Modelling of NO emissions from homogeneous and stratified charge spark ignition engines

Abstract

The basis for modelling NO formation in spark ignition (SI) engines by the so-called thermal mechanism is reviewed, along with a comparison of the coefficients that have been recommended for use in the rate equations over the last 25 years. The importance of considering heat transfer, and a multizone representation of the burned gas, is demonstrated by reference to modelling NO in a homogeneous charge SI engine. The model has then been extended to a stratified charge SI engine, in order to investigate the influence of overall equivalence ratio and degree of stratification on the NO emissions and the engine brake specific fuel consumption. For fixed throttle operation, it is concluded that the best trade-off is with an overall weak mixture that is close to homogeneous. For maximum power output using a slightly rich stoichiometric mixture, the mixture should also be close to homogeneous. However, if the engine is constrained to operate with an overall stoichiometric mixture, then the trade-off between NO emissions and brake specific fuel consumption is with a stratified mixture that is rich at the spark plug.