Influence of three-dimensional in-cylinder flows on cycle-to-cycle variations in a fired stratified DISI engine measured by time-resolved dual-plane PIV

The objective of this study is an improved understanding of the three-dimensional in-cylinder flow and its influence on cycle-to-cycle variations in a fired, direct-injection, spray-guided, spark-ignition engine. The engine was operated at part-load. A triple injection scheme was applied late during compression to generate stratified fuel/air mixtures. These operating conditions are prone to cyclic variations. The large-scale tumble flow caused cycle-to-cycle variations in the spray shape. This was followed by cycle-to-cycle variations in combustion, which was characterized by the indicated mean effective pressure. Time-resolved particle image velocimetry was applied simultaneously in two parallel planes to account for the three-dimensional nature of transient in-cylinder flows. The focus was on the influence of the evolution of the large-scale tumble flow on combustion. The planes were 18 mm apart and located in the central tumble plane and one mid-valve plane. The central tumble plane is located below the fuel injector and spark plug; the mid-valve plane is below the center of one pair of intake and exhaust valves. Correlation analyses were performed to identify the flow features responsible for cycle-to-cycle variations in spray and combustion. By correlating single velocity vectors in the central tumble plane with indicated mean effective pressures, two flow regions important to combustion were identified: A flow above the piston, which had a significant impact on spray evolution, and a flow along the cylinder head, pointing from the injector toward the spark plug. By tracing in-cylinder flows backward in time, a clear correlation between flow features in the central tumble plane and the mid-valve plane was observed.