Modeling Autoignition and Engine Knock Under Spark Ignition Conditions

Abstract

A computer model that is able to predict the occurrence of knock in spark ignition engines has been developed and implemented into the KIVA-3V code. Three major sub-models were used to simulate the overall process, namelythe spark ignition model, combustion model, and end-gas autoignition models. The spark ignition and early flame development is modeled by a particle marker technique to locate the flame kernel. The characteristic-time combustion model is applied to simulate the propagation of the regular flame. The autoignition chemistry in the end-gas was modeled by a reduced chemical kinetics mechanism that is based on the Shell model. The present model was validated by simulating the experimental data in three different engines. The spark ignition and the combustion models were first validated by simulating a premixed Caterpillar engine that was converted to run on propane. Computed cylinder pressure agrees well with the experimental data. To test the performance of the autoignition model, simulations were compared to experimental data of the Sandia Optical Engine, a research engine equipped with four spark plugs and operated with n-butane. The model predicted the pressure history and ignition delay times in the end gas reasonably well. Finally the model was applied to simulate a single cylinder spark-ignition Kohler utility engine that was run at various conditions, including intentional knocking conditions. Different engine speeds were studied and the occurrence of knock was predicted correctly. Furthermore, resonant pressure oscillations in the combustion chamber under knocking conditions were investigated for this engine.