.575

1996 0.401 0.411 0.384 0.535 0.562 0.570

Table 15. MOBILE5 1987 bus BSFC
10

(lb/bhp-hr)

Diesel Gasoline

Transit Intercity School Transit Intercity School

0.479 0.467 0.444 -- -- 0.660 a a

 No sales were assumed for transit and school buses past 1980 a

BSFCs predicted for both diesel and gasoline buses had lower values than those used for

calculation of conversion factors for MOBILE5. Since the transit bus information in MOBILE5 was

based upon data for an older DDC 6V-92TA and a DDC 6V-71N, it is assumed that the newer more

complete data on bus engine BSFC and in-use populations used in this analysis provide a more accurate

picture of transit bus BSFC. While there is no mention of how BSFCs were calculated for other buses

(intercity and school) in Machiele’s report [1], it is also assumed that this analysis provided more

complete data for those classes as well.

III. FUEL ECONOMY

Average truck fuel economy and use of non-engine fuel economy improvement devices were

calculated using the 1992 Truck Inventory and Use Survey (TIUS) Microdata File [5]. Details of those

calculations as well as bus fuel economy calculations can be found in Reference 7, which is a

companion report for this work assignment. Curve fit diesel truck fuel economies are shown in Table

16. Fuel economies used for conversion factors in MOBILE5 for diesel trucks for 1987 (using 1992

estimated fuel economy and annual fuel economy improvement tables from Reference 1) are shown

in Table 17. Average gasoline truck fuel economies from Reference 7 are shown in Table 18.

MOBILE5 gasoline truck fuel economies for 1987 are shown in Table 19. Average bus fuel economies

from Reference 7 are shown in Table 20 and MOBILE5 bus fuel economies for 1987 are shown in

Table 21.

Estimated fuel economies for 1987 Class 2B diesel trucks derived in this study are significantly

lower than the previous estimates used in MOBILE5. It is believed that this a result of TIUS not

directly differentiating between Class 2A and Class 2B, and without doing an analysis similar to what

was done for this study, higher mileage Class 2A vehicles would be averaged with lower mileage Class

2B vehicles. This study used vehicle weight to separate the two subclasses. The other difference in

fuel economy (beyond the estimate in MOBILE5 that diesel vehicles in Classes 3-5 did not exist in this

time period) is that the Class 8 trucks had better fuel economy than previously estimated in MOBILE5.

A significant improvement in fuel economy has been seen in this class between 1982 and 1987 not

previously accounted for in MOBILE5 estimates. Even though fuel economy was not calculated or

used in this study beyond 1996, it is expected that fuel economy improvements due to electronic

controls will result in even further improvements in fuel economy by 1998 in class 8 trucks.
11

Table 16. Diesel truck fuel economy taken from Reference 7

(miles per gallon)

Model Weight Class

Year 2B 3 4 5 6 7 8A 8B

1987 11.69 10.52 9.56 9.12 8.20 7.43 5.96 5.51

1988 11.83 10.65 9.63 9.21 8.25 7.44 6.03 5.59

1989 11.97 10.77 9.70 9.29 8.31 7.45 6.10 5.68

1990 12.11 10.90 9.77 9.38 8.37 7.46 6.17 5.77

1991 12.26 11.03 9.85 9.46 8.42 7.47 6.24 5.86

1992 12.40 11.15 9.92 9.54 8.48 7.48 6.31 5.95

1993 12.54 11.28 9.99 9.63 8.54 7.49 6.38 6.03

1994 12.68 11.41 10.06 9.71 8.59 7.51 6.45 6.12

1995 12.82 11.53 10.13 9.80 8.65 7.52 6.52 6.21

1996 12.96 11.66 10.20 9.88 8.71 7.53 6.59 6.30

Table 17. MOBILE5 1987 diesel truck fuel economy

(miles per gallon)

Weight Class

2B 3-5 6 7 8A 8B

 14.33 -- 8.47 7.60 5.67 5.41 a

 No sales were assumed in classes 3-5 after 1976 a

Table 18. Gasoline truck fuel economy taken from Reference 7

(miles per gallon) 

Model Weight Class

Year 2B 3 4 5 6 7 8A

1987 9.22 8.54 8.32 7.52 7.23 6.83 6.39

1988 9.32 8.63 8.43 7.58 7.33 6.89 6.47

1989 9.42 8.73 8.55 7.63 7.43 6.96 6.54

1990 9.52 8.82 8.66 7.68 7.53 7.03 6.62

1991 9.62 8.92 8.78 7.74 7.63 7.10 6.70

1992 9.73 9.01 8.89 7.79 7.73 7.17 6.77

1993 9.83 9.11 9.01 7.85 7.84 7.24 6.85

1994 9.93 9.20 9.12 7.90 7.94 7.31 6.92

1995 10.03 9.30 9.24 7.95 8.04 7.38 7.00

1996 10.13 9.39 9.35 8.01 8.14 7.45 7.07

Table 19. MOBILE5 1987 gasoline truck fuel economy
12

(miles per gallon)

Weight Class

2B 3-5 6 7 8A

 11.75 6.65 6.70 5.29 5.50

Table 20. Curve fit bus fuel economy from Reference 7

(miles per gallon)

Model Diesel Gasoline

Year Transit Intercity School Transit Intercity School

1987 3.43 4.64 6.29 3.11 3.64 6.18

1988 3.47 4.69 6.28 3.15 3.68 6.21

1989 3.51 4.75 6.27 3.19 3.72 6.24

1990 3.55 4.80 6.25 3.22 3.76 6.27

1991 3.59 4.85 6.24 3.26 3.80 6.30

1992 3.63 4.91 6.23 3.30 3.85 6.33

1993 3.67 4.96 6.22 3.33 3.89 6.37

1994 3.71 5.01 6.20 3.37 3.93 6.40

1995 3.75 5.07 6.19 3.40 3.97 6.42

1996 3.79 5.12 6.18 3.44 4.01 6.45

Table 21. MOBILE5 1987 bus fuel economy

(miles per gallon)

Diesel Gasoline

Transit Intercity School Transit Intercity School

4.26 4.96 9.87 -- -- 7.59 a a

 No sales were assumed for transit and school buses past 1980 a

Gasoline truck fuel economies determined in this study were significantly higher that previous

MOBILE5 estimates [1] (except for class 2B for the same reason as diesel class 2B trucks). Improved

fuel economy in gasoline trucks since 1982 is due to improvements in fuel management and the

introduction of electronic fuel injection which was not accounted for in MOBILE5 estimates.

Bus fuel economies determined in this study were significantly lower than previous MOBILE5

estimates [1] for both transit and school buses. It is assumed that the use of more up-to-date

information on BSFCs and in-use populations used in this study provide a more accurate picture of bus

fuel economy than was previously estimated for MOBILE5.
 Actual conversion factors used in MOBILE5 are aggregated into one heavy-duty 4

conversion factor for gasoline vehicles and one for diesel vehicles. Class specific conversion

factors will be used in MOBILE6.

13

IV. FUEL DENSITIES

Fuel densities were determined from National Institute for Petroleum and Energy Research

(NIPER) publications for both gasoline and diesel. Average gasoline density over the period 1987

through 1996 was 6.173 lb/gal [7] which compared well with the previous value of 6.09 lb/gal used in

MOBILE5. Average diesel fuel density over the period 1987 through 1996 was 7.099 lb/gal [7] which

compared well with the previous value of 7.11 lb/gal used in MOBILE5. 

V. CALCULATION OF CONVERSION FACTORS

Using the equation defining the conversion factor in Section I together with the data described

in Sections II, III and IV of this report, weight class specific conversion factors were calculated for

gasoline and diesel vehicles for model years 1987 through 1996. Diesel truck conversion factors are

shown in Table 22 with values developed for MOBILE5 shown in Table 23. Gasoline truck 4

conversion factors are shown in Table 24 and corresponding conversion factors developed for

MOBILE5 are shown in Table 25. Conversion factors for buses are shown in Table 26 with factors

developed for use in MOBILE5 shown in Table 27.

Table 22. Diesel truck conversion factors

(bhp-hr/mi)

Model Weight Class

Year 2B 3 4 5 6 7 8A 8B

1987 1.105 1.254 1.501 1.690 1.984 2.390 2.971 3.295

1988 1.103 1.253 1.496 1.676 1.979 2.392 2.946 3.263

1989 1.101 1.252 1.491 1.662 1.974 2.394 2.922 3.231

1990 1.099 1.251 1.486 1.649 1.969 2.396 2.898 3.201

1991 1.097 1.250 1.481 1.636 1.964 2.398 2.874 3.171

1992 1.095 1.250 1.476 1.623 1.960 2.400 2.851 3.141

1993 1.094 1.250 1.472 1.610 1.955 2.403 2.828 3.113

1994 1.093 1.250 1.467 1.597 1.951 2.405 2.806 3.085

1995 1.091 1.250 1.463 1.585 1.947 2.407 2.784 3.058

1996 1.090 1.250 1.458 1.573 1.942 2.409 2.763 3.031

Table 23. 1987 - 1996 diesel truck conversion factors developed for MOBILE5

(bhp-hr/mi)
14

Weight Class

2B 3 4 5 6 7 8A 8B

0.919 -- -- -- 1.865 2.127 2.987 3.129 aaa

 No sales were assumed in classes 3-5 after 1976 a

Table 24. Gasoline truck conversion factors

(bhp-hr/mi)

Model Weight Class

Year 2B 3 4 5 6 7 8A

87 1.068 1.152 1.164 1.291 1.311 1.383 1.503

88 1.071 1.151 1.160 1.294 1.310 1.389 1.507

89 1.073 1.150 1.156 1.297 1.310 1.395 1.510

90 1.076 1.150 1.152 1.301 1.309 1.402 1.513

91 1.079 1.149 1.149 1.305 1.309 1.409 1.517

92 1.082 1.149 1.146 1.308 1.309 1.416 1.521

93 1.085 1.149 1.143 1.312 1.309 1.423 1.526

94 1.089 1.149 1.140 1.316 1.309 1.430 1.530

95 1.092 1.149 1.137 1.320 1.310 1.438 1.535

96 1.096 1.150 1.134 1.324 1.311 1.446 1.540

Table 25. 1987 - 1996 gasoline truck conversion factors developed for MOBILE5

(bhp-hr/mi)

Weight Class

2B 3 4 5 6 7 8A

0.809 1.346 1.348 1.342 1.317 1.668 1.627
15

Table 26. Bus conversion factors

(bhp-hr/mi)

Diesel Gasoline

Year Transit Intercity School Transit Intercity School

87 4.595 3.422 2.661 3.195 2.779 1.622

88 4.602 3.415 2.673 3.210 2.773 1.628

89 4.609 3.408 2.685 3.225 2.767 1.633

90 4.617 3.402 2.697 3.241 2.762 1.639

91 4.625 3.395 2.708 3.258 2.757 1.645

92 4.635 3.390 2.771 3.275 2.752 1.651

93 4.645 3.384 2.823 3.294 2.747 1.658

94 4.655 3.379 2.877 3.313 2.743 1.664

95 4.667 3.374 2.932 3.333 2.739 1.671

96 4.679 3.370 2.989 3.354 2.735 1.677

Table 27. 1987-1996 bus conversion factors used in MOBILE5

(bhp-hr/mi)

Diesel Gasoline

Year Transit Intercity School Transit Intercity School

96 3.241 2.890 1.615 -- -- 1.161 a a

 No sales were assumed for transit and school buses past 1980 a

Diesel truck conversion factors derived in this study matched MOBILE5 estimates within 10%.

Gasoline truck conversion factors derived in this study tended to be lower than MOBILE5 estimates

for Classes 3, 4, 7 and 8A due to the fuel economy improvement in those classes as shown by 1992

TIUS data. Both gasoline and diesel class 2B trucks in this study had a higher conversion factor than

that used in MOBILE5 due to the lower fuel economy shown in TIUS when compared to the value

used in MOBILE5. Bus conversion factors showed the greatest variation from MOBILE5 due to the

much lower fuel economy estimated in this report than previously estimated for MOBILE5.

VI. PROJECTION OF CONVERSION FACTORS

 Based upon the analysis in Reference 7, it is reasonable to assume that most of the non-engine

fuel economy improvements available with current technology were already implemented in the U.S.

fleet by the 1996 model year. Therefore, it is assumed that further fuel economy improvements will

be associated with engine technology which will affect both BSFC and fuel economy. BSFC would

decrease and fuel economy would increase, with these effects for the most part offsetting each other

and thus having little impact in the calculated conversion factors. That being the case, conversion

factors for projections beyond the 1996 model year should be similar to those for the 1996 model year.
16

Table 28 gives projected conversion factors for 1997 and later model years for diesel trucks. Projected

conversion factors for 1997 and later model year gasoline trucks are shown in Table 29. Projected

conversion factors for 1997 and later model year buses are shown in Table 30.

Table 28. Diesel truck conversion factor projections for 1997 and later model years

(bhp-hr/mi)

Weight Class

2B 3 4 5 6 7 8A 8B

1.090 1.250 1.458 1.573 1.942 2.409 2.763 3.031

Table 29. Gasoline truck conversion factor projections for 1997 and later model years

(bhp-hr/mi)

Weight Class

2B 3 4 5 6 7 8A

1.096 1.150 1.134 1.324 1.311 1.446 1.540

Table 30. Bus conversion factor projections for 1997 and later model years

(bhp-hr/mi)

Diesel Gasoline

Transit Intercity School Transit Intercity School

4.679 3.370 2.989 3.354 2.735 1.677

It should be noted that several unknowns can change these conversion factors in the future. The

first is that changes in emissions control systems to meet future standards might change the ratio of fuel

economy improvement to BSFC improvement. Second, there has been much debate over off cycle

emissions in heavy-duty engines. As discussed in Section I, emissions can be significantly different

for on-the-road operation than during the emissions certification test cycle. The last caveat is that these

conversion factors are probably most reasonable for in-use NO emissions, since other emissions are x

more a function of transient behavior than the force required to move a truck or bus down the road. 
17

VII. REFERENCES

1. P. Machiele, “Heavy-Duty Vehicle Emission Conversion Factors II - 1962-2000,” EPA

Technical Report EPA-AA-SDSB-89-01, October 1988.

2. M. Kitchen and W. Damico, “Development of Conversion Factors for Heavy-Duty Buses G/BhpHr to G/Mile,” EPA Technical Report EPA-AA-EVRB-92-01, July 1992.

3. J.E. Brown, D.B. Harris and F.G. King, “Comparison of Emission Models with On-Road HeavyDuty Diesel Modal Data,” NTIS PB98-116353, presented at the AWMA “Emission Inventory:

Planning for the Future” conference, October 1997.

4. Dreher and Harley, “A Fuel-Based Inventory for Heavy-Duty Diesel Truck Emissions,” Journal

of the Air & Waste Management Association, April 1998. 

5. “1992 Truck Inventory and Use Survey (TIUS) - Microdata File,” U.S. Department of

Commerce, Economics and Statistics Administration, Bureau of the Census, 1993 (on CDROM).

6. Memo from Phil Heirigs and Larry Caretto of Sierra Research to Mr. David Lax of American

Petroleum Institute, May 20, 1997.

7. L. Browning, “Update of Heavy-Duty Engine Emission Conversion Factors -- Analysis of Fuel

Economy, Non-Engine Fuel Economy Improvements and Fuel Densities,” March 1998.

8. L. Browning, D. Coleman and C. Pera, “Update of Fleet Characterization Data for Use in

MOBILE6,” Acurex Environmental Report 97-105, May 1997.

9. “1995 Transit Passenger Vehicle Fleet Inventory as of January 1, 1995,” American Public Transit

Association, April 1995.

10. “School Bus Fleet -- 1997 Fact Book Issue,” Bobit Publication Management & Maintenance

Magazine for School Transportation Fleets, January 1997.