Modeling Mobile-Source Emissions (2000) / Chapter Skim
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3 Technical Issues Associated with the MOBILE Model
3 Technical Issues Associated with the MOBILE Model
Pages 61-134
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From page 61... ...
HISTORY AND STATUS OF THE MOBILE MODEL History of the MOBILE Model The MOBILE model for estimating on-road vehicle emissions factors (in grams per mile [g/mi]
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From page 62... ...
From one model version to the next, these changes can be either increases or decreases in emissions factors, and the changes are not always in the same direction for all three pollutants (NOX, CO, and VOCs)
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From page 63... ...
Elimination of California vehicle emissions rates (contin ued to model low- and high-altitude emissions)
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From page 64... ...
Source: EPA l999e All of the MOBILES emissions factors are estimated from existing test data, and engineering judgment in the absence of test data. Although there is a detailed User's Guide (EPA 1994)
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From page 65... ...
to refer to the general class of gaseous organic compounds the Tier 2 Model does not use the MOBILES methodology and test data for estimating evaporative emissions. MOBILES will likely increase the fraction of evaporative to tailpipe emissions of VOCs compared with that obtained in MOBILE5b.
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From page 66... ...
The significant changes being incorporated into MOP;TT,F:Hi into t.h~ fallowing dramatically lower basic emissions rates, based on analyses of the Dayton, Ohio I/M program data; Reparation of start and running-exhaust emissions; addition of so-called off-cycle emissions (aggressive driving and airconditioning operation, which are not included in the Federal Test Procedure [FTP]
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From page 67... ...
emissions factor estimates for different roadway types (e.g., highways arterials, locals) ; evaporative diurnal emissions factors estimated *
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From page 68... ...
effects of I/M programs on vehicle emissions; addition of off-cycle NOx emissions for heavy-duty diesel vehicles; effects of in-use fuel sulfur content on all emissions; and effects of national low-emissions vehicle (NLEV) and Tier 2 stanAlthough the MOBILE6 documentation provides numerical results for changes in specific model components, overall changes to average in-use fleet emissions factors will not be known until the full model is released.
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From page 69... ...
These are PARTS estimates particulate matter (PM) emissions factors for onroad vehicles; Complex Model estimates emissions impacts of reformulated fuel compositional changes on 1990 light-duty gasoline vehicles; MOBTOX estimates on-road mobile-source toxic emissions factors; and SPECIATE, and related databases and models provide VOC speciation profiles for complex photochemical grid modeling.
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From page 70... ...
PARTS EPA's PARTS mode! estimates PM emissions factors in g/mi for 12 vehicle classes.
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From page 71... ...
CARB's Predictive Model estimates fuel effects for all on-road light-duty vehicles based on a broader database than that used in the development of the Complex Model; EPA should review and consider
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From page 72... ...
MOBTOX5b applies exhaust and evaporative toxic-adjustment factors for various vehicle classes and technologies to MOBILE5b VOC emissions factors. It estimates emissions factors for benzene, formaldehyde, acetaldehyde, 1,3-butadiene, and methyl tertiary-butyl ether GIBBET.
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From page 73... ...
Unlike the MOBILE model, NONROAD provides activity data as well as emissions factors. Thus, it can specifically estimate emissions inventories for off-road equipment.
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From page 74... ...
EPA should place more emphasis on improving both the emissions factors and activity data in this model. SPECIATE and Emissions Processing Systems Photochemical air-quality models require emissions of specific types of VOCs (e.g., formaldehyde, acetaldehyde, alkenes, aromatics, short-chained alkanes, and higher alkanes)
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From page 75... ...
For specific air-quality modeling applications, users enhance the available speciation profile databases and models for their own use. For example, the Auto/Oil Air Quality Improvement Research Program conducted a large test program to develop speciated automobile emissions rates, and used that information in follow-on air-quality modeling studies.
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From page 76... ...
The primary conclusion of the report was that the model likely overestimated the effects of enhanced I/M programs for both exhaust and evaporative emissions. The second Sierra Research study was focused on an important update in the MOBILE5a model-the use of I/M test data to develop LDV basic exhaust emissions rates (all previous versions of the model were based on EPA's FTP testing)
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From page 77... ...
. The report also included an evaluation of the changes in fleet average emissions factors as predicted by MOBILE4, MOBILE4.1, and MOBILE5a.
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From page 78... ...
Emissions estimates and assumptions for the I/M of No HDVs those with a gross vehicle weight of 8,501 pounds or more.
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From page 79... ...
, and EPA's new high-emitter correction factor result in increasing the base emissions rates in MOBILE6. As discussed in the evaluation section of Chapter 4, there are many reasons to suspect that even IM240 data fall short of truly reflecting the realworId fleet emissions, particularly because it is not truly representing the high-emitter category.
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From page 80... ...
Although using the running-emissions correction factor to correct start emissions is undoubtedly better than applying no correction factor, running-emissions correction factors would be less likely to reflect the increased catalyst light-off temperatures associated with older, higher-emitting vehicles. Higher catalyst light-off temperatures, one of the common characteristics of high emitters, would cause the adjustment for cold-start emissions to be relatively higher than the adjustment for running emissions.
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From page 81... ...
The need to measure vehicle emissions over a range of conditions that simulate actual driving has been recognized since the start of vehicle emissions-control programs. However, the original certification test cycle used only steady-state operation and did not include start emissions.
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From page 82... ...
Thus higher engine loads especially those above the loads tested in the FTP can lead to very large emissions rates. The GAO report on the MOBILE model identified three issues related to the underrepresentation of high-load conditions in the model.
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From page 83... ...
~ 0 200 400 600 800 1000 1200 Elapsed time (seconds) 1400 FIGURE 3-3 FTP driving cycle for light-duty vehicles.
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From page 84... ...
Certification to this new cycle will be phased in between the 2000 and 2004 model years. This test procedure should ensure that vehicle emissions control systems will provide improved emissions control over a wider range of vehicle speeds and loads.
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From page 85... ...
From a standpoint of vehicle certification, the high-acceleration loads in the US06 cycle should ensure that vehicle emissions-control systems would be operative during grade-climbing operations in real-world driving. However, MOBILES will not be able to model the effects of road grade on emissions in a local area.
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From page 86... ...
SCF are a function of vehicle type, model year, and pollutant species. SCFs are applied to emissions rates expressed in terms of grams per mile.
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From page 87... ...
Note that expressing emissions factors in units of g/mi creates a problem in describing vehicles at rest (idling)
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From page 88... ...
Other approaches, including the use of emissions rates in units of grams per second or fuel based emissions rates (e.g., in units of grams of emissions per gram of fuel used) could provide a better approach to emissions modeling, particularly at low speeds.
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From page 89... ...
However, such accounting is not satisfactory in many applications, such as air-quality modeling, because it allocates the start emissions in terms of grams per mile to an entire trip as opposed to the specific start location. Start emissions was the third issue identified in the GAO report (GAO 1997)
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From page 90... ...
phase and the emissions measured over the same 505 second driving cycle without a start. (The cold-start and hot-start emissions measured by this process are subsequently adjusted for soak time in MOBILE6, as described below.)
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From page 91... ...
In MOBILES, such emissions were counted as part of the gram-per-mile emissions associated with overall vehicle operation. Having a separate model for start emissions provides a more accurate representation of these emissions.
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From page 92... ...
However, it was felt that the end result was an improvement in estimating deterioration of the real-world fleet. MOBILE6 In-Use Deterioration Compared to MOBILES Figures 3-6, 3-7, and 3-8 present comparisons of MOBILES and proposed MOBILE6 VOC running-emissions factors for Tier 0 vehicles (EPA 199911.
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From page 93... ...
emissions factors as a function of mileage for 1981 model-year passenger cars. Note that this report usually uses the term VOCs as opposed to HCs to refer to the general class of gaseous organic compounds.
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From page 94... ...
3o ~ T 50 T —= tdOO~ 1 1W 150 1 - O FIGURE 3-7 Comparison of MOBILES and MOBILES hydrocarbon (HC) emissions factors as a function of mileage for 1987 model-year passenger cars.
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From page 95... ...
The simplest exhaust emissions test measures idle emissions. This is usually supplemented by a measurement with a no-load engine speed of 2,500 RPM.
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From page 96... ...
Starting with MOBILES, the TECH model will no longer be iTampering as defined in the MOBILE model is the malfunctioning of one or more emissions- control device due to either deliberate disablement or mechanical failure.
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From page 97... ...
. A similar analysis is applied to start emissions as well.
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From page 98... ...
found very little change in on-road fleet emissions that could be attributable to I/M programs. Wenzel (1999)
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From page 99... ...
Thus, one possible discrepancy between the emissions reduction for the remote-sensing and the IM240 results might be due to differences in operating conditions. "Disappearing" Vehicles I/M programs maintain detailed records on vehicle emissions tests.
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From page 100... ...
In these cases, EPA extrapolated the repair results observed to determine what the emissions results would be, if additional repairs could be performed. For MOBILE6, EPA has based repair effectiveness on the results of the Arizona I/M program for 1981-1993 vehicles.
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From page 101... ...
These significant increases are based on a single study in which 11 mechanics participated, repairing three cars each. The emissions comparison is based on the difference between the repair results of the students and those of the trainer.
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From page 102... ...
It is assumed that drivers will not respond to the illuminated MIL unless there is an I/M program in place or the vehicle repair is still under warranty. EPA believes that it might be possible to use an I/M procedure in some future year they suggest 2001 that has no exhaust emissions measurement, only an OBD check.
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From page 103... ...
Early indications are that MOBILE6 will substantially reduce the emissions-reduction benef~ts from I/M compared with MOBILES (Clean Air Report 1999~. EPA appears to be heading towards the use of OBD systems as an alternative to current I/M programs.
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From page 104... ...
All vehicles were tested over 15 different driving cycles. These included the speed-correction factors and facility driving cycles described for determining the effects of actual in-use operation on emissions.
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From page 105... ...
These regression equations have r2 values between 0.25 and 0.80 depending on the pollutant, vehicle type, and emitter class. For start emissions, a single value is used to give the ratio of emissions with air-conditioning to those without air-conditioning.
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From page 106... ...
The compressor-on fraction is assumed to be zero for a heat index of 65°F and below and 1 for a heat index of 110°F and above. The MOBILES model will allow user input on cloud cover.
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From page 107... ...
The measurement and characterization of evaporative VOC emissions is based almost exclusively on certification test procedures with no real-world measurements. New vehicle evaporative emissions certification to the 2 gram per test standard has been based on the sealed-housing for evaporative-determination (SHED)
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From page 108... ...
culty of correctly determining the distribution of emissions in the vehicle fleet and the level of emissions from the high emitters remains a difficult problem. Characterization of Hot-Soaks Hot-soak emissions are evaporative emissions occurring, by definition, during the first hour following engine shutdown.
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From page 109... ...
FUEL EFFECTS Reformulated Gasoline Effects Use of RFGs can be specified in MOBILES input. The effects of RFG on vehicle emissions are determined mainly by the RVP and oxygenated-fuel parameters, discussed below.
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From page 110... ...
In the assessment of the airquality effects of the program, several issues were identified that are relevant to the crediting of such a program in MOBILE5a: The observable reduction in ambient CO levels that could be attributed to the use of fuel oxygenates was lower by a factor of 2 or 3 than the amount predicted by the MOBILE5a model. The MOBILE5a model predicted CO emissions reductions that were about a factor of 3 larger than other EPA models, notably, a version of the Complex Model developed to represent fuel effects for the on road fleet.
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From page 111... ...
This model, which applies strictly to 1990 technology vehicles, was used by EPA to adjust emissions factors in MOBILES to the national average sulfur content of 330 PPM. Appropriate adjustment was also made for Phase I RFG fuel, which assumed an average sulfur content of 220 ppm.
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From page 112... ...
Figures 3-12 through 3-15 provide an indication of the composite correction factors for a range of vehicle technologies. TABLE 3-7 Estimated Emissions Reductions Due to Reductions in Fuel Sulfur for a Normal-Emitting Tier O Vehicle Reduction in Emissions, % (400 ppm to 50 ppm Sulfur)
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From page 113... ...
RVP Effects The Reid vapor pressure is defined as the fuel vapor pressure in pounds per square inch at 100°F. RVP affects vehicle emission primarily in two ways: the fuel evaporation rate and the exhaust emissions.
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From page 114... ...
tion between RVP and evaporative emissions is obvious, as one expects a greater evaporative emissions rate from a fuel with a higher vapor pressure. The link between RVP and exhaust emissions is less obvious but well documented.
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From page 115... ...
For MOBILE6, EPA has not updated the exhaust RVP correction factors, even though there is a wealth of new test data available at low RVP levels, and most areas of the country are using fuels with RVP less than 9 psi in the summer. Although the RVP effects on exhaust emissions are relatively small, credit should still be supplied to states and local areas that are using low-RVP fuels.
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From page 116... ...
For each category, there exist running-loss emissions rates at four temperatures and four RVPs. The model interpolates these to obtain the correct passing and failing running-loss emissions rates and combines these by weighting them according to their expected occurrence in the fleet.
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From page 117... ...
In MOBILES, TCFs for temperatures above 75°F are combined with RVP correction factors. Below that temperature, the correction factors are simple multiplicative factors.
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From page 118... ...
HEAVY-DUTY VEHICLE EMISSIONS Heavy-duty vehicles are those exceeding 8,500 lbs gross vehicle weight (GVW)
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From page 119... ...
A caveat to the use of these data in developing deterioration rates involves manufacturers observing negative deterioration results. Although testing might indicate engine emissions rates that are lower than the ZML, a manufacturer is not permitted to report a negative deterioration.
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From page 120... ...
Conversion Factors To Estimate Vehicle Emissions For MOBILES, EPA (1998i) has updated the estimates of heavy-duty engine emissions factors currently contained in MOBILE5b.
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From page 121... ...
To improve the flexibility of MOBILE6's emissions factors, EPA has chosen to use emissions rates for each service class as shown in Table 3-9 instead of a single rate as used in MOBILES. For heavy-duty gasoline engines, EPA will continue to have a total emissions rate in MOBILE6.
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From page 122... ...
At the present time, EPA has been unable to obtain recent studies as to the effects of varying altitude on exhaust emissions from heavyduty gasoline vehicles. Therefore, MOBILES will use the same adjustment factors used in MOBILES.
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From page 123... ...
Tables 3-10 and 3-11 show this underestimation of PARTS emissions rates for a range of LDVs. Table 3-10 contrasts the results of a test performed for CRC Project E-24-2 with estimates produced by PARTS for light-duty passenger cars and trucks.
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From page 124... ...
Studies are also needed on the effectiveness of diesel I/M programs and whether smoke I/M programs tend to increase NOX TABLE 3-11 Passenger Car PM Emissions Rates in g/mi from PARTS and from the CRC Projects E-24-1 and CRC E-46 Model Year CRC E-24-1 PARTS Pre-1981 .955 .193 1981- 1985 .474 .025 1986-1990 .444 .006 1991-1997 .028 .004 CRC E-46 PARTS Tier 0 .083 .025 Tier 1 .038 .006 l Source: Norbeck et al. 1998; Cadle et al.
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From page 125... ...
Brake-wear emissions factors in PARTS are assumed to be the same for all vehicle classes, although it could be assumed, as with tire wear, that the number of wheels, the weight of the vehicle, and the driving cycle would be significant contributing factors related to the per mile emissions rate. As with tire wear, the dated reference suggests that the emissions factor is based on older materials and needs to be updated.
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From page 126... ...
Once the vehicle-ciassspecific emissions factors are computed, the model then weights each of them by the corresponding fleet mix fraction and sums the results to produce the fleet emissions factor. Although the user is allowed to enter custom registration distributions, mileage- accumulation rates, and fleet mixes, the model contains default (or for fleet mix, internally calculates)
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From page 127... ...
The new model expands the previous 8 vehicle classes to 28, thus requiring much more detailed fleet characterization data. New mileage-accumulation rates for light-duty vehicles and light- and heavyduty trucks are derived from the 1995 National Personal Travel Survey (NPTS)
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From page 128... ...
The final recommendation deals with the need for longrange planning to guide the future development of the model. Obtain Better Data on High-Emitting Vehicles Establish a long-term testing program to characterize in-use deterioration of representatively aged new-vehicle technology using a driving cycle more representative of actual driving conditions.
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From page 129... ...
Improve the Start-Emissions Database Routine tests of start emissions should be made as part of ongoing measurement programs unless there is confirmation that regression techniques, similar to those used for MOBILES, provide an effective estimation of start emissions. Additional measurements of the effects of ambient temperature, wind speed, and soak time on start emissions should be made to get a better representation of these important factors.
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From page 130... ...
The modeling of repaired vehicles' deterioration should be based on data from actual repaired vehicles. Improve the Emissions Factors for Heavy-Duty Vehicles Emissions factors for HDVs are woefully outdated and there are questions about the conversion of engine dynamometer data into on-road gramper-mile emissions.
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From page 131... ...
The facility-specific speed-correction factors also provide greater distinction in roadway classifications. Start emissions have an improved treatment in MOBILE6; more study should be done to provide additional data for the approach proposed.
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From page 132... ...
Quantifying the uncertainty of the model's estimates. New data have shown much lower emissions rates for such vehicles.
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From page 133... ...
It would require the impacts of reformulated gasolines to be extended to include impacts on emissions from all vehicles and technology groups as well as the impacts on CO emissions. Incorporation of Toxic-Emissions Factors into MOBILE These emission factors are currently in a separate model, MOBTOX.
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From page 134... ...
Users will need improved accuracy and reliability from MOBILE, even as the regulatory setting, vehicle technologies, and fleet characteristics are changing. This poses a daunting challenge for EPA's Office of Transportation and Air Quality (OTAQ)
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