Effects of In-cylinder Flow on Fuel Concentration at the Spark Plug, Engine Performance and Emissions in a DISI Engine

A fiber optic instrumented spark plug was used to make time-resolved measurements of the fuel vapor concentration history near the spark gap in a four-valve DISI engine. Four different bulk flow were investigated. Several early and late injection timings were examined. The fuel concentration at the spark gap was correlated with IMEP. Emissions of CO, HCs, and NOx were related to the type of bulk flow. For both early and late injection the CoVs of fuel concentration were generally lowest for the weakest bulk flow which resulted in a stable stratification. Strong bulk flows convected the inhomogeneities through the measurement area near the spark plug resulting in both large intracycle and cycle-to-cycle variation in equivalence ratio at the time of ignition. With early injection the best engine performance was obtained with an injection timing of 120° aTDC of intake, the result of a mixture formation process that exhibits a tradeoff between reducing piston wetting as injection timing is retarded and increasing the time available for mixing as injection timing is advanced.