ABSTRACT

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.

4. Conclusion

This work investigated the impact of NO on knock onset in a CFR octane rating engine under constant knocking, continuous firing conditions. The influences of engine intake temperature, charge equivalence ratio, and fuel composition on the impact of NO were investigated with 0–800 ppm NO added via the engine intake. Major conclusions of this work are as follows.

• Temperature strongly affected the impact of NO on fuel iso-octane. With an engine intake temperature of 52 °C, NO advanced the knock onset of iso-octane (ϕ = 1.43) only up to 200 ppm, with a peak promotion at 25 ppm, and then inhibited it as more NO was added. The promotion effect became much stronger as temperature increased, where the knock onset is monotonically advanced with increasing levels of NO at the intake temperature of 200 °C.

• The impact of NO on iso-octane autoignition was affected by the mixture equivalence ratio. This effect, however, was complicated by the varying amounts of residual NO at different equivalence ratios and also strongly affected by the charge temperature.

• The impact of NO varied with fuel chemical composition. The seven gasoline surrogate fuels of a similar octane number showed that higher contents of toluene and ethanol led to stronger promotion effects which correlated to an increase of the octane sensitivity in these fuels.