EPA's Proposal for MOBILE6 Air Conditioning Correction Factors March 7, 1997 Abstract The effects of air conditioning currently available in MOBILE5 are generally considered obsolete. However, recent testing shows that vehicle air conditioning can have a significant effect on vehicle emissions, especially Oxides of Nitrogen (NOx). Nearly all the existing data on the effects of air conditioning has been measured over only one driving cycle (the Federal Test Procedure). It is expected that the effect of air conditioning on emissions may be significantly different for different driving cycles at different speeds. As a result, adjustment factors which better represent the effect of air conditioning on vehicle emissions are being proposed for MOBILE6. Summary of the Proposal EPA proposes to develop an air conditioning adjustment factor that depends on roadway facility type and average speed. The adjustment factor will be derived from data collected on vehicles tested with and without air conditioning. This adjustment would be applied to exhaust emission estimates after they have been adjusted for fuel and temperature effects as part of the adjustment for facility type and speed. The magnitude of the air conditioning adjustment factor would be a function of temperature, humidity and usage. Supporting Data EPA plans to test a sample of approximately 25 1990 and newer model year passenger cars in the Ann Arbor laboratory before summer 1997. These vehicles will be tested over a variety of driving cycles representing different road types (facilities) and vehicle speeds. The testing will be conducted at 75 degrees without air conditioning and repeated at 95 degrees with air conditioning turned on. These cycles are the same facility cycles developed to address the effects of real-world driving on emissions. A study done as part of the FTP revision included vehicles tested similarly to the Ann Arbor testing at 95 degrees described above. The FTP revision tests were correlated to tests done in an environmental chamber by the vehicle manufacturers to validate the representativeness of the test procedure now being used. EPA collected data in Phoenix for air conditioning compressor on-time and pressure drop. This data may be useful in determining the effect of temperature on the rate of air conditioning usage. Methodology There will not be sufficient time or resources to have the 25 vehicles planned to be tested in Ann Arbor at both at 95 degrees and 75 degrees with air conditioning on all the inventory cycles and again without air conditioning. The testing at 95 degrees with air conditioning on was chosen to best represent the conditions when air conditioning is used and the testing at 75 degrees with air conditioning off will be used to correlate these results with the other baseline testing used to develop the basic exhaust emission rates. MOBILE6 will already contain an estimate of what the effect of temperature will be on emissions for each cycle. The air conditioning test results will contain both the effects of temperature and air conditioning. To avoid double counting the temperature effect, EPA proposes to use the MOBILE6 temperature correction adjustment effect to remove the effect of temperature from the test results. EPA proposes to apply the temperature adjustment to the base emission rate before adjusting the emissions for air conditioning effects in MOBILE6. Logically, the MOBILE temperature corrections can be applied to the results of the testing at 75 degrees (without air conditioning) to estimate what the emissions would be at 95 degrees without air conditioning. The remaining difference between the 95 degree estimates (without air conditioning) and the actual measurements at 95 degrees with air conditioning will determine the air conditioning effect. To summarize: o Adjust the results measured at 75 degrees (without air conditioning) for each inventory facility cycle to 95 degrees using the MOBILE6 temperature correction factors. o Compare the estimated 95 degree (without air conditioning) emission rate for each inventory facility cycle to the measured emissions at 95 degrees with air conditioning on. o Compute an adjustment factor for each facility, dependent on average cycle speed. This estimate would be stored in MOBILE6 and represent full usage of air conditioning at that temperature. In this way, a MOBILE6 run which indicates 95 degrees and full air conditioning usage would reproduce the effect observed in the EPA testing. Adjustments to Air Conditioning Effects One would expect a change in the air conditioning effect with a change in ambient temperature. For example, the effect of air conditioning at 75 degrees on fleet emissions will likely be different than the effect at 95 degrees. This change is both a function of the number of drivers using air conditioning at various temperatures and the difference in engine load. Humidity would also likely effect both the load on the engine as well as the amount of air conditioning usage. EPA proposes to choose a temperature at which air conditioning usage and effects would be assumed to be minimal (for example, 65 degrees) and assume that the effect of air conditioning at that temperature is zero. The air conditioning effect at temperatures between 65 and 95 degrees would be interpolated between the assumed effect at 65 degrees (zero) and the measured effect at 95 degrees. At temperatures above 95 degrees, the effect would be assumed to be the same as at 95 degrees. In order to allow maximum flexibility, this proposal would also allow direct user input of an air conditioning usage adjustment factor. The user supplied input would override the default values discussed above. The user input would allow a value from 0%-100% for air conditioning effects. The 100% value would represent the air conditioning effect measured during the EPA testing. EPA collected data in Phoenix for air conditioning compressor on-time and pressure drop on vehicles. This data was used to assist in the development of an EPA vehicle certification standard that takes into account the effect of air conditioning operation on emissions. It would be possible from this data to determine the additional load as a function of temperature. As time and resources allow, this data could be used to determine appropriate default values for MOBILE6 or provide guidance to users as to appropriate air conditioning usage input values. Outstanding Issues EPA currently plans to test only 1990 and newer model year passenger cars. MOBILE6 will assume that the effects of air conditioning are similar for all other model years and light-duty trucks unless additional information becomes available.