United States Air and Radiation EPA420-R-02-018
Environmental Protection July 2002
Agency M6.HDE.001
svEPA Update of Heavy-Duty
Emission Levels (Model
Years 1988-2004) for
Use in MOBILE6
> Printed on Recycled Paper
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EPA420-R-02-018
July 2002
Of
for Use in
M6.HDE.001
Christian Lindhjem
Trade Jackson
Megan Beardsley
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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Table of Contents
Introduction -3-
Background -3-
Emissions Testing -6-
Changes for MOBILE6 -7-
Methodology -8-
Results of Analysis -10-
Grams per Brake-horsepower-hour Emission Factors for Use in MOBILE6 -16-
Altitude Adjustment Factors -22-
References -24-
Appendix A
Comments Regarding Heavy-duty Engine Emission Conversion Factors and OTAQ
Responses to Comments -25-
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Introduction
To estimate emissions from heavy-duty vehicles, the MOBILE6 model uses updated
emission factors for 1988-and-later model year vehicles. This report describes the development
of work-specific heavy-duty engine emission factors for hydrocarbons (HC), carbon monoxide
(CO) and oxides of nitrogen (NOx) for model years 1988 through 2004. This final version of
the report includes changes to improve clarity and to fix errors in the draft version. It also
includes comments received on heavy-duty conversion factors.
As in previous versions of the MOBILE model, gram per mile (g/mi) emission factors
used in the model were determined by multiplying the work-specific emission factor (in units of
grams per horsepower-hour (g/bhp-hr)) by a conversion factor which converts work units into
mileage units (bhp-hr/mi). The conversion factors are detailed in MOBILE6 report
M6.HDE.004.[1] Comments on this report are listed in Appendix A, below. Heavy-duty
engine emission factors for model years 2005-and-later are described in MOBILE6 report
M6.EXH.004.[2] Emission factors for particulate matter (PM) are described in the technical
report for MOBILE6.1 .[3]
Note, after the emissions factors described here were developed, EPA discovered that
some heavy-duty engines were equipped with alleged "defeat devices" such that certification
tests did not accurately represent the steady-state NOx emissions. A settlement between the
engine manufacturers and EPA requires additional emissions improvements beyond the standards
described in this report. In MOBILE6, the emission factors described here are adjusted to
account for both the excess NOx emissions due to the alleged defeat devices and the emissions
improvements required in the settlement. The adjustments are described in the MOBILE6 report
M6.HDE.003.[4]
Background
EPA defines heavy-duty vehicles as those vehicles exceeding 8,500 Ibs. gross vehicle
weight (GVW). As noted in Table 1, this broad class of vehicles is divided into those requiring
gasoline or diesel fuels, and is further subdivided into more specific classes based on GVW
categories. EPA uses this more detailed subdivision scheme to account for different
characteristics and general uses of the engines included in each GVW class.
Emissions of air pollutants from heavy-duty vehicles, particularly heavy-duty diesel
vehicles, have come under increased scrutiny in recent years. This increased attention is due to
three main factors: 1) EPA's past emphasis on control of emissions from passenger cars and
light-duty trucks has effectively reduced the proportional contribution of these sources to mobile
source air pollution, and hence has increased the relative significance of heavy-duty emissions; 2)
the public has become increasingly concerned about the human health and environmental
impacts of emissions of parti culate matter and nitrogen oxides, both of which are emitted in
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relatively large amounts from heavy-duty diesel engines; and 3) advances in emission control
technology have increased the cost effectiveness of regulating heavy-duty engines.
Table 1. Heavy-Duty Vehicle Classifications used in MOBILE6
Designation
Description
Gross Vehicle Weight (Ibs.)
Gasoline Vehicles
HDGV (class 2B)
HDGV (class 3)
HDGV (class 4)
HDGV (class 5)
HDGV (class 6)
HDGV (class 7)
HDGV (class 8a)
HDGV (class 8b)*
Diesel Vehicles
HDDV (class 2B)
HDDV (class 3)
HDDV (classes 4)
HDDV (class 5)
HDDV (class 6)
HDDV (class 7)
HDDV (class 8A)
HDDV (class 8B)
Buses
HDGB
HDDB (school)
HDDB (transit & urban)
Light heavy-duty gasoline trucks
Light heavy-duty gasoline trucks
Light heavy-duty gasoline trucks
Light heavy-duty gasoline trucks
Medium heavy-duty gasoline trucks
Medium heavy-duty gasoline trucks
Heavy heavy-duty gasoline trucks
Heavy heavy-duty gasoline trucks
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
Heavy-duty gasoline buses (all
types)
Heavy-duty diesel school buses
Heavy-duty diesel transit & urban
buses
8,501-10,000
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
8,501-10,000
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
*Few HDGVSb exist.
EPA has been regulating air pollutant emissions from heavy-duty gasoline and diesel
vehicles since the 1970s. Since manufacturers of individual types of heavy-duty engines may sell
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these engines to multiple vehicle manufacturers for use in different applications (for both on-
highway and off-highway vehicles), EPA has developed emission standards for heavy-duty
engines instead of vehicles.
In response to the need to further reduce air pollution at the national level, EPA has
finalized a new set of combined emission standards for nitrogen oxides (NOx) and non-methane
hydrocarbons (NMHC, hereafter referred to as HC) from heavy-duty engines, to become effective
in model year 2004 (for diesel) and 2005 (for gasoline), and additional standards to become
effective in 2007. Tables 2 and 3 list the emission standards for heavy-duty gasoline and heavy-
duty diesel vehicles respectively from the mid-1980s until 2004. (The 2005 and 2007 standards
are described in the report M6.EXH.004 [2]).
Table 2. Emission Standards for New Heavy-Duty Gasoline Engines
Model Year
1987 (A)*
(B)*
1988-1990 (A)*
(B)*
1991-1997 (A)*
(B)*
1998-2004 (A)*
(B)*
Pollutant (g/bhp-hr)
Hydrocarbons
(HC)
1.1
1.9
1.1
1.9
1.1
1.9
1.1
1.9
Carbon Monoxide
(CO)
14.4
37.1
14.4
37.1
14.4
37.1
14.4
37.1
Nitrogen oxides
(NOx)
10.6
10.6
6.0
6.0
5.0
5.0
4.0
4.0
Paniculate Matter
(PM)
N/A
N/A
N/A
N/A
* (A) refers to heavy-duty gasoline engines less than 14,000 Ibs. GVW.
(B) refers to heavy-duty gasoline engines greater than 14,000 Ibs. GVW.
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Table 3. Emission Standards for New Heavy-Duty Diesel Engines
Model Year
1985-1987
1988-1989
1990
1991-1992
1993
1994-1995
1996-1997
1998-2003
2004+
Pollutant (g/bhp-hr)
Hydrocarbons
(HC)
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
**2.5 combined
NMHC + NOx
Carbon
Monoxide
(CO)
15.5
15.5
15.5
15.5
15.5
15.5
15.5
15.5
15.5
Nitrogen oxides
(NOx)
10.7
10.7
6.0
5.0
5.0
5.0
5.0
4.0
**2.5 combined
NMHC + NOx
Paniculate Matter
(PM)
None
0.6
0.6
0.25
0.25 truck
0.10 urban bus
0.10 truck
0.07 urban bus
0.10 truck
0.05 urban bus
0.10 truck
0.05 urban bus
0.10 truck
0.05 urban bus
** The 2004 standards apply to all GVW classes, and is defined as a combined non-methane hydrocarbon plus
nitrogen oxides (NMHC + NOx) emission standard of 2.5 g/bhp-hr.
In the above tables, one should note that heavy-duty gasoline emission standards are
GVW-specific, while heavy-duty diesel emission standards apply to all GVWs. Also note that,
for the most part, technical changes to engine design over the years were made in response to
these emission standards. That is, engine design changes rather than emission control technology
per se (e.g., catalytic converters, O2 sensors) have been the primary means of compliance with
heavy-duty engine emission standards to date.
Emissions Testing
Testing of heavy-duty vehicles to determine emissions may be performed in two ways.
The first method involves removing the engine from the test vehicle's chassis (frame), mounting
it on a test stand, and operating the engine on a testing apparatus known as an engine
dynamometer. The second method involves testing the engine while it is still in the vehicle by
operating the entire vehicle on what is known as a chassis dynamometer. The latter method is
very similar to the approach used to test light-duty vehicle and light-duty truck emissions.
Emission levels produced on the engine dynamometer are measured in grams per brake
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horsepower-hour (g/bhp-hr) or grams per kilowatt-hour (g/kW-hr) for a given test cycle, while
emissions produced on a chassis dynamometer are measured in grams per mile (g/mi) or grams
per kilometer (g/km). The results of these emissions tests are used to develop emission factors
for heavy-duty vehicles that are then used in mobile source modeling and inventory development.
Both testing methods have certain limitations. Use of chassis dynamometers allows the
investigator to directly account for the impacts of factors such as load and grade on emissions,
thus providing a better sense of emissions due to real-world driving conditions. However, in-
use emission factors for heavy-duty engines are more difficult to determine than for light-duty
engines because chassis dynamometers capable of testing these heavy, larger vehicles are not
widely available. Furthermore, manufacturers of heavy-duty engines may sell these engines for
use in a variety of applications. Given these factors, the usual test procedure for emission
certification is testing the engine on an engine dynamometer.
Heavy-duty engine testing tends to be very costly. Due to the prohibitive costs involved
in obtaining in-use emissions data on heavy-duty vehicles, very little recent test data existed at
the time MOBILESb, the previous version of the MOBILE model, was developed. Therefore, the
heavy-duty emissions factors in MOBILESb (1996) are the same ones that were developed for
use in MOBILE4 (1989). The 1980 through 1990 model year emissions factors used in the
models are based on data derived from a cooperative test program between EPA and engine
manufacturers, involving 18 heavy-duty gasoline engines (model years 1979 to 1982) and 22
heavy-duty diesel engines (model years 1979 to 1984). In MOBILESb, emissions rates from the
cooperative program were used unless the certification results were higher than those produced
from the test program. In cases where the certification results were greater, that rate was used
instead.
Changes for MOBILE6
Since the release of MOBILESb, very little new data on in-use heavy-duty engines, using
representative driving cycles, have been produced. In lieu of actual data on in-use engines, EPA
proposed the use of test data required by EPA from engine manufacturers for new engine
certification as a surrogate for in-use emissions data.
Under the EPA certification test procedure, manufacturers are required to submit
emissions data on new engines using an engine dynamometer test. The engines are run on a
transient engine dynamometer test cycle (developed from in-use data), and emission results are
given in grams of pollutant per brake horsepower-hour.
Using this EPA engine dynamometer test cycle in the cooperative test program between
EPA and engine manufacturers, the test results indicated that emission-control performance in
heavy-duty vehicles does not suffer from significant deterioration over time. Given that these
test data indicate that emission controls on these engines do not deteriorate greatly, and because
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the EPA engine dynamometer test cycle was developed to closely represent the in-use behavior
of these engines, EPA assumed for this analysis that the emission levels produced by the
certification test procedure are representative of the average in-use emission levels.
Methodology
Engine certification data consist of zero-mile level (ZML) emissions (new engine
emissions) typically given in grams of pollutant per brake horsepower-hour (g/bhp-hr), and
additional g/bhp-hr deterioration at the end of the vehicles "useful life." For heavy-duty diesel
engines, the certification data sets also generally include an intended service class for each engine
model (light, medium, heavy, and bus).
Table 4. Intended service classes and useful life for heavy-duty engines
Engine Class
All heavy-duty gasoline engines
Light heavy-duty diesel engines
Medium heavy-duty diesel engines
Heavy heavy-duty diesel engines and buses
Useful Life
(miles)
110,000
110,000
185,000
290,000*
* Under the 2004-and-later standards, the useful life for Heavy HDDEs is 435,000 miles.
The sum of the ZML and the deterioration at useful life must be less than the emission
standard for each pollutant for the engine model to receive EPA certification. While this is true
for individual engines only if no averaging, banking and/or trading provisions are used to offset
excess emissions, for the purpose of modeling average, in-use emissions, as in MOBILE6, such
programs can be ignored.
For this analysis, the emission levels from the certification data were weighted by engine
sales and rated power to produce average emission levels for gasoline and diesel-fueled heavy-
duty engines, beginning with the 1988 model year and ending with 1995 model year data (the
most current available during this analysis). This calculation was performed for ZML and
deterioration emissions, and is illustrated by the following equation:
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Y,(Sales.*HP.*EL.)
Emission Level (EL) = — —
(Sales t* HP t)
Where:
Sales = Sales of a given engine or engine family
HP = Rated horsepower of given engine or engine family
EL = Certification emission data (ZML or deterioration)
A second method of averaging emission levels was identified; this method involved
simply averaging emission level weighted by engine sales. However, EPA opted to use the
method defined by the above equation because this method accounts for differences in rated
power of various engine models. However, the second method does not produce significantly
different results.
The above calculations were performed using certification and sales data for both
gasoline and diesel heavy-duty engines by engine model year. Separate calculations were
performed for hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and
particulate matter (PM). The draft version of this report included PM estimates that were not
used in MOBILE6.0 (which estimates only HC, CO and NOx.). To avoid confusion, PM
estimates have been removed from this final report. The emission factors used in the draft
MOBILE6.1 model (which does estimate PM) are described elsewhere. [3]
In addition to calculating average emission levels for all heavy-duty diesel engines,
calculations were performed for each of the service classes as well. Heavy-duty gasoline engine
certification reports do not include intended service class specification; therefore, for gasoline a
single average emission level for each model year is given.
There are several peculiarities within the certification data that must be noted by anyone
working with the results provided in this report. Manufacturers often supply multiple emission
results for a given engine family, because tests are often run on engines in the same engine family
that are rated at different power levels. For this analysis, multiple results were averaged by
emission level and rated power to avoid double counting the sales information. Another unusual
characteristic of the certification data is that deterioration rates are sometimes given as multiples
of the zero-mile emission rate and at other times as additive emissions to the zero-mile emission
rate. The emission level results presented in this analysis account for these peculiarities and
provide emission rates at the zero mile level and the incremental increase at the end of useful life.
A third caveat involves the reporting of deterioration rates in certification data reports. A
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manufacturer is not permitted to report a negative deterioration. In cases where the manufacturer
observed negative deterioration results, the certification report states that zero deterioration was
found. Therefore, the average deterioration rates calculated from the certification data inflate the
deterioration that the manufacturers have determined. And lastly, because all engines tested for
certification meet the specifications of the manufacturer, the effects of engine malmaintenance
and tampering on emissions are not included in the analysis.
Results of Analysis
Gasoline Engines
The certification data set for heavy-duty gasoline engines is sparsely populated. Close
examination of the data sets seems to indicate that certification data for engine models which
have been "carried over," or sold in subsequent years, have not been recorded in much of the
certification data that EPA acquired for this analysis. This is especially true for the 1992 and
1993 data where only one major manufacturer's engines were reported for 1992 and another
manufacturer's engines were reported for 1993. As it is quite unlikely that only one
manufacturer sold heavy-duty gasoline engines in a given year, we assume that this lack of sales
and emission data is due to a reporting anomaly. This hypothesis is further supported by the fact
that manufacturers of light-duty vehicles may not be required to re-certify models that carry-over;
it is possible that the reporting assumptions were made in the heavy-duty gasoline database. Due
to the data limitations, there is some concern as to the reliability of the emission level
calculations derived from these data sets, particularly 1992 and 1993.
The results of the current analysis are compared to emission levels reported in
MOBILESb model. Tables 5, 6 and 7 present these comparisons for hydrocarbon, carbon
dioxide, and nitrogen oxide emissions, respectively, by model year (1988 through 1995). Model
years 1992 and 1993 are in italics to signify the greater uncertainty involved with the calculations
in these years, as explained above.
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Table 5. Comparison of MOBILESb and Certification Calculation Results for Emission
Levels of Hydrocarbon from Heavy-Duty Gasoline Engines
Model
Year
1988
1989
1990
1991
1992
1993
1994
1995
Zero Mile Level (g/bhp-hr)
MOBILESb
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
Certification
0.59
0.65
0.35
0.30
0.32
0.29
0.42
0.38
Deterioration (g/bhp-hr at
useful life)
MOBILESb
1.10
1.10
1.10
1.10
1.10
1.10
1.10
1.10
Certification
0.26
0.24
0.25
0.21
0.27
0. 15
0.29
0.23
Table 6. Comparison of MOBILESb and Certification Calculation Results for Emission
Levels of Carbon Monoxide from Heavy-Duty Gasoline Engines
Model
Year
1988
1989
1990
1991
1992
1993
1994
1995
Zero Mile Level (g/bhp-hr)
MOBILESb
12.48
12.48
12.48
12.48
12.48
12.48
12.48
12.48
Certification
12.18
15.65
6.89
6.11
6.59
9.77
1.51
7.69
Deterioration (g/bhp-hr at
useful life)
MOBILESb
7.92
7.92
7.92
7.92
7.92
7.92
7.92
7.92
Certification
2.32
3.12
2.34
1.95
4.35
1.22
3.76
3.50
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Table 7. Comparison of MOBILESb and Certification Calculation Results for Emission
Levels of Nitrogen oxides from Heavy-Duty Gasoline Engines
Model
Year
1988
1989
1990
1991
1992
1993
1994
1995
Zero Mile Level (g/bhp-hr)
MOBILESb
5.82
5.82
4.78
3.99
3.99
3.99
3.99
3.99
Certification
5.10
4.82
3.61
3.52
3. 13
3.58
2.80
2.79
Deterioration (g/bhp-hr at
useful life)
MOBILESb
0.33
0.33
0.55
0.55
0.55
0.55
0.55
0.55
Certification
0.49
0.48
0.29
0.34
0.62
0.00
0.54
0.56
Diesel Engines
The following three tables present the calculated emission level results from this analysis
for hydrocarbons, carbon monoxide, and nitrogen oxides. Each table includes a total average
emission level of the pollutant by model year (1988 through 1995), plus average emission levels
by intended service class. For hydrocarbons, certification data for 1988 through 1994 was used;
the certification data employed in the 1997 heavy-duty engine rule regulatory impact analysis
(containing projected sales) [5] was used for this analysis for purposes of consistency. For
purposes of comparison, each table includes emission levels used in MOBILESb.
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Table 8. Modeled and Calculated Hydrocarbon Emission Levels for Heavy-Duty Diesel Engines
Model Year
1988
1989
1990
1991
1992
1993
1994
Zero Mile Level (g/bhp-hr)
MOBILESb
Modeled Total
1.03
1.03
1.03
1.03
1.03
1.03
1.03
Certification Data Calculations
Total
0.56
0.55
0.52
0.37
0.45
0.35
0.26
Heavy
0.42
0.51
NA
0.29
0.21
0.33
0.22
Med.
0.67
0.65
NA
0.40
0.52
0.38
0.31
Light
0.74
0.54
NA
0.51
0.25
0.31
0.26
Bus
NA
NA
NA
0.62
NA
0.30
0.11
Deterioration (g/bhp-hr at useful life)
MOBILE5b
Modeled Total
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Certification Data Calculations
Total
0.03
0.02
0.01
0.01
0.01
0.01
0.01
Heavy
0.02
0.02
NA
0.00
0.00
0.01
0.02
Med.
0.05
0.04
NA
0.00
0.01
0.01
0.00
Light
0.02
0.02
NA
0.01
0.03
0.01
0.01
Bus
NA
NA
NA
0.01
NA
0.00
0.01
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Table 9. Modeled and Calculated Carbon Monoxide Emission Levels for Heavy-Duty Diesel Engines
Model Year
1988
1989
1990
1991
1992
1993
1994
1995
Zero Mile Level (g/bhp-hr)
MOBILESb
Modeled Total
4.68
4.68
4.68
4.68
4.68
4.68
4.68
4.68
Certification Data Calculations
Total
1.87
0.94
1.81
1.32
1.12
1.56
1.05
1.09
Heavy
1.84
0.84
NA
1.81
0.97
1.85
1.09
1.05
Med.
2.11
1.28
NA
1.22
1.23
1.29
0.77
0.98
Light
1.65
0.78
NA
0.28
0.69
0.98
1.20
1.19
Bus
NA
NA
NA
2.70
NA
2.90
1.01
1.12
Deterioration (g/bhp-hr at useful life)
MOBILE5b
Modeled Total
1.16
1.16
1.16
1.16
1.16
1.16
1.16
1.16
Certification Data Calculations
Total
0.38
0.13
0.13
0.11
0.05
0.12
0.08
0.10
Heavy
0.34
0.10
NA
0.08
0.00
0.08
0.10
0.10
Med.
0.44
0.22
NA
0.25
0.04
0.16
0.11
0.22
Light
0.40
0.08
NA
0.00
0.07
0.22
0.04
0.01
Bus
NA
NA
NA
0.00
NA
0.00
0.01
0.01
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Table 10. Modeled and Calculated Nitrogen Oxide Emission Levels for Heavy-Duty Diesel Engines
Model Year
1988
1989
1990
1991
1992
1993
1994
1995
Zero Mile Level (g/bhp-hr)
MOBILESb
Modeled Total
7.93
7.93
5.64
4.60
4.60
4.60
4.60
4.60
Certification Data Calculations
Total
6.0
5.7
4.9
4.5
4.5
4.5
4.3
4.6
Heavy
6.47
6.08
NA
4.59
4.46
4.53
4.52
4.70
Med.
6.64
6.21
NA
4.51
4.57
4.53
4.56
4.67
Light
4.38
4.29
NA
4.41
4.06
4.37
3.85
4.36
Bus
NA
NA
NA
4.55
NA
4.26
4.70
5.09
Deterioration (g/bhp-hr at useful life)
MOBILE5b
Modeled Total
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Certification Data Calculations
Total
0.2
0.2
0.1
0.1
0.1
0.
0.
0.
Heavy
0.28
0.27
NA
0.11
0.04
0.11
0.12
0.05
Med.
0.14
0.18
NA
0.23
0.08
0.06
0.01
0.03
Light
0.02
0.02
NA
0.03
0.00
0.01
0.00
0.01
Bus
NA
NA
NA
0.10
NA
0.00
0.01
0.01
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The certification data file for 1990 model year heavy-duty diesel engines did not report
different emissions for each of the three service classes or for buses. Therefore, EPA has only
reported a total ZML and a total deterioration rate for this model year.
For NOx emissions, the results of the calculations using the certification data are close to
those produced by the MOBILESb. However, the MOBILESb emission level estimates for HC
and CO are higher than those produced by the certification data-based calculations.
Grams per Brake-horsepower-hour Emission Factors for Use in MOBILE6
After reviewing the results of the above calculations, EPA decided to re-compute the
emission levels and deterioration rates based on specific model year groups. These model year
groups represent changes in EPA emission standards.
Table 11. Model-year groups for heavy-duty gasoline engines, heavy-duty diesel engines
and heavy-duty diesel buses for use in MOBILE6
Heavy-duty gasoline engines
Model Year
Group
1988-1989
1990
1991-1997
1998-2004
Emission Limit
10.6 g/bhp-hrNOx
6.0 g/bhp-hr NOx
5.0g/bhp-hrNOx
4.0 g/bhp-hr NOx*
Heavy-duty diesel engines
Model Year
Group
1988-1989
1990
1991-1993
1994-1997
1998-2003
2004+
Emission Limit
10.7 g/bhp-hr
NOx, 0.6 g/bhp-hr
PM
6.0 g/bhp-hr NOx
5.0 g/bhp-hr NOx,
0.25 g/bhp-hr PM
0.10 g/bhp-hr PM
4.0 g/bhp-hr NOx
2.5 g/bhp-hr HC +
NOx
Heavy-duty diesel Buses
Model Year
Group
1988-1989
1990
1991-1992
1993
1994-1995
1996-1997
1998-2003
2004+
Emission Limit
10.7 g/bhp-hr NOx
5.0 g/bhp-hr NOx
0.25 g/bhp-hr PM
0.10 g/bhp-hr PM
(urban buses only)
0.07 g/bhp-hr PM
(urban buses only)
0.05 g/bhp-hr PM
(urban buses only)
4.0 g/bhp-hr NOx
2.5 g/bhp-hr
HC+NOx
* Complete HDGVs could meet optional lower standards in 2003 and 2004. These were not included in the
MOBILE6 model.
-16-
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The certification data values in the previous tables were averaged across these model year
groups to reduce the impact of the data inconsistencies and caveats described above.
To improve the flexibility of MOBILE6's emission factors, EPA has opted to use the
individual emission rates for each intended service class for heavy-duty diesels instead of a single
emission rate. For heavy-duty gasoline engines, since no separate intended service classes are
defined, MOBILE6 will continue to use a single g/bhp-hr emission rate, although, as for all
heavy-duty emission factors, the g/mile rate will be based on service-class specific conversion
factors (see M6.HDE.004 [1]).
Projections for post-1995 model years were also computed. Tables presenting the re-
computed ZMLs and deterioration rates, as well as explanations of the assumptions used in the
projections, follow. All tables below present deterioration rates as g/bhp-hr at 10,000 miles, for
consistency with the MOBILESb framework. Italicized emission rates are projections.
Heavy-Duty Gasoline Engine Inputs for MOBILE6
The heavy-duty gasoline zero mile levels and deterioration rates for HC, CO and NOx are
presented below in Tables 12 through 14. Note that the heavy-duty gasoline engine emission
rates and deterioration levels will also be applied to a separate heavy-duty gasoline bus category
in the model.
HC projections are based on the assumption that no changes occur beyond the 1997
model year.
NOx projections for 1998+ are based on proportioning the emission rates calculated for
1991-1997 by a ratio of the standard in effect in 1998 (4.0 g/bhp-hr) to the standard in effect for
the 1991-1997 model years (5.0 g/bhp-hr).
Since no standard changes occurred between 1988 and 2004 for CO, EPA has assumed
the emission rate calculated for 1991-1997 applies for the 1998-2003 and 2004+ model year
classes. All deterioration rates remain the same as in the 1991-1997 model year group.
Table 12. Heavy-duty Gasoline Engine Emission Rates for Hydrocarbons
Model
Year Class
1988-1989
1990
1991-1997
Zero Mile Level (g/bhp-hr)
MOBILESb
0.92
0.92
0.92
MOBILE6
0.62
0.35
0.33
Deterioration (g/bhp-hr/
10,000 miles)
MOBILESb
0.10
0.10
0.10
MOBILE6
0.023
0.023
0.021
-17-
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1998-2004
0.92
0.33
0.10
0.021
Table 13. Heavy-duty Gasoline Engine Emission Rates for Carbon Monoxide
Model
Year Class
1988-1989
1990
1991-1997
1998-2004
Zero Mile Level (g/bhp-hr)
MOBILESb
12.48
12.48
12.48
12.48
MOBILE6
13.84
6.89
7.10
7.10
Deterioration (g/bhp-hr/10,000
miles)
MOBILESb
0.72
0.72
0.72
0.72
MOBILE6
0.246
0.213
0.255
0.255
Table 14. Heavy-duty Gasoline Engine Emission Rates for Nitrogen Oxides
Model
Year Class
1988-1989
1990
1991-1997
1998-2004
Zero Mile Level (g/bhp-hr)
MOBILESb
5.82
5.82
3.99
3.19
MOBILE6
4.96
3.61
3.24
2.59
Deterioration (g/bhp-hr/10,000
miles)
MOBILESb
0.05
0.05
0.05
0.05
MOBILE6
0.044
0.026
0.038
0.038
Heavy-Duty Diesel Engine Inputs for MOBILE6
Zero mile levels and deterioration rates for HC, CO, and NOx are presented for heavy-
duty diesel engines in Tables 15 through 17. Since no standard changes have occurred for CO
during the 1988-2004 period, emission projections are assumed to be the same as in the 1994-
1997 model year class.
EPA has assumed that HC and CO zero mile levels and deterioration rates for 1998-2003
engines are the same as for 1994-1997 engines. For NOx, a ratio of 4.0 g/bhp-hr to 5.0 g/bhp-hr
has been used to proportion the 1994-1997 emission rates as a means of projecting 1998-2003
emissions.
The 2004 values are based on the split between HC and NOx described in the 2000
-18-
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heavy-duty regulatory impact analysis. [6]
Table 15. Heavy-duty Diesel Engine Emission Rates of Hydrocarbons
Model Year
Class
1988-1989
1990*
1991-1993
1994-1997
1998-2003
2004+
Zero Mile Level (g/bhp-hr)
MOBILESb
1.03
1.03
1.03
1.03
1.03
1.03
MOBILE6
Heavy
0.47
0.52
0.30
0.22
0.22
0.17
Med.
0.66
0.52
0.40
0.31
0.31
0.17
Light
0.64
0.52
0.47
0.26
0.26
0.14
Deterioration (g/bhp-hr/10,000 miles)
MOBILE5b
0.00
0.00
0.00
0.00
0.00
0.00
MOBILE6
Heavy
0.001
0.000
0.000
0.001
o.oo;
o.oo;
Med.
0.002
0.001
0.001
0.001
0.001
0.001
Light
0.002
0.001
0.001
0.001
0.001
0.001
*1990 data was not available by service class.
Table 16. Heavy-duty Diesel Engine Emission Rates of Carbon Monoxide
Model Year
Class
1988-1989
1990*
1991-1993
1994-1997
1998-2003
2004+
Zero Mile Level (g/bhp-hr)
MOBILE5b
4.68
4.68
4.68
4.68
4.68
4.68
MOBILE6
Heavy
1.34
1.81
1.82
1.07
7.07
7.07
Med.
1.70
1.81
1.26
0.85
0.85
0.85
Light
1.21
1.81
0.40
1.19
1.19
1.19
Deterioration (g/bhp-hr/10,000 miles)
MOBILESb
0.04
0.04
0.04
0.04
0.04
0.04
MOBILE6
Heavy
0.008
0.005
0.003
0.004
0.004
0.004
Med.
0.018
0.007
0.010
0.009
0.009
0.009
Light
0.022
0.012
0.004
0.003
0.003
0.003
*1990 data was not available by service class.
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Table 17. Heavy-duty Diesel Engine Emission Rates of Nitrogen Oxides
Model Year
Class
1988-1989
1990*
1991-1993
1994-1997
1998-2003
2004+
Zero Mile Level (g/bhp-hr)
MOBILESb
7.93
5.64
4.60
4.60
3.68
3.68
MOBILE6
Heavy
6.28
4.85
4.56
4.61
3.68
2.11
Med.
6.43
4.85
4.53
4.61
3.69
2.10
Light
4.34
4.85
4.38
4.08
3.26
1.99
Deterioration (g/bhp-hr/10,000 miles)
MOBILESb
0.00
0.00
0.00
0.00
0.00
0.00
MOBILE6
Heavy
0.010
0.004
0.004
0.003
0.003
0.003
Med.
0.009
0.006
0.007
0.001
0.001
0.001
Light
0.002
0.011
0.003
0.001
0.001
0.001
*1990 data was not available by service class.
Heavy-duty diesel bus engines
In MOBILE6, diesel school buses of model year 1988-and-later use the g/bhp-hr emission
factors for Medium Heavy-duty Diesel engines, listed above.
Projections for diesel urban and transit buses essentially follow the same pattern as
heavy-duty diesels, grouping model years according to changes in emission standards, and
computing future emissions in proportion to the future standards.
Table 18. HD Diesel Transit and Urban Bus Engine Emission Rates of Hydrocarbons
Model
Year Class
1988-1989
1990
1991-1992
1993
1994-1995
1996-1997
1998-2003
2004+
Zero Mile Level (g/bhp-hr)
MOBILE5b
1.03
1.03
1.03
1.03
1.03
7.03
7.03
7.03
MOBILE6
0.47
0.52
0.62
0.30
0.08
0.08
0.08
0.08
Deterioration (g/bhp-hr/10,000
miles)
MOBILE5b
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
MOBILE6
0.001
0.000
0.000
0.000
0.000
0.000
0.000
0.000
-20-
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Table 19. HD Diesel Transit and Urban Bus Engine Emission Rates of Carbon Monoxide
Model
Year Class
1988-1989
1990
1991-1992
1993
1994-1995
1996-1997
1998-2003
2004+
Zero Mile Level (g/bhp-hr)
MOBILESb
4.68
4.68
4.68
4.68
4.68
4.68
4.68
4.68
MOBILE6
1.34
1.81
2.7
2.9
1.06
1.06
1.06
1.06
Deterioration (g/bhp-hr/10,000
miles)
MOBILESb
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
MOBILE6
0.001
0.005
0.000
0.000
0.000
0.000
0.000
0.000
Table 20. HD Diesel Bus Transit and Urban Bus Emission Rates of Nitrogen Oxides
Model
Year Class
1988-1989
1990
1991-1992
1993
1994-1995
1996-1997
1998-2003
2004+
Zero Mile Level (g/bhp-hr)
MOBILE5b
7.93
5.64
4.60
4.60
4.60
4.60
3.68
3.68
MOBILE6
6.28
4.85
4.55
4.26
4.88
4.88
3.90
1.95
Deterioration (g/bhp-hr/10,000
miles)
MOBILESb
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
MOBILE6
0.000
0.004
0.000
0.000
0.000
0.000
0.000
0.000
Conversion of Emission Factors to Grams/Mile
The g/bhp-hr emission factors listed above were multiplied by conversion factors [1] to generate
g/mile emission factors actually used in MOBILE6. Note that no conversion factors were
available for 1987+ HDGVSb, so conversion factors for these model years were generated by
using the ratio of 1986 HDGVSa and HDGVSb conversion factors. Also, the categories of buses
used for heavy-duty bus conversion factors were more detailed than the categories used for
heavy-duty bus emission factors, so composite conversion factors were generated by using a
sales-weighted average of the original conversion factors.
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Altitude Adjustment Factors
The MOBILE6 model will calculate emission factors for heavy-duty vehicles at both low-
and high-altitude. Low-altitude emission factors are based on conditions representative of
approximately 500 feet above mean sea level and high-altitude emission factors represent
conditions of approximately 5,500 feet above sea level.
To update the altitude-specific adjustment factors, EPA sought available test data for
heavy-duty gasoline vehicles and heavy-duty diesel vehicles at "low" and "high" altitude. The
following sections describe the data sources used to determine altitude adjustment factors and the
resulting emission rates.
Heavy-duty Gasoline Vehicles Altitude Adjustment Factors
At the time of this analysis, EPA was unable to identify any new studies of the effects of
varying altitude on exhaust emissions from heavy-duty gasoline vehicles. Therefore, MOBILE6
applies the same altitude adjustment factors for heavy-duty gasoline vehicles that were used in
MOBILES. The high altitude adjustment factors for heavy-duty gasoline vehicles are listed
below in Table 21.
Table 21. Heavy-duty Gasoline Vehicle High Altitude Adjustment Factors
for HC, CO, and NOx
Model Year
1987 and later
Altitude Adjustment Factors
Hydrocarbons
1.855
Carbon Monoxide
3.182
Oxides of Nitrogen
0.818
Heavy-duty Diesel Vehicle Altitude Adjustment Factors
EPA identified a small number of studies evaluating the effects of altitude changes on
emissions of hydrocarbons, carbon monoxide, and oxides of nitrogen. These studies are listed in
Table 25, and full citations are provided in the bibliography. To develop new altitude adjustment
factors for heavy-duty diesel vehicles in MOBILE6, EPA calculated the ratio between the
average emission rate at low altitude and the average emission rate at high altitude. Note that
there was some variability in the altitudes used for testing; however, EPA deemed these
differences and their effects on the reported emission levels to be negligible and used all of the
available data. The ratio between low altitude and high altitude will be used in MOBILE6 for all
heavy-duty diesel categories to characterize the effect of altitude changes on emissions. Table 22
lists reported low-and high altitude emission rates, the average emission rates, and the altitude
adjustment factors for heavy-duty diesel vehicles.
-22-
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Table 22. Heavy-duty Diesel Vehicle High Altitude Adjustment Factors for HC, CO, NOx, and PM
Data Source
EPA-68-03-4044 [7]
EPA-68-03-4044 [7]
ES&TVolume31#4[8]
NFRAQS [9]
NFRAQS [9]
NFRAQS [9]
SAE940669 [10]
SAE961166[11]
SAE961974 [12]
Report
V^iir
1989
1989
1998
1998
1998
1998
1994
1996
1996
Engine Type
EPA Caterpillar 3208
EPA Cummins NTC-350
DDC Series 60
DDC Series 50 6047GK28DD2
DDC Series 50 6047GK28DD3
Navistar DTA-466 E250
DDC Series 60
DDC Series 60
DDCSeries 50
Model
V^iir
1980
1984
1989
1993
1993
1993
1994
1991
1995
Low
0.90
0.95
0.14
0.10
0.10
0.30
0.09
0.10
0.10
High
3.76
1.14
0.15
0.04
0.05
0.20
0.14
0.16
0.06
CO
Low
5.48
2.37
2.80
0.90
0.90
0.90
2.77
2.20
1.60
High
20.90
4.47
4.01
3.13
3.51
1.95
4.42
4.46
2.24
JMOx
Low
9.63
5.21
8.00
4.70
4.70
4.50
4.44
4.70
4.65
High
8.59
4.83
5.13
5.88
8.88
4.43
4.39
4.64
4.97
Low
0.63
0.47
0.42
0.08
0.08
0.22
0.21
0.13
0.08
High
1.30
0.68
0.25
0.13
0.10
0.23
0.32
0.30
0.10
Average Emission Rate: 0.3 1
lest Altitude
Low
500
500
500
500
500
500
800
500
500
High
6000
6000
5280
5280
5280
5280
5540
5280
5280
0.63| 2.21| 5.45| 5.61| 5.75| 0.26| 0.38|
ALTITUDE
ADJUSTMENT
FACTORS
* The PM value was not used in MOBILE6.
ES&T=Environmental Science & Technology
NFRAQS=Northern Front Range Air Quality Study
S AE= Society of Automotive Engineers
HC 2.05 CO 2.46 NOx 1.02 PM 1.47*
-23-
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References
[1] U.S. EPA, Update Heavy-Duty Engine Emission Conversion Factors for MOBILE6:
Analysis ofBSFCs and Calculation of Heavy-Duty Engine Emission Conversion Factors,
M6.HDE. 004, EPA420-R-02-005, January 2002. Prepared for EPA by ARCADIS Geraghty &
Miller, Inc. EPA Contract No. 68-C6-0068 Work Assignment No. 0-03 and 1-02.
[2] Koupal, John. Accounting for the Tier 2 and Heavy-Duty 2005/2007 Requirements in
MOIBLE6. Final Report M6.EXH.004. EPA Office of Transportation and Air Quality, EPA420-
R-01-057. 2001
[3] Glover, Edward L., and Cumberworth, Mitch. MOBILE6.1 Particulate Emission Factor
Model Technical Description DRAFT M6.PM.001, EPA420-R-02-012, March 2002.
[4] Glover, Edward L. Development of Heavy-Duty NOx Off-Cycle Emission Effects for
MOBILE6,M6.HDE.003. EPA420-R-02-004, January 2002
[5] U.S. EPA.. Final Regulatory Impact Analysis: Control of Emissions of Air Pollution from
Highway Heavy-duty Engines. EPA Report # A-95-27, V-B-01, 1997.
[6] U.S. EPA. Regulatory Impact Analysis: Control of Emissions of Air Pollution from
Highway Heavy-Duty Engines. EP A420-R-00-010, July 2000.
[7] Human, D. M. and Ullman, T.L. Simulation of High Altitude Effects on Heavy-duty Diesel
Emissions. EPA Report # EPA-68-03-4044. Southwest Research Institute, San Antonio, TX for
U.S. Environmental Protection Agency, Ann Arbor, 1989.
[8] McCormick, R. L., Ross, J. D. and Graboski, M. S.. Effects of Several Oxygenates on
Regulated Emissions from Heavy-duty Diesel Engines. Environmental Science & Technology,
Volume 31, #4, pgs. 1141-1150. American Chemical Society, Columbus OH. 1997.
[9] Watson, J. G., Fujita, E. Chow, J. C., and Zielinska, B. Northern Front Range Air Quality
Study Final Report. Prepared for the Office of the Vice President for Research and Information
Technology, Colorado State University, Fort Collins, CO. 1998
[10] Chaffin, C. A. and Ullman, T. L. Effects of increased altitude on heavy-duty diesel engine
emissions. SAE International Congress and Exposition, Society of Automotive Engineers, Inc.,
Warrendale, PA. 1994.
[11] Graboski, M.S., Ross, J.D., and McCormick, R. L. Transient emissions from No. 2 diesel
and biodiesel blends in a DDC Series 60 engine. SAE International Spring Fuels and Lubricants
Meeting, Society of Automotive Engineers, Inc., Warrendale, PA. 1996.
[12] Daniels, T. L., McCormick, R. L., Graboski, M.S., Carlson, P.N., Rao, V., and Rice, G. W.
The effect of diesel sulfur content and oxidation catalysts on transient emissions at high altitude
from a 1995 Detroit Diesel Series 50 urban bus engine. " 1996 SAE International Fall Fuels and
Lubricants Meeting and Exposition, Society of Automotive Engineers, Inc., Warrendale, PA.
1996.
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Appendix A
Comments Regarding Heavy-duty Engine Emission Conversion Factors and OTAQ
Responses to Comments
The following comments were submitted to OTAQ regarding Heavy-Duty Engine
Emission Conversion Factors for Mobile 6: Analysis ofBSFCs and Calculation of Heavy-Duty
Engine Emission Conversion Factors., EPA420-P-98-015 and Update Heavy-Duty Engine
Emission Conversion Factors for Mobile 6: Analysis of Fuel Economy, Non-Engine Fuel
Economy Improvements, and Fuel Densities., EPA420-P-98-014. Because the contracts for these
reports had ended, neither of the reports were revised with respect to these comments. However,
we thought it was important to document the comments and our responses, and rather than
creating a separate document, we have included them below. Our responses are in bold.
ARGONNE NATIONAL LABORATORY
9700 S. CASS AVENUE, ARGONNE, ILLINOIS 60439
CENTER for TRANSPORTATION RESEARCH
ESD362/B-215
Telephone 630/252-2819
Fax No. 630/252-3443
E-Mail: mqwang@anl.gov
July 16, 1998
Mr. Terry Newell
Mobile6 Review Comments
U.S. EPA Assessment and Modeling Division
2000 Traverwood Drive
Ann Arbor, MI 48105
Re: Comments on
1. Update Heavy-Duty Engine Emission Conversion Factors for Mobile6: Analysis ofBSFCs and
Calculation of Heavy-Duty Engine Emission Conversion Factors., EPA420-P-98-015
2. Update Heavy-Duty Engine Emission Conversion Factors for Mobile6: Analysis of Fuel
Economy, Non-Engine Fuel Economy Improvements, and Fuel Densities., EPA420-P-98-014
Dear Mr. Newell:
We would like to take this opportunity to comment on the two above listed draft reports recently
released by your office. Please find our comments below. If you have questions regarding our
comments, please feel free to contact us.
1. We note that the first draft report acknowledges that while the conversion factor (CF)
based method is reasonable for estimating NOx emissions for heavy-duty vehicles (HDVs),
-25-
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the method may not be applicable to estimating of emissions of CO, HC, and PM, because
not all emissions of these three pollutants are directly related to the force required to drive a
vehicle. We believe that it is critical to develop and use valid methods to estimate emissions
of HC, CO, and PM, as well as NOx for HDVs.
HDVs are increasingly tested on chassis dynamometers. We believe that, eventually, chassis
testing data can be used to replace engine testing data and CFs in the Mobile model for
estimating HDV on-road emissions. We ourselves have observed that Mobile-estimated
HDV emissions are not sensitive to many factors affecting on-road emissions (factors
including vehicle speed and off-cycle driving conditions), partly because the CF-based
method dilutes the effects of these factors since CFs themselves do not change with them.
Use of vehicle chassis testing data will help Mobile accurately predict emissions under
different driving conditions.
We realize that it is not practical to produce and use vehicle chassis testing data to develop
relationships for use in Mobile6 because of resource and time constraints. For Mobile6
development, we suggest that EPA compare HDV emissions estimated with the CF-based
method to chassis-dynamometer-based testing results, in order to learn the differences
between the two. Subsequently, EPA may use available chassis testing results to adjust CF-
based emission estimates within Mobile6.
OTAQ Response: This is a good idea for validation ofMOBILE6 and development
of future OTAQ models. We will consider this as part of our future
modeling efforts.
2. While BSFC data for MY 1987 - 96 FID engines were obtained from six engine makers,
BSFC data for pre-1988 and post-1995 MY FID engines were estimated with regression
relationships that were established with MY 1987 - 1996 data. Technologies employed on
MY 1987 - 1996 vehicles would show different MPG change patterns over time than for pre-
1987 or post-1996 models. We are not entirely comfortable with the wisdom of using
regression relationships here. If regression relationships have to be used, we suggest that
statistics such as R2 and t-test be presented.
Similarly, HDV MPG for MY 1993 - 96 HDVs was projected with regression relationships
that were developed from data for pre-1993 MY HDVs. Again, we question the wisdom of
using regression relationships to predict MPG, since the implementation of new MPG
improvement technologies and enforcement of new emission standards can invalidate
applications of the relationships. It seems more reasonable to use MPG values rated by
vehicle manufacturers for MY 1993 - 96 to estimate MPG for MY 1993 - 96 HDVs. If the
regression relationships have to be used, their R2 and t-test values should be presented.
OTAQ Response: Unfortunately, MPG values rated by vehicle manufacturers were
not provided in the contractor's work. In the future, OTAQ will
explicitly ask for such statistics.
3. In projecting CFs for post-1996 MY HDVs, it was assumed in the reports that the all
available non-engine MPG improvement technologies were already implemented in the U.S.
by MY 1996, and that MPG improvements for future HDVs will be from engine-related
-26-
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technologies, which affect CFs very little. This assumption takes one step backward from the
1988 report, in which penetrations of non-engine MPG improvements were assumed for MY
1986-2000 HDVs.
We note that the draft reports considered the following non-engine MPG improvement
technologies for post-1997 MY HDVs — aerodynamic improvement devices, drivetrain
optimization, low-profile radial tires, speed control, and fan drives. Additional non-engine
MPG improvement technologies could include: (1) additional gains from tires (e.g. "super
singles"), (2) additional reductions in aerodynamic drag by reducing the radiator profile
through improved cooling, and aerodynamic treatments underneath the HDV, (3) lightweight
materials, This improvement would increase payload, so fuel consumption per ton of cargo
hauled would decline, and, indirectly, (4) hybrid powertrains. These technologies may be
implemented on post-1996 MY HDVs, and their use will certainly help reduce CFs of future
HDVs. We estimate that through improved HDV systems in the near-term (by 2005),
reducing power requirements of a 80,000 Ib GVW Class 8 HDV at 65 mph from 215 hp to
181 hp (a 16% decrease), is achievable. Of course, at lower speeds, gains will be smaller.
Reference: OHVT Technology Roadmap, report DOE/OSTI-11690 published by Office of
Heavy Vehicle Technologies, U.S. Department of Energy, Washington (Oct. 1997). This
includes lower aerodyanamic losses, lower wheel losses, lower drivetrain losses, and reduced
accessory loads. This excludes additional gains from reduced weight (which increases
payload) and hybridization (which would benefit Class 3-6 HDVs more than Class 8 HDVs).
OTAQ Response: OTAQ considers these comments to be valid and useful, and will
keep them in mind for future modeling efforts.
4. As a result of the assumptions made in the draft reports regarding implementation of non-
engine MPG improvement technologies, the draft reports estimate constant CFs for MY 1997
- 2050 HDVs. The constant CF values for future HDVs are questionable. Note that the 1988
EPA report on CFs predicted constant CFs for MY 1986 and beyond. The values for MYs
1986 - 96 were rejected by values predicted in the draft 1998 reports. We believe the same
can occur for the 1998 report in some future year when CFs are updated again. Because of
development and implementation of new non-engine MPG improvement technologies for
future HDVs, we believe that CFs of future HDVs will continue to be lowered. Thus, we
would be more comfortable with an asymptotic trend converging on a future CF value lower
than the projected (nominally, 1999) value at which improvement is frozen. We suggest that
a 10% decrease in power requirements is achievable by 2010 in new Class 7-8 HDVs. This
represents improvements in Class 7-8 HDVs over an average speed of about 40 mph.
Additional gains in Class 3-6 HDV efficiency is possible, mainly through hybridization.
However, these improvements are a result of increased engine efficiency.
OTAQ Response: Predicting the future is difficult. OTAQ's general approach with
MOBILE6 has been a conservative one. Again, we will take
these comments into account in future modeling efforts.
Sincerely,
Michael Wang Chris Saricks Frank Stodolsky
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