Two-Stage Ignition Occurrence in the End Gas and Modeling Its Influence on Engine Knock

The most significant operation limit prohibiting the further reduction of the CO₂ emissions of gasoline engines is the occurrence of knock. Thus, being able to predict the incidence of this phenomenon is of vital importance for the engine process simulation - a tool widely used in the engine development. Common knock models in the 0D/1D simulation are based on the calculation of a pre-reaction state of the unburnt mixture (also called knock integral), which is a simplified approach for modeling the progress of the chemical reactions in the end gas where knock occurs. Simulations of thousands of knocking single working cycles with a model representing the Entrainment model’s unburnt zone were performed using a detailed chemical reaction mechanism. The investigations showed that, at specific boundary conditions, the auto-ignition of the unburnt mixture resulting in knock happens in two stages. It is demonstrated that the commonly used knock integral is not capable of representing this behavior of the detailed chemical mechanism, meaning an improved approach for modeling the progress of the chemical reactions is needed for the calculation of the knock boundary. Based on these findings, a new two-stage knock integral approach capable of reproducing the auto-ignition behavior of the detailed chemical mechanism was developed. For this purpose, an enhanced three zone approach for modeling the influence of various parameters (pressure, temperature, exhaust gas fraction, air-fuel ratio, ethanol content and surrogate composition) on the ignition delay times of the mixture is proposed. Furthermore, a newly developed model for the ignition delay of the low-temperature ignition as a function of the boundary conditions is presented. Finally, the performance of the new two-stage approach is demonstrated and compared with the results achieved by the commonly used single-stage knock integral.