Cycle-to-Cycle Correlations Between Flow Field and Combustion Initiation in an S.I. Engine

The purpose of this study is to identify the interactions by which the flow field induces cyclic combustion variability in S.I. engines, especially with lean mixtures. Two main interacting processes can be identified: convection by the “mean” large-scale velocity field, and wrinkling by the small-scale turbulence. In this work, the large-scale velocity field in the electrode gap is characterized with a single component of the low frequency velocity, and with the spark duration. The small scales are described by a high frequency fluctuation intensity prior to ignition. Two-component LDV measurements demonstrate the strong relation between the spark duration and the velocity amplitude between the electrodes.

Multiple regression followed by an analysis of variance is used to calculate the contribution of each variable to the variation of the initiation duration. This duration is defined as the time from ignition to the 5% burnt fraction point determined from the pressure signal. When the mixture is very lean, the cyclic variations of the high frequency turbulence intensity do not affect the combustion rate during the initiation phase. This implies that the wrinkling accelerates the combustion process only later, during the propagation phase. However, the cyclic variations of large-scale fluid motion in the electrode gap contribute strongly to the cyclic variations of the initial flame. Therefore, the lean burn combustion stability is controlled by the cyclic variations of the mean flow field. This result is observed in three intake configurations, which generate various flow fields and turbulence levels.