Keywords

1 Introduction

Liquid rocket engines are considered the preferred power device for launch vehicles and various spacecraft due to their advantages such as reliable operation and long working time [1]. Liquid hydrogen and liquid oxygen have significantly high specific impulse characteristics, and their combustion products are pollution-free, making them particularly suitable for the upper stage [2, 3]. The sub stage with an independent control system added based on the basic stage of the rocket is called the upper stage, which has the ability to fly independently and work in orbit for a long time. It is widely used in tasks such as orbital gliding and deep space exploration [4,5,6]. With the continuous development of space technology, the duration of astronauts’ deep space exploration missions has increased from 1 to nearly 2000 days [7]. It is difficult to control the evaporation of liquid hydrogen and liquid oxygen propellant and the evaporation gas cannot be used, which greatly restricts the development of the hydrogen-oxygen upper stage [8]. Liquid hydrogen and liquid oxygen propellant has a low boiling point and is easily vaporized into low-temperature gas hydrogen and gas oxygen when heated, resulting in an increase in pressure inside the storage tank. It is necessary to discharge gas hydrogen and gas oxygen to meet the safety pressure standards of the storage tank. In order to avoid fuel waste, gas hydrogen and gas oxygen are introduced into the ICE to burn and provide power.

As early as the 1960s, NASA (National Aeronautics and Space Administration) [9] proposed the concept of the hydrogen-oxygen ICE in the Apollo program. ULA (United Launch Alliance) [10] proposed an integrated vehicle liquid system in 2011, which introduces low-temperature hydrogen and oxygen into the ICE for combustion and work. ULA [11, 12] successively manufactured a single-cylinder ICE, a Wankel engine, and a 6-cylinder ICE using hydrogen-oxygen mixture as fuel, and conducted performance tests. Furuhama et al. [13] found that adopting a fuel strategy with a larger equivalence ratio of 3 to 6 can significantly suppress abnormal combustion in the hydrogen-oxygen ICE. Li et al. [14], based on the two-zone quasi-dimensional model, found that with the increase of the equivalence ratio, the indicated thermal efficiency (ITE) of the ICE increases to 40%. Fu et al. [15] conducted numerical simulation research on the combustion characteristics of the hydrogen ICE under oxygen-pure conditions.

The above research indicates that the application prospects of hydrogen-oxygen ICEs are extremely broad. However, previous research was mostly limited to the structural parameters and operational stability of hydrogen-oxygen ICEs, and there has been no in-depth study on the combustion characteristics of hydrogen-oxygen ICEs. In order to study the combustion characteristics of the hydrogen-oxygen ICE in detail, a three-dimensional geometric model was established and verified based on experimental data. The effects of different equivalence ratios and ignition timing on the combustion characteristics of the hydrogen-oxygen ICE were compared.

2 Model Establishment and Validation

2.1 Geometric Model and Boundary Conditions

This research is based on a commercial hydrogen internal combustion engine, whose geometric model is established by CONVERGE software. 2 mm mesh with adaptive mesh refinement (AMR) was used in this study to improve computational efficiency while ensuring accuracy. The geometric model and its mesh refinement are shown in Fig. 1. Table 1 lists the main technical parameters of the engine.

In order to improve the accuracy of the calculation, the boundary conditions of the engine model are set according to the experimental conditions. The ambient temperature of the model is set to 293 K. The engine speed is 1500 rpm, and the inlet pressure of the intake port and the outlet pressure of the exhaust port are both set to 100 kPa.

Fig. 1.
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The geometric model and mesh refinement.

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Table 1. Main technical parameters of the engine.
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2.2 Selection of Mathematical Models

Choosing a suitable turbulence model can accurately calculate the in-cylinder flow field of the engine. The turbulence situation in the cylinder of this model is predicted using the RNG k-ε model [16]. The SAGE model with detailed chemical reaction mechanisms developed by Li et al. is used in this study [17]. The wall heat transfer loss of the model is characterized and calculated by the wall-function model [18] to describe the influence of various parameters on wall heat transfer loss.

2.3 Experimental Setup and Model Validation

Experimental setup and uncertainty analysis. The entire test bench includes an engine system, a dynamometer system, a hydrogen supply system, and a data acquisition system. The schematic diagram of the test bench is indicated in Fig. 2. There is a PowerLink eddy current dynamometer in the dynamometer system used to control engine speed and load. The hydrogen supply system is composed of a hydrogen bottle group, pressure regulating valve, flame arrester and integrated hydrogen flowmeter (S4–33 A/MT, SevenStar, China). The Tociel 20 N060 thermal flow meter is used to measure the air mass flow rate. The data acquisition system consists of an electronic control unit (ecu), a horiba mexa-730λ lambda analyzer, and a kistler kibox combustion analyzer. The measurement uncertainty of the above experimental parameters is shown in Table 2.

Fig. 2.
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The schematic diagram of the test bench of the engine.

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Table 2. The information of main measurement devices.
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Model validation. In order to demonstrate the accuracy of the engine model, the mean in-cylinder pressure of the model was simulated and compared with the experimental results. Due to limitations in experimental conditions, only hydrogen-air experiments were conducted on this engine. In this study, the engine speed was 1500 rpm and the equivalence ratio was 0.5. According to the results in Fig. 3, the curves of the simulation results and the experimental values are nearly overlapped. The mean cylinder pressure difference under different crankshaft angles is less than 0.2 bar. The above verification result indicates that the simulation results have achieved good consistency with the experimental values, and the predictive ability of the engine model has been verified.

Fig. 3.
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Model validation for in-cylinder pressure.

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In order to validate the accuracy of the hydrogen/oxygen reaction mechanism, the ChemkinPro software was used to calculate the laminar flame velocity. Based on the Gri-mech and Li schemes, the improved CANTERA was used by Kuznetsov et al. to calculate the laminar flame velocity of hydrogen-oxygen mixture under different initial pressures [19]. Figure 4 shows the calculation results of laminar flame velocity using ChemkinPro and compares them with Kuznetsov et al. The results indicate that this mechanism has excellent consistency with the laminar flame velocity results in the literature, and the error is within an acceptable range.

Fig. 4.
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The laminar flame velocity of hydrogen-oxygen mixture.

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3 Result and Discussion

3.1 Combustion Analysis

In this study, the equivalence ratio was taken as 3.4, 3.6, 3.8, 4.0, 4.2, and 4.4, and the ignition timing was set to − 2°CA after top dead center (ATDC) and 0°CA ATDC, respectively. Figure 5 shows the variations of in-cylinder pressure with equivalence ratio at different ignition timing. As shown in Fig. 5, the variations of in-cylinder pressure with equivalence ratio at different ignition timing are consistent. During the compression stroke, the upward movement of the piston causes in-cylinder pressure to gradually increase, reaching a value of 2.2 MPa at TDC. After ignition, in-cylinder pressure rises to peak instantly. This is determined by the combustion characteristics of the hydrogen-oxygen mixture, which is well mixed in the cylinder and burns rapidly after ignition. After ignition, in-cylinder pressure decreases as the combustion of the mixture ends and the cylinder volume increases.

Fig. 5.
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Variations of in-cylinder pressure with equivalence ratio at different ignition timing.

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It is necessary to investigate the peak in-cylinder pressure (Pmax) during the combustion process. Figure 6 shows the variations of Pmax with equivalence ratio at different ignition timing. From Fig. 6, it can be seen that when the equivalence ratio is 3.4 and the ignition timing is 0°CA ATDC, Pmax is 6.0 MPa. On the contrary, when the equivalence ratio is 4.4 and the ignition timing is − 2°CA ATDC, Pmax is 5.3 MPa. When the equivalence ratio is 4.0, Pmax at different ignition timing is similar. It can be observed that as the equivalence ratio continues to increase, Pmax decreases. This is because the increasing equivalence ratio leads to a continuous decrease in the mass of oxygen entering the cylinder, which determines a decrease in the mass of burned hydrogen and leads to a decrease in Pmax. Moreover, the combustion rate of the hydrogen-oxygen mixture is inhibited by excess hydrogen, which reduces Pmax. At the same time, The Pmax is decreased with the delay of ignition timing. This indicates that by increasing the equivalence ratio and adjusting the ignition timing, Pmax is reduced to ensure that it does not exceed the maximum pressure that a typical engine withstand.

Fig. 6.
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Variations of Pmax with equivalence ratio at different ignition timing.

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Due to the way of low-temperature intake, the analysis of the cylinder temperature is particularly crucial. Figure 7 shows the variations of cylinder temperature with equivalence ratio at different ignition timing. When the ignition timing is advanced to − 2°CA ATDC, the equivalence ratio increases from 3.4 to 4.4, and the peak temperature decreases from 2700 to 2550 K. It can also be seen that the trend of in-cylinder temperature change is consistent with that of in-cylinder pressure change. The in-cylinder temperature instantly rises to its peak after ignition and gradually decreases after combustion.

Fig. 7.
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Variations of in-cylinder temperature with equivalence ratio at different ignition timing.

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Figure 8 shows the temperature distribution in the cylinder at the time of intake bottom dead center under different equivalence ratios. From the figure, it can be seen that 150 K gas with different equivalence ratios enters the cylinder, which is cooled by low-temperature gas. After the end of the intake, the temperature distribution in the cylinder tends to be consistent, except that the temperature near the spark plug is higher. This indicates that low-temperature hydrogen has an inhibitory effect on the formation of hot areas in the cylinder.

Fig. 8.
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The temperature distribution in the cylinder under different equivalence ratios.

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3.2 Engine Performance

Indicated mean effective pressure (IMEP) is used to evaluate the operational performance of engines. Figure 9 shows the variations of IMEP with equivalence ratio at different ignition timing. From the figure, it can be seen that under the condition of engine speed of 1500 rpm and intake pressure of 1 bar, IMEP gradually decreases as the equivalence ratio gradually increases. When the ignition timing is postponed to 0°CA ATDC, the equivalence ratio increases from 3.4 to 4.4, and the IMEP decreases from 0.40 to 0.31 MPa.

Fig. 9.
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Variations of IMEP with equivalence ratio at different ignition timing.

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When the equivalence ratio is specific, delaying the ignition timing will increase the IMEP. When the equivalence ratio is 4, the IMEP at ignition timing of − 2°CA ATDC and 0°CA ATDC are 0.32 MPa and 0.34 MPa, respectively. This is because when the ignition timing is delayed to 0°CA ATDC, the hydrogen-oxygen mixture burns and releases heat instantly at TDC, completing the constant volume combustion process. On the contrary, when the ignition timing is − 2°CA ATDC, the combustion process is completed before reaching the compression TDC, resulting in the negative work during the compression stroke.

ITE is a parameter that measures the effectiveness of converting thermal energy into indicated work and is used to evaluate the economic performance of engines. Figure 10 shows the variations of ITE with equivalence ratio at different ignition timing. From the figure, it can be seen that the ITE at ignition timing of − 2°CA ATDC and 0°CA ATDC are around 21% and 22%, respectively. When the ignition timing is 0°CA ATDC, the maximum ITE is 22.5% at the equivalence ratio of 3.4, and the minimum ITE is 21.7% at the equivalence ratio of 4.0. It can be observed that at different ignition timing, ITE first decreases and then slightly increases with the increasing equivalence ratio. This is because as the equivalence ratio increases, the mass of oxygen entering the cylinder to participate in the reaction decreases, resulting in a decrease in the mass of hydrogen burned. However, the mass of hydrogen entering the cylinder increases, and the heat release of fuel per unit time decreases, resulting in a decrease in ITE. Furthermore, for a specific equivalence ratio, delaying ignition timing increases the ITE. As the ignition timing is further delayed to TDC, the ITE is improved due to the extremely fast combustion speed of the hydrogen-oxygen mixture which instantly burns at the TDC.

Fig. 10.
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Variations of ITE with equivalence ratio at different ignition timing.

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4 Conclusion

In this study, the CONVERGE software is used to numerically study the combustion characteristics of the port fuel injection hydrogen-oxygen ICE at different ignition timing as a function of equivalence ratio. The main conclusions are as follows:

  1. (1)

    The strategy of using a large equivalence ratio can effectively reduce the peak temperature and pressure in the cylinder, and as the equivalence ratio gradually increases, the peak temperature and pressure in the cylinder further decrease.

  2. (2)

    Preliminary results indicate that low-temperature intake is beneficial for the uniform distribution of temperature inside the cylinder and reduces the formation of hot areas inside the cylinder.

  3. (3)

    When the equivalence ratio is specified and the ignition timing is delayed, the IMEP is increased. On the contrary, when the ignition timing is specified, the IMEP decreases with the increase of the equivalence ratio.

  4. (4)

    The variations of the ITE with equivalence ratio are irregular. When the equivalence ratio is specified, the ITE increases with the delay of ignition timing.