Unburned Mixture Heat-Transfer Surface vs Mass Fraction Burned Correlation for Application in Single- and Two-Zone Models of SI Engine Cycle 2022-01-0379

Assessment of the boundaries for knock in spark ignition engines (related with self-ignition of unburned charge) is required for development of the engine control and charge composition when designing the gas engines with variable fuel compositions. In this work the extension of the single-zone model of the SI engine cycle is proposed. It includes simulation of the fresh charge temperature and pre-flame reactions based on detailed chemical kinetic mechanism of self-ignition. The heat release is modelled based on Wiebe function; the heat transfer is modelled by the Woschni correlation. To extend the capabilities of the single-zone models the closure problem of the heat-transfer surface of unburned charge to combustion chamber walls based on mass fraction burned is addressed in frame of energy balance equation for unburned charge. For model validation the simulation results are compared with the predictions of the two-zone model, which directly tracks the flame front propagation through combustion chamber (pancake and pent-roof). Results of the performed simulations give the correlation for heat-transfer surface of unburned charge as function of mass-fraction burned. In a series of the simulations the computational efficiency and accuracy of the proposed correlation is shown for modeling of the self-ignition onset in frame of single-zone model. Also proposed correlation allows accurately compute the unburned and burned gas heat transfer areas and heat fluxes to walls in frame of two-zone model. In cases when thermal balance and NO_x predictions are of concern, proposed correlation also provides significant speed up of the two-zone model simulations in comparison with explicit flame tracking techniques.