United States Air and Radiation EPA420-P-98-014
Environmental Protection May 1998
Agency
v>EPA Update Heavy-Duty
Engine Emission
Conversion Factors for
MOBILES:
Analysis of Fuel
Economy, Non-Engine
Fuel Economy
Improvements, and Fuel
Densities
> Printed on Recycled Paper
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UPDATE HEAVY-DUTY
ENGINE EMISSION
CONVERSION FACTORS
FOR MOBILE6
Analysis of Fuel Economy, Non-Engine
Fuel Economy Improvements and
Fuel Densities
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 Fuel Economy,
Non-Engine Fuel Economy
Improvements and Fuel
Densities
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 USEPA 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) =
BSFC (Ib/bhp-hr) x Fuel Economy (mi/gal)
where BSFC is brake specific fuel consumption.
It is the intent of Work Assignments 0-03 and 1-02 to update these conversion factors for all
weight classes listed in Table 1. Since the last update calculated conversion factors 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 for model years 1997 through 2050.
This report discusses the analysis of fuel economy for model years 1987 through 1996 and fuel
density for gasoline and diesel. Furthermore, it examines the use of non-engine fuel economy
improvement devices for forecasting conversion factors in the future.
This report first discusses the data sets used in analyzing fuel economy and fuel density, then
describes analysis methodology and results. Further details of the analyses can be found in the
appendices. A second report, "Update Heavy-Duty Engine Emission Conversion Factors for
MOBILE6: Analysis of BSFCs and Calculation of Heavy-Duty Engine Emission Conversion Factors,"
discusses the analysis of brake specific fuel consumption data and provides the calculation of updated
engine emission conversion factors.
II. DATA SETS
A. Truck Fuel Economy and Non-Engine Fuel Economy Improvements
Average truck fuel economy and non-engine fuel economy improvements were calculated using
the 1992 Truck Inventory and Use Survey (TIUS) Microdata File [2]. The 1992 TIUS survey was
1 Numbers in brackets refer to references listed in Section V.
1
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conducted during the 1992-1993 time frame by the U.S. Bureau of the Census. The database, which
was supplied on CD-ROM, compiles a statistically significant sample of on-road light-duty and heavy-
duty trucks. The data includes the attributes of age, gross vehicle weight, fuel type, fuel economy,
average operating weight, travel type fraction, and mileage accumulation during 1992 for each truck
surveyed. The Census Bureau has also assigned an expansion factor on each record to extrapolate the
information in their database to represent the entire US truck population. The data also includes
information on use of non-engine fuel economy improvements such as aerodynamic devices, drive train
optimization, radial tires, governors and variable fan drives. This data set was used for both gasoline
and diesel trucks.
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
B. Bus Fuel Economy
Data on in-use bus fuel economy was not as readily available as that for trucks. Counts of
transit buses by model year and engine type was obtained from the American Public Transit
Association (APTA) 7995 Transit Passenger Vehicle Fleet Inventory [3]. Fuel economy for the
various common engine types was taken from a National Renewable Energy Laboratory (NREL) study
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of transit buses [4]. The APTM996 Transit Fact Book [5] was used to confirm calculations against
average fuel economy figures.
For school buses, limited data from the National Transportation Statistics 1997[6] together
with data from a school bus vehicle demonstration program [7] and school bus type counts by model
year from School Bus Fleet Magazine [8] were used to characterize gasoline and diesel school bus fuel
economy. Diesel intercity bus fuel economy was estimated from comparisons of similar buses with
DDC 6V-92TA engines (the most common engine prior to 1994) during a central business district
(CBD) cycle and an commuter cycle (COM) [9].
Gasoline fuel economies for transit and intercity buses by model year could not be located.
Since these represent a small portion of the inventory, previous work by Machiele [1] was used to
estimate gasoline transit and intercity bus fuel economies. Further discussion of these assumptions and
calculations are described in Section ni(B).
C. Fuel Density
Fuel densities were determined from National Institute for Petroleum and Energy Research
(NIPER) Petroleum Product Surveys (PPS) for years 1987 through 1996 [10-26]. These documents
list diesel and summer and winter gasoline properties.
III. METHODOLOGY
Methodologies to determine fuel economy, non-engine fuel economy improvement penetration
and fuel density data are presented below.
A. TIUS Methodology
To provide the best analysis of the TIUS data for the purposes needed by this work assignment,
ARCADIS Geraghty & Miller manipulated the TIUS data on a record-by-record basis. Pertinent data
from the TIUS data file TI92MDF.DAT was converted into a comma-delimited file using a C program
(TIHDCF.C), which is listed in the appendices. The comma-delimited file was then appended to a
dBASE file (TIUSHDCF.DBF) with the structure presented in Table 2. Two additional fields were
added to TIUSHDCF.DBF to further help in the manipulation of the data for this work assignment.
They are listed in Table 3.
The TIUS data set contains 247,282 records. These records were separated into the various
truck weight classes listed in Table 1 using the TIUS gross vehicle weight class (TIUGVW), the fuel
type (ENGTYP), and the average operating weight (AVGWT). The parameters TIUGVW and
AVGWT were used to determine weight class since these parameters are cross checked by the Census
Bureau and gave consistent results in terms of fuel economy versus weight class. Since TIUGVW does
not differentiate between classes 2A (6,001 - 8,500 Ibs) and 2B (8,501 -10,000 Ibs), AVGWT was used
to determine which trucks were class 2B. Records which did not fall into one of the classes defined
in Table 1, were incomplete, or used a fuel other than gasoline or diesel, were eliminated. In addition,
since the last model year of data included model years 1982 and older, these data were also eliminated
as they could not be assigned to a specific model year. This resulted in 59,046 records for the analysis.
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Results defined by 2 or less records were also deleted. The data were then used to characterize average
fuel economy, travel fractions, average operating weight, vehicle miles traveled (VMT) and penetration
of non-engine fuel economy improvements for the classes of vehicles listed in Table 1.
Table 2. TIUSHDCF.DBF data structure
Field Name
Description
EXPANF
MDLYR
AVGWT
ENGTYP
PKCID
AERODN
AXLRAT
ECOENG
RADIAL
GOVNOR
VARFAN
OTHFUEL
ANNMIL
MPG
PLOCAL
PSHORT
PSMED
PLMED
PLONG
TIUGVW
PKGVW
PKRWGT
Expansion factor
Model year
Average operating weight
Fuel type
Engine size code
Aerodynamic device?
Optimized axle ratio?
Fuel economy engine?
Radial tires?
Road speed governor?
Variable fan drives?
Other fuel conservation features?
Annual Mileage during 1992
Fuel Economy
% of mileage for trips < 50 miles from home
% of mileage for trips 50-100 miles from home
% of mileage for trips 100-200 miles from home
% of mileage for trips 200-500 miles from home
% of mileage for trips > 500 miles from home
TIUS gross vehicle weight class
Polk gross vehicle weight class
Polk registered weight
Table 3. Additional fields in TIUSHDCF.DBF
Field Name
WGTCLASS
TRIP CODE
Description
Vehicle class description
Trip type description
Trip types were broken into four trip categories as shown in Table 4 for further analysis. It was
believed that fuel economies would be different for trucks that operated locally to those that operated
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in long-haul applications. Vehicle characteristics versus trip type were determined from records in
which over 75 percent of the VMT represented that trip type. All values were averaged by vehicle
miles traveled (registrations times annual average mileage). The program to manipulate the database,
HDCF.PRG, is listed in the appendices.
Table 4. Trip type descriptions
Trip Type
Local
Short
Medium
Long
Description
Trips less than 50 miles from home base
Trips between 50 and 100 miles from home base
Trips between 100 and 200 miles from home base
Trips over 200 miles from home base
A regression analysis was performed for fuel economies by model year for each weight class
and a power curve fit (y = axb) was generated to extrapolate values beyond 1992. Curve fits for each
weight class are shown in Table 5. TIUS provided the most complete set of in-use fuel economy data
for trucks, but since it only described trucks for 1992 model year and older, the fuel economy curves
needed to be extrapolated to provide data for model years 1993 through 1996. In all cases the
equations resulted in about a 1% improvement in fuel economy per year which seemed reasonable
given current truck fuel economy trends. TIUS provided no data for Class 8B gasoline trucks and
therefore no fuel economies were calculated for that class. No extrapolation beyond 1996 was done
for fuel economy since BSFCs beyond 1996 were not available. Future projections of conversion
factors were made based upon conversion factors calculated between 1987 and 1996, similar to the
methodology applied by Machiele [1].
Table 5. Curve fits of fuel economy
(y is fuel economy in mpg and x is [model year - 1900])
Weight
Class
2B
3
4
5
6
7
8A
8B
Gasoline
y = 0.1253x°-9624
y = 0.1157x°-9632
y = 0.0409xL1902
y = 0.4416x°-6348
y = 0.0338xL2015
y = 0.1277x°-8909
y = 0.0647xL0285
Diesel
y = 0.1072xL0506
y = 0.0989xL045
y = 0.502x°-6598
y = 0.2474x°-8078
y = 0.5336x°-6117
y = 4.0206x°-1374
y = 0.15485x°-8194
y = 0.0119x13742
Non-engine fuel economy improvement penetration versus model year for model years 1983
through 1992 were curve fit using a logarithmic curve (y = a + b*ln(x)). Usage of non-engine fuel
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economy improvements for the 1996 model year were then calculated using the curve fits and
compared against MOBILE4 estimates [1]. Discussion of these results can be found in Section IV.
Raw averaged TIUS data for each weight class and fuel can be found in the appendices (Tables A-l
through A-15). Blank entries indicate no data.
B. Bus Fuel Economy Methodology
Diesel transit bus fuel economy is highly dependent on the type of engine used. Prior to 1993,
68 to 87% of the diesel bus inventory used the DDC 6V-92TA two-stroke engine. The Cummins L-10
four-stroke engine was the second most used engine in transit buses during that time period. The
Cummins L-10 has approximately 14% better fuel economy than the DDC 6V-92TA [4]. In 1992,
DDC introduced the Series 50 four stroke engine for the bus market with approximately 16% better
fuel economy than the 6V-92TA [27]. Due to more stringent emission regulations, the 6V-92 is being
phased out and will not be built after 1998 for the on-road market. The penetration of the four stroke
engines into the bus market each model year is a larger driver of average fleet fuel economy than the
minor changes that occur from year to year in a given engine line. Thus to calculate fuel economy for
this work assignment, bus engine counts for model years 1987 through 1995 were taken from the
APTA 7995 Transit Passenger Vehicle Fleet Inventory [2]. These are listed in Table 6. As transit
buses are defined in the Code of Federal Regulations (Title 40 §86.093-2) as having a load capacity of
15 passengers or more, buses that held fewer than 15 passengers were not counted. In addition, trolleys
and streetcars also were not counted. The numbers in Table 6 represent active buses for model years
1987 through 1994 and purchases for 1995.
Table 6. Diesel transit bus inventory by engine type
(U.S. in-service population)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
DDC
Series 50 6V-92TA
2189
1826
2983
2910
1979
1394
257 1473
1604 243
1370 200
8V-92TA
33
5
102
34
1
50
12
11
Cummins
L-10
355
683
239
1087
189
365
361
603
333
Other
Engines
238
142
96
204
180
78
148
28
21
Average fuel economies for the DDC 6V-92TA and the Cummins L-10 were derived from a
transit bus study done by NREL [4] and are listed in Tables 7 and 8 respectively. Average fuel
economies were determined by weighting each transit district average diesel fuel economy by the fleet
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mileage. This resulted in a 14% increase in fuel economy for the four stroke L-10 over the two stroke
6V-92TA.
Comparisons of certification BSFCs for the 6V-92 and the Series 50 showed a 16%
improvement in fuel economy for the newer four stroke Series 50. The DDC 8V-92TA was assumed
to have the same fuel economy as the DDC 6V-92TA since the DDC 8V-92TA has slightly better
BSFC but is usually used in heavier buses. The other engines in Table 6 were also mostly four stroke
engines (mostly Caterpillar 3306). Using this information, fuel economies for two-stroke buses were
estimated to be 3.4 mpg (DDC 6V-92 and 8V-92) and four-stroke buses were estimated to be 3.9 mpg
(DDC Series 50, Cummins L-10 and others). Fuel economy by model year for diesel transit buses was
then weighted by the vehicle counts listed in Table 6.
Table 7. Determination of average diesel transit bus fuel economies
for the DDC 6V-92TA (Taken from Reference 4)
Transit District
Houston TX
Miami FL
Peoria IL
Minneapolis/St. Paul MN
No. of
Buses
5
5
O
5
Fleet
Miles
(miles)
282,881
380,453
225,377
266,338
VMT Weighted Average
Average
Fuel Economy
(mpg)
3.63
3.32
3.51
3.14
3.39
Table 8. Determination of average diesel transit bus fuel economies
for the Cummins L-10 (Taken from Reference 4)
Transit District
Portland OR
Miami FL
No. of
Buses
5
5
Fleet
Miles
(miles)
203,007
330,342
VMT Weighted Average
Average
Fuel Economy
(mpg)
4.30
3.61
3.87
Intercity bus fuel economy was estimated from transit bus fuel economy by applying the percent
increase in fuel economy between a transit bus operating on the central business district (CBD) driving
cycle and the commuter (COM) cycle. Intercity buses are similar to transit buses, but stop less and
usually travel at higher speeds. Since intercity buses travel freeways and arterials between cities, the
COM driving cycle is a good representation of intercity bus use. The CBD is used to represent in-city
driving by transit buses. Battelle Columbus Laboratories tested six transit buses with 6V-92TA
engines on both the CBD and COM cycles [9]. Averaged results from that study is shown in Table 9.
7
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Diesel buses driven on the COM cycle had a 35.2% increase in fuel economy over that for the same
bus driven on the CBD cycle. Thus fuel economies for diesel transit buses by model year were then
multiplied by 1.352 to determine intercity bus fuel economies.
Gasoline school bus fuel economies were calculated from fuel usage and vehicle-mile statistics
for school buses from the National Transportation Statistics 1997 [6]. Gasoline school buses werel
assumed to be mostly Type A&B2. To calculate diesel school bus fuel economy for Type A&B, the
ratio of diesel to gasoline fuel economies for school buses was determined from a 1988 report on
conversion factors [1] and applied to fuel economies calculated for gasoline Type A&B school buses.
This resulted in an estimate of 8.2 mpg for Type A&B diesel school buses. Fuel economies for Type
C and D buses were taken from a California Energy Commission school bus demonstration program
[7]. Average fuel economy for Type C & D buses from that study was approximately 6.0 mpg. Using
these estimates together with the school bus populations by vehicle type from School Bus Fleet [8]
(shown in Table 10), diesel school bus fuel economy was calculated.
Table 9. Fuel economy difference between CBD and COM driving cycles
(Taken from Reference 9)
Driving
Cycle
CBD
COM
Miles
between
Stops
0.142
4.000
Average
Speed
(mph)
12.9
46.5
Top
Speed
(mph)
20
55
Ratio of COM to CBD fuel economy
Fuel
Economy
(mpg)
3.69
4.99
1.352
Table 10. Diesel school bus inventory by model year and type
(Taken from Reference 8)
Model
Year
90
91
92
93
94
95
96
A&B
2225
3756
3820
3535
3215
2216
2225
School Bus
C
23670
21370
16444
18928
21005
20861
22016
Type
D
6286
6864
5444
6734
7321
9671
9270
Total
32181
31990
25708
29197
31541
32748
33511
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.
8
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The gasoline transit bus inventory amounted to approximately 0.5% of the diesel transit bus
inventory [3]. To calculate gasoline transit bus fuel economies, the ratio of diesel to gasoline fuel
economies for transit buses was determined from a 1988 report on conversion factors [1]. That report
indicated that gasoline transit buses fuel economies were approximately 90.8% of diesel transit bus fuel
economies. This factor was applied to the previously calculated diesel transit bus fuel economies to
determine gasoline transit bus fuel economies. A similar procedure was used for gasoline intercity
buses, but in this case, the ratio was determined between gasoline transit and intercity buses. It was
determined from Reference 9 that gasoline intercity buses had 16.7% better fuel economy than gasoline
transit buses. Thus gasoline transit bus fuel economy by model year was multiplied by 1.167 to
determine gasoline intercity bus fuel economies.
C. Fuel Density Methodology
1. Gasoline
Gasoline American Petroleum Institute (API) gravity was extracted from NIPER publications
on summer and winter motor gasoline properties. It was assumed that all heavy-duty gasoline trucks
use regular unleaded gasoline. Low altitude values were used for all years. Summer and winter values
were averaged (added together and divided by two) for each year in question. Fuel densities in pounds
per gallon were then calculated from API gravity using the following formula [28]:
141.5x8.328
Fuel Density (Ibs/gal) =
(131.5+API)
2. Diesel
Diesel API gravity was extracted from NIPER publications on diesel properties. It was
assumed that all heavy-duty diesel trucks and buses use #2 diesel fuel. Nationwide average values were
used to calculate fuel densities for each year. Fuel densities in pounds per gallon were then calculated
from API gravity using the above formula.
IV. RESULTS
A. Truck Fuel Economy
Average heavy-duty gasoline truck operation by weight class during 1992 (1992 TIUS data)
is presented in Table 11. Both fuel economy and average operating weight are VMT weighted
averages. In all weight classes except 4, over 60% of the VMT occurred in trips within 50 miles of
the home base of the vehicle (Local). All weight classes except 4 had over 80% of the VMT within
100 miles from the home base of the vehicle (Local + Short). Class 4 vehicles had only 66% of the
VMT within 100 miles from the home base.
Gasoline truck fuel economy was calculated for 1987 through 1996 model year trucks using
the curve fits listed in Table 5 (derived from TIUS data). The results are shown in Table 12.
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Table 13 shows average heavy-duty diesel truck operation by weight class during 1992 (1992
TIUS data). Both fuel economy and average operating weight are VMT weighted averages. As with
gasoline vehicles, local operation (Local) was compared with long-haul (Long) operation to determine
trends in fuel economy. Data in Tables 11 and 13 indicate that diesel trucks tended to operate over a
greater radius from home base than gasoline trucks. Weight classes 2B through 7 drove over 40% of
their VMT on trips within 50 miles of the home base of the vehicle (Local). These trips accounted for
only 23% and 7% of their VMT for weight classes 8A and 8B, respectively. Class 8A had almost 50%
of VMT in trips over 200 miles from the home base (Long), while class 8B drove over 70% of the
VMT in trips over 200 miles from the home base. The TIUS data also show that for class 8 trucks, fuel
economy for local trips was approximately equal to fuel economy for long-haul trips. It is expected that
these results would be different for trucks with newer, electronically-controlled engines.
Table 11. Heavy-duty gasoline vehicle averages in 1992
(taken from 1992 TIUS[2])
Weight
Class
2B
O
4
5
6
7
8A
VMT
(Mil Miles)
3283.02
4194.68
1224.18
765.40
1301.57
443.19
165.04
Travel Fraction (%)
Local
64.3
65.9
46.4
72.4
68.9
76.6
73.0
Short
25.7
16.2
19.8
19.3
17.1
10.6
19.3
Med
8.8
7.4
7.4
5.9
2.3
10.2
5.3
Long
1.2
10.4
26.5
2.5
11.7
2.6
2.4
FEa
(mpg)
9.2
9.0
8.2
7.4
7.3
6.7
6.6
Wgtb
(Ibs)
9490
11997
15274
17877
22289
29068
39838
a Average weight class fuel economy in miles per gallon
b Average weight class operating weight in pounds
Table 12. Projected gasoline heavy-duty vehicle fuel economies (mpg)
Model
Year
87
88
89
90
91
92
93
94
95
96
Weight Class
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
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
10
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Table 14 shows calculated fuel economy for model year 1987 through 1996 diesel trucks,
derived from the curve fits listed in Table 5.
Table 13. Heavy-duty diesel vehicle averages in 1992
(taken from 1992 TIUS [2])
Wgt
Class
2B
3
4
5
6
7
8A
8B
VMT
Mil Miles
1857.59
3751.85
1479.63
1857.42
5492.44
4768.90
25088.28
65513.19
Travel Fractions (%)
Local
55.0
49.2
47.2
55.9
47.9
44.0
22.7
7.3
Short
31.4
30.2
37.2
18.8
28.8
22.6
10.7
8.8
Med
9.0
8.5
8.1
9.3
12.8
10.2
9.9
9.4
Long
4.5
12.1
7.5
15.9
10.5
23.2
56.6
74.4
Fuel Economy
Ave
11.9
11.2
9.7
9.4
8.2
7.4
6.0
5.7
Local
12.0
12.1
9.5
9.4
8.0
7.7
6.0
5.6
1 (mPg)
Long
9.6
8.5
6.6
6.0
5.7
Average Weight1" (Ibs)
Ave
9591
12219
15123
17814
22935
29906
48881
75784
Local
9544
12262
15185
17812
22829
30074
47622
76575
Long
17916
23366
30350
49900
75063
a Average weight class fuel economy in miles per gallon
b Average weight class operating weight in pounds
Table 14. Projected diesel heavy-duty vehicle fuel economies (mpg)
Model
Year
87
88
89
90
91
92
93
94
95
96
Weight Class
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
5
9.12
9.21
9.29
9.38
9.46
9.54
9.63
9.71
9.80
9.88
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
8B
5.51
5.59
5.68
5.77
5.86
5.95
6.03
6.12
6.21
6.30
B. Bus Fuel Economy
Calculated fuel economies for transit, intercity and school buses are shown in Table 15. The
average for the calculated fuel economy from Table 15 for diesel transit buses for model years 1987
through 1995 is 3.61 mpg. This is reasonably close to the 3.68 mpg for all diesel transit buses in
operation in 1994 calculated from data given in the 1996 Transit Fact Book [5] and therefore seems
reasonable.
11
-------
Table 15. Estimated bus fuel economies (mpg)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
Diesel
Transit
3.43
3.47
3.51
3.55
3.59
3.63
3.67
3.71
3.75
3.79
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
Gasoline
Transit
3.11
3.15
3.19
3.22
3.26
3.30
3.33
3.37
3.40
3.44
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
C. Use of Non-Engine Fuel Economy Improvement Devices
For previous versions of MOBILE, projections of conversion factors for future model years
were determined by examining increased use estimates of fuel economy improvement devices that
were not engine related (aerodynamic devices, drive train optimization, radial tires, speed control and
variable speed fan drives). It was thought that if the fuel economy of an engine line improved due to
engine improvements (such as better fuel injection control, combustion optimization, turbocharging),
these changes would be reflected both in the fuel economy of the vehicle and the BSFC of the engine,
and that these effects would more or less offset one another. However, non-engine related fuel
economy improvement devices could improve the fuel economy of the vehicle without affecting
engine BSFC. Since improving fuel economy without a corresponding reduction in BSFC would
decrease conversion factors, these non-engine fuel economy improvement devices could affect
conversion factors for future model years and need to be taken into account.
As part of this study, the 1992 TIUS data was used to determine the extent to which non-engine
related devices were used by the various weight classes in the U.S. heavy-duty vehicle fleet.
Regression analyses were performed on data for model years 1983 through 1992 to determine use
trends of these devices and project those trends to the 1996 model year. These devices are the most
beneficial on longer-haul, higher speed trips. Therefore, if the number of trucks that use these devices
is less than the number of trucks that operate on long-haul trips, one can assume that there may be
increased use of these devices in the future, which would affect truck fleet fuel economy and thus
conversion factors. To test this assumption, predicted use of non-engine fuel economy improvement
devices were compared against the VMT fraction of long-haul trips.
Table 16 shows the percent of use of non-engine fuel economy improvement devices for heavy-
duty gasoline trucks. As may be seen in this table, in all classes except 3 and 4, data projected out for
1996 model year trucks shows that, in fact, the use percent of non-engine related devices exceeds the
12
-------
percent of trucks that operate on long-haul trips. Thus, it is unlikely that there will be further increased
use of these devices past the 1996 model year and therefore need not be considered in conversion factor
calculations for future model years. Since class 3 and 4 vehicles still spend most of their travel in
shorter trips, it is not likely that there will be much increased use of these devices in those weight
classes over the 1996 model year levels, either.
A similar trend in the use of non-engine related devices is illustrated in Table 17 for 1996
model year heavy-duty diesel trucks. For diesel trucks, however, the percent of use of non-engine fuel
economy improvement devices for the 1996 model year greatly exceeds the long-haul travel fraction
for all weight classes. Thus, it is unlikely that there will be much further use of non-engine fuel
economy improvement devices in diesel trucks beyond those already in use on 1996 model year trucks.
Therefore, increased use of these devices need not be figured into calculations of conversion factors
beyond the 1996 model year.
Table 16. Estimated percent of use of non-engine fuel economy improvements
in each weight class of 1996 model year heavy-duty gasoline vehicles
Weight Class
Long-Haul VMT Fraction
Aero Devices
TIUS
MOBILE4
Drive Train Optimization
TIUS
MOBILE4
Radial Tires
TIUS
MOBILE4
Speed Control
TIUS
MOBILE4
Fan Drives
TIUS
MOBILE4
2B
1%
18%
0%
22%
27%
96%
67%
12%
13%
18%
0%
3
10%
9%
0%
12%
27%
100%
67%
5%
13%
9%
0%
4
27%
11%
0%
32%
27%
77%
67%
10%
13%
18%
0%
5
3%
33%
7%
23%
27%
73%
14%
62%
4%
26%
90%
6
12%
34%
7%
39%
27%
100%
14%
32%
4%
10%
90%
7
3%
24%
7%
37%
27%
86%
14%
44%
4%
25%
90%
8A
2%
38%
7%
31%
27%
91%
14%
42%
4%
15%
90%
D.
Fuel Densities
Fuel densities for unleaded gasoline, taken from the NIPER publications, are shown in Table
18. Fuel densities for #2 diesel are shown in Table 19. These fuel densities are similar to those used
in MOBILE4 emission factor calculations [1].
13
-------
Table 17. Estimated percent of use of non-engine fuel economy improvements
in each weight class of 1996 model year heavy-duty diesel vehicles
Weight Class
Long-Haul VMT Fraction
Aero Devices
TIUS
MOBILE4
Drive Train Optimization
TIUS
MOBILE4
Radial Tires
TIUS
MOBILE4
Speed Control
TIUS
MOBILE4
Fan Drives
TIUS
MOBILE4
2B
5%
17%
0%
30%
27%
91%
67%
39%
13%
41%
0%
3
12%
21%
0%
40%
27%
91%
67%
28%
13%
42%
0%
4
8%
18%
0%
38%
27%
100%
67%
41%
13%
28%
0%
5
16%
17%
7%
56%
27%
100%
14%
35%
4%
40%
90%
6
11%
28%
7%
46%
27%
92%
14%
41%
4%
46%
90%
7
23%
48%
7%
59%
27%
94%
14%
41%
4%
46%
90%
8A
57%
87%
7%
80%
27%
90%
14%
71%
4%
80%
90%
SB
74%
100%
32%
100%
27%
95%
50%
81%
14%
85%
100%
Table 18. Gasoline Fuel Densities
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
API Gravity
Winter
62.3
62.5
61.8
62.2
61.8
61.2
61.2
60.8
59.4
60.2
Summer
59.2
58.9
58.2
58.2
58.0
57.4
56.1
55.7
56.1
56.9
Average
60.75
60.70
60.00
60.20
59.90
59.30
58.65
58.25
57.75
58.55
Density
Ib/gal
6.130
6.131
6.154
6.147
6.157
6.176
6.197
6.210
6.227
6.201
Average 6.173
MOBILE4 6.09
14
-------
Table 19. Diesel Fuel Densities
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
API
Gravity
34.2
34.5
33.8
34.3
34.0
33.7
34.3
35.3
35.4
35.6
Density
Ib/gal
7.112
7.099
7.129
7.107
7.120
7.133
7.107
7.065
7.061
7.052
Average 7.099
MOBILE4 7.11
V. REFERENCES
1. P. Machiele, "Heavy-Duty Vehicle Emission Conversion Factors n -1962-2000," EPA report no.
EPA-AA-SDSB-89-01, October 1988.
2. "1992 Truck Inventory and Use Survey (TIUS) - Mcrodata File," U.S. Department of Commerce,
Economics and Statistics Administration, Bureau of the Census, 1993 (on CD-ROM).
3. "1995 Transit Passenger Vehicle Fleet Inventory as of January 1, 1995," American Public Transit
Association, April 1995.
4. K. Chandler et. al, "Alternative Fuel Transit Bus Evaluation Program Results," SAE paper no.
961082, May 1996.
5. "1996 Transit Fact Book," American Public Transit Association, January 1996.
6. "National Transportation Statistics 1997," Bureau of Transportation Statistics, U.S. Department
of Transportation, 1997.
7. C. Colucci and A. Hill, "School Bus Program: Transition to Alternative Fuels," SAE paper no.
952747, January 1995.
8. "School Bus Fleet - 1997 Fact Book Issue," Bobit Publication Management & Maintenance
Magazine for School Transportation Fleets, January 1997.
9. G. Francis, "Transit Bus Fuel Economy Research,: SAE paper no. 831185, August 1983.
15
-------
10. C. Dickson and P. Woodward, "Diesel Fuel Oils, 1986," NIPER publication 147 PPS 86/5, October
1986.
11 C. Dickson and P. Woodward, "Diesel Fuel Oils, 1988," NIPER publication 157 PPS 88/5, October
1988.
12. C. Dickson and P. Woodward, "Diesel Fuel Oils, 1990," NIPER publication 167 PPS 90/5, October
1990.
13. C. Dickson, "Diesel Fuel Oils, 1992," NIPER publication 177 PPS 92/5, October 1992.
14. C. Dickson and G. Sturm, Jr., "Diesel Fuel Oils, 1994," NIPER publication 187 PPS 94/5,
December 1994.
15. C. Dickson and G. Sturm, Jr., "Diesel Fuel Oils, 1996," NIPER publication 197 PPS 96/5, October
1996.
16. C. Dickson and P. Woodward, "Motor Gasolines, Summer 1987," NIPER publication 153 PPS
88/1, March 1988.
17. C. Dickson and P. Woodward, "Motor Gasolines, Winter 1987-88," NIPER publication 155 PPS
88/3, June 1988.
18. C. Dickson and P. Woodward, "Motor Gasolines, Summer 1989," NIPER publication 163 PPS
90/1, February 1990.
19. C. Dickson and P. Woodward, "Motor Gasolines, Winter 1989-90," NIPER publication 165 PPS
90/3, June 1990.
20. C. Dickson and P. Woodward, "Motor Gasolines, Summer 1991," NIPER publication 173 PPS
92/1, February 1992.
21. C. Dickson and P. Woodward, "Motor Gasolines, Winter 1991-92," NIPER publication 175 PPS
92/3, June 1992.
22. C. Dickson and G. Sturm, Jr., "Motor Gasolines, Summer 1993," NIPER publication 183 PPS 94/1,
July 1994.
23. C. Dickson and G. Sturm, Jr., "Motor Gasolines, Winter 1993-1994," NIPER publication 185 PPS
94/3, August 1994.
24. C. Dickson and G. Sturm, Jr., "Motor Gasolines, Summer 1995," NIPER publication 193 PPS 96/1,
February 1996.
25. C. Dickson and G. Sturm, Jr., "Motor Gasolines, Winter 1995-1996," NIPER publication 195 PPS
96/3, July 1996.
26. C. Dickson and G. Sturm, Jr., "Motor Gasolines, Summer 1996," NIPER publication 198 PPS 97/1,
January 1997.
27. Personal communication with DDC.
16
-------
28. T. Baumeister and L. Marks, Standard Handbook for Mechanical Engineers., seventh edition,
McGraw-Hill Book Company, 1967.
17
-------
APPENDIX
-------
TIHDCF.C
/* TIHDCF
Converts the TIUS dataset TI92MDF.DAT to a comma delimited file for
importing into dBASE file TIUSHDCF
*/
#include
#include
#define comma 44
char buffer [625] ;
char bufout [110] ;
FILE *fin,*fout;
int count;
long n;
int idex, odex;
void main()
{
fin = f open ("E: TI92MDF.DAT" , "rb") ;
fout = fopen ("C: TIHDCF1.DAT" , "wb") ;
n = 0;
while (n < 125000) {
fgets (buffer, 625, fin) ;
odex = 0 ;
idex = 14 ;
/* EXPANF 15-21 */
for (count=l; count <=7; count++) {
bufout [odex] = buf fer [idex] ;
odex++ ;
idex++ ;
}
bufout [odex] = comma;
odex++;
/* MDLYR 24-25 */
idex = 23 ;
for (count=l; count <=2; count++) {
bufout [odex] = buf fer [idex] ;
odex++ ;
idex++ ;
}
bufout [odex] = comma;
odex++;
/* AVGWT 99-104 */
idex = 98;
for (count=l; count <=6; count++) {
bufout [odex] = buf fer [idex] ;
odex++ ;
idex++ ;
}
bufout [odex] = comma;
odex++;
/* EngTyp 112 */
idex = 111;
bufout [odex] = buf fer [idex] ;
odex++;
bufout [odex] = comma;
odex++;
/* PKCID 114-115 */
A-1
-------
TIHDCF.C
idex = 113;
for (count=l; count <=2; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* AERODN 119 */
idex = 118;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* AXLRAT 120 */
idex++;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* ECOENG 121 */
idex++;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* RADIAL 123 */
idex = 122;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* GOVNOR 124 */
idex++;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* VARFAN 125 */
idex++;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* OTHFUEL 126 */
idex++;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* ANNMIL 155-160 */
idex = 154;
for (count=l; count <=6; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
A-2
-------
TIHDCF.C
/* MPG 170-172 */
idex = 169;
for (count=l; count <=3; count++) {
bufout [odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PLOCAL 183-185 */
idex = 182;
for (count=l; count <=3; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PSHORT 186-188 */
for (count=l; count <=3; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PSMED 189-191 */
for (count=l; count <=3; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PLMED 192-194 */
for (count=l; count <=3; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PLONG 195-197 */
for (count=l; count <=3; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* TIUGVW 421-422 */
idex = 420;
for (count=l; count <=2; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
A-3
-------
TIHDCF.C
/* PKGVW 423 */
bufout[odex] = buffer[idex] ;
odex++;
idex++;
bufout[odex] = comma;
odex++;
/* PKRWGT 424-429 */
for (count=l; count < = 6; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
bufout[odex] = '\n';
odex++;
bufout[odex] = '\0';
fputs(bufout,fout);
fclose(fout);
puts("\nfile 1 written");
fout = fopen("C:TIHDCF2.DAT","wb");
while (fgets(buffer,625,fin)) {
odex = 0;
idex = 14;
/* EXPANF 15-21 */
for (count=l; count <=7; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
bufout[odex] = comma;
odex++;
/* MDLYR 24-25 */
idex = 23;
for (count=l; count <=2; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
bufout[odex] = comma;
odex++;
/* AVGWT 99-104 */
idex = 98;
for (count=l; count <=6; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
bufout[odex] = comma;
odex++;
/* EngTyp 112 */
idex = 111;
bufout[odex] = buffer[idex] ;
odex++;
bufout[odex] = comma;
odex++;
/* PKCID 114-115 */
idex = 113;
for (count=l; count <=2; count++) {
bufout[odex] = buffer[idex];
A-4
-------
TIHDCF.C
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* AERODN 119 */
idex = 118;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* AXLRAT 120 */
idex++;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* ECOENG 121 */
idex++;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* RADIAL 123 */
idex = 122;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* GOVNOR 124 */
idex++;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* VARFAN 125 */
idex++;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* OTHFUEL 126 */
idex++;
bufout[odex] = buffer[idex];
odex++;
bufout[odex] = comma;
odex++;
/* ANNMIL 155-160 */
idex = 154;
for (count=l; count <=6; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* MPG 170-172 */
idex = 169;
for (count=l; count <=3; count++) {
A-5
-------
TIHDCF.C
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PLOCAL 183-185 */
idex = 182;
for (count=l; count <=3; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PSHORT 186-188 */
for (count=l; count <=3; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PSMED 189-191 */
for (count=l; count <=3; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PLMED 192-194 */
for (count=l; count <=3; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PLONG 195-197 */
for (count=l; count <=3; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* TIUGVW 421-422 */
idex = 420;
for (count=l; count <=2; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = comma;
odex++;
/* PKGVW 423 */
bufout[odex] = buffer[idex];
odex++;
A-6
-------
TIHDCF.C
idex++;
bufout[odex] = comma;
odex++;
/* PKRWGT 424-429 */
for (count=l; count <=6; count++) {
bufout[odex] = buffer[idex];
odex++;
idex++;
}
bufout[odex] = '\n';
odex++;
bufout[odex] = '\0';
fputs(bufout,fout);
}
fclose(fout);
puts("\nfile 2 written");
fclose(fin);
A-7
------- |