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This paper describes the use of a modified quasi-dimensional spark-ignition engine simulation code to predict the extent of cycle-to-cycle variations in combustion. The modifications primarily relate to the combustion model and include the following: 1. A flame kernel model was developed and implemented to avoid choosing the initial flame size and temperature arbitrarily. 2. Instead of the usual assumption of the flame being spherical, ellipsoidal flame shapes are permitted in the model when the gas velocity in the vicinity of the spark plug during kernel development is high. Changes in flame shape influence the flame front area and the interaction of the enflamed volume with the combustion chamber walls. 3. The flame center shifts due to convection by the gas flow in the cylinder. This influences the flame front area through the interaction between the enflamed volume and the combustion chamber walls. 4. Turbulence intensity is not uniform in cylinder, and varies cycle-to-cycle.

A model for predicting the performance and emissions characteristics of Wankel engines has been developed and tested. Each chamber is treated as an open thermodynamic system and the effects of turbulent flame propagation, quench layer formation, gas motion, heat transfer and seal leakage are included. The experimental tests were carried out on a Toyo Kogyo 12B engine under both motoring and firing conditions and values for the effective seal leakage area and turbulent heat transfer coefficient were deduced. The agreement between the predicted and measured performances was reasonable. Parametric studies of the effects of reductions in seal leakage and heat transfer were carried out and the results are presented.

This paper investigates the effects of the variations of turbulence characteristics and initial flame kernel center position on the cyclic combustion variations by means of quasi-dimensional turbulent entrainment combustion model. The turbulence intensity and turbulence integral length scale at spark ignition time in the model are determined by maximizing the agreement between the predicted and measured results such as pressure diagrams, mass fraction burned etc. With different values of the turbulence intensity and turbulence integral length scale at spark ignition time, the calculation of the cyclic combustion variations for the engine is carried out. In addition, the prediction of the effect of different flame kernel center positions on the cyclic combustion variations is also studied. Finally, some conclusions are drawn out about the importance of turbulence and initial flame kernel center position on the cyclic combustion variations for spark-ignition engine.