Full Length ArticleLean combustion of stratified hydrogen in a constant volume chamber
Introduction
To improve the fuel economy, the lean-burn combustion mode has gained research attention, particularly for spark-ignition (SI) engines. Because engine load is controlled by the amount of air intake, negative work is produced when an engine operates under low-load conditions. However, a lean burn can reduce the negative pumping work by opening the throttle valve, even under low-load conditions. Furthermore, the specific heat ratio of a lean mixture is larger than that in the stoichiometric combustion mode. As is known, increasing the specific heat ratio increases the Otto-cycle efficiency owing to the dilution of the in-cylinder mixture [1]. Therefore, the lean-burn technique can realize a high-efficiency SI engine by reducing the negative pumping work and increasing the specific heat ratio of the in-cylinder mixture [2]. Even though lean-mixture operation can improve the fuel economy, the reduction reaction of the nitrogen oxide (NOX) emissions is inefficient in the three-way catalyst (TWC) because of the large amount of oxygen in the exhaust gas [3]. To reduce both the fuel consumption and the emissions using the lean-burn combustion, the lean limit must be extended by adopting advanced in-cylinder or additional techniques. Owing to the dilution of the in-cylinder gas, the thermal NOX could be decreased by reducing the mean in-cylinder temperature. Therefore, zero NOX emissions are achievable by realizing a high air excess ratio [4], [5]. In conclusion, the in-cylinder mixture must be diluted for achieving high efficiency and reducing the raw NOX emissions.
To extend the lean limit, various techniques have been developed such as advanced ignition systems, combustion methods, and fuel switching. However, a lean burn can deteriorate the combustion stability when the mixture level approaches the lean limit [6]. Moreover, as the fuel mixture becomes leaner, the laminar flame speed gradually decreases. Ayala et al. [7] showed that the initial burn duration of a fuel mixture gradually increases as it becomes leaner. Owing to the lengthening of the initial burn duration, the net indicated engine efficiency was decreased with a high coefficient of variation of the Indicated mean effective pressure (IMEP) at a high air excess ratio. To alleviate the unstable operation, a possible solution is to increase the ignition energy by forming a stable ignition kernel at the early stage of combustion [8], [9]. Hwang et al. [10] investigated the effect of microwave-assisted plasma ignition on the lean limit extension in a constant-volume chamber (CVC). Additional 700-W microwaves could extend the lean limit from an equivalence ratio of 0.6 to 0.5. Kewal et al. [11] conducted a laser ignition method for hydrogen-air combustion in a constant volume combustion chamber (CVCC) up to the air excess ratio of 5.0. The increased ignition energy promotes the speed of flame propagation but the peak pressure was almost constant. Moritz et al. [12] conducted lean combustion in a prechamber ignition system. The ignition system was allowed to operate in highly diluted mixtures with an air excess ratio of 1.6. Gong et al. [13] numerically calculate the effect of the twin spark plug location on the methanol direct injection to enhance the combustion speed in a lean-burn SI engine. The advanced ignition system could operate in the highly diluted mixtures with high combustion stability [14]. However, the lean limit could not be extended up to the state at which thermal NOX is not formed. In addition to employing advanced ignition systems, the adoption of different combustion methodologies can improve combustion stability. The stratified charge combustion (SCC) can achieve high combustion stability by stratifying the fuel mixture in the in-cylinder chamber. Unlike the stoichiometric combustion, in the SCC, the fuel is injected during the compression stroke [15], and a locally rich mixture is formed the periphery of the spark plug. The flammable mixture is ignited by the spark discharge, and the generated flame propagates toward the lean mixture near the cylinder wall. Mindaugas et al. [16] compared three combustion modes—stoichiometric, lean-burn, and stratified—and the results showed that the stratified mode achieves the leanest mixture formation among the modes. Although the SCC can reduce the fuel consumption, the combustion stability and characteristics of the particulate matter (PM) emissions are worsened by the locally rich mixture [17], [18]. The stratified mixture in the SCC is maintained for an optimum duration between the injection and spark-discharge times to realize the best efficiency [19]. Even though SCC can extend the lean limit, the combustion stability is susceptible to the mixture formation time (MFT), which is the time between injection and ignition. Shudo et al [20]. varied fuel ratio of stratification in a CVCC to elucidate the effect of cooling loss ratio in a hydrogen SCC. The highly stratified case decreased the heat loss characteristic and the mixture formation time also affect the cooling loss ratio. To maximize the combustion efficiency by a stable SCC operation, Park et al. [21] adopted the inter-injection SI strategy in which ignition discharge occurs during fuel injection. Although most of the injected fuel participates in the combustion process, a large amount of PM is generated. Lee et al. [22] conducted SCC by using a late intake valve closing operation. The reinforced in-cylinder motion and the decreased ambient pressure improve the homogeneity of the mixture with a reduced spray contraction, thereby decreasing the PM emissions. Park et al. [23] visualized the n-butane with SCC in a constant volume combustion chamber (CVCC) to find the area where PM is mainly generated. The result showed that most luminous flames were measured in a spray recirculation zone which has a rich-fuel mixture. To optimize characteristics of efficiency or emission, the CVCC experiment results show that the ignition timing should be advanced or retarded respectively and should not be located in the middle timing. By adopting in-cylinder techniques, stable operation and low PM emissions are achievable. However, the SCC-based PM emissions are still a major problem for clean engine operation. To ensure stable and efficient engine operation without PM emissions, fuel switching from carbon-based fuels to a low-carbon-content fuel must be investigated for a lean burn in the SCC [24].
Fuel switching from conventional fuels to low-carbon fuels is a well-known strategy for decreasing carbon-based harmful emissions [25]. However, low-carbon fuels, such as auto-gas, alcohol fuel, and carbon–neutral bio-fuels, cannot completely mitigate the carbon-based emissions [26]. Therefore, hydrogen could be an ideal solution for internal combustion engines, particularly in the SCC. Chemically, hydrogen has a wider flammable limit than the conventional fuels for SI engines [27]. In addition to the chemical advantages of using hydrogen as a fuel for combustion, the emissions are decreased owing to decarbonization of the fuel [28]. The combustion products of hydrogen do not include carbon dioxide, which is the main source of greenhouse gases. Although using hydrogen, the carbon-related emissions can be eliminated, the NOX emissions may be increased owing to the high adiabatic flame temperature. Yu et al. [29] studied hydrogen addition in a gasoline engine according to the exhaust gas recirculation (EGR) strategy for the mitigation of NOX and carbon-based emissions. Although the EGR strategy significantly reduces NOX when a large amount of hydrogen is used, NOX is still emitted owing to the high in-cylinder temperature. In contrast, hydrogen SCC can occur in a much lean-mixture state in which thermal NOX is not produced. Owing to the high dilution of the in-cylinder mixture, the increase in the in-cylinder temperature is reduced, and hence, the peak temperature is decreased. Wallner et al. [30] introduced two operation modes for a hydrogen-fueled internal combustion engine for the mitigation of NOX emissions: the TWC operation mode with a stoichiometric area and the lean combustion mode, which does not emit raw NOX. To mitigate the NOX emissions without a TWC, a lean limit extension is required. Furthermore, if hydrogen is used for SCC, PM emissions are avoided even in the case of spark discharge when the MFT is short. In conclusion, the adoption of hydrogen in the SCC may extend the lean limit without the deterioration of both the combustion stability and emission characteristics.
The extension of the lean limit is inevitable for improving the fuel economy and emission characteristics in the SCC operation. By applying hydrogen as a fuel, the lean limit extension might be achievable without emissions, particularly PM emissions. For a lean burn, hydrogen is used as a fuel mostly in the homogeneous lean-burn combustion [31]. The addition of hydrogen to the combustion process can improve both the lean limit and combustion stability [32]. Hydrogen is mostly adopted as a supplement fuel for combustion engines as a type of dual-fuel because of the advantages of hydrogen combustion and emission characteristics. Gong et al. [33] experimentally conducted dual-fuel combustion with hydrogen and methanol and the lean limit was extended with hydrogen enrichment. Moreover, the emission characteristics were improved when the hydrogen portion was increased especially for hydrocarbon emission [34]. In the SCC, the MFT is one of the key parameters for realizing high-efficiency energy conversion through combustion. Moreover, the ambient density gradually varies with the injection timing because injection occurs during the compression strokes. The ambient density plays a major role in the spray and flame structures. Therefore, the MFT and ambient density are important variables for SCC. In this study, to understand energy conversion through hydrogen SCC, the effects of MFT and ambient density were thoroughly investigated using an optical measurement system. Most research focus on hydrogen combustion under lean-mixture conditions because hydrogen has the advantages of a low minimum ignition energy and a wide flammable range. In hydrogen SCC, combustion starts in a hydrogen-rich area and propagates toward a lean-hydrogen area. To optimize the injection and ignition timings for hydrogen SCC, the effects of the MFT and the ambient density on the combustion characteristics must be elucidated. In addition, it is commonly believed that a hydrogen flame does not emit in the visible region wavelength range during combustion. However, in this study, a luminous flame was measured when a hydrogen-rich area was formed near the injector with a short MFT. In this study, we mainly focused on the combustion characteristics of hydrogen in the SCC by varying the MFT and ambient density. The visible spectrum of the hydrogen flame was also investigated through high-speed (HS) optical measurements of the spatial flame images. In summary, the characteristics of the SCC with hydrogen are presented based on optical measurements of the hydrogen visible flame. In addition, the effectiveness of the injection and ignition timings with the ambient density variation is comprehensively presented.
Section snippets
Experimental setup
Fig. 1 shows the experimental setup for the hydrogen SCC in a CVC. The CVC has a volume of 1.39 L and its temperature was electrically heated by an external heater. The optical window has a 100 mm diameter. A piezo-actuated injector with a hollow-cone shaped spray structure was adopted. More detailed information on the injector is described in preliminary research [35]. Because the spray-guided (SG) SCC was applied, the spark plug is located near the injector. The detailed layout of the
Effect of MFT on hydrogen SCC
Fig. 6 shows the effect of the MFT on RTHR with fixed ambient pressure of 0.1 MPa. As the MFT increases, the RTHR first shows an increasing tendency. However, it subsequently decreases after an MFT of 2560 . In conclusion, the hydrogen chemical energy is most efficiently converted into pressure energy when the MFT is 2560 . This RTHR result suggests that even though hydrogen has a wide flammable range, optimization of the MFT is needed. Contrary to the prediction, the pressure energy is
Conclusion
To realize hydrogen SCC, a hydrogen monofueled combustion experiment was conducted in a CVC. Additionally, various optical combustion measurements (direct flame, Schlieren, and OH* chemiluminescence imaging) were performed to understand the characteristics of the hydrogen SCC. The key findings of the hydrogen SCC are presented as follows.
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The local equivalence ratio in the periphery of the spark plug was the main cause of the combustion characteristics. Moreover, the turbulent intensity produced
CRediT authorship contribution statement
Sanguk Lee: Conceptualization, Methodology, Software, Investigation, Formal analysis, Visualization, Writing - original draft. Gyeonggon Kim: Investigation, Writing - review & editing. Choongsik Bae: Conceptualization, Resources, Supervision, Writing - review & editing.
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.
Acknowledgments
This paper was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MIST) (NO. 2021R1A2C2008711). The author would like to thank the Hyundai Motor Chung Mong-Koo Foundation for its financial support.
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