Comparative assessment of stoichiometric and lean combustion modes in boosted spark-ignition engine fueled with syngas

https://doi.org/10.1016/j.enconman.2021.114224Get rights and content

Highlights

  • Comparison of stoichiometric and lean combustion in syngas spark-ignition engines.

  • High compression ratios and intake boosting improved engine power and efficiency.

  • Lean combustion improved engine efficiency owing to low heat transfer losses.

  • Stoichiometric operation for high power and lean operation for high efficiency.

Abstract

Synthesis gas (syngas) is considered an intermediate step between conventional carbon-based fuels and future hydrogen-based fuels. Spark-ignition (SI) engines are suitable for converting the chemical energy of syngas for small-scale electrical power generation. However, syngas-fueled SI engines have lower power outputs and thermal efficiencies than SI engines fueled with conventional fuels such as gasoline and natural gas do. The objective of this study was to compare the stoichiometric and lean combustion modes in a single-cylinder SI engine to determine the optimal combustion mode for the development of a syngas engine generator with high power and high efficiency. A high gross indicate power was achieved in the stoichiometric combustion mode by increasing the intake pressure, owing to the increase in the volumetric efficiency and syngas fuel input. The gross indicated thermal efficiency (ITE) improved as the compression ratio was increased from 10:1 to 17.1:1, owing to the high peak heat release rate and short combustion duration. In the lean combustion mode, high gross ITEs were achieved by increasing the excess air ratio to 2.5, but the additional increase led to low combustion efficiencies. However, the gross indicated power decreased with an increase in the excess air ratio. The low gross indicated power was increased through intake boosting. Based on a parametric study, the optimal compression ratio for the stoichiometric combustion mode was selected to be 15:1. Pre-ignition occurred in the stoichiometric combustion mode at a compression ratio of 17.1:1 and an intake pressure of 0.16 MPa. Engine operation with a high compression ratio of 17.1:1 was possible in the lean combustion mode owing to the low combustion temperature. The gross ITE in the lean combustion mode was 18.4% higher than that in the stoichiometric combustion mode, mainly because of a significant reduction in the heat transfer loss. However, the gross indicated power in the lean combustion mode was 25.6% lower than that in the stoichiometric combustion mode.

Introduction

With growing concerns regarding climate change due to the increase in greenhouse gas emissions, interest in the use of low-carbon and hydrogen (H2)-based fuels is increasing. Synthesis gas (syngas) is considered an intermediate step in the transition from carbon-based fuels to H2-based fuels [1]. Syngas is a fuel-gas mixture composed mainly of H2 and carbon monoxide (CO). Syngas can be produced directly through chemical processes such as steam methane reforming, coal gasification, and biomass gasification, or obtained indirectly as a by-product of these processes [2]. Various energy systems have been developed to convert the chemical energy of syngas effectively into electrical power [3]. Among these systems, an internal combustion engine is a suitable option for producing heat and electricity simultaneously [4]. Considering the recent increase in the demand for small-scale distributed power generation, the combination of internal combustion engine systems with gasification technology for syngas production is regarded as a promising solution [5].

A compression-ignition (CI) engine is an obvious candidate for producing electrical power because of its ability to burn syngas with various fuel compositions [6]. Another merit of the CI engine is the use of a high compression ratio, which ensures high thermal efficiency [7]. As it is difficult to achieve autoignition of syngas in CI engines without intake heating, dual-fuel combustion is normally adopted to ignite the syngas [8]. In dual-fuel combustion, syngas is supplied with air during the intake stroke, and diesel is injected into the syngas-air mixture during the compression stroke to initiate syngas combustion. Dual-fuel combustion is controlled by the diesel injection timing and the ratio of diesel to syngas [9]. As syngas can be substituted for diesel fuel, dual-fuel engines can reduce diesel consumption. Sahoo et al. conducted an experimental study on a diesel-syngas dual-fuel engine with minor modifications to the existing diesel engine design and operating parameters [10]. Saving in diesel were assessed by varying the syngas composition. As the H2 proportion increased in the syngas, the diesel consumption decreased. A high diesel replacement of 72% was achieved with only H2 at 80% engine load. The possibility of increasing the engine thermal efficiency through dual-fuel combustion has also been reported [11], [12]. Rinaldini et al. experimentally investigated diesel-syngas dual-fuel combustion in a CI engine equipped with a common-rail injection system [12]. The dual-fuel operation with syngas and diesel improved the brake thermal efficiency compared with that under operation with only diesel; the improvement was remarkable at higher loads. This can be explained by the faster combustion in the dual-fuel mode during the combustion-development stages, compared with that in the diesel mode. Although the aforementioned diesel-syngas dual-fuel engines have certain advantages, they also have some drawbacks. The syngas substitution rate is limited to a certain level as additional syngas introduction cannot simultaneously ensure high performance and low emissions [1], [13]. The fundamental problem with dual-fuel engines is that diesel injection is necessary to initiate syngas combustion. However, engine operation with only syngas is desirable considering the growing demand for environmentally friendly electricity production [14].

A homogeneous charge compression ignition (HCCI) engine is a potentially viable energy system for electricity generation because it can be operated using syngas alone. A high intake temperature, which is usually achieved via gasifier heating or intake heating, and a high compression ratio are required in an HCCI engine to ignite the syngas-air mixture owing to the high autoignition temperature of this mixture [15]. HCCI engines are not only capable of 100% syngas operation but can also be adapted for a wide range of fuel compositions [16]. However, it is difficult to control the combustion timing in an HCCI engine, because the combustion is controlled primarily by the chemical kinetics of syngas [17]. Therefore, some concerns exist regarding the control of combustion timing in response to variations in the syngas composition. Power de-rating is also an important issue in HCCI engines [18]. Bhaduri et al. assessed HCCI operation with syngas, which was obtained from a two-stage downdraft gasifier, in a modified diesel engine [19]. Under variations in the syngas composition reflecting the actual HCCI operation, stable combustion was maintained by controlling the syngas flow rate. The authors reported that an expensive and sophisticated in-cylinder pressure sensor was required to control the combustion phasing in the HCCI engine. However, the engine operation was confined to low loads owing to the high pressure rise rate. Therefore, high-load extension is required to commercialize HCCI engines for electricity generation.

A spark-ignition (SI) engine is a technically viable solution for electricity generation via syngas combustion, because it can compensate for the aforementioned shortcomings of CI and HCCI engines. SI engines can be operated with syngas alone because syngas combustion is controlled by spark timings [20]. Commercial SI engines, such as gasoline and natural gas engines, can be operated directly as syngas SI engines with minimal modifications [21], [22]. Raman et al. tested the load and efficiency of an SI engine fueled with syngas obtained from fuel wood [21]. The fuel intake manifold and hydraulic governor of a natural gas SI-based engine, which had a compression ratio of 12:1, were modified for syngas operation. A maximum electrical power of 73 kW with an overall efficiency of 21% was achieved by the syngas SI engine. The authors also conducted SI engine operation with natural gas at high loads. With the same compression ratio, the efficiency of the syngas operation (21%) was lower than that of the natural gas operation (24%). Indrawan et al. compared the overall efficiency and electrical power achieved under natural gas and syngas operation in a two-cylinder SI engine [22]. The efficiencies with syngas and natural gas were 21% and 23%, respectively, at an electrical power of 5 kW. While a maximum load of 7 kW could be produced under natural gas operation, the syngas engine generated a maximum load of 5 kW. A common finding with regard to syngas SI engines is that they have lower power outputs and thermal efficiencies than SI engines fueled with conventional fuels such as gasoline and natural gas do.

Although SI engines are suitable for commercialization of syngas engine generators through simple modifications, the development of a dedicated engine design for syngas combustion is required to increase the efficiency and power output. One common strategy for improving the performance of SI engines is to increase the compression ratio [23]. As syngas has a high knock resistance, a high compression ratio can be implemented in syngas SI engines [24]. Oh et al. experimentally investigated the performance and emissions of a naturally aspirated SI engine fueled with syngas over a wide range of compression ratios [25]. The syngas composition was 70% H2, 15% CO, and 15% carbon dioxide (CO2) by volume; thus, the fuel had a high proportion of H2. The gross indicated thermal efficiency (ITE) increased from 46% to 51% with lean combustion when the compression ratio was varied from 10:1 to 17:1. However, the gross ITE decreased to 38% as the engine operation was changed to stoichiometric combustion at the highest compression ratio of 17:1, owing to the spark timing retardation for preventing backfire. While the achievable maximum gross indicated mean effective pressure (IMEP) increased with increasing compression ratio until a compression ratio of 14:1, the additional increase in compression ratio reduced the maximum gross IMEP. Another effective method to improve the thermal efficiency of SI engines is lean combustion. As the low ignition energy requirement of H2 in syngas can extend the lean flammability limit, lean combustion with syngas enhances thermal efficiency [26]. Arroyo et al. analyzed the combustion and performance of syngas operation in a gasoline SI engine without any optimization of the syngas [27]. Although the thermal efficiencies increased under syngas operation with lean combustion, the optimal excess air ratio for the thermal efficiency was influenced by the syngas composition. The highest thermal efficiency was achieved at an excess air ratio of 1.4 with syngas 2, which was composed of 40% H2, 39% CO, 11% methane (CH4), and 10% CO2 by volume; however, the optimal excess air ratio was 1.2 with syngas 1, which was composed of 23% H2, 23% CO, 26% CH4, and 28% CO2 by volume. Ran et al. compared ethanol, natural gas, and syngas as fuels for lean combustion in a naturally aspirated SI engine [28]. Compared with the ethanol and natural gas operations, the syngas operation had an extended lean misfire limit; it also involved an excess air ratio of 1.3–4.0. Among those of the fuels tested, the ITE of syngas was the highest, at 35%, at an excess air ratio of 2.0. Despite the high efficiency, engine operation with syngas led to the lowest net IMEP owing to the low volumetric efficiency under syngas operation and the lower heating value of syngas.

While a dedicated engine design based on syngas properties can increase efficiency and power output, most studies have investigated the performance of syngas SI engines with minor modifications to existing commercial engines. A high compression ratio and lean combustion can be employed in syngas SI engines to improve their performance, because syngas has a high knock resistance and low minimum energy requirement, as mentioned earlier. Lean combustion is effective in improving engine thermal efficiency owing to the high ratio of specific heats, but the power output is reduced because of the reduction in fuel input into the cylinder at a given intake pressure [25], [27]. The objective of this study was to compare the stoichiometric and lean combustion modes in a boosted SI engine over various compression ratios, to determine the optimal combustion mode for the development of a high-performance syngas engine generator. To compensate for the low power output associated with syngas engine operation, intake boosting, which has not been considered in previous studies, was employed. SI engines fueled with syngas have been operated under naturally aspirated conditions in the literature [20], [21], [22], [23], [24], [25], [26], [27], [28]. In this study, optimal compression ratio, intake manifold pressure, and excess air ratio for stoichiometric and lean combustion modes were determined under boosted conditions. In addition, an energy balance analysis was conducted at the optimum engine operation of the two combustion modes to investigate the sources of energy loss and their contributions.

Section snippets

Experimental setup

An experimental investigation was performed under stoichiometric and lean combustion conditions in a single-cylinder SI engine that was modified from a six-cylinder diesel engine. A diesel injector of the first cylinder head was removed, and a spark plug (NGK, BKR6ES) was installed on the diesel injector hole located at the center of the combustion chamber. The other five cylinders were deactivated by drilling holes in the pistons. The intake and exhaust manifolds of the base engine were

Optimization of stoichiometric combustion

An SI engine fueled with syngas has a lower power output than one fueled with conventional fuels, such as natural gas and gasoline, owing to the low volumetric efficiency under syngas engine operation and the low heating value of syngas. A large portion of the cylinder volume is occupied by the syngas. To achieve a high power, it is more desirable to operate a syngas SI engine under stoichiometric combustion than under lean combustion. However, an important issue related to stoichiometric

Conclusion

SI engines are advantageous for commercialization of syngas engine generators with minimal changes in the existing engines. However, previous studies have shown that syngas SI engines have lower power outputs and thermal efficiencies than equivalent SI engines operating on conventional fuels such as gasoline and natural gas do. Therefore, a dedicated engine design for syngas combustion is required to address the poor performance of syngas SI engines. The objective of this study was to compare

CRediT authorship contribution statement

Hyunwook Park: Conceptualization, Formal analysis, Writing - original draft. Junsun Lee: Investigation, Data curation. Narankhuu Jamsran: Investigation, Data curation. Seungmook Oh: Supervision. Changup Kim: Methodology, Resources. Yonggyu Lee: Visualization. Kernyong Kang: .

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

The authors would like to acknowledge the Creative Allied Project grant (No. CAP-16-06-KIER) of the National Research Council of Science and Technology (NST) funded by the Ministry of Science and ICT of Korea.

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