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.

Introduction

Knock in spark ignition (SI) engines fundamentally limits engine efficiency, and is a process that results from the interaction of the fuel autoignition chemistry and the thermo-chemical conditions inside the engine cylinder. For reciprocating engines, these in-cylinder conditions commonly involve the products of combustion as part of the autoigniting mixture due to the presence of residual gases. The increasing use of exhaust gas recirculation (EGR) to control engine emissions and to enable advanced, low temperature combustion [1] also makes the residual combustion products even more significant in the engine autoignition process. Amongst the various combustion products, nitric oxide (NO) has been reported to have a pronounced and complex impact. NO was reported to promote autoignition (and therefore knock) of iso-octane in SI engines [2–۵] and in homogeneous charge compression ignition (HCCI) engines [6–۹]. For example, Sheppard et al. [3] reported that knock onset of iso-octane in an optical SI engine was monotonically advanced with increasing NO addition up to 387 ppm. Contino et al. [9] studied the impact of NO (from 0 to approximately 500 ppm) in an HCCI engine and found that the CA50 (50% mass fraction burnt point) of iso-octane was consistently advanced as more NO was added. This promoting effect of NO was, however, generally observed at low levels of NO addition or high autoignition temperatures, with high levels of NO addition at low temperatures found to retard autoignition [2,3,6,7,10]. For example, Prabhu et al. [11] conducted a motored engine study using PRF81 (iso-octane/n-heptane mixture at 81:19 ratio by volume), and reported that for intake temperatures below 70 °C, NO addition up to 100 ppm promoted the fuel autoignition as indicated by the CO formation, but higher levels of NO addition inhibited the reactivity. A similar, non-monotonic impact was reported by Dubreuil et al. in an HCCI engine [6] fueled with n-heptane and the mixtures of n-heptane/iso-octane and n-heptane/toluene with research octane numbers (RON) of 25 and 24, respectively. They found that the firststage ignition (low temperature heat release) was advanced with NO addition up to 100 ppm but was delayed by further NO addition. The second-stage ignition (hot ignition) was also advanced with NO addition up to 100 ppm but remained almost unaffected at higher NO levels. The impact of NO has also been reported to be fuel dependent. Sheppard et al. [3] reported that NO tended to suppress the autoignition of fuels with strong negative temperature coefficient (NTC) behaviors (e.g. paraffins), but promote the autoignition of fuels with weak NTC behaviors (e.g. aromatics). More fundamental experiments have also been conducted in flow reactors, jet stirred reactors, and a rapid compression machine to understand the chemistry between NO and C1- C5 hydrocarbons [12–۱۹], n-heptane, iso-octane and toluene [6,20,21].