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

doi:10.1016/j.fuel.2018.08.039

Fuel

Volume 235, 1 January 2019, Pages 495-503

https://doi.org/10.1016/j.fuel.2018.08.039 Get rights and content

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.

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], [3], [4], [5] and in homogeneous charge compression ignition (HCCI) engines [6], [7], [8], [9]. 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 first-stage 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], [13], [14], [15], [16], [17], [18], [19], n-heptane, iso-octane and toluene [6], [20], [21]. Similar to the engine studies, the impact of NO in these experiments exhibited a complex dependence on temperature, NO concentration and fuel composition [21].

Such investigations show that the impact of NO in SI engines is complex and challenging to study experimentally. This is in part because NO in the residual gases varies with engine operating conditions. Techniques such as skip-firing [2], [3] and Ar-O₂ combustion [4] have been used to remove the impact of residual NO at the cost of altering the engine combustion environment. Also, production engines are often unable to withstand continuous knocking combustion, and the borderline knocking condition commonly used in these engines makes it difficult to examine the impact of NO on engine autoignition.

To address such issues, this study therefore uses a Cooperative Fuel Research (CFR) engine, which is the standard octane rating engine [22], [23] and is designed for continuous knocking operation. Fuel rich conditions are a focus in order to suppress combustion generated NO, thereby enabling study of the impact of NO addition via the engine intake. Although these conditions differ from normal, stoichiometric SI engine operation, they permit continuous firing and thus maintain more realistic temperatures for the cylinder walls and residual gases, both of which are important to autoignition. The adjustable compression ratio of the CFR engine further allows knocking to be studied over a wide range of operating conditions, particularly the varied intake temperatures, equivalence ratios and fuel compositions that are considered in this work.

Section snippets

Engine setup

A Waukesha CFR engine is used in this work. The engine is in the standard setup for octane number testing with minor modification for this study. Engine specifications are shown in Table 1. A schematic of the setup is shown in Fig. 1. Ambient air is dehumidified by a chiller operating at 3.5 ± 1.5 °C. NO (10 vol% in N₂) is injected into the air flow upstream of a surge tank. Pure O₂ (99.999%) is added at the same location to maintain the intake O₂ at the same level as air (20.95%). The gases

Variation of the impact of NO with equivalence ratio (IAT = 52 °C)

The impact of NO on iso-octane autoignition is first studied at equivalence ratios at 0.91, 1.0, 1.12 (standard RON condition) and 1.43 with the intake air temperature fixed at 52 °C. Figure 5 shows the changes of knock onset timings from the 0 ppm NO baseline cases as a function of NO addition levels. For ϕ = 0.91, 1.0 and 1.21, NO addition delays the knock timing for all cases relative to the baseline. The maximum delay occurs at the higher levels of NO addition, which is 0.4–1.2 CAD later

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

Acknowledgement

This project is supported by the Australian Research Council, Australia (DP140100846).

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Full Length ArticleThe impact of nitric oxide on knock in the octane rating engine

Abstract

Introduction

Section snippets

Engine setup

Variation of the impact of NO with equivalence ratio (IAT = 52 °C)

Conclusion

Acknowledgement

Proc Combust Inst

Proc Combust Inst

Proc Combust Inst

Combust Flame

Proc Combust Inst

Combust Flame

Proc Combust Inst

Proc Combust Inst

Combust Flame

Proc Combust Inst

Combust Flame

Combust Flame

Fuel

Combust Flame

Combust Flame

Prog Energy Combust Sci

SAE Trans

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

Variation of the impact of NO with equivalence ratio (IAT = 52 °C)