Knock modelling in spark-ignition engines and a study of the effect of combustion instability on knock
One of the limiting factors for improved performance of the spark ignited internal combustion engine is the phenomenon of knock. The present investigation makes a survey of the available models for auto-ignition for introduction into an existing phenomenological combustion model. A mathematical model of knock, based on a degenerate branched chain mechanism for the prediction of autoignition delay time, was considered and introduced into the combustion model. Experiments were carried out on a single cylinder variable compression ratio engine to validate the model. The interaction of acoustic waves with unsteady combustion leading to unstable combustion and thus triggering knock is considered. This work examines the relationship between the variation in the power of the frequency component corresponding to the natural frequency of the combustion chamber, and the occurrence of knock. A model based on the relaxation oscillation phenomenon is developed to calculate the oscillating frequency of the flame front. This model was developed on the basis that the gas contents of the combustion chamber in an internal combustion engine can be considered as a 'lumped parameter' spring mass damper system. Analysis of the frequency spectrum of flame ionization data show a favourable comparison with the predictions from the model, thus, opening the possibility of future work on correlating the acoustic instability in the combustion chamber to knock in spark ignition engines.