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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.

Approximately 245 passenger cars were involved in a Cooperative Test Program (CTP) conducted by EPA and industry. The purpose of this program was to examine ways of improving the effectiveness of I/M tests, specifically for the 1981 and later vehicles with closed loop fuel metering. Detroit area vehicles failing the Michigan Auto Exhaust Testing (AET) program were recruited and investigated to determine the cause of the failures. Complete FTP emission tests were run as received, and at each step of the repair sequences. A variety of I/M preconditioning methods were evaluated to understand false failures. GM's portion of the CTP was sixty vehicles, half 1981-1982, and half 1983 and later model years. During the time period of the CTP, the AET failure rates were 14.5% for the 1981-1982 model years and 6.6% for the 1983 and later model years. Although all sixty vehicles had failed the Michigan AET, thirty-three (55%) passed “as-received” when tested in the 6M laboratory.

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 (EPA) and the California Air Resources Board (CARB) are currently examining various proposals to modify the test procedures used to measure and regulate hydrocarbon evaporative emissions. The purpose of the proposed modifications would be to approximate the ambient conditions that vehicles can encounter on high temperature days, when many exceedances of the ozone air quality standard can occur. EPA has also developed an evaporative emissions model to support its evaluation of the proposed test procedure changes. GM has conducted tests to assess the performance of evaporative emissions control systems using the elevated temperature conditions under consideration by EPA. GM's tests used a real-time temperature cycle that extended over 24 hour time periods. The data from GM's tests indicate that on a real-time basis, evaporative emissions control systems perform better than predicted by the EPA 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.