1996-05-01

A Study of Cycle-to-Cycle Variations in SI Engines Using a Modified Quasi-Dimensional Model 961187

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. An effective turbulence intensity is defined to determine the turbulent burning speed. The resulting model was used to predict the extent of cycle-to-cycle combustion variations using a set of fiber optic spark plug probe data taken in a modern SI engine as input. The gas convection velocity and the turbulence intensity in the vicinity of the spark plug for every cycle are required as input data in the computation of the flame development process. Thus, the effect of variations in these two parameters can be calculated with the model; other influencing factors are total mass in the cylinder, cylinder pressure and temperature, and residual gas fraction - all of which are related to the previous cycle in the simulation. The computation results for the distributions of different mass fraction burned durations (0-2%, 0-10% and 0-90%) for 600 cycles are compared with the experimentally determined values, and the effects of variations in turbulence intensity, flame center shift in the combustion chamber, and the residual gas fraction are examined.

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