United States Air and Radiation EPA420-R-02-005
Environmental Protection January 2002
Agency M6.HDE.004
svEPA Update Heavy-Duty
Engine Emission
Conversion Factors for
MOBILES
Analysis of BSFCs and
Calculation of Heavy-Duty
Engine Emission
Conversion Factors
$5b Printed on Recycled
Paper
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EPA420-R-02-005
January 2002
Heavy-Duty
for
Analysis of BSFCs and Calculation of
Heavy-Duty Engine Emission
Conversion Factors
M6.HDE.004
Assessment and Modeling Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
Prepared for EPA by
ARCADIS Geraghty & Miller, Inc.
EPA Contract No. 68-C6-0068
Work Assignment No. 0-03 and 1-02
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.
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UPDATE HEAVY-DUTY
ENGINE EMISSION
CONVERSION FACTORS
FOR MOBILE6
Analysis of BSFCs and Calculation of
Heavy-Duty Engine Emission
Conversion Factors
05 May 1998
PREPARED FOR
U.S. Environmental Protection Agency
Motor Vehicle Emissions Laboratory
2565 Plymouth Road
Ann Arbor, Michigan 48105
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Update Heavy-Duty Engine
Emission Conversion
Factors for MOBILES
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
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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.
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I. INTRODUCTION
The USEPA highway emission factor model, MOBILESa, calculates average in-use emission
factors for hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOX) for eight
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 brake-
horsepower-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 [I]1.
The conversion factors used in MOBILESa were calculated from the following expression:
Fuel Density (Ib/gal)
Conversion Factor (bhp-hr/mi) = (1.1)
BSFC (Ib/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 B SFC for the certification test cycle in which the emissions factors
were generated and fuel economy for the in-use duty cycle. Flistorically, 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
Numbers in brackets refer to references in Section VII of this report.
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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 NOx-fuel economy trade-off than previous
mechanically inj ected 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 MOBILES and the ones calculated in
this report are most likely only reasonable estimates of in-use NOX emissions. Emissions of CO and
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 MOBILES 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 MOBILES 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
m.
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
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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 B SFC or conversion factor for that
class are calculated.
Table 1. Vehicle weight classes
Designation
HDGV (class 2B)
HDGV (class 3)
HDGV (class 4)
HDGV (class 5)
HDGV (class 6)
HDGV (class 7)
HDGV (class 8A)
HDGV (class 8B)
HDGTB
HDGSB
HDGCB
HDDV (class 2B)
HDDV (class 3)
HDDV (class 4)
HDDV (class 5)
HDDV (class 6)
HDDV (class 7)
HDDV (class 8A)
HDDV (class 8B)
HDDTB
HDDSB
HDDCB
Description
Light heavy-duty gasoline vehicles
Light heavy-duty gasoline vehicles
Heavy heavy-duty gasoline vehicles
Heavy heavy-duty gasoline vehicles
Heavy heavy-duty gasoline vehicles
Heavy heavy-duty gasoline vehicles
Heavy heavy-duty gasoline vehicles
Heavy heavy-duty gasoline vehicles
Gasoline transit buses
Gasoline school buses
Gasoline intercity buses
Light heavy-duty diesel trucks
Light heavy-duty diesel trucks
Light heavy-duty diesel trucks
Light heavy-duty diesel trucks
Medium heavy-duty diesel trucks
Medium heavy-duty diesel trucks
Heavy heavy-duty diesel trucks
Heavy heavy-duty diesel trucks
Diesel transit buses
Diesel school buses
Diesel intercity buses
Gross Vehicle
Weight Ob)
8501-10,000
10,001-14,000
14,001-16,000
16,001-19,500
19,501-26,000
26,001-33,000
33,001-60,000
>60,000
all
all
all
8501-10,000
10,001-14,000
14,001-16,000
16,001-19,500
19,501-26,000
26,001-33,000
33,001-60,000
>60,000
all
all
all
order to weight the B SFC data for bus engines, population data was taken from a number of
Transit bus engine populations for model years 1987 through 1995 were taken from the
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APT A 7995 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
(Ib/bhp-hr)
Model
Year
1988
1989
1990
1991
1992
1993
1994
1995
2B
0
0
0
0
0
0
0
0
553
536
545
504
494
527
516
511
3
0.544
0.528
0.535
0.491
0.491
0.521
0.500
0.504
0
0
0
0
0
0
0
0
4
478
508
504
485
450
498
490
502
Weight
5
0.465
0.460
0.453
0.460
0.450
0.469
0.444
0.467
Class
6
0.444
0.432
0.432
0.416
0.450
0.418
0.431
0.427
0
0
0
0
0
0
0
0
7
414
410
397
397
395
430
415
426
8A
0.403
0.397
0.397
0.388
0.400
0.429
0.392
0.392
SB
0.395
0.385
0.375
0.385
0.407
0.387
0.371
0.373
Table 3. Sales-weighted BSFC for gasoline trucks
(Ib/bhp-hr)
Model
Year
1988
1989
1990
1991
1992
1993
1994
1995
0
0
0
0
0
0
0
0
2B
.611
.614
.607
.602
.588
.570
.570
.565
0
0
0
0
0
0
0
0
3
626
613
610
602
595
577
587
585
4
0.642
0.627
0.611
0.602
0.604
0.589
0.608
0.591
Weight Class
5
0
0
0
0
0
0
0
0
640
627
607
601
606
596
607
587
6
0.642
0.641
0.638
0.600
0.602
0.597
0.604
0.589
0
0
0
0
0
0
0
0
7
640
644
639
599
600
600
602
578
8A
0.638
0.616
0.621
0.598
0.600
0.600
0.600
0.557
Table 4. Diesel transit bus inventory by engine type
(U.S. in-service population)
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Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
DDC
Series 50 6V-92 8V-92
2189 33
1826 5
2983 102
2910 34
1979 1
1394 50
257 1473 12
1604 243 11
1370 200
Cummins
L-10
355
683
239
1087
189
365
361
603
333
Other
Engines
238
142
96
204
180
78
148
28
21
Table 5. Gasoline transit bus inventory by engine manufacturer
(U.S. in-service population)
Model
Year
87
88
89
90
91
92
93
94
Manufacturer
Chrysler Ford
3
2
1 12
19
24
7
4
36
GM
1
7
3
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Table 6. School bus inventory by bus type2
(U.S. in-service population)
MY
90
91
92
93
94
95
96
A&B
2225
3756
3820
3535
3215
2216
2225
Diesel
C
23670
21370
16444
18928
21005
20861
22016
D
6286
6864
5444
6734
7321
9671
9270
Gasoline
A&B
3575
3554
2856
3244
3504
3638
3723
Table 7. Intercity diesel bus engine assumptions by model year
(% of U.S. in-service population)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
DDC
Series 60 6V-92TA
60%
60%
60%
60%
60%
60%
15% 60%
50% 30%
75% 15%
8V-92TA
30%
30%
30%
30%
30%
30%
15%
10%
Cummins
L-10
10%
10%
10%
10%
10%
10%
10%
10%
10%
2 Types A & B are generally smaller school buses with the engine in the front. Types C
and D are generally larger school buses, Type C has a front engine and Type D has an engine in
the rear or midship.
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Table 8. Sales- weighted bus BSFC
(Ib/bhp-hr)
Model
Year
1988
1989
1990
1991
1992
1993
1994
1995
Diesel
Transit
0.427
0.451
0.432
0.438
0.447
0.440
0.399
0.402
Intercity
0.427
0.451
0.432
0.438
0.447
0.440
0.399
0.407
School
0.421
0.420
0.410
0.407
0.396
0.391
Gasoline
Transit
0.550
0.600
0.615
0.598
0.598
0.541
0.544
Intercity
0.610
0.601
0.604
0.595
0.585
0.569
0.568
0.569
School
0.600
0.595
0.590
0.585
0.580
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 andafter 19953. These
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 MOBILES [1] shown in Table 11. As shown by this
comparison, the curve fits produced reasonable values when compared to MOBILES estimates.
Table 9. Curve fit equations for BSFCs by weight class and fuel
Class
2B
3
4
5
6
7
8A
8B
Transit
Intercity
School
Gasoline
y = -0.721 l*ln(x) + 3. 8473
y = -0.5656*ln(x) + 3.1535
y = -0.5583 *ln(x) + 3. 1319
y = -0.5435*ln(x) + 3.0630
y = -0.7339*ln(x) + 3.9284
y = -0.8224*ln(x) + 4.3266
y = -0.768 l*ln(x) + 4.0725
N/A
y = -0.8652*ln(x) + 4.4842
y = -0.495 l*ln(x) + 2.8221
y = -0.4648*ln(x) + 2.6918
Diesel
y = -0.4806*ln(x) + 2.6959
y = -0.51 83 *ln(x) + 2.8529
y = -0.1780*ln(x)+ 1.2897
y = -0.0349*ln(x) + 0.6162
y = -0.1706*ln(x)+ 1.1985
y = -0.0863 *ln(x) + 0.7854
y = -0.1141*ln(x) + 0.9107
y = -0.2003 *ln(x)+ 1.2858
y = -0.5058*ln(x) + 2.7092
y = - 0.3648*ln(x) + 2.0764
y = -0.531 l*ln(x) + 2.8123
y = BSFC (Ib/bhp-hr)
x = MY-1900
3 Sales data was only available for model years 1988 through 1995.
7
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Table 10. Curve fit diesel truck BSFC
(Ib/bhp-hr)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
2B
0.550
0.544
0.539
0.533
0.528
0.523
0.518
0.512
0.507
0.502
3
0.538
0.532
0.526
0.521
0.515
0.509
0.504
0.498
0.493
0.487
4
0.495
0.493
0.491
0.489
0.487
0.485
0.483
0.481
0.479
0.477
Weight
5
0.460
0.460
0.460
0.459
0.459
0.458
0.458
0.458
0.457
0.457
Class
6
0.437
0.435
0.433
0.431
0.429
0.427
0.425
0.423
0.422
0.420
7
0.400
0.399
0.398
0.397
0.396
0.395
0.394
0.393
0.392
0.391
8A
0.401
0.400
0.399
0.397
0.396
0.395
0.394
0.392
0.391
0.390
SB
0.391
0.389
0.387
0.384
0.382
0.380
0.378
0.376
0.374
0.372
Table 11. MOBILES 1987 diesel truck BSFC
(Ib/bhp-hr)
Weight Class
2B
0.54
3-5
0.51
6
0.45
7
0.44
8A
0.41
SB
0.39
Curve fit BSFCs for gasoline trucks are shown in Table 12 and estimated BSFCs from
MOBILES 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 MOBILES 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 MOBILES conversion factors [1] for 1987 buses are shown in Table
15.
8
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Table 12. Curve fit gasoline truck BSFCs
(Ib/bhp-hr)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
2B
0.627
0.619
0.611
0.602
0.595
0.587
0.579
0.571
0.563
0.556
3
0.628
0.621
0.615
0.608
0.602
0.596
0.590
0.584
0.578
0.572
4
0.638
0.631
0.625
0.618
0.612
0.606
0.600
0.594
0.588
0.582
Weight Class
5
0.636
0.630
0.624
0.618
0.612
0.606
0.600
0.594
0.588
0.582
6
0.651
0.642
0.634
0.626
0.618
0.610
0.602
0.594
0.586
0.579
7
0.654
0.644
0.635
0.626
0.617
0.608
0.599
0.590
0.581
0.573
8A
0.642
0.633
0.625
0.616
0.608
0.599
0.591
0.583
0.575
0.567
Table 13. MOBILES 1987 gasoline truck BSFC
(Ib/bhp-hr)
Weight Class
2B
0.62
3-5
0.62
6
0.66
7
0.65
8A
0.63
Table 14. Curve fit bus BSFCs
(Ib/bhp-hr)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
Transit
0.450
0.445
0.439
0.433
0.428
0.422
0.417
0.411
0.406
0.401
Diesel
Intercity
0.447
0.443
0.439
0.435
0.431
0.427
0.423
0.419
0.415
0.411
School
0.424
0.423
0.422
0.421
0.420
0.411
0.404
0.398
0.391
0.384
Transit
0.620
0.610
0.601
0.591
0.581
0.572
0.563
0.553
0.544
0.535
Gasoline
Intercity
0.611
0.605
0.600
0.594
0.589
0.583
0.578
0.573
0.567
0.562
School
0.616
0.611
0.604
0.600
0.595
0.590
0.585
0.580
0.575
0.570
Table 15. MOBILES 1987 bus BSFC
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(Ib/bhp-hr)
Diesel
Transit Intercity School
0.479 0.467 0.444
Gasoline
Transit Intercity School
0.660
a No sales were assumed for transit and school buses past 1980
BSFCs predicted for both diesel and gasoline buses had lower values than those used for
calculation of conversion factors for MOBILES. Since the transit bus information in MOBILES 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 howBSFCs 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 MOBILES 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.
MOBILES gasoline truck fuel economies for 1987 are shown in Table 19. Average bus fuel economies
from Reference 7 are shown in Table 20 and MOBILES 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 MOBILES. 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 MOBILES 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 MOBILES.
A significant improvement in fuel economy has been seen in this class between 1982 and 1987 not
previously accounted for in MOBILES 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.
10
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Table 16. Diesel truck fuel economy taken from Reference 7
(miles per gallon)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
2B
11.69
11.83
11.97
12.11
12.26
12.40
12.54
12.68
12.82
12.96
3
10.52
10.65
10.77
10.90
11.03
11.15
11.28
11.41
11.53
11.66
4
9.56
9.63
9.70
9.77
9.85
9.92
9.99
10.06
10.13
10.20
Weight
5
9.12
9.21
9.29
9.38
9.46
9.54
9.63
9.71
9.80
9.88
Class
6
8.20
8.25
8.31
8.37
8.42
8.48
8.54
8.59
8.65
8.71
7
7.43
7.44
7.45
7.46
7.47
7.48
7.49
7.51
7.52
7.53
8A
5.96
6.03
6.10
6.17
6.24
6.31
6.38
6.45
6.52
6.59
SB
5.51
5.59
5.68
5.77
5.86
5.95
6.03
6.12
6.21
6.30
Table 17. MOBILES 1987 diesel truck fuel economy
(miles per gallon)
2B 3-5
14.33
Weight Class
6 7
8.47 7.60
8A
5.67
8B
5.41
a No sales were assumed in classes 3-5 after 1976
Table 18. Gasoline truck fuel economy taken from Reference 7
(miles per gallon)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
2B
9.22
9.32
9.42
9.52
9.62
9.73
9.83
9.93
10.03
10.13
3
8.54
8.63
8.73
8.82
8.92
9.01
9.11
9.20
9.30
9.39
4
8.32
8.43
8.55
8.66
8.78
8.89
9.01
9.12
9.24
9.35
Weight Class
5
7.52
7.58
7.63
7.68
7.74
7.79
7.85
7.90
7.95
8.01
6
7.23
7.33
7.43
7.53
7.63
7.73
7.84
7.94
8.04
8.14
7
6.83
6.89
6.96
7.03
7.10
7.17
7.24
7.31
7.38
7.45
8A
6.39
6.47
6.54
6.62
6.70
6.77
6.85
6.92
7.00
7.07
Table 19. MOBILES 1987 gasoline truck fuel economy
11
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(miles per gallon)
Weight Class
2B 3-5 6 7
11.75 6.65 6.70 5.29
8A
5.50
Table 20. Curve fit bus fuel economy from Reference 7
(miles per gallon)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
Transit
3.43
3.47
3.51
3.55
3.59
3.63
3.67
3.71
3.75
3.79
Diesel
Intercity
4.64
4.69
4.75
4.80
4.85
4.91
4.96
5.01
5.07
5.12
School
6.29
6.28
6.27
6.25
6.24
6.23
6.22
6.20
6.19
6.18
Transit
3.11
3.15
3.19
3.22
3.26
3.30
3.33
3.37
3.40
3.44
Gasoline
Intercity
3.64
3.68
3.72
3.76
3.80
3.85
3.89
3.93
3.97
4.01
School
6.18
6.21
6.24
6.27
6.30
6.33
6.37
6.40
6.42
6.45
Table 21. MOBILES 1987 bus fuel economy
(miles per gallon)
Diesel
Transit Intercity School
4.26 4.96 9.87
Gasoline
Transit Intercity School
7.59
' No sales were assumed for transit and school buses past 1980
Gasoline truck fuel economies determined in this study were significantly higher that previous
MOBILES 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 MOBILES estimates.
Bus fuel economies determined in this study were significantly lower than previous MOBILES
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 MOBILES.
12
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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 Ib/gal [7] which compared well with the previous value of 6.09 Ib/gal used in
MOBILES. Average diesel fuel density over the period 1987 through 1996 was 7.099 Ib/gal [7] which
compared well with the previous value of 7.11 Ib/gal used in MOBILES.
V.
CALCULATION OF CONVERSION FACTORS
Using the equation defining the conversion factor in Section I together with the data described
in Sections n, m 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 MOBILES4 shown in Table 23. Gasoline truck
conversion factors are shown in Table 24 and corresponding conversion factors developed for
MOBILES are shown in Table 25. Conversion factors for buses are shown in Table 26 with factors
developed for use in MOBILES shown in Table 27.
Table 22. Diesel truck conversion factors
(bhp-hr/mi)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
2B
1.105
1.103
1.101
1.099
1.097
1.095
1.094
1.093
1.091
1.090
3
1.254
1.253
1.252
1.251
1.250
1.250
1.250
1.250
1.250
1.250
4
1.501
1.496
1.491
1.486
1.481
1.476
1.472
1.467
1.463
1.458
Weight
5
1.690
1.676
1.662
1.649
1.636
1.623
1.610
1.597
1.585
1.573
Class
6
1.984
1.979
1.974
1.969
1.964
1.960
1.955
1.951
1.947
1.942
7
2.390
2.392
2.394
2.396
2.398
2.400
2.403
2.405
2.407
2.409
8A
2.971
2.946
2.922
2.898
2.874
2.851
2.828
2.806
2.784
2.763
SB
3.295
3.263
3.231
3.201
3.171
3.141
3.113
3.085
3.058
3.031
Table 23. 1987 - 1996 diesel truck conversion factors developed for MOBILES
(bhp-hr/mi)
4 Actual conversion factors used in MOBILES are aggregated into one heavy-duty
conversion factor for gasoline vehicles and one for diesel vehicles. Class specific conversion
factors will be used in MOBILE6.
13
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2B
0.919
3
a
Weight Class
456
1.865
7
2.127
8A
2.987
SB
3.129
' No sales were assumed in classes 3-5 after 1976
Table 24. Gasoline truck conversion factors
(bhp-hr/mi)
Model
Year
87
88
89
90
91
92
93
94
95
96
2B
1.068
1.071
1.073
1.076
1.079
1.082
1.085
1.089
1.092
1.096
3
1.152
1.151
1.150
1.150
1.149
1.149
1.149
1.149
1.149
1.150
Weight Class
4 5
1.164
1.160
1.156
1.152
1.149
1.146
1.143
1.140
1.137
1.134
1.291
1.294
1.297
1.301
1.305
1.308
1.312
1.316
1.320
1.324
6
1.311
1.310
1.310
1.309
1.309
1.309
1.309
1.309
1.310
1.311
7
1.383
1.389
1.395
1.402
1.409
1.416
1.423
1.430
1.438
1.446
8A
1.503
1.507
1.510
1.513
1.517
1.521
1.526
1.530
1.535
1.540
Table 25. 1987 - 1996 gasoline truck conversion factors developed for MOBILES
(bhp-hr/mi)
2B
Weight Class
456
8A
0.809 1.346 1.348 1.342 1.317 1.668 1.627
14
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Table 26. Bus conversion factors
(bhp-hr/mi)
Year
87
88
89
90
91
92
93
94
95
96
Transit
4.595
4.602
4.609
4.617
4.625
4.635
4.645
4.655
4.667
4.679
Diesel
Intercity
3.422
3.415
3.408
3.402
3.395
3.390
3.384
3.379
3.374
3.370
School
2.661
2.673
2.685
2.697
2.708
2.771
2.823
2.877
2.932
2.989
Transit
3.195
3.210
3.225
3.241
3.258
3.275
3.294
3.313
3.333
3.354
Gasoline
Intercity
2.779
2.773
2.767
2.762
2.757
2.752
2.747
2.743
2.739
2.735
School
1.622
1.628
1.633
1.639
1.645
1.651
1.658
1.664
1.671
1.677
Table 27. 1987-1996 bus conversion factors used in MOBILES
(bhp-hr/mi)
Year
96
Diesel
Transit Intercity School
3.241 2.890 1.615
Gasoline
Transit Intercity School
1.161
' No sales were assumed for transit and school buses past 1980
Diesel truck conversion factors derived in this study matched MOBILES estimates within 10%.
Gasoline truck conversion factors derived in this study tended to be lower than MOBILES 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 MOBILES due to the lower fuel economy shown in TIUS when compared to the value
used in MOBILES. Bus conversion factors showed the greatest variation from MOBILES due to the
much lower fuel economy estimated in this report than previously estimated for MOBILES.
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 proj ections beyond the 1996 model year should be similar to those for the 1996 model year.
15
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Table 28 gives proj ected conversion factors for 1997 and later model years for diesel trucks. Proj ected
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)
2B
1.090
3
1.250
4
1.458
Weight
5
1.573
Class
6
1.942
7
2.409
8A
2.763
SB
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
1.096 1.150 1.134 1.324 1.311
7 8A
1.446 1.540
Table 30. Bus conversion factor projections for 1997 and later model years
(bhp-hr/mi)
Diesel
Transit Intercity School
4.679 3.370 2.989
Gasoline
Transit Intercity School
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 NOX emissions, since other emissions are
more a function of transient behavior than the force required to move a truck or bus down the road.
16
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VII. REFERENCES
1. P. Machiele, "Heavy-Duty Vehicle Emission Conversion Factors H - 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/Bhp-
Hr to G/Mile," EPA Technical Report EPA-AA-EVRB-92-01, July 1992.
3. I.E. Brown, D.B. Harris and F.G. King, "Comparison of Emission Models with On-Road Heavy-
Duty 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 CD-
ROM).
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.
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