An Experimental Study of Fuel Composition and Combustion Chamber Deposit Effects on Emissions from a Spark Ignition Engine 950740

Six chemically defined fuels have been run in a 2.0 liter production engine, in both a deposit-free and deposited state, at five different operating (speed/load) conditions. The fuel used to produce the deposits minimized the development of deposits in the intake system, so that primarily the effect of combustion chamber deposits were considered in this study. The six chemically defined fuels investigated were based on an iso-octane and toluene mix (base fuel). Methyl tertiary butyl ether, cyclohexane, diisobutylene, 1,2,4 trimethylbenzene and ethylbenzene were added to this base fuel to form five tertiary fuel mixtures. Engine-out emissions of speciated hydrocarbons, NO_x, CO and CO₂ were measured for the base fuel and five tertiary fuel mixtures in both the deposit-free and deposited state.

NO_x emissions were seen to increase relative to the base fuel for the cyclohexane and ethylbenzene tertiary fuel mixtures. Nine major hydrocarbon emissions were found with engine runs on the base fuel, representing the partial oxidation products of iso-octane and toluene. The five tertiary fuel mixtures affected the distribution of the hydrocarbon emissions and in some case added new emissions. Most oxidation products were found to decrease with increasing load. Unburned fuel represented one-third to one-half of the hydrocarbon emissions.

After the development of combustion chamber deposits approximately a 30% increase in NO_x emissions was observed. Also, unburned hydrocarbon emissions were found to increase approximately 25% with combustion chamber deposits. The extent to which combustion chamber deposits affected hydrocarbon emissions was found to be dependent on fuel structure. These findings indicate that the interaction of fuel structure with combustion chamber deposits should be considered in the development of reformulated gasolines if targeted emissions are to be maintained over the life of a vehicle.