HYDROCARBON OXIDATION IN THE EXHAUST PORT AND RUNNER OF A SPARK-IGNITION ENGINE

An exhaust gas quenching technique was used to study the relationship of the fuel type to both the evolution and the extent of oxidation of the HC species in the exhaust port/runner system of an SI engine at light load condition. The fuel set consisted of gasoline, several alkanes (methane, ethane, propane, butane, iso-octane), an alkene (ethene), and an aromatic (toluene). There are significant fuel-to-fuel differences in cylinder-out HC emissions, which ranged from similar to 500 ppm C-1 (for ethene) to similar to 3350 ppm C-1 (for toluene). There is no significant fuel dependence on the percentage of the cylinder-out HC oxidized in the exhaust port/runner system, which ranges from 35% to 45%. Most of the reduction in total HC during passage through the exhaust system occurs in the port, although the distribution of IIC species changes throughout the port/runner system. A large portion of the runner-out HC emissions consists of the fuel species: the fuel fraction is similar to 80%-95% in methane, ethene and toluene, and similar to 40%-70% for the nonmethane alkanes. For the latter, the dominant nonfuel species are alkenes with a carbon number lower than or equal to the fuel carbon number. Because of the much higher specific ozone reactivity of the alkenes, the total HC reduction in the exhaust port/runner system is accompanied by an increase in reactivity which results in a smaller reduction in ozone reactivity for the alkane fuels than would be expected based on total HC emissions alone.