Investigation of cyclic combustion variability & development of a predictive knock controller

Abstract

In order to gain better insights into the knocking phenomenon, various thermodynamic investigations have been performed. The influence of temperature and mixture inhomogeneities on knock occurrence was evaluated. Two different evaluations were performed: one global investigation of unburnt cells with the highest temperatures and lowest air-fuel equivalence ratios and one local investigation of the temperature and charge velocity around the spark plug gap. Both revealed no direct correlation of inhomogeneities to the occurrence of knock. Subsequently, the reactivity parameter 𝜀 of the detonation diagram was applied to a 0D simulation environment to determine 𝜀 for single working cycles to predict the knock tendency based on this detonation diagram parameter. The results showed that the general knock tendency could be predicted but with low accuracy. Further, a post-processing method was developed to identify auto-igniting volumes (so-called hotspots) separated from the spark-ignited flame front in 3D CFD simulations. Lastly, cycle-to-cycle variations were further investigated by applying a knock frequency calculation approach that includes cyclic variations to 0D simulations. Based on the simulation results, precise prediction of the knock frequency was possible. Following the successful knock frequency prediction, a control approach was developed that adjusts the spark timing based on the knock frequency.