Investigation of thermodynamic and chemical influences on knock for the working process calculation

Abstract

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 working process calculation – 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 unburnt zone where knock occurs. Simulations performed at in-cylinder conditions using a detailed chemical reaction mechanism have shown that, at specific boundary conditions, the auto-ignition of the unburnt mixture resulting in knock happens in two stages. It is demonstrated that the 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. Furthermore, an enhanced approach for modeling the influence of various parameters on the ignition delay times of the mixture is presented. Additionally, thermodynamic investigations demonstrate the interrelation of engine proneness to knock expressed by the position of 50% MFB at the knock boundary and unburnt mass flowing out of the piston top land that is supposed to have an effect on knock.