United States

Environmental Protection

Agency

Air and Radiation EPA420-P-98-014

May 1998

Update Heavy-Duty

Engine Emission

Conversion Factors for

MOBILE6:

Analysis of Fuel

Economy, Non-Engine

Fuel Economy

Improvements, and Fuel

Densities

 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 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

Update Heavy-Duty Engine

Emission Conversion

Factors for MOBILE6

Analysis of Fuel Economy,

Non-Engine Fuel Economy

Improvements and Fuel

Densities
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 listed in Section V. 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 USEPA 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) = ---------------------------------------------------------

 BSFC (lb/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
2

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 heavyduty 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 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

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) 1995 Transit Passenger Vehicle Fleet Inventory [3]. Fuel economy for the

various common engine types was taken from a National Renewable Energy Laboratory (NREL) study
3

of transit buses [4]. The APTA 1996 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 III(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 lbs) and 2B (8,501 -10,000 lbs), 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.
4

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 Expansion factor

MDLYR Model year

AVGWT Average operating weight 

ENGTYP Fuel type

PKCID Engine size code

AERODN Aerodynamic device?

AXLRAT Optimized axle ratio?

ECOENG Fuel economy engine?

RADIAL Radial tires?

GOVNOR Road speed governor?

VARFAN Variable fan drives?

OTHFUEL Other fuel conservation features?

ANNMIL Annual Mileage during 1992

MPG Fuel Economy

PLOCAL % of mileage for trips < 50 miles from home

PSHORT % of mileage for trips 50-100 miles from home

PSMED % of mileage for trips 100-200 miles from home

PLMED % of mileage for trips 200-500 miles from home

PLONG % of mileage for trips > 500 miles from home

TIUGVW TIUS gross vehicle weight class

PKGVW Polk gross vehicle weight class

PKRWGT Polk registered weight

Table 3. Additional fields in TIUSHDCF.DBF

Field Name Description

WGTCLASS Vehicle class description

TRIP CODE 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
5

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 Description

Local Trips less than 50 miles from home base

Short Trips between 50 and 100 miles from home base

Medium Trips between 100 and 200 miles from home base

Long 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 = ax ) was generated to extrapolate values beyond 1992. Curve fits for each b

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 Gasoline Diesel

2B y = 0.1253x y = 0.1072x 0.9624 1.0506

3 y = 0.1157x y = 0.0989x 0.9632 1.045

4 y = 0.0409x y = 0.502x 1.1902 0.6598

5 y = 0.4416x y = 0.2474x 0.6348 0.8078

6 y = 0.0338x y = 0.5336x 1.2015 0.6117

7 y = 0.1277x y = 4.0206x 0.8909 0.1374

8A y = 0.0647x y = 0.15485x 1.0285 0.8194

8B y = 0.0119x -- 1.3742

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
6

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-1

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 1995 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 DDC Cummins Other

Year Series 50 6V-92TA 8V-92TA 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

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
7

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) 

Fleet Average

No. of Miles Fuel Economy

Transit District Buses (miles) (mpg)

Houston TX 5 282,881 3.63

Miami FL 5 380,453 3.32

Peoria IL 3 225,377 3.51

Minneapolis/St. Paul MN 5 266,338 3.14

 VMT Weighted Average 3.39

Table 8. Determination of average diesel transit bus fuel economies

for the Cummins L-10 (Taken from Reference 4) 

Fleet Average

No. of Miles Fuel Economy

Transit District Buses (miles) (mpg)

Portland OR 5 203,007 4.30

Miami FL 5 330,342 3.61

 VMT Weighted Average 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.
 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.

8

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&B . To calculate diesel school bus fuel economy for Type A&B, the 2

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 between Speed Speed Economy

Cycle Stops

Miles Average Top Fuel

(mph) (mph) (mpg)

CBD 0.142 12.9 20 3.69

COM 4.000 46.5 55 4.99

Ratio of COM to CBD fuel economy 1.352

Table 10. Diesel school bus inventory by model year and type

(Taken from Reference 8)

Model School Bus Type

Year A&B C D Total

90 2225 23670 6286 32181

91 3756 21370 6864 31990

92 3820 16444 5444 25708

93 3535 18928 6734 29197

94 3215 21005 7321 31541

95 2216 20861 9671 32748

96 2225 22016 9270 33511
9

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]:

 141.5 x 8.328

Fuel Density (lbs/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.
10

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 VMT Travel Fraction (%) FE Wgt a b

Class (Mil Miles) Local Short Med Long (mpg) (lbs)

2B 3283.02 64.3 25.7 8.8 1.2 9.2 9490

3 4194.68 65.9 16.2 7.4 10.4 9.0 11997

4 1224.18 46.4 19.8 7.4 26.5 8.2 15274

5 765.40 72.4 19.3 5.9 2.5 7.4 17877

6 1301.57 68.9 17.1 2.3 11.7 7.3 22289

7 443.19 76.6 10.6 10.2 2.6 6.7 29068

8A 165.04 73.0 19.3 5.3 2.4 6.6 39838

 Average weight class fuel economy in miles per gallon a

 Average weight class operating weight in pounds b

Table 12. Projected gasoline heavy-duty vehicle fuel economies (mpg)

Model Weight Class

Year 2B 3 4 5 6 7 8A

87 9.22 8.54 8.32 7.52 7.23 6.83 6.39

88 9.32 8.63 8.43 7.58 7.33 6.89 6.47

89 9.42 8.73 8.55 7.63 7.43 6.96 6.54

90 9.52 8.82 8.66 7.68 7.53 7.03 6.62

91 9.62 8.92 8.78 7.74 7.63 7.10 6.70

92 9.73 9.01 8.89 7.79 7.73 7.17 6.77

93 9.83 9.11 9.01 7.85 7.84 7.24 6.85

94 9.93 9.20 9.12 7.90 7.94 7.31 6.92

95 10.03 9.30 9.24 7.95 8.04 7.38 7.00

96 10.13 9.39 9.35 8.01 8.14 7.45 7.07
11

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 VMT Travel Fractions (%) Fuel Economy (mpg) Average Weight (lbs) a b

Class Mil Miles Local Short Med Long Ave Local Long Ave Local Long

2B 1857.59 55.0 31.4 9.0 4.5 11.9 12.0 9591 9544

3 3751.85 49.2 30.2 8.5 12.1 11.2 12.1 12219 12262

4 1479.63 47.2 37.2 8.1 7.5 9.7 9.5 15123 15185

5 1857.42 55.9 18.8 9.3 15.9 9.4 9.4 9.6 17814 17812 17916

6 5492.44 47.9 28.8 12.8 10.5 8.2 8.0 8.5 22935 22829 23366

7 4768.90 44.0 22.6 10.2 23.2 7.4 7.7 6.6 29906 30074 30350

8A 25088.28 22.7 10.7 9.9 56.6 6.0 6.0 6.0 48881 47622 49900

8B 65513.19 7.3 8.8 9.4 74.4 5.7 5.6 5.7 75784 76575 75063

 Average weight class fuel economy in miles per gallon a

 Average weight class operating weight in pounds b

Table 14. Projected diesel heavy-duty vehicle fuel economies (mpg)

Model Weight Class

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

87 11.69 10.52 9.56 9.12 8.20 7.43 5.96 5.51

88 11.83 10.65 9.63 9.21 8.25 7.44 6.03 5.59

89 11.97 10.77 9.70 9.29 8.31 7.45 6.10 5.68

90 12.11 10.90 9.77 9.38 8.37 7.46 6.17 5.77

91 12.26 11.03 9.85 9.46 8.42 7.47 6.24 5.86

92 12.40 11.15 9.92 9.54 8.48 7.48 6.31 5.95

93 12.54 11.28 9.99 9.63 8.54 7.49 6.38 6.03

94 12.68 11.41 10.06 9.71 8.59 7.51 6.45 6.12

95 12.82 11.53 10.13 9.80 8.65 7.52 6.52 6.21

96 12.96 11.66 10.20 9.88 8.71 7.53 6.59 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. 
12

Table 15. Estimated bus fuel economies (mpg)

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

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 heavyduty 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
13

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 2B 3 4 5 6 7 8A

Long-Haul VMT Fraction 1% 10% 27% 3% 12% 3% 2%

Aero Devices

 TIUS 18% 9% 11% 33% 34% 24% 38%

 MOBILE4 0% 0% 0% 7% 7% 7% 7%

Drive Train Optimization

 TIUS 22% 12% 32% 23% 39% 37% 31%

 MOBILE4 27% 27% 27% 27% 27% 27% 27%

Radial Tires

 TIUS 96% 100% 77% 73% 100% 86% 91%

 MOBILE4 67% 67% 67% 14% 14% 14% 14%

Speed Control

 TIUS 12% 5% 10% 62% 32% 44% 42%

 MOBILE4 13% 13% 13% 4% 4% 4% 4%

Fan Drives

 TIUS 18% 9% 18% 26% 10% 25% 15%

 MOBILE4 0% 0% 0% 90% 90% 90% 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].
14

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 2B 3 4 5 6 7 8A 8B

Long-Haul VMT Fraction 5% 12% 8% 16% 11% 23% 57% 74%

Aero Devices

 TIUS 17% 21% 18% 17% 28% 48% 87% 100%

 MOBILE4 0% 0% 0% 7% 7% 7% 7% 32%

Drive Train Optimization

 TIUS 30% 40% 38% 56% 46% 59% 80% 100%

 MOBILE4 27% 27% 27% 27% 27% 27% 27% 27%

Radial Tires

 TIUS 91% 91% 100% 100% 92% 94% 90% 95%

 MOBILE4 67% 67% 67% 14% 14% 14% 14% 50%

Speed Control

 TIUS 39% 28% 41% 35% 41% 41% 71% 81%

 MOBILE4 13% 13% 13% 4% 4% 4% 4% 14%

Fan Drives

 TIUS 41% 42% 28% 40% 46% 46% 80% 85%

 MOBILE4 0% 0% 0% 90% 90% 90% 90% 100%

Table 18. Gasoline Fuel Densities

API Gravity Density

Year Winter Summer Average lb/gal

1987 62.3 59.2 60.75 6.130

1988 62.5 58.9 60.70 6.131

1989 61.8 58.2 60.00 6.154

1990 62.2 58.2 60.20 6.147

1991 61.8 58.0 59.90 6.157

1992 61.2 57.4 59.30 6.176

1993 61.2 56.1 58.65 6.197

1994 60.8 55.7 58.25 6.210

1995 59.4 56.1 57.75 6.227

1996 60.2 56.9 58.55 6.201

Average 6.173

MOBILE4 6.09
15

Table 19. Diesel Fuel Densities

API Density

Year Gravity lb/gal

1987 34.2 7.112

1988 34.5 7.099

1989 33.8 7.129

1990 34.3 7.107

1991 34.0 7.120

1992 33.7 7.133

1993 34.3 7.107

1994 35.3 7.065

1995 35.4 7.061

1996 35.6 7.052

Average 7.099

MOBILE4 7.11

V. REFERENCES

1. P. Machiele, “Heavy-Duty Vehicle Emission Conversion Factors II - 1962-2000,” EPA report no.

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

2. “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).

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.
16

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.
17

28. T. Baumeister and L. Marks, Standard Handbook for Mechanical Engineers, seventh edition,

McGraw-Hill Book Company, 1967.
APPENDIX
TIHDCF.C

A-1

/* TIHDCF

 Converts the TIUS dataset TI92MDF.DAT to a comma delimited file for

importing into dBASE file TIUSHDCF

*/

#include <stdio.h>

#include <ctype.h>

#define comma 44

char buffer[625];

char bufout[110];

FILE *fin,*fout;

int count;

long n;

int idex,odex;

void main()

{

 fin = fopen("E:TI92MDF.DAT","rb");

 fout = fopen("C:TIHDCF1.DAT","wb");

 n = 0;

 while (n < 125000) {

 fgets(buffer,625,fin);

 n++;

 odex = 0;

 idex = 14;

 /* EXPANF 15-21 */

 for (count=1; count <=7; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* MDLYR 24-25 */

 idex = 23;

 for (count=1; count <=2; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* AVGWT 99-104 */

 idex = 98;

 for (count=1; 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 */
TIHDCF.C

A-2

 idex = 113;

 for (count=1; 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=1; count <=6; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;
TIHDCF.C

A-3

 /* MPG 170-172 */

 idex = 169;

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PLOCAL 183-185 */

 idex = 182;

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PSHORT 186-188 */

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PSMED 189-191 */

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PLMED 192-194 */

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PLONG 195-197 */

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* TIUGVW 421-422 */

 idex = 420;

 for (count=1; count <=2; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;
TIHDCF.C

A-4

 /* PKGVW 423 */

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 bufout[odex] = comma;

 odex++;

 /* PKRWGT 424-429 */

 for (count=1; 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=1; count <=7; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* MDLYR 24-25 */

 idex = 23;

 for (count=1; count <=2; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* AVGWT 99-104 */

 idex = 98;

 for (count=1; 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=1; count <=2; count++) {

 bufout[odex] = buffer[idex];
TIHDCF.C

A-5

 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=1; count <=6; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* MPG 170-172 */

 idex = 169;

 for (count=1; count <=3; count++) {
TIHDCF.C

A-6

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PLOCAL 183-185 */

 idex = 182;

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PSHORT 186-188 */

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PSMED 189-191 */

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PLMED 192-194 */

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PLONG 195-197 */

 for (count=1; count <=3; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* TIUGVW 421-422 */

 idex = 420;

 for (count=1; count <=2; count++) {

 bufout[odex] = buffer[idex];

 odex++;

 idex++;

 }

 bufout[odex] = comma;

 odex++;

 /* PKGVW 423 */

 bufout[odex] = buffer[idex];

 odex++;
TIHDCF.C

A-7

 idex++;

 bufout[odex] = comma;

 odex++;

 /* PKRWGT 424-429 */

 for (count=1; 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);

}