EFFECT OF FUEL STRUCTURE ON EMISSIONS FROM A SPARK-IGNITED ENGINE

Seven pure fuels (methane, ethane, propane, n-butane, isopentane, isooctane, and toluene) have been run in a single-cylinder production-type engine at four operating points. Measurements of the emitted engine-out hydrocarbons, NOx, CO, and CO2 have been made at each condition and show effects partially explainable by changes in flame temperature and H/C ratio of the fuel. The results show that both the total engine-out hydrocarbon emissions and the distribution of individual hydrocarbon species in the exhaust gas are sensitive to the fuel used. In the case of total hydrocarbon emissions, ethane produces the lowest and toluene the highest (4X ethane) under all operating conditions. The percentage contribution of unburned fuel to the hydrocarbon emissions varies from 95% for methane fuel to 50% or below for isooctane. These percentages are affected by engine operating parameters, particularly speed and spark timing. Olefins constitute the majority of the remaining emitted hydrocarbons for alkane fuels. Toluene also emits a large percentage of unburned fuel (80%) but little olefin (1%). Benzene (6%) and benzaldehyde (6%) are major nonfuel hydrocarbon emissions from toluene but are not observed to an appreciable extent from the alkane fuels (< 0.3%).