Off-Stoichiometry Operation of an SI Engine-A Model of Formation and Control of Nitric Oxide

The highest concentrations of NO appear in the exhaust of a spark ignition engine when it is operated slightly on the lean side of stoichiometry. A suggested method of control therefore is to run the engine at an off-stoichiometric air/fuel ratio where the nitric oxide emission is low. Operating the engine in the fuel rich region entails high emissions of carbon monoxide and unburned hydrocarbons and is not compatible with the idea of conserving fuel energy. Therefore, it is advisable to run the engine in the lean mixture region.

A number of models have been developed in the past to predict NO emission levels by the use of the rate kinetic reactions:

In these the oxygen atom concentration has been assumed to be that at equilibrium and this has been justified on the basis that the rate of heat release is greater than the rate of NO formation. However, in the lean mixture range (fuel/air equivalence ratio ≃ 0.80:0.60) experimental observations have indicated higher concentrations than those predicted by such models. Now it is known that the oxygen atom concentration in the combustion chamber of an engine can be higher than the equilibrium concentration, particularly in the lean mixture region. Such higher concentrations of [O] can yield higher concentrations of NO.

This paper describes a model which takes account of this super-concentration of oxygen atoms in the flame front in order to predict NO emission levels. Comparisons are made with measured results and the agreement found to be good. The combustion duration period is considered as a critical parameter in the control of NO in the lean mixture range.