Mixture Preparation Effects on Ignition and Combustion in a Direct-Injection Spark-Ignition Engine 962013
Planar instantaneous fuel concentration measurements were made by laser-induced fluorescence of 3-pentanone in the spark gap just prior to ignition in a direct-injection spark-ignition engine operating at a light load, highly stratified condition. The distribution of the average equivalence ratio in a circle of 1.9 mm diameter centered on the spark plug showed that a large fraction of the cycles had an equivalence ratio below the lean limit, yet acceptable combustion was achieved in those cycles. Further, weak correlation was found between the local average equivalence ratio near the spark plug and the time required to achieved a 100 kPa pressure rise above the motoring pressure, as well as other parameters which characterize the early stages of combustion. The cause for this behavior is assessed to be mixture motion during the spark discharge which continually convects fresh mixture through the spark gap during breakdown. Such high mixture motion is inherent in direct injection systems because of the requirements placed on mixing for full load operation, and the influence of the spray on the flowfield for light load conditions where the injection and ignition event may overlap. The implication is that good ignition of stratified charges is not necessarily a main difficulty, particularly if engine development includes detailed in-cylinder measurements.