Optimization and knock modeling of a gas fueled spark ignition engine

The final design of an engine is normally the best compromise between various requirements achieved through lengthy and costly processes involving the use of experimental and analytical approaches.; Analytical techniques, primarily in the form of computer codes, are intended to reduce the time and resources needed for this purpose and provide an insight into the complex interacting processes which occur in the engine. The availability of such codes for the prediction of engine combustion and knock is limited and their scope of application is narrow mainly due to the complexity of the processes involved.; The present contribution describes a predictive procedure for the onset of knock and associated performance parameters of a spark ignition engine fueled with common gaseous fuels such as methane. The two-zone predictive combustion model is based on an estimate of the effective duration of the combustion period and the mass burning rate for any set of operating conditions. The unburned end gas preignition chemical reaction activity is described by a detailed chemical reaction kinetic scheme. The variation with time during a cycle of the value of a dimensionless knock criterion, K, is calculated. This parameter relates the total energy released within the end gas due to autoignition reaction activity per unit of the corresponding instantaneous volume relative to the total energy release per cylinder volume that would take place normally due to regular flame propagation. When knocking is encountered the value of K builds up to a sufficiently high value that exceeds an experimentally established acceptable limit. Under normal operating conditions, its value remains throughout the combustion period at comparatively low levels.; There is a need to provide effective guidelines for predicting the highest limit for acceptable power output for any spark ignition engine when operating on gaseous fuels. This is especially the case in relation to the encountering of knock that can impose a severe barrier to engine output and the efficient use of some fuel resources in engines. The results of a parametric study that investigates the influence of operational variables on engine performance and the knock intensity are presented. A relatively simple procedure for choosing an optimum operating condition close to an initial setting is also outlined.; A deterministic gradient based model and a simple genetic algorithm are described and shown to be capable of predicting analytically the changes in specified operating parameters to produce optimum output while maintaining knock free operation. A penalty based transformation method is used to convert the constrained engine optimization problem into an unconstrained one. The concepts of these two optimization methods are presented in detail with a number of typical illustrative examples.; There is no universal agreement on a satisfactory procedure for knock rating of gaseous fuels and their mixtures in spark ignition engines. The suitability of different procedures used in the past by different researchers is examined both analytically and experimentally. It is shown that a rating procedure based on a scale that uses n-butane-methane mixtures offers much promise.