Modeling the Effects of Mixture Composition on Cyclic Variability

In spark ignited engine maximum thermal efficiency is found with lean mixtures. The authors' models for optimizing engine design show a preference for burning at the lean limit which to date has been found from experimental measurements. Hence ignoring combustion variability in the modeling can cause significant error in engine performance and emissions at or close to the lean limit. To aid in optimizing engine design, a model is needed that allows the inclusion of variability in the search for lean operation solutions. Here a useful addition to modeling is presented - a physically based lean limit model that can allow the lean limit to be set as non-dimensional COV of IMEP or as a variance in IMEP.

The current work focuses on predicting the increase of cyclic variability due to the dilution of the mixture, whether by EGR and residual gas or by excess air. The model has two steps - the growth and shape of the flame kernel and the subsequent propagation of the flame that inherits features attributed to the kernel. It is shown that the speed and shape of the kernel growth influences initial and total burning periods and can predict the large increases in cyclic variability. While pressure fluctuations resulting from cyclic variability cannot be avoided, because of the stochastic nature of turbulence present in the cylinder, its effect can be reduced by fast burning processes.