Analysis of Cyclic Fluctuations of Charge Motion and Mixture Formation in a DISI Engine in Stratified Operation

Engine processes are subject to cyclic fluctuations, which a have direct effect on the operating and emission behavior of the engine. The fluctuations in direct injection gasoline engines are induced and superimposed by the flow and the injection. In stratified operation they can cause serious operating problems, such as misfiring. The current state of knowledge on the formation and causes of cyclic fluctuations is rather limited, which can be attributed to the complex nature of flow instabilities.

The current investigation analyzes the cyclic fluctuations of the in-cylinder charge motion and the mixture formation in a direct injection gasoline engine using laser-optical diagnostics and numerical 3D-calculation. Optical measurement techniques and pressure indication are used to measure flow, mixture formation, and combustion processes of the individual cycles. The 3D-calculations are performed with Large Eddy Simulation (LES) in order to model large-scale turbulence and cyclic fluctuations. The results of the calculation are used to analyze the formation of cyclic fluctuations and their effect on injection and mixture formation.

The results show that the intensity of the cyclic fluctuations increases significantly during the injection event, due to the interaction between the cyclic fluctuating in-cylinder flow and the injection. These experimental findings are confirmed by computations, which when combined with reproducible injection predict strongly differing mixture states at the ignition point with significant asymmetries. These asymmetries with their broad scatter range of mixture formation can explain the stochastic misfires that occur in the engine.