A review of water injection application on spark-ignition engines

https://doi.org/10.1016/j.fuproc.2021.106956Get rights and content

Highlights

  • A comprehensive review of water injection applied on spark-ignition engines was conducted.

  • Different water injection methods were introduced and compared.

  • Water's atomization and vaporization process were discussed.

  • Physical and chemical effects of water injection on engine performance were introduced.

  • Future research directions of water injection were proposed.

Abstract

The increasingly stringent emission and fuel economy regulations force the advancement of internal combustion engines towards higher power density and lower emissions. Knock and thermal limits are bottlenecks that prevent improvement of the spark ignition (SI) engines' thermal efficiency. Due to its significant knock mitigation and cooling abilities, water injection technology has great potentials in enhancing thermal efficiency of SI engines and thus, regains research attention in recent years. This paper aims to present a comprehensive review of water injection applications on SI engines. Various methods for the implementation of water injection in SI engines are introduced and compared. Methods to maximize the working efficiency of water injection and its detailed physical and chemical effects on combustion and emissions are discussed. It was found that, with different purposes, water injection can improve the combustion and performance of SI engines. Typically, better combustion phasing and thus higher thermal efficiency can be achieved especially under high load conditions. Emissions such as nitrogen oxides and soot can be largely suppressed. However, crucial issues such as water supply and wall wetting still restrict wide application of water injection technology. The detailed kinetic mechanisms of chemical effects and coupling of physical and chemical effects are needed to be investigated.

Introduction

Admittedly, there is a global focus and enthusiasm on electric vehicles. However, internal combustion engines (ICEs) will still continue to be the dominant source of propulsion power for road transport in the near future [1]. Specifically, spark-ignition (SI) engines are the main power source for light transportation sector worldwide except for some parts of Europe [2].

The increasingly stringent emission legislations combined with the consumers' requirements for power performance and drivability put forward stricter demands for SI engines to achieve higher thermal efficiency. The current development trend of SI engines towards higher power density and better fuel economy is mainly realized by high-boost and downsizing technologies, which are seriously constrained by knock and the thermal limits of components. Various solutions have been hence proposed, such as exhaust gas recirculation (EGR), Miller cycle, variable compression ratio (VCR), water injection, duel-fuel injection, hybridization, and inspection and maintenance [3,4].

Among them, water injection shows significant ability in knock suppression and cooling. The earliest history of water addition in ICEs can be traced back to the early 20th century [5]. Afterwards, as an effective means for knock suppression, water injection was widely adopted in aircrafts and racing cars to obtain temporary power enhancement [[6], [7], [8]]. With the emergence of intercooler (also called charge air cooler), people's enthusiasm for water injection technology gradually declined. Recently water injection has been applied on the mass produced cars by BMW, which achieved substantial power boost and fuel economy improvement [9,10].

As discussed above, knock and thermal limits are the primary obstacles to further enhance the thermal efficiency of SI engines. Since water is an effective cooling and anti-knock agent, water injection technology attracts attention again. Due to the impending needs for enhancing the engine performance, substantial studies have been conducted to investigate the potential of water injection with the foci on achieving a higher compression ratio (CR), improving the working efficiency and extending the antiknock area of SI engines. Fig. 1 shows the statistical analysis results of the application of water injection in SI engines based on published papers, which covered different CR, engine speed, water/fuel (W/F) ratio, indicated mean effective pressure (IMEP) and injection methods [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]]. It can be seen from Fig. 1-(a) that most studies adopted W/F ratios lower than 1. Only a few researches attempted to raise water injection quantity and adopted up to 5 W/F ratios. This indicated that proper water injection quantity is lower than a certain value. On one hand, the vaporization of water is limited. On the other hand, the water supply and engine proper working life should be taken into account. It can be seen from Fig. 1-(b) that water injection allows CR to be increased up to 14. However, most researches adopted CRs ranging from 10 to 11. Fig. 1-(c) indicated that for port fuel injection (PFI) engine, direct water injection (DWI) shows poor working efficiency, achieving IMEPs below 10 bar. However, the range of IMEP can be significantly increased up to 22 bar by using port water injection (PWI). For gasoline direct injection (GDI) engines, it is better to use DWI at low and medium engine speeds to achieve higher IMEP. When at high engine speeds, PWI shows better effects in raising IMEP. This could be related to the vaporization process of water. On the whole, GDI engines showed higher power increase potential with the aid of water injection compared with PFI engines.

It is proved that water injection could effectively inhibit knock combustion [11,15,16,18,19,21,22,24,[27], [28], [29]]. Therefore, in high knock propensity areas (commonly low speed, high load conditions or high speed, high/full load conditions) water injection allows more advance of spark timing to achieve optimal combustion phasing and thus higher thermal efficiency [25]. Additionally, as an effective cooling agent, water helps eliminate the need for fuel enrichment under high load conditions, which is used to thermally protect pistons and catalytic converters [10]. These merits all contribute to the improvements in power performance, fuel economy and emission control, as summarized in Fig. 2. Fig. 2 shows that the most important contribution of water injection technology is its charge cooling effect, which reduces the density of the intake air, allowing more fuel to be burned to obtain more power. Water injection has been widely applied on aero-engines before the advent of intercoolers, which are well documented [6,30]. Currently, the increasingly stringent emission laws make highly downsized, boosted SI engines popular in transportation sector. The use of higher CR is one of the most important means to further enhance the thermal efficiency of SI engines. However, it is seriously restricted by knock combustion onset [31]. Water injection technology helps to cool the air fuel mixture, thus significantly reducing the knock propensity. Lanzafame and Brusca [27,32] proved that water injection led to the increase of octane number on a single cylinder Cooperative fuel research (CFR) engine using the same fuel. Therefore, the use of higher CR enabled by water injection is one solution to further enhance the thermal efficiency of SI engines without the need for adopting higher octane number fuel than the-state-of-art. Another way to improve the thermal efficiency by using water injection's anti-knock effect is to apply larger spark advance. It is worth mentioning that, with constant spark timing the cooling and dilution effect of water injection leads to delayed combustion phasing and elongated combustion. The deteriorated combustion hence induces lower thermal efficiency and thus poor fuel economy and/or power performance. In terms of emissions, NOx emissions are commonly reduced while HC and CO emissions are worsened. Therefore, in order to effectively utilize the anti-knock advantage of water injection technology, advancing of spark timing is required when CR is kept unchanged. In fact, the enhancing effect of water injection on combustion can be realized only if the engine is running in the high knock propensity area. Under high risk of knock combustion conditions, spark advance is commonly reduced to suppress detonation onset, which leads to poor combustion phasing and hence lower thermal efficiency. With the aid of water injection, much larger spark advance can be achieved to realize optimal combustion phasing, leading to better fuel economy and/or power performance. In addition, under full load and high speed conditions, knock combustion is not the only critical issue to be concerned. Another important problem is the high thermal stress on pistons and catalytic converters. Normally, rich combustion is adopted to lower the exhaust gas temperature under high load conditions. The excessively injected fuel is used to cool the end gas. Being a high efficient cooling agent, water has the potential to eliminate the need for fuel enrichment operation and obtain stoichiometric combustion at broader engine map. This leads to the improvement of fuel economy, power performance and even emission control. Additionally, higher boost pressure is possible for further increase of the engine power density. Fig. 3 shows the engine efficiency improvements of water injection from literatures [10,12,16,18,19,21,24,25,[33], [34], [35], [36], [37], [38], [39]]. It can be seen that both PWI and DWI have promotion effects on engine efficiency. Moreover, the effect of PWI is more prominent than DWI. Recently, more scholars tend to investigate PWI. Another reason can be attributed to the lower costs and control complexity compared with DWI.

There are a few reviews on the water injection application in ICEs [[40], [41], [42]]. However, the mechanisms of water injection especially the vaporization process and chemical effects of water on combustion are rarely discussed. Understanding the kinetic mechanism of water on SI engine combustion process is crucial for further exploring its potential in improving engine performance. This paper aims to review the recent studies on the mechanisms of water injection on SI engine performance. The water injection concept here specifically refers to the SI engines that are fueled with pure gasoline and equipped with PWI or DWI system, which have attracted the most research attention. The remaining of this review is organized as follows. Section 2 introduces and compares different methods for the implementation of water injection in SI engines. Then effect of water injection on mixture formation is introduced. Section 3 introduces thermal and chemical effects of water injection on SI engines. Next, 4 Effects of water injection on combustion, 5 Effect of water injection on emissions introduce the effects of water injection on combustion and emission, respectively. Finally, challenges and future research directions are discussed in section 6 and conclusions are made in section 7.

Section snippets

Effect of water injection on mixture formation

Due to its high enthalpy of vaporization, water is proved to be a good cooling and anti-knock agent in historical usage. It was found that the significant cooling effect of water is the primary contributor to the knock mitigation of water injection technology. When water is added into the engine, fuel enrichment under high load could be eliminated to achieve better fuel economy. In addition, more spark advance can be applied to achieve better combustion phasing, which leads to improvement of

Cooling effect

With reference to Table 2, the main thermodynamic properties of water are compared with that of gasoline [15]. Water presents six times higher heat of vaporization compared to gasoline fuel. Therefore, water acts as a cooling agent to effectively reduce the charge temperature in SI engines, which could inhibit detonation and eliminate fuel enrichment operation. The charge cooling effect of water injection is well documented in investigations [19,23,28,49,50]. Berni et al. [28] reported that,

Effects of water injection on combustion

Water affects combustion mainly through its thermal, dilution and chemical effects. Some parameters are remarkably critical to SI engine combustion, such as the minimum ignition energy, auto ignition delay time, and burning velocities.

Combustion in SI engine is premixed combustion, which means its combustion velocity is determined by flame propagation speed. According to knocking theory, raising the flame propagation speed helps reduce knock propensity. For the SI engines, burning velocity is

Effect of water injection on emissions

To protect the environment and meet more stringent regulatory, tail-pipe emissions are always a critical issue. The regulated exhaust emissions comprise of carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM) [85]. Existing measures to control these emissions include post-treatment measures, such as TWC, selective catalytic reduction (SCR) systems, and particulate filters.

The use of water as an emission control agent in combustion applications was proposed

Challenges

Although water injection has shown great potential in enhancing the thermal efficiency and emission control of SI engines, it is still not a matured technique for mass production automobiles due to the following three challenges. Firstly, suitable water injection control and water supply systems should be developed for complex engine operating conditions. Attempts have been made to address the water supply issue [14,34,[107], [108], [109]]. There are currently four methods to maintain the water

Conclusions

This article systematically reviews the mechanisms of water injection on SI engine performance, including different water injection methods, water evaporation process, thermal and chemical effects of water injection, and water injection effects on combustion and emissions performance of SI engines. The main conclusions are drawn as follows:

  • 1.

    Due to its remarkable cooling and anti-knock abilities, water injection technology reduces the heat stress and knock propensity of SI engines, leading to

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 52076065).

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