A REFINED ANALYTICAL COMBUSTION MODEL FOR EVALUATING THE EFFECTS OF EGR PERCENTAGE ON IMPROVEMENT OF KNOCK CHARACTERISTICS OF NATURAL GAS IN SPARK IGNITION ENGINE

Accurate modeling of engine knock onset is needed for control of SI engine combustion and increase of thermal efficiency. This contribution presents a refined model for analysis of engine knock when using natural gas fuel and EGR. The model is used to compare the effectiveness of EGR to other knock suppression methods such as lean-bum combustion, compression ratio reduction, and ignition timing retardation. The model consists of two zones: a burned combustion products region and an unburned reactants comprising the end gas region, separated by a flame front of negligible thickness. A mass burning rate is derived from a turbulent combustion model. FORTRAN code as programming software is used for combustion simulation. Operating conditions which affect an engine's tendency to knock are discussed. The model was validated by comparison to experimental data. Results show that EGR addition is more effective at suppressing knock, while maintaining high thermal efficiency and output work, compared to other knock suppression techniques such as inlet pressure and temperature, equivalence ratio, spark timing, or compression ratio.