The Effect of Air/Fuel Ratio on Wide Open Throttle HC Emissions from a Spark-Ignition Engine

Currently most automotive manufacturers calibrate for rich air/fuel ratios at wide open throttle which produces lower exhaust gas temperatures. Future federal emissions regulations may require less enrichment under these conditions. This study was undertaken to address the question of what happens to engine-out hydrocarbon emissions with different air/fuel ratios at wide open throttle. Tests were run on a single cylinder research engine with a two valve combustion chamber at a compression ratio of 9:1. The test matrix included three air/fuel ratios (10.5, 12.5 and 14.5) and two speeds (1500 and 3000 rpm) at wide open throttle as well as three air/fuel ratios (12.5, 14.6 and 16.5) at a part load condition (1500 rpm, 3.8 bar IMEP). The exhaust was sampled and analyzed for both total and speciated hydrocarbons. The speciation analysis provided concentration data for hydrocarbons present in the exhaust containing twelve or fewer carbon atoms. The specific reactivity (g Ozone/g NMOG) of the exhaust was calculated for each operating condition. The results show that the total HC emissions decrease significantly with leaner air/fuel ratios as expected. The specific reactivity of the emissions increased slightly with leaner operating conditions at both wide open throttle and part load conditions. The specific reactivity was slightly higher at wide open throttle compared to the part load condition but was not dependent on engine speed.