United States Air and Radiation EPA420-P-99-012
Environmental Protection April 1999
Agency M6.FUL.004
vvEPA Modeling Emission
Factors for Compressed
Natural Gas Vehicles
> Printed on Recycled Paper
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EPA420-P-99-012
April 1999
for
M6.FUL004
Janet Kremer
Assessment and Modeling Division
Office of Mobile Sources
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 which 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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-DRAFT-
Modeling Emission Factors for Compressed Natural Gas Vehicles
Report Number
M6.FUL.004
Janet C. Kremer
U.S. E.P.A. Assessment and Modeling Division
April 1999
1.0 Introduction
Under the Clean Air Act (CAA)Amendments of 1990, states which have serious, severe, or
extreme ozone non-attainment areas, and areas with carbon monoxide design values greater than
16 ppm must establish a Clean Fuel Fleet (CFF) Program. Under the CFF program, a certain
percentage of light-duty vehicles, light-duty trucks, and heavy-duty vehicles acquired by certain
fleet owners located in covered areas will be required to meet clean-fuel emissions standards.
Clean Fuel Fleet emissions standards are lower than those set by the CAA. Many states have
incorporated compressed natural gas vehicles (CNGVs) into their CFF programs, and need to
model the emission factors and benefits obtained through the addition of CNGVs to the in-use
vehicle fleets in their states.
This document examines the emissions benefits of CNGVs and how these benefits will be
modeled in MOBILE6. It also describes the data and methodology used to determine these
benefits.
2.0 Background
The current version of the MOBILE model, MOBILES, allows for two fuel types, gasoline and
diesel. The model allows for variation in gasoline fuel by permitting a user to specify different
Reid vapor pressures and oxygen content. It also estimates emissions of vehicles operating on
reformulated gasoline. MOBILES also recognizes vehicles that certify to lower emission
standards, such as Low Emitting Vehicles (LEVs), Ultra Low Emitting Vehicles (ULEVs), and
Zero Emitting Vehicles (ZEVs). MOBILES assumes that all of these vehicles are used and
accumulate mileage in the same way as other vehicles of the same age, and that all (except
ZEVs) are fueled by gasoline.
MOBILE6 will allow users to specifically model the exhaust and evaporative emission benefits
of certified Original Equipment Manufacturer (OEM) and certified conversion CNGVs,
operating only on CNG fuel. Since any on-board supply of gasoline raises evaporative emissions
issues and the magnitude of such emissions will be a function of details of the vehicle design and
usage patterns, we are reserving development and release of guidance on estimating emission
reduction benefits of hybrid or dual-fuel vehicles (such as those that may have a small supply of
gasoline on board for emergency use) until better information on their design and likely in-use
emissions performance is available. Users will be able to include both light- and heavy-duty
CNG vehicles and trucks in their modeling.
3.0 Data
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The data used in this analysis were provided in a report entitled "Comparison of Off-Cycle and
Cold-Start Emissions from Dedicated NGVs and Gasoline Vehicles" by Engine, Fuel, and
Emissions Engineering, Incorporated (EF&EE).1 EF&EE was contracted by the Gas Research
Institute (GRI) to conduct an automotive emission testing program that would compare emissions
from CNGVs to emissions from their gasoline fueled counterparts. The study consisted of
twelve vehicles, six CNGVs and six gasoline counterparts. For each fuel type there were two
light-duty vehicles, two light-duty truck class 1 (LDT1, up to 6000 Ib GVW) , and two light-duty
truck class 2 (LDT2, 6001-8500 Ib GVW). Exhaust emissions were measured using several
different procedures, but this analysis considered only the data obtained through testing using the
Federal Test Procedure (FTP).
4.0 Light-duty Analysis
4.1 Methodology
Many CNGVs can be certified to the ULEV emissions standards set by the Environmental
Protection Agency (EPA). Manufacturers have tested CNG vehicles that have had emission rates
lower than the ULEV standard. In order to insure that CNG vehicles are credited with the correct
modeling benefit, EPA performed a comparison between the CNGV emission data and the
ULEV emission factors proposed for use in MOBILE6. The CNG vehicles in the data set were
relatively new and had low mileage accumulation, therefore they were compared with the
emission factors for normal emitting ULEV vehicles at the same mileage. This comparison was
performed for both the light-duty vehicles (LDV) and light-duty trucks (LDT) in the data sample.
Table 1 contains the estimated emission values for M6 ULEVs, average measured emissions data
from CNGVs, and the percent change between the two vehicle types. The standard deviation for
CNGV emissions is also given for the readers information. Due to the limited sample size, p-
values were not calculated.
Table 1: Emission Values and Percent Change for Predicted M6 ULEVs and Measured CNGVs
Vehicle Type
and Pollutant
LDV - NMHC
-CO
-NOx
LDT1 -NMHC
-CO
-NOx
LDT2 - NMHC
-CO
-NOx
Estimated M6
ULEV
Emissions
.026
.544
.156
.026
.544
.156
.03
.781
.265
Average
Emissions from
CNGVs
.025
.62
.058
.03
.23
.122
.03
1.09
.473
Percent Change
3.9% decrease
13. 9% increase
62.8% decrease
15. 4% increase
57.7% decrease
2 1.8% decrease
N/A
39.5% increase
78. 5% increase
Standard Deviation
for CNGV
Emissions
.008
.444
.055
.007
.046
.107
.018
.548
.228
1 A copy of this report can be obtained by contacting Gas Research Institute, 8600 West
Bryn Mawr Avenue, Chicago, Illinois 60631-3562
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4.2 Light-Duty Vehicle Analysis
The average emissions at an average mileage for light-duty vehicles in the data set were
compared to predicted MOBILE6 ULEV emission factors at the same mileage. Figure 1 shows
the graphical comparison. Figure 1 shows a slight decrease in CNG non-methane hydrocarbons
(NMHC) and a decrease in oxides of Nitrogen (NOx). An increase in carbon monoxide (CO)
emissions for CNG vehicles, relative to gasoline vehicles, was observed using this comparison.
Emissions for LDV M6 ULEVs vs Data
0.700 -T-
0.600 -
|T 0.500 --
E
| 0.400 -
&
« 0.300 --
o
•| 0.200 -
LLJ
0.100 4-
o.ooo
!
DLDV
! !
M6 UL_NMHC Data
CNG NMHC
M6 UL CO Data CNG CO M6 UL NOx Data CNG NOx
Figure 1: Emissions for LDV ULEVs M6 vs CNG Data
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4.3 Light-Duty Truck Analysis
The data set used contained two weight classes of light-duty trucks, Light-Duty Truck 1 (LDT1)
and Light-Duty Truck 2 (LDT2). The average emissions for each pollutant from each truck class
were compared to the estimated MOBILE6 ULEV emission factors for that truck class at the
same milage. Figures 2 and 3 show the comparison.
Figure 2: Emissions for LDT1 M6 ULEVs vs CNG Data
Emissions for LDT1 M6 ULEVs vs Data
0.600 -r
0.500 -
1 0.400
J, 0.300
in
% 0.200
in
w 0.100 +
0.000
DLDT1
M6UL NMHC
Data
CNG NMHC
M6 UL CO
Data CNG CO M6 UL NOx Data CNG NOx
Figure 3: Emissions for LDT2 M6 ULEVs vs Data
Emissions for LDT2 M6 ULEVs vs Data
1.200 -r
1.000 -
= 0.800
"Si
ra 0.600
in
0.400
E
LLJ
0.200 --
0.000
DLDT2
M6 UL NMHC
Data
CNG NMHC
M6 UL CO Data CNG CO M6 UL NOx Data CNG NOx
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Figure 2 shows a decrease in CNGNMHC, CO, and NOx emissions. Figure 3 shows no change
in CNG NMHC, while there is a noticeable increase in CO and NOx emissions. It is hard to
justify the inconsistencies seen in these comparisons. It may be due to the small sample size
(only two CNG vehicles of each vehicle type), or a problem with the vehicles themselves.
4.4 Results and Conclusions
The analysis shows some inconsistencies in the benefits associated with CNG vehicles. Based on
this analysis, MOBILE6 will set NMHC, CO and NOx exhaust emissions for light-duty CNG
vehicles and trucks equal to those of gasoline ULEV vehicles and trucks. Also, since there are
no evaporative emissions associated with gaseous fueled vehicles (such as CNGVs), MOBILE6
will calculate zero evaporative emissions for CNG vehicles.
5.0 Heavy-duty Vehicles
5.1 Methodology
Some heavy-duty CNG engines are used in buses, sanitation trucks, long-distance hauling trucks,
etc. and are replacing their diesel and gasoline counterparts. EPA will assume that these heavy-
duty CNG vehicles operate and accumulate mileage in the same fashion as heavy-duty diesel and
gasoline trucks. There was no in-use heavy-duty emissions data available at the time this
analysis was completed, therefore the following methodology was developed to generate
emission factors for MOBILE6
EPA proposes that emission factors for certified heavy-duty CNG vehicles are equal to the
emission factors of heavy-duty diesel engines certified to the year 2004 emission standard of
2.5g/bhp-hr for NOx and NMHC combined. In MOBILE6, the NOx exhaust benefit associated
with heavy-duty CNG vehicles will be determined by which fuel type vehicle and model year it is
replacing in the fleet. For example, if a heavy-duty CNG vehicle is replacing a heavy-duty
gasoline vehicle, current heavy-duty gasoline vehicles have a less stringent emission standard
than heavy-duty diesel vehicles meeting the 2004 standard, therefore the benefit is given by the
difference between the 2004 standard and the standard to which the vehicle it is replacing is
certified. Also, there is an evaporative benefit associated with replacing a heavy-duty gasoline
vehicle with a heavy-duty CNG vehicle. The same method applies if a heavy-duty CNG vehicle
is replacing a heavy-duty diesel vehicle, except that diesel vehicles already exclude evaporative
emissions. There will be no CO or NMHC benefit associated with heavy-duty CNG vehicles.
6.0 CNG Vehicles in MOBILE6
6.1 Start vs Running
MOBILE6 will estimate emission factors for start and running emissions separately for light-duty
vehicles. Thus, the emissions estimates for light-duty CNGVs had to be separated into start and
running components. EPA tried to use FTP bag data to distinguish start and running emissions,
as was done for light-duty gasoline vehicles (as described for gasoline vehicles in M6.STE.002),
but the results were inconsistent. This may be due to the limited sample size or it may be that the
technology and combustion cycle of CNGVs are different enough from conventional vehicles
that applying this methodology is incorrect. Because of the inconsistency, emissions from light-
duty CNG vehicles will be split into start and running using the same proportions used to
separate emissions for ULEV vehicles in MOBILE6 as documented in EPA report number
M6.EXH.006.
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MOBILE6 does not split heavy-duty diesel emissions into separate start and running emission
factors. Hence, heavy-duty CNGV emissions also will not be separated into start and running.
6.2 Correction Factors
Several correction factors are applied to vehicle emissions in MOBILE6. These include off-cycle
corrections, air conditioning corrections and fuel corrections. The same off-cycle and air
conditioning correction factors that are applied to gasoline ULEVs will be applied to CNG
vehicles. CNG vehicles, including heavy-duty, will not be affected by any fuel corrections.
6.3 Inspection and Maintenance. On-Board Diagnostics, and Tampering Assumptions
There are several assumptions made in MOBILE6 concerning the effects that inspection and
maintenance(I/M) programs, on-board diagnostics, and tampering have on the emissions of such
vehicles. The same assumptions that apply for gasoline ULEVs will apply for CNG vehicles in
MOBILE6.
7.0 Conclusion
There are some emissions benefits associated with Compressed Natural Gas vehicles. MOBILE6
will model these benefits for light-duty and heavy-duty vehicles. For light-duty vehicles, CNG
vehicle emissions will be modeled as being equal to the emissions of a ULEV vehicle for CO and
NOx, and NMHC emissions. For heavy-duty vehicles, CNG vehicle NOx emissions will be
equivalent to the emissions of heavy-duty diesel vehicle at the federal 2004 standard of 2.5
g/bhp-hr for NOx and NMHC. There will be no CO or NMHC benefit associated with heavy-
duty CNG vehicles. There will be an evaporative benefit associated with heavy-duty CNG
vehicles that replace heavy-duty gasoline vehicles.
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