Optimum operating conditions to get out of knock in a spark ignition engine operating on gaseous fuels



A method is presented to predict the onset of knock and associated performance of a spark ignition engine when primarily fuelled with gaseous fuels such as methane. It gives guidelines for choosing the best operating conditions to obtain maximum power just before the onset of knock. A two-zone predictive combustion model was developed 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 of the value of a formulated dimensionless knock parameter K is calculated. This parameter relates the total energy released within the end gas due to autoignition reaction activity per unit of corresponding instantaneous volume relative to 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 acceptable limit. Under normal operating conditions, the value remains throughout the combustion period at comparatively low levels.