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Closed-loop emission control systems were adopted by most motor vehicle manufacturers on a wide-spread basis in model year 1981, in order to meet the 1981 and later Federal exhaust emission standards. General Motors and the Environmental Protection Agency have conducted test programs to quantify the in-use performance of these closed-loop systems. Over 4000 vehicles (passenger cars and light duty trucks) have been evaluated covering the 1983 through 1988 model years at both low and high altitude locations. For model year 1986 and later, average emission levels, adjusted to the 50,000 mile point, are below the applicable EPA standards. The GM and EPA data indicate average emission levels have decreased during the 1980's and reflect the increased penetration of fuel injection, improvements in catalytic converter performance, and improved system and component reliability.

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.

EPA and GM test programs have quantified the in-use emission performance of the GM closed-loop emission control systems. In-use exhaust emission levels, adjusted to the 50,000 mile point, show averages under the standards for the 1986 model year cars. In-use evaporative emission levels are under the standard, on average, for the 1985 and 1986 model year cars. Fuel injection systems have inherent evaporative emission performance advantages over carburetor systems. Average exhaust emission levels for the national fleet will continue to decrease as the closed-loop fleet replaces the older models. Predictions of future inventories are lower if the GM data reflecting the 1981+ emission improvements are used instead of the estimates currently in the MOBILE3 model.

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 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.