The effects of hydrogen addition on engine power and emission in DME premixed charge compression ignition engine

https://doi.org/10.1016/j.ijhydene.2012.09.177Get rights and content

Abstract

Premixed-charge compression-ignition (PCCI) combustion of dimethyl-ether (DME) with double injection strategy was investigated in a single-cylinder compression-ignition engine. DME main-injection was replaced by hydrogen to reduce carbon dioxide emissions. To study the effect of hydrogen, the injected amount of hydrogen was increased. Engine performance and emission of DME PCCI combustion were compared to those of hydrogen–DME PCCI combustion. In the DME PCCI engine operation, DME was injected directly into the cylinder at −120 crank angle degrees (°CA) after top dead center (aTDC) to simulate homogeneous charge at first, and then DME was injected secondly with varied second injection timing. In this case, DME injection timing in the second stage affected the engine performance and emissions. Delayed combustion phase showed a higher indicated mean effective pressure (IMEP), while it increased NOx emission when DME second injection is retarded. In the hydrogen–DME PCCI, hydrogen was injected at intake port with fixed injection timing. DME injection timing in hydrogen–DME PCCI combustion was also varied from −120 °CA to TDC, as in the DME PCCI engine operation. The total supplied heating value was fixed at 400 J for all cases. DME injection timing determined the start of combustion for the hydrogen–DME PCCI. With increasing the amount of hydrogen, exhaust emissions were reduced. Hydrogen–DME PCCI engine was operated with minimum amount of DME via the hydrogen addition and DME injection timing control. The optimized DME injection timing, −30 °CA aTDC, resulted in a lower exhaust emission-operation, while maintaining a higher IMEP.

Highlights

► Second injection of DME in DICI engine fueled with H2 and DME. ► Increasing fuel ratio of H2 resulted in improved H2–DME engine efficiency. ► With increasing fuel ratio of H2 engine-out emissions could be reduced. ► Controlling the DME injection timing regulated combustion phasing of H2–DME PCCI.

Introduction

With stringent government regulations on engine-out emissions, the interests in the clean energy source have been growing steadily. Reducing the CO2 emission also has become the key priority in the engine manufacturing industry. As CO2 emission is highly related to the engine efficiency, clean compression-ignition (CI) engines could be the solution in the near future due to its high efficiency characteristics. However, CI engines by nature emit large amounts of nitrogen oxides (NOx) and particulate matter (PM). As a solution, homogeneous-charge compression-ignition (HCCI) combustion has been suggested to reduce the NOx and PM emissions [1]. In the first step of HCCI, diesel fuel is injected into the intake manifold or directly into the cylinder in the early stage to make a homogeneous mixture, hence the term HCCI referred as premixed-charge compression-ignition (PCCI). The PCCI concept has several challenges such as high carbon monoxide (CO) and hydrocarbon (HC) emissions as well as its narrow operating range [2]. With PCCI engines, controlling the combustion phase is critical to regulate the engine performance, as combustion is controlled by the chemical reaction at a certain temperature and pressure. Previous works suggest that the multiple-injection strategy is one of the solutions to control the combustion phase in PCCI [2]. Such multiple-injection system in PCCI combustion was realized by injecting the fuel not only through the intake port but also into the cylinder directly. Toyota Motor Corporation introduced uniform-bulky-combustion system by employing a double injection strategy with a single fuel [1]. This strategy generates a mixture by the combination of an early injection and late injection aTDC. The system controls the first injection timing, injection quantity, intake temperature, and boosting pressure to prevent any high temperature reactions, and then delivers a second injection to initiate the high temperature reaction aTDC. Delaying the combustion phase near to TDC generally leads to a higher power compared to the PCCI combustion due to reduced negative work. Some researchers injected diesel in the early stage using two side injectors, and injected diesel secondly through the center injector [1]. The concept of multi-stage diesel combustion was first introduced to reduce the NOx and PM emission, as the delayed second injection timing aTDC reduces NOx emission without misfire [1]. Although the strategy achieved significant reduction of NOx emission, it stills suffers from high CO and HC emissions. To control the combustion phase of PCCI, the study of dual-fuel engine also has been reported [3], [4], [5], [6], [7], [8], [9]. The dual fuel concept employs two different fuels: a low-resistant and a high-resistant fuel to auto-ignition. Dual-fuel PCCI makes premixed mixture by using direct injection and port injection to trigger the ignition timing and heat release rate during the combustion [8], [9]. In this paper, double injection and dual-fuel strategies were introduced to control the combustion phase in PCCI engine. As the engine fuels, hydrogen and dimethyl-ether (DME) were used.

The concept of using hydrogen as an alternative fuel for CI engine has gained a lot of attraction recently. Hydrogen has many advantages over conventional fuels for internal combustion engine: it is a very clean energy source, its amount is practically unlimited, and it is considered as a high octane-numbered fuel. Therefore it is very easy to implement hydrogen into the conventional spark ignition engine with a relatively higher compression ratio [10], [11], [12]. The easiest way to introduce the hydrogen internal combustion engine is using a gasoline engine with a hydrogen injection system. Hence, the hydrogen internal combustion engines with spark ignition source are investigated by some researchers [13], [14], [15], [16], [17]. Hydrogen fuel, however, usually requires a subsidiary ignition source due to its high auto-ignition temperature, posing a practical problem to be used directly into a conventional CI engine [4], [5], [18], [19]. Nevertheless, by using a compression ratio over 20, it is possible to operate the hydrogen CI engine without any supplementary spark source. Some researchers have reported that a compression ratio of 17 is enough to auto-ignite hydrogen when the engine is operated at high temperature. However, cold start of hydrogen CI engine was not possible due to many misfiring events [20], [21]. For a stable operation, it is necessary to make a high temperature condition by employing a high compression ratio, intake charge heating, or charge boosting [21]. On the other hand, a high compression ratio could generate noise and cause knocking, and intake charge heating has risks of backfire or pre-ignition. To use hydrogen as a fuel in a conventional diesel engine without such risks, hydrogen should be used as an additive fuel or with other ignition sources.

DME is considered as one of the promising fuels that can substitute diesel. DME has several advantages as an ignition promoter in the CI engine. The cetane number of DME is higher than that of diesel. Therefore, DME has a low auto-ignition temperature and DME can auto-ignite easily at low compression ratio. As the DME is injected, it can be instantaneously vaporized due to its low boiling temperature [22]. DME is an oxygenated fuel without any C–C bond, hence, soot formation is negligible [22]. However, due to the lower enthalpy of formation of DME than that of diesel, the injection volume of DME needs to be increased. The lower viscosity of DME causes the wearing of any moving parts. In addition, selecting the right sealing materials is one of the most critical issues as DME tends to corrode rubbery materials [23]. DME has two-stage ignition during the auto-ignition process. DME and air mixture is first subjected to an increasing pressure and temperature as it is compressed by the piston, causing two stages of heat release: a low temperature reaction and a high temperature reaction. The heat release value of DME at the low temperature reaction is larger than that of petroleum. Furthermore, in the DME low temperature reaction step, there are several radicals that participate in the hydrogen oxidation reactions, such as hydrogen peroxide and hydroxide radicals. In particular, hydrogen peroxide has a significant effect on promoting the auto-ignition of hydrogen. The thermal decomposition reaction of hydrogen peroxide is the most important reaction for the production of hydroxide radicals which promotes hydrogen auto-ignition [24]. Due to these characteristics of DME combustion, DME is recommended as a fuel for the hydrogen-DME CI engine.

The objective of this study was to use DME and hydrogen as the fuels for PCCI engines. A double injection strategy, with DME as the second injection fuel, was employed to control the combustion phase. The effect of second injection timing along with the hydrogen to DME ratio on the engine performance and emissions was studied and optimized.

Section snippets

Experimental setup

The test engine is a single-cylinder direct-injection compression ignition engine equipped with a common-rail injection system and a port fuel injection system. The bore and stroke dimensions were 83 mm and 92 mm, respectively, and the displacement volume of a cylinder was 498 cm3. The compression ratio was 17.3:1. In general, DME fueled engine was investigated under lower compression ratio than conventional diesel engine [25], [26] However, Kajitani et al. investigated the effect of

DME PCCI engine

The effect of second DME injection timing was investigated on a PCCI combustion engine with the double DME injection scheme. Total of 400 J of DME was injected directly into the cylinder per cycle. The characteristics of two different injection schemes, DME single injection and DME double injection, were investigated. For the DME single injection, all DME was injected at −120 °CA aTDC. In the case of DME double injection, the first injection angle was set at −120 °CA aTDC, and the second

Conclusion

Dimethyl ether (DME) partially premixed charge compression ignition (PCCI), and hydrogen–DME PCCI compression ignition combustions were compared in a single-cylinder direct injection compression ignition engine. Hydrogen was injected at the intake manifold with an injection pressure of 0.5 MPa. Hydrogen injection timing was fixed to −210 °CA aTDC. DME was injected directly into the cylinder through common-rail injection system with an injection pressure of 30 MPa. DME was injected at −120 °CA

Acknowledgment

This work was supported by the Energy Efficiency & Resources (2010T100100440) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge Economy.

References (34)

  • T. Shudo et al.

    Hydrogen as an ignition-controlling agent for HCCI combustion engine by suppressing the low-temperature oxidation

    Int J Hydrogen Energy

    (2007)
  • H. Akagawa et al.

    Approaches to solve problems of the premixed lean diesel combustion control

    (1999)
  • S. Kook et al.

    Combustion control using two-stage diesel fuel injection in a single-cylinder PCCI engine

    (2004)
  • K. Inagaki et al.

    Dual-fuel PCI combustion controlled by in-cylinder stratification

    (2006)
  • Y. Narioka et al.

    HCCI combustion characteristics of hydrogen and hydrogen-rich natural gas reforme supported by DME supplement

    (2006)
  • T. Shudo et al.

    HCCI combustion of hydrogen, carbon monoxide and dimethyl ether

    (2002)
  • K. Yeom et al.

    Gasoline – di-methyl ether homogeneous charge compression ignition engine

    Energy Fuels

    (2007)
  • Cited by (0)

    View full text