Predictions of Cyclic Variability in an SI Engine and Comparisons with Experimental Data

An investigation of cyclic variability in a spark ignition engine is reported. Specifically, the predictions of an engine code have been compared with experimental data obtained using a well-characterized SI engine. The engine used for the experimental work and modeled in the code is the single cylinder research engine developed at Sandia National Laboratories and now operating at Drexel University. The data used for comparison were cylinder pressure histories for 110 engine cycles gathered during operation at a single engine operating condition. The code allows the various factors that could influence cyclic variability to be examined independently. Specifically, a model has been used to independently examine the effects of variations in equivalence ratio and of the turbulence intensity on cycle-to-cycle variations in the peak cylinder pressure, the crankangle of occurrence of peak pressure, the flame development angle, and the rapid burning angle. The equivalence ratio affects flame stretch and the distribution of flame wrinkling scales. The turbulence intensity affects both of these plus the maximum and minimum flame wrinkling scales. It is shown that variations in the equivalence ratio dominates cyclic variability in the peak cylinder pressure but that variations in the turbulence intensity have a dominant effect on the crankangle of occurrence of peak pressure, the flame development angle, and the rapid burning angle. Physical arguments are provided to explain why these conclusions are expected for the fast burn engine examined in this study.