Hydrogen energy share enhancement in a heavy duty diesel engine under RCCI combustion fueled with natural gas and diesel oil

https://doi.org/10.1016/j.ijhydene.2020.04.263Get rights and content

Highlights

  • Reduction in fuel consumption by enhancing hydrogen energy share.

  • No exposure to diesel knocks.

  • Achieving the gross indicated efficiency over 50%.

  • No significant engine power losses.

  • Satisfying EURO VI level for CO and UHC, and EPA level for Formaldehyde.

Abstract

The aim of this study is to enhance hydrogen energy share in a RCCI engine. The engine under consideration is fueled with diesel oil and natural gas enriched with hydrogen or syngas and is set to operate at 9.4 bar gross indicated mean effective pressure (Mid- Load). The syngas used in this study consists of hydrogen and carbon monoxide which are mixed together on a volumetric ratio of 80:20. A fixed amount of diesel oil is injected per cycle into the combustion chamber of the RCCI engine. Based on two different strategies, hydrogen or syngas mixed with exhaust gas recirculation are admitted gradually along with natural gas while ensuring that always the low temperature combustion concept is fulfilled. The RCCI engine operation is simulated through commercial software coupled with chemical kinetics solver. The simulation results show that without any engine diesel knock occurrence, by adding hydrogen to natural gas, the share of hydrogen energy could be increased up to 40.43% while the engine power output is reduced only by about 1%. Also, syngas addition to natural gas causes that the share of hydrogen energy could be increased up to 27.05% while improves the engine power more than 4%. At the same time, by considering two mentioned strategies, the overall hydrocarbon fuel consumption per cycle can be reduced by up to 46.60% and 33.86%, respectively. Moreover, having the gross indicated efficiency of well over 50% and significant reduction in the engine emissions compared to RCCI combustion fueled solely with natural gas and diesel oil are achievable.

Introduction

Internal combustion engines (ICs), classified as spark ignition (SI) engines and compression ignition (CI) engines, are used generating mechanical power from the chemical energy of conventional fuels. Among these two main types of engines, CI engines known as diesel engines are favorable due to their higher fuel efficiency, durability, low emissions, and high power/weight ratio are widely used for transportation (land, sea, and air), power generation, and industrial purposes. Although diesel engines had a key role in developing of the modern industrials and the life of humans, however, in recent decades, their performance was associated with serious concerns related to the health of humans, animals, materials, aquatic ecosystems, and so on. The major side effects which result from the CI engines' combustion process and classify as air pollutant are the oxides of nitrogen (NOx), sulfur oxides (SOx), carbon monoxide (CO), unburned hydrocarbons (UHC), and particulate matter (PM). In order to meet ever increasing stiffer emission regulations on mentioned emissions that are proposed by organizations such as United States Environmental Protection Agency and European emission standards, the present conventional diesel combustion is becoming more unacceptable. Therefore, in order to meet the demand by reducing the diesel engine's emissions and satisfy the aforementioned emission regulations, some abatement technologies such as after– treatment systems were used [1]. Using these technologies would hamper the fuel efficiency, increases the maintenance, operation and installation costs of the diesel engine. Therefore, new combustion strategies known as premixed low temperature combustion (PLTC) were introduced to reduce the conventional diesel combustion emissions. Homogeneous charge compression ignition (HCCI) combustion, premixed charge compression ignition (PCCI) combustion, and reactivity– controlled compression ignition (RCCI) combustion are the known types of PLTC strategy. The lake of control in the combustion duration and the combustion phasing are the main challenge of HCCI and PCCI combustion strategies. Introducing RCCI combustion strategy would easily overcome obstacles, using two fuels with different auto–ignition characteristics where one is low reactive fuel and the other is high reactive fuel. In RCCI combustion, during the intake stroke, low reactive fuel with lower cetane number is mixed with air through the intake port. Thus, initially a low reactive charge is induced in the combustion chamber. Then, higher reactive fuel with higher cetane number is injected to the combustion chamber in order to ignite the existing premixed charge. In recent years, a wide variety of fuels were used in the RCCI combustion studies such as gasoline/diesel fuel [2], hydrated ethanol/diesel fuel [3], natural gas/biodiesel fuel [4], natural gas/diesel fuel [5,6] and [[7]], methanol/diesel fuel [8], ethanol/diesel fuel [8], n- butanol/diesel fuel [8], isobutanol/isobutanol with addition of di-Tert butyl peroxide [9], and so on.

According to annual energy consumption forecast [10] which is depicted in Fig. 1, natural gas (NG) share of the market is increasing relative to other fossil fuels and energy sources due to its capability of reducing exhaust emissions with minimal after– treatment requirements especially with tightened legislations on air pollution. Also, the natural gas is abundant and pricewise is cheaper as well and can be stored and used as compressed natural gas (CNG) or liquefied natural gas (LNG) in vehicles or simply off gas line grid for stationary heavy– duty diesel engines, thus, as a low reactive alternative fuel together with diesel oil as a high reactive fuel has become a very attractive feasible option for researchers.

Generally, the natural gas compositions include methane, ethane, propane, butane, pentane, hexane, and other elements such as carbon dioxide, and nitrogen at different proportions. In natural gas, methane has the significant percentage compared to the other mentioned components sometimes as high as 89.8% depending on the gas field. Therefore, it is common practice to consider Methane for Natural gas. It should be noted that methane has a relatively high auto– ignition temperature compared to diesel oil (540–630 °C [11]), therefore, the reactivity difference between methane and diesel oil is high. Thus, it is expected that methane characteristics cause that the combustion duration increases and the fuel energy release in the engine combustion chamber to be slower. As a result, the peak pressure rise rate as an indicative of diesel knock reduces especially at higher loads.

On the other hand, hydrogen with large amount of energy compared with conventional hydrocarbon (HC) fuels can easily be used to meet the global energy demand. But unfortunately, hydrogen does not exist as a separate element in nature and has to be extracted by different methods include dark or photo fermentation of biomass, from natural gas through methyl formate hydrolysis, and from water which is economically not feasible yet. In comparison with all other HC fuels, hydrogen has a wide flammability range, a small quenching distance, a relatively high auto ignition temperature (585 °C), very high diffusivity, and a high flame speed [11]. Also it requires very low energy for ignition [11]. Due to the higher self– ignition temperature of hydrogen, an external source such as spark plug or glow plug is required to initiate the combustion in internal combustion engines (ICEs). Thus, hydrogen can readily be used as a sole fuel in a spark ignition (SI) engine. In contrast, in a compression ignition (CI) engine, since the obtainable compression pressure and temperature during the compression stroke is not enough to ignite the air– hydrogen mixture due to its higher self– ignition temperature; hence, hydrogen cannot be used as a sole fuel in a CI engine [11]. Recently, based on some researches, the use of hydrogen as an additive to some different fuels in ICEs (SI and CI engines) has been studied extensively. In the RCCI combustion field, the hydrogen addition effects on the engine performance and emissions were studied such as hydrogen addition to landfill gas (LFG)/diesel oil RCCI engine [12], hydrogen addition to NG/diesel oil RCCI engine [13,14], hydrogen addition to compressed natural gas (CNG)/biodiesel RCCI engine [15], hydrogen addition to NG/Dimethyl Ether (DME) RCCI engine [16], and so on. Also, in a naturally aspirated direct– injection dual fuel CI engine, the effect of the use of hydrogen– NG blend on the engine emissions was evaluated compared with diesel fuel only [17]. In a SI engine fueled with NG, the effect of hydrogen addition on the engine performance and emissions were studied [18]. Moreover, cycle–by–cycle variations in a SI engine fueled with natural gas– hydrogen blends combined with exhaust gas recirculation (EGR) was assessed [19]. Furthermore, some experimental and simulation studies on ignition delays of lean mixtures of methane–hydrogen–oxygen–argon [20], laminar burning characteristics of premixed methane–hydrogen–air flames [21], and laminar burning velocities of lean mixtures of NG–hydrogen–air were studied [22].

Although hydrogen as a green fuel has desirable properties and it can be used in ICEs as a sole fuel or an additive to other fuels [11], however, the use of hydrogen in an engine is limited. A primary problem restricting use of hydrogen is its fast flame speed compared to other hydrocarbon fuels such as natural gas and diesel oil. Hence, especially in CI engines, this characteristic leads to rapid combustion and faster heat release rate in the engines combustion chamber. Thus, faster heat release rate due to hydrogen presence as an additive in air– fuel mixture leads to higher in– cylinder temperature and higher nitrogen oxides (NOx) emission. Therefore, in the engines that uses hydrogen as a sole fuel or additive, the use of EGR is proposed as a key role in reducing the in– cylinder temperature and the NOx reduction, but, it was proven that EGR causes to reduce the engine's volumetric efficiency [11].

Another dilemma in using hydrogen as a sole fuel or an additive in ICEs for transportation purposes is its storage challenge and related safety issues. In order to overcome this problem in ICEs, especially in CI engines, hydrogen can be produced on– board through a catalytic fuel reformer [23]. By injecting a HC fuel into the catalytic fuel reformer that installed over the engine exhaust pipes, hydrogen is produced. Of course, along with hydrogen production, a significant amount of carbon monoxide is also produced and the engine efficiency enhances due to recovering the exhaust energy. The product of the catalytic fuel reformer (i.e. hydrogen along with carbon monoxide) is known as reformer gas or synthetic gas or syngas. It is important that, the produced syngas compositions (i.e. the proportion of hydrogen and carbon monoxide) can significantly vary with the engine operating conditions.

Some researches show that the presence of carbon monoxide in hydrogen– air–fuel mixture has potential to reduce the laminar burning speed of the mixture resulted from the hydrogen presence [24]. Therefore, the combustion duration can increase by slowing down the heat release rate and NOx emission can reduce due to reduction in the in– cylinder peak temperature. It should be noted that, although the use of higher percentages of carbon monoxide in the intake mixture causes to reduce NOx emission further, but at the same time, the engine volumetric and thermal efficiencies are reduced [24].

By recognizing the potential of carbon monoxide to overcome the problem of using hydrogen in ICEs, the use of syngas, which consists of hydrogen and carbon monoxide with different proportion, as an additive to some different fuels in SI and CI engines has become attractive to researchers. For example, in a base diesel engine, the conventional diesel combustion mode was changed to dual fuel mode and the effects of the use of syngas with different compositions of hydrogen and carbon monoxide on the engine performance, combustion, and emissions were assessed [24]. Also, in the RCCI combustion field, reformer gas or syngas addition to NG/diesel oil RCCI engine shown that the combustion efficiency enhanced and the engine emissions reduced [23]. In addition, reformer gas and nitrogen addition to NG/diesel oil RCCI engine caused to extend the engine loads range with low emissions [25].

Regardless of the hydrogen's storage challenge which is somehow like natural gas, that is either compressed H2 or liquefied H2, LH2, another way to produce the syngas is its generation through mixing pure hydrogen and pure carbon monoxide that are supplied from two separate storage cylinders. In this method, in contrast to catalytic fuel reformer, the proportion of hydrogen and carbon monoxide in the simulated syngas can be maintained throughout the engine operation [24].

As mentioned above, the use of EGR and also carbon monoxide along with hydrogen, in the other word syngas, can overcome problem of faster heat release rate due to the presence of hydrogen in an air–fuel mixture. Therefore, by implementing two different strategies, this study considers introducing hydrogen energy share (HES) in a heavy– duty diesel engine under RCCI combustion fueled with natural gas/diesel oil at 9.4 bar gross indicated mean effective pressure (IMEP) for mid– load range. These strategies include (a) hydrogen addition to the intake charge with the use of EGR and (b) the simulated syngas addition, 80:20 volumetric proportion of hydrogen (H2): carbon monoxide (CO), to the intake charge with the use of EGR. While assessing the results from the two mentioned strategies, the targets of this study are maintaining the engine load, extending diesel knock limitation, and reducing the engine emissions while the share of hydrogen energy increases and the hydrocarbon fuel consumption reduces.

Section snippets

Engine specifications and computational model

In the present study, in order to simulate RCCI combustion in a heavy– duty diesel engine, Caterpillar single– cylinder engine model 3401E SCOTE with bathtub piston bowl profile [5] is used. In the heavy– duty diesel engine under investigation, common rail injection technology is used to supply diesel oil to the fuel injectors and NG enriched with hydrogen or syngas is injected through the intake port. Based on Walker et al. experimental work [5], the engine specifications are presented in

Implementing two different strategies

In this simulation study, the engine is set to operate at 9.4 bar gross IMEP which is in Mid–Load range [5]. Therefore, in order to reduce the hydrocarbon fuel consumption, to enhance the share of hydrogen energy, and to have complete combustion in Caterpillar heavy– duty diesel engine, two different strategies are implemented:

Results and discussion

Based on the engine operating conditions mentioned in Table 6, Table 8, hydrogen or syngas addition to the intake premixed charge (i.e. air and methane mixture) causes to occur very important changes in the RCCI combustion characteristics as follow:

Conclusions

In the current study, for a single cylinder heavy– duty diesel engine operating under RCCI combustion fueled with diesel oil/natural gas enriched with hydrogen and syngas (containing 80:20 volumetric proportion of H2:CO) at 9.4 bar gross IMEP (Mid- Load), the following remarks can be concluded:

  • 1)

    By adding hydrogen to natural gas along with proper EGR management, without the exposure to diesel knock, the hydrogen energy share percentage can be increased by up to 40.43%. The overall hydrocarbon

Acknowledgments

Authors would like to thank Dr. Vahid Esfahanian from the University of Tehran, Department of Mechanical Engineering, for giving the authors permission to use an original licensed copy of the AVL Fire software to conduct the simulation in this present work.

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