United States Air and Radiation EPA420-R-01-041
Environmental Protection July 2001
Agency M6.EXH.011
xvEPA MOBILE6 Estimates of
Exhaust Emissions for
1994-and-later Light
Duty Diesel Cars and
Trucks
$5b Printed on Recycled
Paper
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EPA420-R-01-041
July 2001
of for
Light Duty Trucks
M6.EXH.001
Megan Beardsley
David Brzezinski
Venkatesh Rao
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
NOTICE
This technical report does not necessarily represent final EPA decisions or positions.
It is intended to present technical analysis of issues using data that are currently available.
The purpose in the release of such reports is to facilitate the exchange of
technical information and to inform the public of technical developments which
may form the basis for a final EPA decision, position, or regulatory action.
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MOBILE6 Estimates of Exhaust Emissions for
1994-and-later Light Duty Diesel Cars and Trucks
INTRODUCTION
Light duty diesel cars and trucks form a very small part of the highway vehicle fleet, and
emissions data on these vehicles is extremely limited. For these reasons, in developing
MOBILE6, EPA chose not to update all the light duty diesel (LDD) emission factors. MOBILE6
will generally use the same LDD emission factors as used in MOBILES. The MOBILES
emission factors are separated into start and running emission factors (diesel vehicles have
negligible evaporative emissions) as described in MOBILE6 report M6.EXH.005.
However, in the time since MOBILES was developed, several new standards for light
duty diesels have come into effect. This makes it necessary to update the MOBILE emission
rates for 1994-and-later light duty vehicles. The new standards includes Tier 1 standards that
came into effect in 1994, and Low Emission Vehicle (LEV) standards that came into effect in
2001. Additional factors will be phased in starting in 2004 (Tier 2). This paper explains how
emission factors for light duty diesel vehicles meeting these new standards will be modeled in
MOBILE6.
APPROACH
Only a small amount of in-use data is available on emission rates from Tier 1 and newer
diesel vehicles and trucks1'2. And, unfortunately, much of this data was gathered under non-
Federal Test Procedure conditions or on vehicles that are classified in MOBILE6 as light heavy-
duty diesel vehicles. Thus, we have used the simple approach of applying ratios of the
appropriate standards to the Tier 0 estimates of emission rates for these classes of vehicles in
MOBILES.
Table 1 summarizes the relevant Tier 1 and LEV standards for Light Duty Diesel vehicles
and trucks. In general, these standards apply to both diesel and gasoline powered vehicles.
While MOBILE6 has four light duty gasoline truck classes to correspond to the four truck
classes in the vehicle standards, MOBILE6 will have only two light-duty diesel truck classes
(light light-duty trucks (LDDT12) which includes diesel LDT1 and LDT2, and heavier light-duty
trucks (LDDT34) which includes diesel LDT3 and LDT4). Diesel fleet sales data3 indicates that
there have not been (and are not projected to be) any significant LDDT1, LDDT2, or LDDT3
sales since model year 1988. Therefore, for simplicity, the LDT2 emission standard will be used
to compute emissions for the MOBILE6 LDDT12 vehicle class and the LDT4 emission standard
will be use to calculate emissions for the MOBILE6 LDDT34 vehicle class.
Not all Tier2 vehicles are required to meet the same emission standard. Instead, Tier 2
emission standards are phased in by bin, that is, each bin has a specific emission standard, and
1
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each year, manufacturers must assure that a certain fraction of the fleet falls into that bin. In
MOBILE6, the base emission rate is calculated using the weighted average emission standard for
a specific model year.
In MOBILE6, the basic emission rate for Tier 1 and later vehicles will be calculated using
a ratio of the listed standards and the MOBILES-based emission factor for Tier 0 vehicles. The
ratio will be applied as follows:
Tier "X" BERs = Tier 0 BER
Tier"X" Standard
Tier 0 Standard
Where:
Tier "X" BER =
Tier 0 BER =
the basic emission rate for start or running emissions for a given
vehicle class, standard and pollutant. It is a function of vehicle
mileage.
the basic emission rate for start or running emissions for a given
pollutant for vehicles of a given class meeting the "Tier 0"
standard. It is a function of vehicle mileage.
Tier "X" Standard = the emission level for a given standard, vehicle class and pollutant
as listed in Tables 1, or the average Tier 2 standard applying to the
model year, vehicle class and pollutant.
Tier 0 Standard =
the Tier 0 standard for a given vehicle class and pollutant, as listed
in Table 1.
As a simplification, in cases where the standard is listed in non-methane organic gases rather
than non-methane hydrocarbons (organic gases include aldehydes), the speciation is ignored for
the purpose of calculating base emissions; this may slightly overestimate hydrocarbon emissions
for LEVs. Also, for Tier 2 LDDTs, the model ratios "50K" and "full useful life" standards,
which also may lead to a slight underestimate of Tier 2 emissions for these vehicles. We believe
these approximations are appropriate for the MOBILE6 model because these vehicles make up
such a small fraction of the total highway vehicle fleet. However, in future highway vehicle
models, we hope to replace these estimates with estimates based on emissions data from these
advanced light duty diesel vehicles.
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Table 1
Light-Duty Exhaust Emissions Standards4
Vehicle
Tvne
LDDV
LDDT12 (c)
LDDT34 (c)
Emission
Standard
TierO
Tier 1
TLEV
LEV
ULEV
Tier 2 (e)
TierO
Tier 1
TLEV
LEV
ULEV
Tier 2 (e)
TierO
Tier 1
TLEV
LEV
ULEV
Tier 2 (e)
NMHC
p/mi
0.34 (a)
0.25
0.125(b)
0.075 (b)
0.040 (b)
0.67
0.40
0.20 (b)
0.13 (b)
0.07 (b)
0.67
0.56
0.28 (b)
0.28 (b)
0.167(b)
CO
p/mi
3.40
3.40
3.40
3.40
1.70
10.00
5.50
5.50
5.50
2.80
10.00
7.30
7.30
7.30
3.70
NOx
p/mi
1.00
l.OO(d)
0.40
0.20
0.20
1.70
0.97
0.90
0.50
0.50
1.70
1.53
1.50
1.50
0.80
Vehicle Age
50k Standards
50k Standards
50k Standards
50k Standards
50k Standards
50k Standards
Full Life Standards
Full Life Standards
Full Life Standards
Full Life Standards
Full Life Standards
50k Standards
Full Life Standards
Full Life Standards
Full Life Standards
Full Life Standards
Full Life Standards
50k Standards
Notes: (a) Standard is in THC, but NMHC equivalent is used for these computations.
(b) NMOG
(c) LDT2 standards used for LDDT12; LDT4 used for LDDT34
(d) The Tier 1 NOx standard listed here applies to diesel vehicles only.
(e) Tier 2 standards are described in M6.EXH.004.5
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REFERENCES
1. J.G. Watson, E. Fujita, J.C. Chow, B. Zielinska, L.W. Richards, W. Neff, D. Dietrich,
"Northern Front Range Air Quality Study Final Report", Colorado State University, 1998.
2. "Investigation of Exhaust Emissions from Light Heavy-Duty Vehicles as a Function of
Payload," Prepared for U.S. EPA by Center for Environmental Research and Technology,
Quarterly Progress Report Number 1, September 1999.
3. "Fleet Characterization Data for MOBILE6," Tracie R. Jackson, MOBILE6 Report
M6.FLT.007, April 1999.
4. "Exhaust Emission Certification Standards." U.S. EPA Office of Mobile Sources, EPA420-
B-98-001. March 24, 1998.
5. "Accounting for the Tier 2 and Heavy-Duty 2005/2007 requirements in MOBILE6" John
Koupal, MOBILE6 Report M6.EXH.004. June 2001.
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