A Model for Prediction of Knock in the Cycle Simulation by Detail Characterization of Fuel and Temperature Stratification

Development of SI engines to further increase engine efficiency is strongly affected by the occurrence of engine knock. Engine knock has been widely investigated over the years and the main promoting parameters have been identified as load (temperature and pressure), mixture composition, engine speed, characteristic of the fuel, combustion chamber design, and etc. In this paper a new model for predicting engine knock in 0-D environment is presented. The model is based on the well-known approach of using a Livengood and Wu knock integral. Ignition delay data that are supplied to the knock integral are for specific fuel calculated by detail chemical kinetics and are comprised of low temperature heat release ignition delay and high temperature heat release ignition delay. Next, the cycle to cycle variations of engine and temperature stratification of the end gas have to be taken into account. For temperature stratification a new model is developed which is based on the detail analysis of specific CFD results of several engines at different operating conditions. The validation of the knock prediction model was made by comparisons of the simulation results with the experimental data, for two different fuels (n-heptane and gasoline), showing promising ability of the model to predict the tendency of knocking. The results also showed that the cyclic variability of the knock occurrence and intensity in SI combustion is not only caused by the differences in pressure profile caused by cycle to cycle variations, but also by some other factors.