Elsevier

Energy

Volume 158, 1 September 2018, Pages 899-910
Energy

Study on pollutants formation under knocking combustion conditions using an optical single cylinder SI research engine

https://doi.org/10.1016/j.energy.2018.06.063Get rights and content

Highlights

  • Experimental investigation of knocking combustion and cycle resolved emissions.

  • Effect of knock index on NO formation.

  • Utilization of instantaneous flame images to interpret emissions formation.

  • Effect of knock and heavy knock conditions on cyclic emission variability.

Abstract

The aim of this experimental study is to investigate the pollutants formation and cyclic emission variability under knocking combustion conditions. An optical single cylinder SI research engine and fast response analyzers were employed to experimentally correlate knocking combustion characteristics with cyclic resolved emissions from cycle to cycle. High-speed natural light photography imaging and simultaneous in-cylinder pressure measurements were obtained from the optical research engine to interpret emissions formation under knocking combustion. The test protocol included the investigation of the effect of various engine parameters such as ignition timing and mixture air-fuel equivalence ratio on knocking combustion and pollutant formation. Results showed that at stoichiometric conditions by advancing spark timing from MBT to knock intensity equal to 6 bar, instantaneous NO and HC emissions are increased by up to 60% compared to the MBT operating conditions. A further increase of knock intensity at the limits of pre-ignition region was found to significantly drop NO emissions. Conversely, it was found that when knocking combustion occurs at lean conditions, NO emissions are enhanced as knock intensity is increased.

Introduction

Gasoline engine technology has entered one of the most exciting periods in its long history: downsizing, hybridization and stricter emission standards determine the future of SI engines [1]. Advanced engine requirements for high power density and low fuel consumption can be achieved through high boost with direct injection [2], engine start&stop [3], exhaust waste heat recovery [4] and lean burn operation [5]. Implementation of this technology in SI engines increases the potential possibility that under certain operating conditions auto-ignition or pre-ignition can occur [6]. Knock not only limits engine thermal efficiency but it also affects the formed emissions during combustion. As pollutants formation is directly linked to the combustion process, it is critical to understand the effect of abnormal combustion cycles on emissions performance and variability. To this end, meeting fuel economy requirements and future emission standards in high-efficiency SI engines requires a well understanding of knock and its effect on engine emissions.

Knock is as old as SI engine itself [7] and takes its name from the metallic ‘pinging’ noise that auto-ignition (spontaneous combustion) creates before the piston reaches TDC. Knock can be divided into two main groups: light to medium knock and super knock. Light to medium knock limits compression ratio and as a consequent the engine thermal efficiency is also limited due to the end-gas auto-ignition. Auto-ignition is the fast discharge of chemical energy contained in the end-gas, which is the final fraction of the air-fuel mixture that enters the cylinder but without inclusion into the flame front reaction [8,9]. On the other hand, super knock limits raising the boost pressure and the engine power density due to detonation, also known as pre-ignition [10,11]. Pre-ignition occurs earlier to auto-ignition and leads to a fast and violent combustion that can potentially damage the engine. Apart of the great number of studies on knocking combustion, the correlation between knock intensity, heat transfer, fuel chemistry, pressure oscillations and oil droplets is not well understood [6]. Nowadays, the main research focuses on super knock due to the high boost technology applied in high power density downsized engines, which usually occurs under low speed and high load engine operating conditions.

Fuel and cylinder charge properties as well as engine calibration can affect combustion characteristics and simultaneously affect the formed emissions. Methods for suppressing engine knock and controlling emissions involve the optimization of those parameters. Conventional suppression methods of engine knock include control strategies for retarding spark timing [12], raising fuel octane number by using additives [13,14] and enriching mixture [15] when knock occurs. Recent studies correlated the effect of cooled EGR with the reduction of knocked cycles and fuel consumption. Diana et al. presented a knock free operation of a stoichiometric SI engine at full load conditions with a compression ratio of 12.5 by using cooled EGR [8]. Same trend was also observed in another experimental study where it was shown that the resistance to knock is strongly increased by cooled EGR [16]. It was also found that utilization of EGR can significantly reduce emissions by up to 90% [8] [17], [18], while HC emissions increase across the EGR range [19], [20]. Air-fuel ratio can also affect engine knock limit. Although leaner mixtures benefit fuel consumption, retarded combustion is required to avoid knock [9], while over-fuelling increases the knock limits of the engine [21]. In another study, it is reported that the EGR and the lean burn operation exhibit lower average knock indexes. Due to the longer combustion duration by utilizing EGR and lean burn, heat transfer is enhanced and end-gas temperature is decreased [22]. Conversely, another experimental work using a single cylinder research engine that was supplied with ON 75 proved that lean mixtures have the earliest onset of knock, and the highest knock intensity [23]. Furthermore it was noted that lean mixtures are particularly sensitive to the charge heating during compression. Overall, this research implies that excess air at high loads does not suppress knock.

More than 700 research studies exist in literature on the research area of engine knock [6]. The main research studies that deal with engine knock can be categorized on knock detection, numerical simulation, optical diagnostics, theoretical studies, engine optimization and fuel/oil properties. Although knocking combustion can significantly affect formed emissions, the effect of knocking combustion on pollutants formation is at an early research stage and there is a significant lack of studies in this research field.

Pollutants formation during engine combustion is correlated both with mixture properties such as air-fuel equivalence ratio, residual gas fraction and fuel properties as well as with the combustion type. At normal combustion conditions (deflagration), NOx formation is primarily controlled by oxygen availability under lean or rich conditions while within the stoichiometric window formed NOx concentration is correlated with the combustion burn rate [24]. Under no knocking combustion, previous studies showed that as in-cylinder peak pressure is increased, NOx emissions are proportionally increased [[24], [25], [26]]. However under knocking combustion, the emissions formation mechanism can significantly change. As knock intensity is increased, residence time of the residual gases in the burned zone is significantly decreased and peak temperature can be enhanced. On the one hand, under super knock conditions combustion duration is decreased to less than 2° CA which means that combustion is almost isochoric, post-combustion residence time is trivial but peak temperature and pressure are much higher than in normal SI combustion. Such a type of combustion presents many similarities with HCCI combustion that is known for the ultra-low NOx emissions and the relatively higher CO and HC emissions [27]. On the other hand, under light knock conditions NOx concentration that is formed in the post-flame zone during normal combustion is expected to be increased due to the high temperatures of end gas auto-ignition. However, the above described trends have not been explored in literature and more research is required to well understand the relationship between pollutants formation and knock index.

A great number of research studies investigate the main mechanisms of pollutants formation, described in detailed elsewhere [28]. The most well-known NOx formation mechanism is the thermal one, co-called as extended Zeldovich mechanism which is responsible for the majority of the formed NO in the post flame gas zone [29,30]. Apart from the thermal mechanism, the prompt NO mechanism [31] can be significant at stoichiometric and richer mixtures [32,33], the N2H mechanism becomes important at slightly rich conditions [34] and finally the N2O chemical pathway appears at slightly lean conditions [29,35]. Regarding carbon monoxide, CO formation is mainly kinetically controlled by water gas shift reaction [36]. Finally, hydrocarbons (HC) are the consequence of incomplete combustion of hydrocarbon fuel [37]. In most studies, simplified chemical mechanisms are utilized to predict pollutants formation [32,38]. Recently, Karvountzis et al. [39,40] proposed the use of a detailed chemical kinetics model to predict pollutants formation in an SI engine, which operates in the post-combustion zone. The latter is the most accurate state-of-the art emission model existing in literature, as the validation against experimental cycle resolved emission values for both NO and CO emissions under various engine operating conditions showed errors less than 10% and compared to simplified emissions models improvements were higher than 50% [41]. However the accuracy of this emission model under knocking conditions has not been explored, due to the lack of experimental data.

Although the relationships between combustion characteristics and pollutant formation under normal combustion have been explored in the past, the effect of knocking combustion on pollutants formation is not well understood. Few studies in the past presented an engine model that can predict knock limit and NOx emissions [42] [43], [44]; however those models do not present the pollutant formation under knocking combustion conditions. The only known relationship is that knock results to an increase in CO and HC emissions due to incomplete combustion [45]. Furthermore, there is a lack of experimental data at this research field, as there are no experimental studies presenting the correlations between knock index and emissions performance on a cycle to cycle base. Simultaneously, the effect of pressure oscillations and hot spots (occurring during knocking combustion) on pollutants formation is also unknown. In the past, fast response analyzers were successfully employed to measure cyclic emission variability under normal combustion conditions [46]. Optical diagnostics have been also utilized to deeper understand the end gas auto-ignition mechanism and explore the detonation limits at pre-ignitive cycles [10]. However, as authors are aware, a study that employs both fast response analyzers to detect emissions from cycle to cycle and optical diagnostics to correlate combustion characteristics with cycle resolved emissions doesn’t not exist in literature.

This experimental work studies the correlation between pollutants formation and knock index, primarily NO and HC, under various air-fuel equivalence ratio and spark ignition timings using an optical single cylinder SI engine. Fast response analyzers were employed to measure the cycle resolved NO and HC emissions. Images of the visible light from combustion heat were captured through an overhead window using a high speed camera. The aim of this study is to give insights between the type of combustion (deflagration, end gas auto-ignition and detonation) and the formed emissions and determine by which extent emissions are increased under knock. The output of this study can be applicable to new engine design requirements for more efficient and less pollute SI engines. To the best knowledge of the authors, this is a unique study of its kind.

Section snippets

Optical engine

A customized single cylinder research engine with a distinct optical arrangement was employed in this study [21], [47,48]. The basic engine characteristics of the engine are presented in Table 1. The bottom-end of the engine is based on a commercial Lister-Petter TSI with a combined full-bore overhead access and a semi-traditional poppet-valve valvetrain. Due to the full-bore overhead optical access, the glass of the engine head was designed to withstand peak combustion pressures up to 150 bar.

Effect of spark ignition timing

The phenomenon of knock is controllable by the spark advance as advancing or retarding the spark timing can increase or decrease the knock severity or intensity respectively [50]. Although knock varies substantially from cycle to cycle and doesn’t occur at each combustion cycle, an average combustion cycle can present the ‘mean’ picture of knock intensity. Various spark timings lead to different heat release rate histories, which can be linked to end-gas pressure and temperature conditions that

Conclusions

This study explores pollutants formation under knocking combustion conditions using an optical single cylinder SI engine. A novel experimental setup has been utilized including fast response analyzers for NO and HC emissions, natural light photography and in-cylinder pressure recordings. The effect of two engine operating parameters where explored such as spark ignition timing and air-fuel equivalence ratio.

The effect of ignition timing on pollutants formation under various knock index

Acknowledgement

The authors would like to thank the personnel of the Centre of Advanced Powertrain and Fuels (CAPF) at Brunel University London for their support in the experimental part of this work.

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      Therefore, the post-combustion processes could be reduced due to the lowered exhaust temperatures, and the rate of the oxidation reaction of the unburned mixtures may be reduced, which may result in increased THC emissions. As the knocking intensity increases, the THC emissions were noted to increase in a study by Karvountzis-Kontakiotis et al. [33]. However, the THC emissions observed in this study did not remarkably increase when knocking occurred.

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