The impact of fuel distribution on cyclic combustion variations in a natural gas fueled spark-ignition engine

Combustion variations that occur between individual cycles during spark ignition engine operation are a major cause of elevated emissions, depressed fuel economy, and power fluctuations in internal combustion engines. Reducing the level of cyclic variations in natural gas engines is of particular importance as these engines typically operate at significantly higher compression ratios and with wider equivalence ratio limits than their gasoline counterparts. The role of mixture inhomogeneity in generating cyclic combustion variations is little understood due to the difficulty in determining the fuel distribution within the cylinder during operation.;The purpose of this work was to measure the level of mixture inhomogeneity in the cylinder of a natural gas engine and correlate these results with the level of cyclic combustion variations as determined from cylinder pressure data. The distribution of fuel within the cylinder was measured in three different planes, as a function of fuel injection timing and crank angle. Processed Planar Laser Induced Fluorescence (PLIF) images provide a two dimensional measure of the equivalence ratio in each of the given planes as a function of crank angle and fuel injection timing. The mixture formation process and the state of the mixture just prior to ignition were analyzed statistically to provide information pertaining to the bulk distribution of the fuel and the magnitude of small scale inhomogeneities.;The state of the mixture in terms of small scale inhomogeneities, bulk maldistribution of fuel, and cyclic variations in total fuel content and distribution were found to correlate well with the level of cyclic combustion variations in the engine. More specifically, increases in the inhomogeneity of the cylinder charge were found to correlate closely with the variation of ignition delay. Variations in the overall burn duration and maximum cylinder pressure were found to be most closely related to fluctuations in the total amount of fuel present in the cylinder in a given cycle. This study has also led to the determination of an injection timing which produces the most uniform fuel distribution for this specific engine.