Critical Factors Affecting Automotive Sulfate Emissions

Two recently completed experimental programs provide data on four critical factors affecting automotive sulfate emissions: catalyst selectivity, catalyst age, driving mode, and gasoline sulfur level. A fifth factor, excess O₂ level is discussed, but was not studied experimentally.

In the first program, 20 1975 California model cars each accumulated 80 000 km on fuel containing 300 ppm sulfur. The cars used three different catalyst systems and were tested under four different test conditions: Federal Test Procedure (FTP), the Fuel Economy Test (FET), the Sulfate Emission Test (SET), and 80 km/h cruise. The results showed that SO₄⁼ emissions from one of the systems were about three times higher than SO₄⁼ emissions from the other two systems. SO₄⁼ emissions from all systems reached a peak within the first 10 000 km of the test and declined thereafter. Averaged over the 80 000 km of the test, FTP SO₄⁼ emissions from all 20 cars were half the peak value measured at 6 400 km and half EPA's previous estimate for SO₄⁼ emission from these types of vehicles. Data obtained on the SET cycle indicate that EPA's recent estimate of 20 mg SO₄⁼/km for catalyst cars with air pumps operating on 300 ppm sulfur gasoline may also be high. Finally, SO₄⁼ emissions as a function of driving cycle were found to be:

In the second program, four catalyst systems were tested on fuels containing 24 to 469 ppm sulfur using SET, 64 km/h cruise, and FTP test conditions. Under all test conditions, SO₄⁼ emissions were found to be a linear function of gasoline sulfur content, though in some cases, SO₄⁼ emission differences between catalyst systems were larger than the effect of changes in fuel sulfur content for a given catalyst system. SO₄⁼ emissions as a function of catalyst system type were found to be: