Study on the E ﬀ ect of Second Injection Timing on the Engine Performances of a Gasoline / Hydrogen SI Engine with Split Hydrogen Direct Injecting

: Split hydrogen direct injection (SHDI) has been proved capable of better e ﬃ ciency and fewer emissions. Therefore, to investigate SHDI deeply, a numerical study on the e ﬀ ect of second injection timing was presented at a gasoline / hydrogen spark ignition (SI) engine with SHDI. With an excess air ratio of 1.5, ﬁve di ﬀ erent second injection timings achieved ﬁve kinds of hydrogen mixture distribution (HMD), which was the main factor a ﬀ ecting the engine performances. With SHDI, since the HMD is manageable, the engine can achieve better e ﬃ ciency and fewer emissions. When the second injection timing was 105 ◦ crank angle (CA) before top dead center (BTDC), the P max was the highest and the position of the P max was the earliest. Compared with the single hydrogen direct injection (HDI), the NO X , CO and HC emissions with SHDI were reduced by 20%, 40% and 72% respectively.


Introduction
With the improvement of the world's industrial level, the consumption of fossil fuels is increasing year by year and the energy crisis is increasingly intensified. At the same time, the burning of fossil fuels also leads to a large number of harmful emissions, such as NO X , HC, PM, etc. Therefore, how to improve the efficiency of engines and reduce fuel consumption and reduce harmful emissions has become a technical problem for all countries. Finding clean, efficient and renewable fuels is one of the ways to alleviate the energy crisis and environmental problems.
Hydrogen is considered one of the most promising alternatives to fossil fuels because it is widely available and renewable. However, the pure hydrogen engine still has big technical bottlenecks to be solved [1]. Hydrogen is more suitable as a kind of blending fuel for the spark ignition (SI) engine. Firstly, hydrogen has lower ignition energy than gasoline. Secondly, the flame propagation speed of hydrogen is faster, which can speed up the combustion and increase the engine efficiency [2]. Thirdly, hydrogen has a shorter wall quenching distance, which can reduce hydrocarbon (HC) emission [3]. Hydrogen has a wider flammability limit than gasoline, so it is easier to achieve lean burn combustion with hydrogen.
Ji et al. studied a hydrogen/gasoline engine with hydrogen injected into the intake-port and gasoline injected into the cylinder directly [4][5][6][7][8][9][10][11]. The results show that the performance was improved by adding hydrogen, especially under the lean burn condition [4,5]. By injecting hydrogen into the engine, the lean burn limit can be extended and the brake mean effective pressure can be increased [6,7]. The emissions were reduced by injecting hydrogen into the engine. The addition of hydrogen greatly reduced HC and PM emissions but increased NO X emissions [8]. In the cold starting condition, injecting hydrogen can reduce emissions during the cold starting process, but it will lead to an increase of NO X emission [9]. By adding hydrogen, the idle speed and the fuel consumption can be reduced [10]. Hydrogen addition cannot improve the engine performance at high loads [11].
Huang et al. investigated the natural gas-hydrogen SI engine with mixture direct injection [12][13][14][15][16]. They found that, with hydrogen addition, the combustion of the engine becomes faster and the emissions are fewer. The early flame growth is more stable and faster under the lean burn condition after adding hydrogen.
Yu et al. have done a lot of research on a single hydrogen direct injection (HDI) SI engine in the past few years. According to their studies, the addition of hydrogen can accelerate the flame propagation speed in the cylinder, advance the optimal ignition advance angle and make the combustion completer and more stable, which can significantly reduce the engine's cycle-by-cycle variations [17][18][19][20][21]. With hydrogen, the engine could achieve low HC and CO emissions. Hydrogen addition increases the combustion temperature, which leads to an increase of NO X emissions [22][23][24]. To solve the problem of high NO X emission after injecting hydrogen into the engine, exhaust gas recirculation (EGR) was used to restrain the increase of NO X [25].
In [26], Li et al. did a comparative study of homogenous hydrogen mixture and stratified hydrogen mixture and found that the stratified hydrogen mixture had higher thermal efficiency and lower HC and CO emissions than the homogenous hydrogen mixture, especially under the lean combustion condition and found that the main factor affecting the performance of the hydrogen/gasoline SI engine was the hydrogen mixture distribution (HMD). When the HMD is homogenous, the combustion is complete and the emissions are reduced. When the HMD is stratified, the combustion can be accelerated and efficiency is higher. To achieve lower emissions and higher efficiency, a new kind of injection mode was found. In [27], Li et al. studied the effects of split injection of hydrogen in the cylinder on engine performance and found that split hydrogen direct injection (SHDI) could achieve the best performance in all kinds of hydrogen injection modes. SHDI could form a better HMD. The first injection could form a homogenous HMD, and the second injection could form a stratified HMD. As a result, the SHDI achieved lower emissions and higher efficiency.
The HMD in the cylinder was hard to measure by experiment. Numerical simulation is a good way to investigate the principles of HMD. In recent years, many numerical studies of hydrogen blended SI engines have been done. Gong et al. have published several papers about a hydrogen/methanol SI engine [28][29][30][31][32][33]. With hydrogen addition, cylinder pressure increased and all are emissions reduced.
In [34], Shang et al. studied the effect of hydrogen addition to a hydrogen\n-butanol SI engine. With more hydrogen addition, HC, CO, acetaldehyde and formaldehyde emissions were all reduced continually.
There are few studies investigating the effects of HMD on a hydrogen\gasoline SI engine with SHDI. In [35], we investigated the split injection proportion of SHDI and found that HMD is the main factor affecting engine performance. To deeply investigate how SHDI achieves better engine performance, a numerical study on the effect of second injection timing was presented. Five different second injection timings achieved five different kinds of HMD in a SI engine with SHDI. The results proved that SHDI can achieve better efficiency and fewer emissions.

Computational Model
The parameters of the engine are shown in Table 1. Figure 1 shows the 3D computation model in CONVERGE (V2.3, Convergent Science, Madison, Wisconsin, USA). The grid document is the same as in our former research and was tested for grid independence in [34] to ensure the accuracy and ability of the model to meet the requirements. The parameters of the engine are shown in Table 1. Figure 1 shows the 3D computation model 89 in CONVERGE (V2.3, Convergent Science, Madison, Wisconsin, USA). The grid document is the 90 same as in our former research and was tested for grid independence in [34] to ensure the accuracy 91 and ability of the model to meet the requirements. 92  Table 2 shows mathematical models of our model. The intake mixture was regarded as a 96 homogeneous mixture with a certain proportion of oxygen, nitrogen and gasoline [36]. Six inflow 97 boundaries were set to inject hydrogen directly [37]. 98 The combusting mechanism was a combination of a skeleton chemical reaction mechanism 99 (48 components, 152 reactions) [38] and a detailed chemical reaction mechanism of hydrogen (10 100 components, 21 reactions) [39]. 101  Table 2 shows mathematical models of our model. The intake mixture was regarded as a homogeneous mixture with a certain proportion of oxygen, nitrogen and gasoline [36]. Six inflow boundaries were set to inject hydrogen directly [37]. The combusting mechanism was a combination of a skeleton chemical reaction mechanism (48 components, 152 reactions) [38] and a detailed chemical reaction mechanism of hydrogen (10 components, 21 reactions) [39]. Table 3 shows the initial parameters. In this paper, five second injection timings from 75 • CA BTDC to 135 • CA BTDC were selected under the condition of 1200 rpm, a throttle opening of 10% and an excess air ratio of 1.5. The direct injection pressure was set at 5 MPa, and the ignition timing was set at 15 • CA BTDC.
There were two kinds of contrast tests: single HDI and pure gasoline. The 120 • CA BTDC was proved to be the best injection timing for single HDI. Therefore, 120 • CA BTDC was selected as the injecting timing of single HDI. For SHDI, the first hydrogen injection timing was 300 • CA BTDC and the hydrogen mass of the two injections was the same. Under these conditions, the engine can work stably. The hydrogen energy fraction was set at 20%. Table 4 show the experimental setup scheme and measurement instruments. Figure 3 shows the errors between the calculations and the experiment. The condition with single HDI was under the conditions of 1200 rpm, a throttle opening of 10%, a direct injection pressure of 5 MPa, an ignition timing of 15 • CA BTDC, a direct injection timing of 120 • CA BTDC, a hydrogen energy fraction of 20% and an excess air ratio of 1.5.

Figure 2 and
As shown in the figures, the simulation results are in good agreement with the experimental data.     Figure 4 shows the change of the HMD at ignition timing with different second injection timings. The HMD of single HDI is rich in a small zone in the cylinder, and the HMD of SHDI is more homogenous.  Hydrogen addition improves the efficiency of the engine in three main aspects: accelerating the ignition, accelerating the combustion rate and completing combustion [22]. Single HDI cannot do well in all aspects at the same time. Therefore, single HDI must balance the aspects to have high efficiency.

Hydrogen Mixture Distribution
As shown in Figure 4, the HMD of single HDI is rich near the spark plug, which can accelerate the ignition but cannot do well in other aspects. However, the HMD of SHDI is not only rich near the spark plug but also homogenous in other zones. By injecting hydrogen twice, SHDI can do well in all aspects. The rich mixture near the spark plug can accelerate the ignition, and the homogenous HMD can accelerate the combustion rate and complete the combustion. Since the first injection formed a more homogenous hydrogen mixture, the second injection should form a rich zone near the spark plug. As shown in the figure, early injection timing made the mixture too homogenous and late injecting timing made the mixture too rich on one side of the cylinder. As a result, there is a best second injection timing. In this work, it was 105 • CA BTDC.
The too rich HMD of single HDI would cause high emissions. When it is combusting, the zone with rich mixture would have higher temperature and produce lots of NO X emissions. Furthermore, the CO and HC emissions cannot be burned completely since the hydrogen is too rich in a small zone and cannot do well in completing the combustion. However, SHDI reduces a great deal of emissions compared to single direct hydrogen injection. On the one hand, more homogenous HMD would reduce the peak temperature, which reduces NO X emissions. On the other hand, with SHDI, the hydrogen affects a large zone in the cylinder to complete the combustion, which leads to low HC and CO emissions. In brief, SHDI can achieve low emissions. Figure 5 shows the change in cylinder pressure with different second injection timings. With hydrogen addition, the cylinder pressure increases obviously. Hydrogen addition improves the engine efficiency in three main ways: accelerating the ignition, speeding up the combustion rate and completing the combustion [22]. As the second injection timing advances, the cylinder pressure climbs to the highest value measured with a second injection timing of 105 • CA BTDC. This is because early injecting timing would make the mixture too homogenous and late injecting timing would make the mixture too rich on one side of the cylinder. Since the best injection timing was set for single HDI, the pressure curve is higher than some pressure curves of SHDI but lower than the pressure curves for the second injection timings of 90 • CA BTDC, 105 • CA BTDC and 120 • CA BTDC. Split hydrogen direct injection can form better HMD.  Figures 6 and 7 show the change of P max and the position of P max with different second injection timings. When the second injection timing is 105 • CA BTDC, P max is the highest and the position of P max is the earliest. Compared with gasoline, at the second injection timing of 105 • CA BTDC, the P max increases by 33% and the position of the P max advances by 9 • CA. With the second injection timing of 105 • CA BTDC, the P max increases by 2.3% and the position of P max advances by 1.2 • CA compared to single HDI.  Figure 8 shows the change in the heat release rate with different second injection timings. The heat release rate with hydrogen is more concentrated and advanced. As shown in Figure 8, the ignition with single HDI is the fastest. However, the most advanced maximum heat release rate is achieved by SHDI with a second hydrogen injection timing of 105 • CA BTDC. This means that single hydrogen direct injection can only do well in accelerating the ignition, and SHDI can further speed up the combustion rate to improve the efficiency.   Figure 9 shows the change of NOX emissions with different second injection timings. The 210 temperature impacts NOX emissions significantly. Since the flame temperature of hydrogen is 211 higher than gasoline, the NOX emissions increase by 140% with hydrogen addition compared to 212 gasoline [22]. Since the HMD of SHDI is more homogenous, the maximum temperature is less than 213 that of single HDI [27]. Therefore, the NOX emissions with SHDI are fewer by an average of 20% 214 compared to single HDI. As the second injection timing advances, the NOX emissions fluctuate in 215 a narrow band.  Figure 9 shows the change of NO X emissions with different second injection timings. The temperature impacts NO X emissions significantly. Since the flame temperature of hydrogen is higher than gasoline, the NO X emissions increase by 140% with hydrogen addition compared to gasoline [22]. Since the HMD of SHDI is more homogenous, the maximum temperature is less than that of single HDI [27]. Therefore, the NO X emissions with SHDI are fewer by an average of 20% compared to single HDI. As the second injection timing advances, the NO X emissions fluctuate in a narrow band. 224 Figure 10 shows the change in the conditions of NOX emissions and the change in temperature 225 after combustion with different second injection timings, which could indicate a similarity between 226 NOX emissions and temperature. With single HDI, the peak temperature is obviously higher, and 227 the NOX emissions are greater than those of SHDI. With SHDI, the homogenous HMD could reduce 228 the size of the high temperature zone and produce fewer NOX emissions than single HDI. 229 Figures 11 and 12 show the change in CO and HC emissions with different second injection 230

Emissions
timings. Due to completed combustion with hydrogen addition, CO and HC emissions are 231 obviously reduced [22]. Since the HMD of single HDI is too rich and cannot affect most of the zone 232  Figure 10 shows the change in the conditions of NO X emissions and the change in temperature after combustion with different second injection timings, which could indicate a similarity between NO X emissions and temperature. With single HDI, the peak temperature is obviously higher, and the NO X emissions are greater than those of SHDI. With SHDI, the homogenous HMD could reduce the size of the high temperature zone and produce fewer NO X emissions than single HDI.   Figures 11 and 12 show the change in CO and HC emissions with different second injection timings. Due to completed combustion with hydrogen addition, CO and HC emissions are obviously reduced [22]. Since the HMD of single HDI is too rich and cannot affect most of the zone in the cylinder, single HDI cannot make full use of hydrogen to limit CO and HC emissions. However, the HMD of SHDI is more homogenous and hydrogen can affect the majority zone of the cylinder. As a result, SHDI produces fewer CO and HC emissions than does single HDI [27]. Compared with single HDI, CO and HC emissions with SHDI are respectively reduced by 40% and 72%. As the second injection timing advances, the CO and HC emissions continue to decline. When the second injection timing is 135 • CA BTDC, the CO and HC emissions respectively decrease by 20% and 40% compared to the second injection timing of 75 • CA BTDC. As the injection of hydrogen makes the HMD more homogenous earlier, the hydrogen can affect a larger zone.

Conclusions
In this study, by building a model that was validated in CONVERGE, the effects of second injection timing on HMD, combustion and emissions were investigated. With SHDI, the engine can simultaneously increase its efficiency and reduce emissions without additional cost. The main conclusions are as follows.

1.
SHDI can form a better HMD. The HMD of SHDI is not only rich near the spark plug but also homogenous in other zones. Therefore, combustion can be accelerated and completed. As a result, SHDI can achieve better engine performance.

2.
With hydrogen addition, cylinder pressure increases obviously. The best second injection timing is 105 • CA BTDC. This is because early injecting timing would make the mixture too homogenous and late injection timing would make the mixture too rich on one side of the cylinder. When the second injection timing is 105 • CA BTDC, P max is the highest and the position of P max is earliest of all values measured. 3. NO X emissions increase by 140% after hydrogen addition compared to gasoline. NO X emissions with SHDI are reduced by an average of 20% compared to single HDI. The main reason is that the HMD of SHDI is more homogenous and the maximum temperature is lower compared to single HDI. As the second injection timing advances, the NO X emissions change a little. 4.
CO and HC emissions are respectively reduced by 60% and 95% after hydrogen addition compared to gasoline. This is because the HMD of SHDI is more homogenous than that of HDI and hydrogen can affect the majority zone of the cylinder. Compared with single HDI, the CO and HC emissions with SHDI are respectively reduced by 40% and 72%. As the second injection timing advances, the CO and HC emissions continue to decline. When the second injection timing is 135 • CA BTDC, the CO and HC emissions respectively decrease by 20% and 40% compared to the emissions associated with a second injection timing of 75 • CA BTDC.

Conflicts of Interest:
The authors declare no conflict of interest.