Stratified-Charge Fuel Preparation Influence on the Misfire Rate of a DISI Engine

The influence of mixture preparation on misfires at idle in a Direct Injection Spark Ignition (DISI) engine was investigated. A wall-guided DISI engine was run at idle conditions in a stratified charge mode (750 rpm / 90 kPa MAP). Images of the mixture composition at the spark plug were taken at spark timing using Planar Laser Induced Fluorescence (PLIF) for several different End-of-Injection (EOI) timings and spark timings. Cylinder pressure data were acquired simultaneously with the images to identify misfire cycles. The misfire rate was found to increase as the EOI timing was advanced from the optimal timing, defined by maximum stability and lowest ISFC. Images show that the misfire rate at a particular operating condition can be correlated to the fuel distribution and the location of the stratified charge in the engine. Cycles that showed a lower amount of stratification (overmixing) and/or high gradients in fuel concentration near the spark plug were the least stable. At the most advanced EOI timings, the charge was shown to have overpenetrated and moved past the spark plug in its trajectory, increasing the misfire rate. Retarding the spark timing yielded the same result as advancing the EOI. Advancing the spark timing resulted in a charge that appeared to still be in the process of exiting the bowl during the spark event and increased the misfire rate. Individual misfire cycles were also compared to average firing cycles. These cycles displayed both a large amount of variation from the mean and very poor charge formation, in terms of location and fuel concentration.