ON KNOCKING PREDICTION IN SPARK-IGNITION ENGINES

The paper presents a theoretical model for knocking prediction in spark ignition engines (SI engines). The model mimics the combustion chamber by means of two zones-one in front and one behind the flame front. The latter is characterized by a one-dimensional temperature distribution which emerges due to the Mache effect. The heat transfer is accounted for. High-temperature chemical reactions are described via a chemical kinetic mechanism composed in the present work by combining several already validated detailed mechanisms. Flame velocity predictions by the new mechanism are compared to experimental data. Low-temperature preflame reactions are predicted via an already validated chemical kinetic mechanism. Knocking is treated as the self-ignition of the end gas in front of the flame front, as a result of compression by expanding combustion products and the moving piston. Pressure, temperature, flame front propagation, and completeness of combustion histories are predicted under conditions of both normal and knocking operation. Concentrations of species as well as preflame heat release histories are calculated during knocking operation. The mass fraction of the unburnt fuel at the moment of knocking onset is predicted as a function of the angular crank shaft position corresponding to ignition, crank shaft angular velocity and compression ratio. The effect of methanol admixture on gasoline knocking is investigated. The anti-knocking effect of methanol blended with gasoline is explained and quantitative information regarding its anti-knocking ability in such a blend is presented. The theoretical results are compared to experimental data and the agreement is fairly good.