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 li ght load condition. The fuel set consisted of gasoline, several alkane s (methane, ethane, propane, butane, iso-octane), an alkene (ethene), and an aromatic (toluene). There are significant fuel-to-fuel differen ces 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 n o 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 exha ust system occurs in the port, although the distribution of IIC specie s changes throughout the port/runner system. A large portion of the ru nner-out HC emissions consists of the fuel species: the fuel fraction is similar to 80%-95% in methane, ethene and toluene, and similar to 4 0%-70% for the nonmethane alkanes. For the latter, the dominant nonfue l species are alkenes with a carbon number lower than or equal to the fuel carbon number. Because of the much higher specific ozone reactivi ty of the alkenes, the total HC reduction in the exhaust port/runner s ystem is accompanied by an increase in reactivity which results in a s maller reduction in ozone reactivity for the alkane fuels than would b e expected based on total HC emissions alone.