Cyclic Variation in an SI Engine Due to the Random Motion of the Flame Kernel

This paper reports an investigation of the association between flame kernel movement and cyclic variability and assesses the relative importance of this phenomenon, with all other parameters that show a cyclic variability held constant. The flame is assumed to be subjected to a “random walk” by the fluctuating velocity component of the flow field as long as it is of the order of or smaller than the integral scale. However, the mean velocity also imposes prefered convection directions on the flame kernel motion. Two-point LDA (Laser Doppler Anemometry) measurements of mean velocity, turbulence intensity and integral length scale are used as input data to the simulations. A quasi-dimensional computer code with a moving flame center position is used to simulate the influence of these two components on the performance of an S I engine with a tumble-based combustion system. The simulations are compared with engine tests using the variation in combustion duration, indicated mean effective pressure (imep), maximum pressure and location of maximum pressure as a measure of the cyclic variability. The standard deviation of these variables for the simulations amounted to approximately 25% of the standard deviation determined from the engine tests at stoichiometric conditions.