LES study of cycle-to-cycle variations in a spark ignition engine

Multi-cycle Large Eddy Simulation (LES) of flow and combustion in a single cylinder engine set-up are presented in order to validate the ability of this technique to reproduce cyclic combustion variability in a spark ignition engine. The simulated engine operating point is part of a larger experimental database acquired at IFP and specifically designed to validate LES techniques in terms of cycle-to-cycle prediction. To characterize individual cycles, the database combines acquisitions of pressure and temperature in different locations of the engine set-up with optical diagnostics in the intake pipe and the cylinder. This new experimental database combined with LES is a powerful tool to study cycle-to-cycle variations (CCV). The operating points include points with low and high CCV levels. The LES includes the whole engine set-up, and covers 25 consecutive four-stroke engine cycles for a reference operating point with low CCV. The results show that LES is able to reproduce the experimental findings, and illustrate how LES can help explain the sources of CCV. In the present case it is shown that CCV are essentially due to velocity fluctuations at the spark plug, which induce variations of the early flame kernel growth and of the overall combustion duration.

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