EPA's Proposal for MOBILE6 Heavy-Duty Emission Factors March 14, 1997 Standard Plan The methodology for producing emission estimates for MOBILE5a could be followed for the next generation of the mobile source emission inventory. Succinctly, this methodology determines a gram per mile emission rate by multiplying a work specific emission level in units of grams per horsepower-hour (or kilowatt-hour) by a conversion factor of work to mileage. It is our intent to make available specific emission factors for truck classes 2b, 3, 4, 5, 6, 7, 8a, 8b, urban buses, commercial buses, and school buses. We will attempt to complete the disaggregation of truck class to the extent that it can be supported with information or available data. It may be necessary to combine some of the classes due to the paucity of relevant data for those categories. Emission Levels In lieu of actual data on in-use engines, we will use the data that engine manufacturers provide for certification of new engines built in or after 1988 for MOBILE6. The emission levels are given in grams of pollutant per horsepower-hour (g/hp-hr) work from the certification data and could be weighted by engine sales and horsepower to produce one average emission level for diesel and gasoline-fueled heavy-duty engines. The certification data supplies zero mile levels and deterioration factors at the end of the useful life. The zero mile level plus the deterioration must be under the emission standard unless the averaging, banking, and trading provisions are used to offset excess emissions. The calculated deterioration factors would be used with these mileage standards to produce a deterioration for all mileages. A cooperative EPA and engine manufacturers program of some 20 diesel engines showed no significant deterioration in HC and NOx emissions performance and provided emission factors for pre- 1988 engines. Eighteen heavy-duty gasoline were also tested in that study, and the emission estimates will be retained for those engines as well. The engines in this study were produced before the most stringent emission standards were in effect. This emission level estimates would be retained for MOBILE6 for pre- 1988 heavy-duty vehicles. Given the current schedule of M6, our proposal is to update the 1988 and later heavy-duty vehicle emissions rates only with newer certification data. If the schedule changes to allow more time, we would pursue utilizing the data being collected this year and next year under the programs described below. The EPA engine dynamometer test cycle was developed to closely represent the in-use behavior of these engines, so it is assumed that the emission level produced by the certification test procedure is representative of the average in-use emission level. The certification data also supplies an intended service class of light , medium, heavy, and bus heavy-duty engines, so in addition to having specific conversion factors it may be appropriate to have specific emission levels by service class. The intended service classes are restricted to "light-heavy-duty", "medium-heavy-duty", and "heavy-heavy-duty" restricting the level of disaggregation that could occur. Conversion Factors Previous work using the US Census Bureau's Truck In-Use Survey (TIUS) fuel consumption in gallons per mile and pounds per work unit by weight class will be repeated using the latest information. Combining the two estimates from TIUS with appropriate fuel densities produces a conversion factor to convert the emission level data into a useful unit for emission inventory estimations. Data Acquisition Plan If data becomes available in time for the MOBILE6 update, it would be used to produce emission rate estimates for the heavy-duty truck classes. Several proposed and ongoing programs would need to be reasonably complete by the end of 1997 in order to include new estimates. In order to reduce the complexity in modelling emissions with a large variety of cycles, future test programs should use a common list of test cycles. California Air Resources Board has been instrumenting trucks to determine typical operating conditions and cycles for use in future heavy-duty emission testing in the chassis configuration. The cycles should cover the range of typical operation allowing for adjustment of speed or congestion level by facility (freeway, arterial, etc.). If all future emission testing incorporates these cycles, then emission rates can be calculated directly from the data similar to the methods used for light-duty vehicles. Any future testing on these cycles should include a range of inertial loads for each vehicle. Also, chassis dynamometers may allow for testing to be performed to predict the impact of slope on emissions. Cold start emissions may not be as important for most heavy-duty engines because few are catalyst equipped. Certainly gasoline engines may behave much differently than diesel engines so some cold start testing should be performed as well. A number of in-use emission testing programs are expected to be completed this year funded by New York, California, and EPA. This testing should include different inertial loadings and a variety of engine types and certification levels. The range of vehicles used will focus primarily on post-1988 technology when emission standards began to encourage engine design changes. To deal with transportation measures such as stoplights and freeway ramps, West Virginia University researchers with funding from the Department of Energy have been collecting emissions data using test cycles that might reasonably simulate these types of driving conditions. Using the second-by-second data, the impact on emissions of adding stoplights or freeway ramps might be modelled successfully also. Power Demand\ Steady-State Approach An approach that could produce emission estimates with less effort uses emission data collected under loaded steady-state conditions to predict emission results under any conditions. We have had initial success in modelling transient hydrocarbon and nitrogen oxide emissions using loaded steady-state emissions measurements. We would like to continue to pursue this approach because emission measurements at steady-state are typically easier and cheaper to obtain. Future emission testing programs should consider testing over various steady-state conditions to consider this approach to emissions modelling. Emission rates from testing at various power levels under steady-state conditions could be applied to the power demanded over a given stretch of road for a variety of heavy-duty trucks. Distributions of speeds, accelerations, and inertial loads could be accounted. Road level grade could also be incorporated. Under this proposal particulate emissions rates may need to be adjusted to reflect transient operation using the data obtained from representative cycle testing. We are inviting comment on these approaches and specifically to issues that must be considered when designing a test program and calculating emission factors.