Experimentally Supported Modeling of Cycle-to-Cycle Variations of SI Engine Using Cycle-Simulation Model 2014-01-1069

The paper presents modeling of cycle-to-cycle variations (CCV) of a SI engine by using the modified cycle-simulation model. The presented research has been performed on CFR engine fueled by gasoline. Experimental in-cylinder pressure traces of 300 cycles have been processed for several operating points representing the spark sweep which captured the operating points with low and high CCV. The cycle-simulation model applied in this study uses significantly improved turbulence and combustion model that have been implemented into the cycle-simulation code. Developed k-ε turbulence model and the quasi-dimensional combustion model based on the fractal theory have been applied. New quasi-dimensional ignition model was developed and integrated into the fractal combustion model in order to simulate the early flame kernel growth including the detailed modeling of spark plug geometry, electric spark phenomenon, heat transfer, in-cylinder flow around spark plug and the flame kernel interaction with the turbulence. Introduction of specific perturbations of turbulence production constant enables the oscillations of in-cylinder turbulence and spark gap velocity from cycle-to-cycle. The obtained simulation results of operating points with different ignition timing show very good agreement with the experimental cycle-to-cycle combustion data. The statistical analysis of IMEP and normalized heat release and their comparison with the experimental data demonstrate the capability of the presented model to capture the effects of low and high CCV in combustion at different ignition timings. This study confirms the fact that the stochastic nature of in-cylinder fluid motion is the major source of CCV in combustion.