Dual nature of hydrogen combustion knock

https://doi.org/10.1016/j.ijhydene.2013.07.036Get rights and content

Highlights

  • A hydrogen fueled spark ignited CFR engine was applied for investigation.

  • Study on hydrogen knock was conducted vs. variable engine compression ratio.

  • Light and heavy knock were recognized and examined.

  • Intensity of the light knock depends on temperature of hydrogen–air mixture.

  • Intensity of the heavy knock is associated with mass of hydrogen self-ignited.

Abstract

Combustion knock is abnormal combustion taking place in an internal combustion spark ignited engine. It might be particularly observed in the engine at the end of combustion when the air–fuel mixture residue can be self-ignited due to exceeding auto-ignition temperature of this mixture. However, while hydrogen is combusted the knock can also occur as a result of non-auto-ignited combustion events. Investigation on knock, presented in the manuscript, was conducted in a hydrogen fueled spark ignited single cylinder engine with variable compression ratio. To express in numbers intensity of the combustion knock the in-cylinder pressure pulsations were used as a credible metrics. On the basis of analysis of these pulsations the hydrogen knock was distinguished as light and heavy one depending on its origin. The light knock is generated by combustion instabilities, which are a source for generating pressure waves inside the engine cylinder. The heavy knock results from hydrogen auto-ignition at the end of combustion. Its intensity is several times higher in comparison to the light knock. These observations were additionally confirmed by analysis of heat release rate. Finally, the light and the heavy knock were characterized by average amplitude of the pulsations from the entire test series of hundreds and several thousands kPa, respectively.

Introduction

Combustion knock is a typical abnormal combustion occurring in the internal combustion (IC) spark ignited (SI) reciprocating engine. Following the definition [1] the knock derives its name from strange sharp noise resembling breaking glass and is generated as a result of engine body vibration under the action of pressure waves propagating across the cylinder combustion chamber. Sometimes, similar to knock sound can be heard from the engine while the air–fuel mixture is pre-ignited by hot spots. This effect is associated with surface ignition and cannot be treated as the pure knock coming from fuel auto-ignition, however, it is also considered as abnormal combustion.

The pressure waves that result from combustion knock occur at frequencies that are acoustic vibration modes of the chamber geometry [2], [3]. It is generally accepted that gasoline knock in the SI engine comes from unburnt end-gas auto-ignition resulting from pressure–temperature increase while the spark controlled flame propagates throughout the cylinder space. As observed, knock onset strongly depends on fuel physical–chemical properties, thus, to characterize the fuel resistance to knock, the anti-knock rating was introduced. The knock ratings, which are currently in common use, are both the Research and Motored Octane Number (RON, MON). They accurately characterize knock resistance of liquid fuels. As far as hydrogen is concerned as an engine fuel the RON and the MON cannot properly examine its resistance to the knock. Due to relatively high auto-ignition temperature of 858 K at NTP for hydrogen (for gasoline approximately 530 K), it can be considered as a fuel extremely resistant to knock onset. It is confirmed by the RON for hydrogen, which is relatively high [4]. On the other hand, the hydrogen fueled engine at the same working conditions can generate stronger combustion knock if compared to the gasoline engine. Thus, for gaseous fuels another anti-knock index called Methane Number (MN) was introduced and applied to describe their resistance to knock [5], [6]. The MN for hydrogen is 0, that means hydrogen is highly prone to generate the knock. Hence, following this criteria, hydrogen is also treated as the worst fuel. Karim and his group did several research tests on hydrogen combustion in the CFR engine and observed the knock was generated in the engine with compression ratio varying from 6 to 14 [7], [8]. As they found the most severe knock occurred for the stoichiometric air–hydrogen mixture. They proposed leaning the hydrogen–air mixture as remedy to reduce knock intensity. Verhelst et al. expressed their doubt on hydrogen knock onset at its classic gasoline-like form at low compression ratio (6–8) and they encouraged to continue research in this area [9], [10].

On the basis of the presented contradictions White et al. [11] also recommended further thorough investigation concentrating on identifying causes for knocking phenomena in the hydrogen fueled engine.

The main target of the research tasks conducted in this field was to identify causes contributing to knock onset in the hydrogen fueled engine. The second aim was to find correlation between knock intensity and the engine compression ratio. As known, knock is the abnormal combustion that depends on several physical–chemical properties of the fuel and thermodynamic parameters of the environment such as initial temperature and pressure, fuel density, air to fuel premixing ratio etc. Among physical–chemical hydrogen combustion properties there are some which are associated with environmental parameters as temperature, pressure and air to hydrogen ratio. With respect to potential knock they are specified as follows:

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    ignition delay time – strongly influenced by initial temperature of the air–hydrogen combustible mixture at ignition,

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    laminar flame speed (LFS) – is in function with both air–hydrogen ratio and initial temperature,

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    adiabatic flame temperature – mainly depends on hydrogen to air ratio,

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    minimum ignition energy (MIE).

The MIE, adiabatic flame temperature has their extreme values at nearly stoichiometric air to hydrogen ratio [9], [12], [13]. Furthermore, the most intensified knock also takes place in case the air–hydrogen mixture is stoichiometric [4], [8], [14], [15]. Thus, to reduce knock intensity, several measures as leaning the combustible mixture and applying EGR are introduced [10], [16], [17]. One can expect, the knock intensity is correlated with initial temperature of the combustible mixture at its ignition forced by the spark discharge. While changing the engine compression ratio, both initial temperature, pressure and mixture density at ignition have been simultaneously changed. That resulted in different initial conditions for starting hydrogen combustion, in particular for flame kernel development and further flame propagation across the in-cylinder chamber. It is almost impossible to separate these parameters and identify their each individual impact on knock intensity in the IC engine. Hence, impact of compression ratio on knock intensity has to be considered as a combined effect of initial temperature, pressure and density, which cannot be changed independently and investigated separately, as it was discussed in Ref. [15]. Additionally, with increase in compression ratio the squeezing effect by piston significantly increases. It leads to higher turbulence in the cylinder combustion chamber and contributes the flame to be more wrinkled that resulted in faster flame propagation. Thus, turbulence intensity in the engine cylinder at ignition timing should be considered as an additional parameter influencing on the knock intensity by the compression ratio [18]. Summing up, experimental results presented here concern impact of the engine compression ratio on the knock intensity while other engine operating parameters as the spark timing, hydrogen dose, hydrogen to air ratio, engine coolant temperature and engine speed are held constant. The tests were conducted on the CFR 1 Waukesha research engine, which is designed for determining fuel octane rating, what provides premises for comparing these results with results obtained from other R&D centers possessing the same engine.

The tests were conducted under parameters as follows:

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    Fuel applied: industrial compressed hydrogen,

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    Mixture preparation: port fueled, premixed,

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    Mixture composition: stoichiometric (Equivalence Ratio ER = 1),

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    Load: maximal at WOT with the H2 dose unchanged in all tests,

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    Spark timing: 0° (at TDC),

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    Compression ratio was varying from 6 to 14,

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    Engine speed 900 rpm.

Section snippets

Test bed description

The engine used for this research is a single cylinder CFR (Cooperative Fuels Research) engine manufactured by the Waukesha Motor Company. The engine was chosen for its versatility and robustness of construction which is important because of the intended study of combustion knock. A specialized attribute of this engine is the ability to vary the compression ratio without disassembling the engine. Characteristics of the test bed and engine are shown in Fig. 1 and listed in Table 1 respectively.

Results and discussion

Among various methods for determining the knock intensity the method based on analysis the in-cylinder combustion pressure pulsations appears as the most accurate and credible as far as anyone assumes that knock origin comes from these pressure pulsations. The exemplary in-cylinder combustion pressure history typical for the knock in the hydrogen fueled engine at CR = 12 is plotted in Fig. 2(a). Fig. 2(b) shows the in-cylinder pressure pulsations obtained after filtering with a high-pass

Conclusions

  • 1.

    Hydrogen combusted in the spark-ignited reciprocating engine generates knock which can be divided into two types depending on its origin and intensity as follows:

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      light knock – generated by fast and instable combustion initiated by spark discharge. Light knock starts with start of combustion. The average amplitude APPP of pressure pulsations is usually in the range between 20 and 100 kPa.

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      heavy knock – resulted from unburnt hydrogen auto-ignition at the end of combustion. At the knock border the

Acknowledgments

We would like to acknowledge the support of Michigan Technological University's Mechanical Engineering – Engineering Mechanics Department and the Research Office for support of this work.

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