Assessment of the Knock Prediction Capabilities with Single-Zone Thermodynamic Model of SI Engine and Detailed Chemical Kinetic Mechanisms of Fuel Combustion

Assessment of the boundaries for self-ignition of unburned charge in spark ignition engines (also related to knock) is required for development of the engine concepts and controls with respect to charge composition, spark advance and valve timing when designing the gas engines with wide range of the fuel compositions and converting compression ignition engines to gas engines. In this paper the combination of the single-zone model of the SI engine and chemical kinetics modeling is evaluated as a rapid prototyping tool for prediction of the self-ignition of the unburned charge in SI engine. The single-zone model simulates the cylinder pressure history based on Wiebe heat release function. The simulation of the self-ignition of the unburned charge is performed with coupled solution of the system of ordinary differential equations for temperature and species concentration with detailed chemical kinetic mechanism. Three fuels were considered: primary reference fuel, methane, hydrogen. The performance of the proposed approach for self-ignition simulation is evaluated based on comparison with the predictions of the multi-zone model which tracks the flame front propagation, computes the combustion products temperature based on chemical equilibrium assumptions, applies Wiebe heat release function. Results of the performed simulations show that proposed combination of the single-zone model and chemical kinetic simulation provides the following accuracy of the self-ignition simulation against multi-zone model: about 1 c.a.d. earlier for PRF, about 1.5 c.a.d. earlier for methane, about 3 c.a.d. earlier for hydrogen. It is shown, that observed single-zone model performance is due to neglection of dissociation of combustion products. Modification of the single-zone model is proposed, which reduces the deviations from multi-zone model to below 0.5 c.a.d. for all simulated fuels and conditions. Based on the performed simulations, it can be concluded that the single zone model and chemical kinetics simulations can be a valuable tool for rapid qualitative assessment of knock limits in SI engine.