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In recent years various parties have proposed new evaporative emission test procedures focused on controlling “excess” evaporative emissions, on hot “ozone prone” days. Studies by General Motors established the need for real-time measurements of daily emissions from parked vehicles and of “running losses” from vehicles that are driven to quantify and control the mobile source contribution to VOC inventory. “Resting losses” are shown to be a previously unidentified major source of hydrocarbon emissions. This paper describes the theories, data and development of GM's Real-Time Test Procedure.

The state of California has adopted a “real-time” evaporative emission procedure that will be used starting with the 1995 model year. This test, which focuses on high temperature conditions, and measures all sources of evaporative emissions, represents a very stringent requirement. Non-fuel background emissions, i.e., paint, adhesives, and even air conditioning refrigerant, can be a significant fraction of the total measured emissions. California has included a provision in their regulations that allows for the subtraction of background emissions, using a methodology to be developed in the future. This paper reviews the history of non-fuel emission regulations, provides data showing the magnitude of the problem using the new real-time procedure, and suggests a methodology for establishing a new background emission test procedure.

The major sources of variability of an exhaust emission test on the 1975 Federal Test Procedure (FTP) are discussed. Vehicle, driver and the ambient conditions affect hydrocarbon (HC) and carbon monoxide (CO) variability significantly. On the other hand, oxides of nitrogen (NOx) and carbon dioxide (CO2) are influenced more by the differences in vehicle loading. However, the importance of any other source of variability cannot be ignored, especially when a comparison is made between two tests. Various diagnostic aids such as the “Total Torque Tester”, a “Driver Evaluator”, an “Exhaust Bag Cross-Check”, and a “Repeatable Car”, which are used for the purpose of a better correlation between measurement systems on different test sites, are described. Designed experiments were conducted on vehicles whose emission levels were at or near the standards of 0.41 grams per mile HC, 3.4 grams per mile CO and 2.0 grams per mile NOx.

The Environmental Protection Agency published a paper in November of 1985 (“Study of Gasoline Volatility and Hydrocarbon Emissions from Motor Vehicles”, EPA-AA-SDSB-85-5) suggesting that the evaporative emission test fuel be modified to reflect current “in-use” fuel characteristics. It was shown that higher evaporative emissions resulted from current vehicles when tested on higher RVP fuels. Vehicle tank fuel volatility decreases as the lighter ends in the fuel evaporate. As fuel is used in vehicle operation, the remainder in the tank becomes less volatile. The evaporative emission test procedure specifies that the test be conducted with the tank at 40% of capacity. At this level, one would expect the fuel to have “weathered” and be of less volatility than originally dispensed. This factor was not included in the EPA data.

The E-35 “Running Loss” program was planned in the fall of 1996, and conducted in the summer of 1997, as the third part of a series of Coordinating Research Council (CRC) sponsored evaporative emission test programs. One hundred and fifty vehicles (half cars - half light duty trucks) were recruited at a local I/M lane, and tested for running loss emissions at the ATL Facility in Mesa, AZ. The previous CRC programs had studied hot soak, and then diurnal emissions. Running loss emissions were measured in a Running Loss SHED (RL-SHED) for a 25 minute, 7.5 mile trip on a hot summer day (95°F). Vehicles from model years 1971 through 1991 were tested. A wide range in emission levels was observed - from a low of 0.13 g/mile to 43 g/mile. The test results were not able to establish whether car emissions are different, or the same, as light duty trucks. The major causes of the high emissions were liquid leaks on carburetor equipped models.