Characteristic Time Analysis of SI Knock with Retarded Combustion Phasing in Boosted Engines

This study investigates the use of a characteristic reaction time as a possible method to speed up automotive knock calculations. In an earlier study of HCCI combustion it was found that for ignition at TDC, the ignition delay time at TDC conditions was required to be approximately 10 crank angle degrees (CAD), regardless of engine speed. In this study the analysis has been applied to knock in SI engines over a wide range of engine operating conditions including boosted operation and retarded combustion phasing, typical of high load operation of turbocharged engines. Representative pressure curves were used as input to a detailed kinetics calculation for a gasoline surrogate fuel mechanism with 312 species. The same detailed mechanism was used to compile a data set with traditional constant volume ignition delays evaluated at the peak pressure conditions in the end gas assuming adiabatic compression. For conditions with relatively early combustion phasing, the kinetic calculation predicted that auto ignition would occur at peak pressure if the ignition delay at peak pressure was in the range of 7 to 10 CAD. For retarded conditions however, the ignition delay threshold for auto-ignition at peak pressure was significantly longer and ranged from 18 to 35 CAD, due to the more gradual approach to peak pressure for retarded phasing. Additionally, at engine speeds where the compression path passed through the NTC kinetics region there was an increased tendency to knock due to significant heat release prior to final auto-ignition. This also increased the threshold ignition delay for knock. While a simple correlation of the results was not possible, the analysis points out the strong influence of retarded timing and NTC behavior on knock.