Experimental Investigation of the Origin of Cyclic Fluctuations in a DISI Engine by Means of Advanced Laser Induced Exciplex Fluorescence Measurements

Cyclic fluctuations of the in-cylinder processes in a Direct Injection Spark Ignition (DISI) engine may strongly affect the engine operation causing misfires or variations in the indicated mean effective pressure (imep). Particularly misfires prevent compliance with current or future exhaust emission legislations. Nevertheless, the origin of cyclic fluctuations is not well understood since fluctuations of in-cylinder air flow, fuel injection and wall interaction have to be considered. This paper focusses on a detailed experimental analysis of the origin of cyclic fluctuations in a DISI engine with an air guided combustion process by means of advanced Laser Induced Exciplex Fluorescence (LIEF) measurements. It reveals that cycle-to-cycle variations primarily originate from the air/fuel ratio at the spark plug. Simultaneous measurements of the flow fields of liquid and vaporized fuel show that the cycle-to-cycle variations in the mixture preparation are mainly induced by the in-cylinder air flow and not by the injection. This is also corroborated by droplet temperature measurement.