Engine performance and optimum injection timing for 4-cylinder direct injection hydrogen fueled engine

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Abstract

This paper presents the engine performance and optimum injection timing for 4-cylinder direct injection hydrogen fueled engine. The 4-cylinder direct injection hydrogen engine model was developed utilizing the GT-Power commercial software. This model employed one dimensional gas dynamics to represent the flow and heat transfer in the components of engine model. Sequential pulse injectors are adopted to inject hydrogen gas fuel within the compression stroke. Injection timing was varied from 110° before top dead center (BTDC) until top dead center (TDC) timing. Engine speed was varied from 2000 rpm to 6000 rpm, while the equivalence ratio was varied from 0.2 to 1.0. The validation was performed with the existing previous experimental results. The negative effects of the interaction between ignition timing and injection duration was highlighted and clarified. The results showed that optimum injection timing and engine performance are related strongly to the air fuel ratio and engine speed. The acquired results show that the air fuel ratio and engine speed are strongly influence on the optimum injection timing and engine performance. It can be seen that the indicated efficiency increases with increases of AFR while decreases of engine speed. The power and torque increases with the decreases of AFR and engine speed. The indicated specific fuel consumption (ISFC) decreases with increases of AFR from rich conditions to lean while decreases of engine speed. The injection timing of 60° BTDC was the overall optimum injection timing with a compromise.

Introduction

With increasing concern about the energy shortage and environmental protection, research on improving engine fuel economy, hydrogen fueled engine is being developed into a hydrogen fueled engine with manifold injection, direct injection or duel injection according to the fuel supply method [1], [2], [3], [4]. Of course, the hydrogen fueled engine with direct injection can fundamentally keep backfires from occurring so it can be utilized as a high powered hydrogen power system if the reliability of high pressure direct injection valve is secured [5], [6]. Hydrogen gas is characterized by a rapid combustion speed, wide combustible limit and low minimum ignition energy. Such characteristics play a role to decrease engine cycle variation for the safety of combustion. However, it is frequently observed that the values of cycle variation for hydrogen-fueled engines with direct injection are higher than those of hydrogen-fueled engines with manifold injection or those of gasoline engines, due to a decrease in the mixing period by direct injection in the process of compressing hydrogen gas [7], [8], [9]. In today’s modern world, where new technologies are introduced every day, transportation’s energy use is increasing rapidly. Fossil fuel particularly petroleum fuel is the major contributor to energy production and the primary fuel for transportation. Rapidly depleting reserves of petroleum and decreasing air quality raise questions about the future. As world awareness about environment protection increases so does the search for alternative to petroleum fuels.

Hydrogen can be used as a clean alternative to petroleum fuels and its use as a vehicle fuel is promising in the effects to establish environmentally friendly mobility systems. So far, extensive studies investigated hydrogen fueled internal combustion engines (H2ICE) with external mixture formation fuel delivery system [10], [11]. However, the operation of these engines subjected to abnormal combustion, such as pre-ignition, backfire and knocking. Moreover, the power outputs of these hydrogen engines are about 30% less than those of gasoline engines [12]. Therefore the premixed-charge spark ignition engines fueled with hydrogen can be used for significantly limited operation range [13]. It is a common conclusion achieved by many researchers that abnormal combustion can be controlled by direct injection (DI) of hydrogen in-side the cylinder [13], [14], [15]. Direct injection H2ICE requires optimized operation strategies that enable the availability of high power output as well as the abolition of critical exhaust gas emission in combination with high efficiencies. Several parameters need to be optimized. Optimization of spark timing, valve timing, combustion chamber geometry, injection parameters such as injection timing, injection duration, injection pressure and nozzle hole numbers/arrangement, swirls intensity, etc. are indeed important to achieve an engine performance level competitive to that in the modern direct-injection diesel engines [15].

Injection timing plays a critical role in the phasing of the combustion, and hence the emissions and torque production. Therefore, extensive number of studies indicated the significance of optimization for ignition timing [15], [16], [17]. White et al. [18] suggested that late injection can minimize the residence time that a combustible mixture is exposed to in-cylinder hot spots and allow for improved mixing of the intake air with the residual gases. This selection can control pre-ignition problem. The main challenge for selecting the proper ignition timing that is in-cylinder injection requires hydrogen–air mixing in a very short time. For early injection (i.e., coincident with inlet valve closure (IVC)), maximum available mixing times range from approximately 20 ms to 4 ms across the speed range 1000–5000 rpm. In practice, to avoid pre-ignition, start of injection (SOI) is retarded with respect to IVC and mixing times are further reduced. Regarding the behavior of the performance characteristic with ignition timing, there are several contradictories in the literature. Eichlseder et al. [3] found that at low loads (or low equivalence ratio (θ)), indicated efficiency (IE) increases with retard of SOI. The increase was shown to be due to the decrease in the compression work caused by differences in mixture gas properties and charge mass with retarded SOI. The authors also found their study at high loads, IE first increases and then decreases with retard of SOI. The reversing trend is assumed to be a consequence of an unfavorable mixture formation. However, Kim et al. [17] reported the contradictory results to Eichlseder et al. [3] results, where they find that, for both low and high loads, indicated efficiency decreases monotonically with retard of SOI. These contradictory findings may be a result of differences in mixture formation [18]. Much effort has been devoted to optimize the injection timing which is ranging from IVC until the top dead center (i.e. within the compression stroke). However, Mohammadi et al. [15] optimized the injection timing for three ranges:

  • during the intake stroke, where they prevented backfire. However, thermal efficiency and output power are limited by knock due to reduction in volumetric efficiency;

  • at compression stroke, where they prevented knock and gives an increase in thermal efficiency and maximum output power; and

  • at later stage of compression stroke, where they achieved thermal efficiency higher than 38.9% and brake mean effective pressure 0.95 MPa.

This study attempts to optimize injection timing that gives the best performance of a 4-cylinders direct injection. The 4-cylinder direct injection hydrogen fueled engine model is developed for this purpose. The effects of air fuel ratio, engine speed on engine performance and the optimum injection timing on the engine performance such as indicated efficiency, indicated specific fuel consumption, power and torque for direct injection hydrogen fueled engine.

Section snippets

Engine performance parameters

The brake mean effective pressure (BMEP) can be defined as the ratio of the brake work per cycle Wb to the cylinder volume displaced per cycle Vd, and it can be expressed as [19]:BMEP=WbVdEq. (1) can be rewrite for the four stroke engine as:BMEP=2PbNVdwhere Pb is the brake power, and N is the rotational speed.

Brake efficiency (ηb) can be defined as the ratio of the brake power Pb to the engine fuel energy as:ηb=Pbm˙f(LHV)where m˙f is the fuel mass flow rate and LHV is the lower heating value of

Model validation

The experimental results obtained from Mohammadi et al. [15] were used for the purpose of validation in this study. Engine specifications of Mohammadi et al. [15] and present single cylinder direct injection engine model are listed in Table 4. For the purpose of validation, single cylinder direct injection engine model converted to 4-cylinder direct injection model. Fig. 4 shows the single cylinder direct injection engine model. Engine speed and AFR were fixed at 1200 rpm and 57.216 (φ = 0.6)

Results and discussion

A lean mixture is one in which the amount of fuel is less than stoichiometric mixture. This leads to fairly easy to get an engine start. Furthermore, the combustion reaction will be more complete. Additionally, the final combustion temperature is lower reducing the amount of pollutants. The air–fuel ratio (AFR) was varied from rich limit (AFR = 27.464:1 based on mass where the equivalence ratio (φ = 1.2) to a very lean limit (AFR = 171.65 where (φ = 0.2) and engine speed varied from 2500 rpm to 4500 rpm.

Conclusion

A computational model was developed for 4-cylinder direct injection hydrogen fueled engine. The influence of air fuel ratio and engine speed with injection timing was investigated. The main results are summarized as follows:

  • (i)

    At very lean conditions with low engine speeds, acceptable BMEP can be reached, while it is unacceptable for higher speeds. Lean operation leads to small values of BMEP compared with rich conditions.

  • (ii)

    Maximum brake thermal efficiency can be reached at mixture composition in

Acknowledgement

The authors would like to express their deep gratitude to Universiti Malaysia Pahang (UMP) for provided the laboratory facilities and financial support under project No. RDU090393.

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