1. Introduction
Under the double troubles of energy shortage and global warming, many countries have put forward carbon-reduction targets and carried out research on alternative fuels for engines. In order to minimize internal combustion engine carbon emissions and achieve clean combustion, it is crucial to employ low- or zero-carbon, clean, and renewable alternative fuels [
1].
Researchers constantly explore various alternative fuels, such as natural gas, H
2, biofuel, alcohol [
2,
3,
4,
5,
6], etc., as possible alternatives to current transport fuels in order to reduce total carbon emissions during transport. Due to its absence of carbon elements and its advantages, such as fast combustion rate, large diffusion coefficient, and wide ignition limit [
7,
8], hydrogen is considered the cleanest and ideal alternative energy source [
9]. H
2 can be utilized as a single fuel, which is usually called a “Hydrogen fuel internal combustion engine”. Extensive study has been undertaken on the engine’s performance [
10,
11,
12,
13,
14,
15]. Compared with traditional engines, it has high combustion efficiency and no harmful emissions, such as carbon smoke, CO, and CO
2. Hydrogen can be added to the engine as an alternative fuel and the engine can switch between the two fuels according to demand, with high flexibility to adapt to different working loads and driving conditions. Existing engine infrastructure can be utilized without the need for a dedicated high-pressure hydrogen supply system, thereby reducing costs.
Hydrogen has been used extensively as a clean energy source throughout the last few decades. Yuriy Gutarevych et al. [
16] found that adding hydrogen improves fuel economy and reduces CO emissions in gasoline engines. Karagoz et al. [
17] found that adding hydrogen improved the thermal efficiency and reduced the power of gasoline engines, with CO and THC emissions approaching zero, but NO
x emissions increased by up to 99.5%. Masaki Naruke et al. [
18] showed that hydrogen doping can effectively extend the lean burn limit of gasoline engines and improve thermal efficiency. Due to their great efficiency, potent power production, longevity, and dependability, as well as combustion economy, diesel engines are widely employed in a variety of industries. However, their CO
2 and particulate emissions are relatively high [
19,
20] and secondary fuel can be added to reduce emissions while maintaining power. Adding hydrogen to diesel engines has been a new idea in recent years. The higher natural temperature and lower ignition energy of hydrogen enable diesel engines to use diesel as a pilot to ignite hydrogen, which can improve the performance of diesel engines by utilizing the advantages of zero-carbon cleanliness of hydrogen and combining the high efficiency and good economy of diesel engines.
The prior literature shows that hydrogen-doped diesel-engine combustion is an efficient technique to decrease emissions. Szwaja et al. [
21] investigated the impact of mixing hydrogen on engine performance and the research results showed that mixing hydrogen in a diesel engine could make the fuel mixture more uniform and burn better. Jafarmadar et al. [
22] investigated how the dual-fuel engine’s performance changed and discovered that, as the hydrogen mixture grew, so did the average effective pressure also increase. Menna et al.’s [
23] research of a hydrogen–diesel dual-fuel engine revealed that combining hydrogen with diesel increased NO
x pollution while lowering CO
2 emissions. Yang Zhenzhong [
24] studied the emission characteristics of CO, CO
2, and HC in different proportions of hydrogen–diesel hybrid fuels and found that hydrogen–diesel dual-fuel helped to lower the specific emission of CO, CO
2, and HC. Tsujimura et al. [
10] discussed the effect of hydrogen mixing on the performance of single-cylinder diesel engines and the findings indicated that the thermal efficiency decreased at low load and increased at high load. Adnan et al. [
25] studied the effect of engine speed on hydrogen doping in diesel engines and the results of the tests indicated that when the engine is running at medium to low speeds, CO emissions are reduced. As the engine speed increases, the highest cylinder temperature increases, leading to an enhancement in CO and NO
x. Naber et al. [
26,
27] studied the spontaneous combustion characteristics of hydrogen gas in diesel engines and the findings suggested that there is a prominent correlation between the natural characteristics of hydrogen gas and gas temperature and ignition time. Liang Li et al. [
28] discussed the impact of hydrogen doping on the efficiency of engines and discovered that power, combustion temperature, and cylinder pressure all rose at a 10% hydrogen substitution rate. The higher the temperature, the more NO
x emissions. They also found that under high load, hydrogen doping led to a more significant increase in NO
x.
A dual-fuel engine’s cylinder combustion can be improved by the addition of a particular quantity of hydrogen, effectively reducing some emissions but worsening NO
x emissions. Based on this, the introduction of the EGR technique can lessen NO
x emissions [
29]. By utilizing EGR technology, diesel engines’ NO
x emissions may be successfully reduced and the knocking phenomenon brought on by hydrogen combustion in diesel engines can be prevented [
30,
31]. In an inquiry into emissions of hydrogen-diesel dual-fuel engines, Dimitriou [
32] et al. discovered that EGR technology significantly affects NO
x emission control at moderate load. Yu Xiumin et al. [
33] analyzed the consequences of collaborative control of hydrogen quantity and EGR on engine NO
x emission and it was declared that when the hydrogen substitution rate was high, the effect of EGR on NO
x emission reduction was more obvious. Wu [
34] reported that the application of a 20% hydrogen blending ratio and 40% EGR in a diesel engine reduces NO
x and smoke emissions at the same time, as opposed to the use of pure diesel fuel. The results obtained display that injection parameters also have a big impact on the combustion and emission performance of a dual-fuel engine. Miyamoto et al. [
35] found that premature fuel injection results in an important surge in NO
x production and a quick rise in combustion chamber pressure. Additionally, Tomita et al. [
36] also confirmed that the average effective pressure and NO
x will raise due to premature diesel injection. Gaurav, Tripathi et al. [
37] examined the impact of the ignition advance angle on a hydrogen–diesel dual-fuel engine, and the conclusions proved that with the advance of ignition timing, the combustion of mixed fuel was more complete, resulting in a reduction in HC and CO, but an increase in NO
x. Osama H. Ghazal [
38] examined the shape of injection parameters and hydrogen mixing on diesel-engine performance and the results showed that, with the earlier injection time, engine pressure rise rate increased, in-cylinder combustion pressure and temperature raised, and NO
x increased. The main injection timing has an enormous effect on the time, directly affecting engine performance and exhaust emissions. When hydrogen is burned in a diesel engine, the adjustment of the main injection timing can further optimize the combustion process to improve combustion efficiency and reduce emissions.
According to the past research findings, many researchers have explained the advantages of hydrogen combustion in traditional engines. However, it is necessary to further analyze the effect of the hydrogen substitution rate on the capability of a dual-fuel engine under different loads, and take EGR technology and fuel-injection control to solve the emission problems caused by the increase in the hydrogen substitution rate under partial loads. In this paper, the dual-fuel integrated controller independently developed by the team can facilitate the precise injection control of diesel and hydrogen, along with the flexible switching of single-fuel and dual-fuel modes, and realize the collaborative integrated control of the fuel injection and air system. By controlling the ratio of two fuels under different loads, the maximum hydrogen substitution rate is determined and the detrimental impact of hydrogen doping on the functional mode of the diesel engine is studied. At the same time, the influence of EGR rate and main minimum injection timing on the performance of hydrogen–diesel dual-fuel engines is studied, which provides a theoretical basis for further optimization and control of engine performance and emissions.
2. Test Device and Test Method
2.1. Test Device
The engine parameters used in the test are shown in
Table 1. The two fuels used in the test are 0# diesel and 99.5% hydrogen, and the physical and chemical characteristics of the two fuels used in the test are shown in
Table 2 [
11,
39]. The schematic diagram of the test bench is shown in
Figure 1, the physical object is shown in
Figure 2, and the main test and control equipment are shown in
Table 3.
As is shown in
Figure 1, the hydrogen from the hydrogen cylinder group (1) is depressurized and reaches the hydrogen common rail (8). It is then injected into the intake manifold by the hydrogen nozzle (9) and mixed with air before entering the combustion chamber. The diesel oil is pressurized by the high-pressure oil pump (13) to reach the diesel high-pressure common rail (14) and, then, directly injected into the combustion chamber by the diesel nozzle (15). After being pressurized by the turbocharger (19), the air flows through the intercooler (20) to the intake manifold. The mixture of air and hydrogen enters the cylinder and is ignited by diesel fuel. After combustion, part of the exhaust gas flows through the exhaust pipe and is discharged through the turbine. The other part of the exhaust gas can flow back to the intake pipe through the EGR cooler (23) and EGR valve (22) and enter the cylinder again to participate in combustion. Adjust the control parameters required by the dual fuel ECU (27) through the upper computer (31), output control signals to control the hydrogen nozzle (9), diesel nozzle (15), and EGR valve (22), and further adjust the amount of hydrogen, diesel, and exhaust gas injected into the cylinder. The AVL combustion analyzer (28) collects cylinder pressure through the cylinder pressure sensor (32) and calculates parameters such as heat release rate and temperature. Finally, the emission analyzer (26), dual fuel ECU (27), and AVL combustion analyzer (28) are read through the data collector (30).
2.2. Test Method
When studying the consequence of hydrogen substitution rate on dual-fuel engine economy, burn and emission properties under various loads, due to the characteristics of hydrogen combustion speed, high calorific value, and large diffusion coefficient under high loads lead to rough in-cylinder combustion, increased heat load, and increased NOx emission, etc. Therefore, in the test process, the maximum exhaust temperature and maximum explosion pressure are mainly used as constraint conditions to obtain the maximum hydrogen substitution rate under each load and record the data, so as to avoid problems such as rough combustion in the cylinder and high emissions.
During the test, the engine cooling water temperature is controlled at (80 ± 0.5) °C, and test data collection is done after the engine is running steadily. The main test method is to conduct a test with a replacement rate of 0 after a fixed speed and load, then open the hydrogen supply valve, adjust the hydrogen supply pressure to 0.4 MPa, switch the dual-fuel mode, gradually increase the hydrogen-injection pulse width, and increase the hydrogen gas supply. With the self-developed dual-fuel ECU as the control core, the fuel-injection quantity of the original engine under the current hydrogen-doping amount is obtained according to the total energy-conservation control strategy, so as to realize the accurate injection control of diesel and hydrogen. When the maximum hydrogen substitution rate of this working condition is increased, three groups of data are collected at each working condition and averaged and the data are recorded. Then, the hydrogen pulse width is gradually reduced to 0 and the hydrogen gas supply valve is closed and switched to the next working condition. The above steps are repeated to carry out the test and record the data, thus obtaining the maximum hydrogen substitution rate under different working conditions.
The impacts of EGR technology and injection timing on the economy, combustion characteristics, and emission characteristics of a diesel engine blended with hydrogen were studied. During the test, the maximum hydrogen substitution rate corresponding to a high load at 1800 rpm was chosen. First, fix the diesel injection strategy and adjust the EGR valve. The data under different EGR rates were recorded. Then the EGR valve is closed, the injection timing of the diesel main injection is gradually adjusted from −8 °CA ATDC to 0 °CA ATDC, and the data of different main injection timings are recorded. The impact of the aforementioned variables on engine performance is analyzed next. During the emission test, the sampling frequency of the AVL FTIR i60 multicomponent gas analyzer is 1 Hz and the sampling time is 10 s.
2.3. Measurement Calculation Formula
The hydrogen substitution rate represents the percentage of the energy delivered by hydrogen to the energy jointly provided by the two fuels at the same operating point, and the EGR rate represents the ratio of the amount of recycled exhaust gas to the total intake of air sucked into the cylinder. The calculation formula for the hydrogen substitution rate is shown in (1) and the calculation formula for the EGR rate of exhaust-gas recirculation is shown in (2). In order to characterize the economy of a hydrogen–diesel dual-fuel engine with the same calorific value, the hydrogen consumption is converted into diesel consumption through the calorific values of hydrogen and diesel, and the converted diesel consumption plus the pilot diesel consumption is the total fuel consumption under the dual-fuel setting. The BSFC
equ is equal to the total fuel consumption compared to the effective work it does and is a member of the key metrics for assessing engine fuel economy. The calculation formula is shown in (3).
In the formula,
is the diesel consumption, kg/h;
is the hydrogen consumption, kg/h;
= 120.9 MJ/kg is the low calorific value of hydrogen gas;
= 42.5 MJ/kg is the low calorific value of diesel fuel; and
is the hydrogen substitution rate, %.
In the equation,
is the volume fraction of CO
2 in the intake air diluted by EGR;
is the volume fraction of CO
2 in the exhaust gas; and
is the EGR rate.
In the formula, is BSFCequ, g/(kW·h) and is the engine power, kW.