The impact of nitric oxide on knock in the octane rating engine

Nitric oxide (NO) is a trace species that is always present in reciprocating engines, and can significantly affect fuel autoignition. This work presents a systematic investigation of the impact of NO on fuel autoignition in a standard, octane rating engine. Knock onset timing is investigated over a wide range of equivalence ratios, intake temperatures, and fuel compositions with increasing levels of NO added via the engine intake. NO is observed to both promote and retard autoignition in different cases. In particular, NO added via the engine intake can often promote autoignition when the engine is operated at sufficiently rich conditions such that there is negligible, combustion-induced residual NO in the fresh charge. Increasing the intake air temperature with iso-octane fuelling further enhances NO's promoting effect. The promoting effect of NO is also found to be stronger for fuels containing higher toluene and ethanol content rather than paraffins, suggesting that the autoignition of fuels with higher octane sensitivity is also more sensitive to NO addition. These observed impacts of NO are discussed using a current understanding of the interaction chemistry between NO and the studied fuels. This suggests that new, fuel-specific NO mechanisms are required as an integral part of the kinetic modelling of engine combustion.