Charge stratification has long been considered the best feasible means of burning overall weak mixtures. This offers the attractions of fuel economy and relatively low levels of pollutants emitted from the engine. In this paper, two idealised models of charge stratification are considered theoretically, with the emphasis on nitric oxide emissions.
The first model represents a linear variation of fuel-air equivalence ratio, from a fuel rich mixture of the first element to burn, to a fuel weak mixture of the last element to burn. The effects of the slope of the equivalence ratio to mass fraction burned and the mean value of the equivalence ratio on nitric oxide emissions are analysed and discussed.
The second model represents a step variation in the equivalence ratio, with two different zones each having a constant value of the equivalence ratio, and each being homogeneous within itself. The nearest to this model in practice is the double chamber engine, with a fuel rich primary chamber and a fuel weak secondary chamber. The theory analyses and discusses in detail the effects of the ratio of the two equivalence ratios, the ratio of the primary to secondary charge mass, and the overall mean equivalence ratio on exhaust emissions of nitric oxide. Furthermore, the intake and compression strokes of such a system prior to ignition are considered, showing the continuous variations of equivalence ratios.