Autoignition in engines modelling and experiments

This section discusses the problems of combining models of engine combustion and of end gas autoignition. But prior to this, some effects observed in engines which complicate this endeavour, involving the carryover of reactive species from one cycle to another, are diseussed. 

1 Carry-over of combustion products between cycles 

Run-on occurs when the engine continues running even after the spark has been switched off. It is caused by autoignition induced by piston compression only. This occurs when active partial oxidation products from 

These authors have discussed the main reactions causing this behaviour in qualitative terms, but, as yet, no quantitative modelling studies have been reported. Little is known about the interaction of NO and NO2 with organic radicals at elevated temperatures. The reactions mainly responsible for the enhancement of the oxidation chemistry at low NO concentrations are probably, 

In the low-temperature oxidation chemistry, formation of HO2 is effectively a terminating step. Reaction (1) regenerates an active radical, OH, and reduces the termination rate. 

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In 1985 Leppard [156] reported engine measurements, for stoichiometric ethane-air, of pressure and end gas temperature, the latter derived from the energy equation. The occurrence of autoignition agreed closely with prediction based on an earlier chemical model of Westbrook and Dryer [52]. From their engine experiments, Cowart et al. [59] also compared, for iso-octane and -pentane, the predictions of the simplified models of Hu and Keck [75] and Chun et al. [157], and the more detailed kinetic predictions of Westbrook et al. [158]. These were found to simulate the time of knock occurrence if the kinetic data were re-calibrated. This, and the subsequent work of Brussovansky et al. [76], showed the need for accurate allowances for heat transfer and piston blow-by, because of their important effect on the derived end gas temperature. Where end gas temperature can be measured directly this problem is circumvented. 

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