EPA's Proposal for MOBILE6 Modeling Evaporative Emissions Based on Real-Time-Diurnal Data March 14, 1997 Abstract In earlier versions of EPA's highway vehicle emission factor model, EPA modeled diurnal evaporative emissions based on the results from one-hour (accelerated) diurnal tests. EPA has recently developed and begun using a 72-hour real-time diurnal (RTD) test during which the ambient temperatures cycles over a 24§ F range. Results from abbreviated versions of this test (24, 33, and 38 hours) as well as from the full 72 hours will be used in MOBILE6 to model three distinct types of in-use evaporative emissions: pressure driven vapor leaks (i.e., resulting from rising ambient temperatures, but not necessarily limited to just those nine hours of rising temperatures), liquid leaks, and permeation losses (i.e., resting losses minus liquid leaks). Supporting Data 1. EPA Testing Programs EPA's testing contractor (ATL) has conducted five (5) work assignments performing RTD tests (in Phoenix and Southbend) on a total of 119. At least one RTD test was performed on each vehicle using 6.7 psi RVP fuel with the ambient (i.e., SHED) temperatures cycling between 72§ and 96§ F. Additionally, RTD tests were performed on most of those 119 vehicles using a second fuel (9.0 psi RVP) as well as over different temperatures cycles (i.e., temperatures cycling between 60§ and 84§ F and temperatures cycling between 82§ and 106§ F). The distribution of the 119 EPA test vehicles is:  6 1974-80 passenger cars  95 1981-94 passenger cars  3 1971-80 trucks  15 1981-95 trucks 2. Coordinating Research Council (CRC) Testing A 24-hour RTD test was performed on each of 151 LDVs and LDTs in "As Received" condition. Also, 24 of those test vehicles received a RTD test following restorative maintenance. All of the vehicles were tested using single fuel RVP (6.7 to 7.0 psi) and a single temperature cycle (cycling between 72§ and 96§ F). The distribution of the CRC fleet is:  39 1971-77 passenger cars  12 1971-77 trucks  50 1980-85 trucks  50 1986-91 trucks Methodology 1. The recruitment methods used for most of the vehicles in the EPA sample were designed to recruit a larger proportion of vehicles that had problems with their evaporative control systems. Specifically, a purge and a pressure test were used to screen the candidate vehicles. This resulted a larger proportion of the test vehicles failing either a purge test or pressure test than did the in-use fleet. Weighting factors will be developed to compensate for this recruitment bias. 2. Vehicles with liquid leaks will be characterized separately from the rest of the vehicles in the sample. One reason for this approach is that the 24-hour emissions from vehicles with liquid leaks can exceed the emissions of corresponding vehicles by one to two orders of magnitude. It must be determined how to define this category of liquid "leakers". A likely approach would be to consider a 24-hour RTD performed on a properly functioning uncontrolled vehicle. For such a vehicle, we would expect the cumulative HC emissions on a 24-hour RTD test to be less that 30 grams (using 6.7 psi RVP fuel with the ambient temperatures cycling between 72§ and 96§ F). Therefore, defining the liquid "leaker" vehicles to be those with total HC emissions of at least 50 grams (on such a test) seems to be a reasonable approach. 3. Preliminary observations of the RTD data suggest that, for virtually all of the tests (regardless of the temperature cycle, fuel RVP, or vehicle type), the hourly HC evaporative emissions had stabilized and were relatively constant for hours 19 through 24. This suggests that the average hourly emissions during the final six (6) hours of the RTD cycle corresponds to what EPA had named as hourly "resting loss" emissions (i.e., permeation losses plus liquid leaks). 4. First, "correct" the hourly resting loss emissions (calculated in the previous item) for temperature. Then, subtract these temperature-corrected hourly resting loss emissions from the hourly RTD emissions. This difference should be the hourly evaporative emissions that result from the daily rise in temperature (i.e., the "diurnal" emissions). This approach will produce calculated hourly diurnal emissions that will approach zero as the SHED temperature drops to near the starting temperature. Also, this approach will allow calculation of partial diurnals. Outstanding Issues 1. Weighting factors need to be developed to compensate for the intentional recruitment bias within the sample of EPA test vehicles. 2. The definition of the category of liquid "leaker" vehicles must be finalized. 3. The distribution of the liquid "leakers" within the fleet must be determined. For example: Is the distribution a function of vehicle age and/or technology (e.g., fuel tank design)? Is the distribution best represented using a Weibull function, or should we just depend on the observations of liquid "leakers" in the CRC sample? 4. Starting with the 1996 model year, vehicles are being certified using the 72-hour RTD test instead of the one-hour accelerated diurnal test. We currently have no data on the in-use evaporative emissions (neither magnitude nor deterioration) of the vehicles certified by this test. Estimates of the evaporative emissions of these vehicles must be made. 5. The definition of "multiple day diurnals" must be finalized. (How many 24-hour cycles constitute a "multiple day diurnal?") Also, the data available to model "multiple day diurnals" is limited (i.e., significantly fewer 72-hour RTDs performed than 24- or 33- or 38-hour RTDs). 6. The definition of "partial diurnals" must be finalized. 7. The appropriate technology or model year groupings (if any) are must be determined. Some parameters will definitely be included in stratifying the data due to their effects on diurnal and resting loss emissions:  Fuel Metering Type -- Carbureted versus Fuel Injected  Canister Type * -- Open Bottom -- Closed Bottom -- Partially Open Bottom  Fuel Tank Type * -- Plastic -- Coated Plastic -- Metal  Evaporative Standard to Which the Vehicle Was Certified -- Model Year Groupings (possibly independent of technology)  Fuel RVP  Temperature Cycles  Portions of Temperature Cycle (i.e., partial diurnals) -- Emissions During Hours of Increasing versus Decreasing Temperatures * Note that while both fuel tank type and canister type will be significant factors in the RTD emissions, we may be unable to use them due either to being unable to classify the test vehicles or to being unable to determine the distribution of these parameters within the in-use fleet. Other parameters might be included in stratifying the data if there is an indication that they might affect diurnal or resting loss emissions:  Vehicle Type -- LDVs versus LDTs versus HDGVs  Fuel Injection Type -- PFI versus TBI  Model Year Ranges 8. The I/M effects on evaporative emissions must be addressed.