3.1. Effect of Diesel/Ammonia Injection Pressure on Combustion and Emission Characteristics of Low-Speed Engines
This section investigates the simulation study under the conditions of a 100% load, a 95% ammonia energy substitution rate, and injection timings of 2.5 °CA BTDC for diesel injection and 2 °CA BTDC for ammonia injection in the diesel/ammonia dual-fuel engine. Simulations are conducted at injection pressures of 500 bar, 600 bar, 700 bar and 800 bar (consistent for both diesel and ammonia).
Figure 5 illustrates the comparative analysis of the in-cylinder pressure and heat release rate, under different diesel/ammonia injection pressure conditions. It can be observed from the figures that as the injection pressure gradually increases, the in-cylinder pressure and heat release rate both show an increasing trend, and the peaks of the pressure and heat release rate are shifted earlier in the phase. This phenomenon is mainly attributed to the higher injection pressure promoting more effective fuel atomization, thereby accelerating the combustion rate.
The effects of injection pressure on the CA10 (the crankshaft angle at the beginning of combustion), the CA50 (the crankshaft angle at the middle of combustion), and the combustion duration are shown in
Figure 6.
Figure 6 illustrates the influence of injection pressure on the CA10 (the crank angle at 10% of heat release), the CA50 (the crank angle at 50% of heat release), and the combustion duration. The results indicate that the CA10 shows no significant variation with increasing injection pressure, while the CA50 and combustion duration exhibit an overall trend of initially decreasing followed by an increase. At an injection pressure of 500 bar, the CA10, CA50 and combustion duration reach their maximum values. This could be attributed to the lower injection pressure resulting in a slower fuel injection rate, requiring more time for fuel–air mixing to form a combustible mixture, thereby prolonging the combustion duration. Moderate increases in injection pressure help shorten this process, improving combustion efficiency.
However, the results also demonstrate that the influence of injection pressure on the CA50 and combustion duration gradually diminishes with further pressure increases. At an injection pressure of 700 bar, the combustion process reaches its optimal state. However, when the injection pressure continues to rise to 800 bar, both the CA50 and combustion duration increase. This may be due to excessively high injection pressures accelerating the fuel injection rate, causing some fuel to adhere to the cylinder walls, deteriorating the combustion process, and resulting in a delay in the combustion phase and prolongation of the combustion duration.
Figure 7 compares the in-cylinder mixture distribution under different injection pressure conditions. At lower injection pressures (e.g., 500 bar), the mixture concentration distribution is more dispersed, indicating inadequate fuel–air mixing, and thus reducing the combustion efficiency. As the injection pressure increases to 600 bar and 700 bar, the mixture concentration distribution tends to be more uniform, indicating an improvement in fuel atomization at higher injection pressures, promoting the formation of a more homogeneous mixture. However, at the highest injection pressure (800 bar), some fuel may have adhered to the cylinder walls, potentially affecting mixture uniformity and combustion efficiency.
Figure 8 depicts variations in the average indicated mean effective pressure (IMEP), the thermal efficiency (ITE), and the indicated specific fuel consumption (ISFC) under different operating conditions, where the ISFC is calculated by converting the consumption of ammonia into the equivalent consumption of diesel fuel based on energy.
However, the results in the figure also show that its effect on the CA50 and combustion duration diminishes with a further increase in injection pressure. The combustion process is optimized at an injection pressure of 700 bar. When the injection pressure continues to increase to 800 bar, the CA50 and combustion duration increase, which may be attributed to the high injection pressure accelerating the fuel injection velocity, resulting in part of the fuel adhering to the cylinder wall, deteriorating the combustion process, shifting the combustion phase backwards, and lengthening the combustion duration.
Figure 9 compares the distribution of in-cylinder temperatures under different injection pressure conditions. It can be observed that as the injection pressure increases, the area of high-temperature regions within the cylinder gradually expands, and the temperature distribution becomes more uniform. This is primarily attributed to the enhancement of the injection pressure, which improves the degree of fuel–air mixing, thereby promoting more effective combustion.
Figure 10 illustrates the variation trends of major emissions under different injection pressures. The results indicate a pattern of increasing and then decreasing NO
X emissions with increasing injection pressures. This trend may be closely related to the distribution of temperature within the combustion chamber and changes in local oxidation-reduction states. At lower injection pressures, inadequate atomization may lead to uneven fuel dispersion, resulting in an uneven distribution of temperatures and oxygen concentrations in localized areas, thus promoting NO
X formation. Conversely, as the injection pressure increases, improved fuel atomization and more uniform mixture formation leads to a more even combustion process, effectively reducing local high-temperature areas and decreasing NO
X generation.
For emissions of N2O, CO2, and unburnt NH3, the emission trends show an initial decrease followed by an increase. This is mainly attributed to the initial improvement in fuel atomization quality with increasing injection pressures, which enhances fuel–air mixing and promotes more complete combustion, thereby reducing the generation of N2O and CO2 to some extent and decreasing unburnt NH3 emissions. However, the increase in unburnt NH3 emissions at 800 bar may be due to excessively fast injection speeds, causing some ammonia gas to not fully mix with air before the exhaust process begins and participate in combustion reactions.
Figure 11 illustrates the distribution of cylinder NO
X under different injection pressure conditions. Analysis of the contour maps reveals that with the gradual increase in injection pressure, both the concentration and distribution area of NO
X initially show an increasing trend, followed by a leveling off for this trend. The area of high-concentration NO
X slightly expands, indicating that within a certain range, increasing the injection pressure improves combustion conditions and increases NO
X generation. However, further increasing the injection pressure to 800 bar does not significantly affect NO
X generation.
3.2. Effect of Diesel/Ammonia Injection Timings on Combustion and Emission Characteristics of Low-Speed Engines
Figure 12 depicts the effects of different ammonia injection timings on the cylinder pressure and heat release rate under the condition that the injection interval between diesel and ammonia is fixed at 0.5 °CA. It can be observed from the figure that, with a delay in the ammonia injection timing, the cylinder pressure shows an overall decreasing trend. This phenomenon can be attributed to two main factors: firstly, an earlier ammonia injection time contributes to improving the mixture of diesel and ammonia in the cylinder, thereby promoting a more effective combustion process; secondly, an earlier injection time causes the fuel to start burning and releasing energy before the top dead center, increasing the rate of pressure rise, and consequently leading to an increase in cylinder pressure.
Figure 13 illustrates the variation in engine combustion characteristics under different ammonia injection timings. Upon observing the results, it is evident that with the delay in injection timing before reaching the top dead center (TDC), both the start of combustion and the center of the heat release shift towards later crankshaft angles. One reason for this change is that the delayed injection timing leads to a delayed mixing of combustion gases with air, thus delaying the start of combustion accordingly. Another reason is that the combustion propagation slows down due to the delayed injection timing, causing the center of the heat release to move backwards and prolonging the combustion duration.
When the injection timing occurs after the top dead center, there is a significant backward shift in both the start of combustion and the center of the heat release, while the combustion duration decreases. This phenomenon is attributed to the fact that combustion mainly occurs during the piston’s downward stroke, during which cylinder temperatures and pressures are higher, facilitating accelerated combustion.
Figure 14 depicts the effects of different ammonia injection timings on the engine’s mean effective pressure, the indicated thermal efficiency and the indicated specific fuel consumption. The analysis results reveal that, as the ammonia injection time is delayed, both the IMEP and ITE exhibit fluctuations but show an overall decreasing trend. Before the ammonia injection time approaches the TDC, the decrease in IMEP and ITE is relatively slow.
After reaching the top dead center, there is a significant decrease in both the mean effective pressure and the indicated thermal efficiency. Meanwhile, the trend of the indicated specific fuel consumption (ISFC) is opposite to that of the mean effective pressure and the indicated thermal efficiency. As the ammonia injection time is delayed, the ISFC gradually increases, especially when the ammonia injection time reaches the top dead center, there is a significant rise in the ISFC, followed by a stabilization. The main reasons for this change include: the delayed ammonia injection time leads to insufficient time for thorough mixing and atomization of ignition fuel and ammonia fuel, thereby reducing combustion efficiency, and resulting in a decrease in the mean effective pressure and the indicated thermal efficiency; simultaneously, at the start of combustion, the piston is positioned lower, causing a cooling effect on the engine cylinder’s components such as the piston and cylinder wall, which leads to partial thermal energy loss, and further affects engine performance. Overall, injecting ammonia before 1 °CA BTDC can essentially achieve the original engine combustion performance.
Combining
Figure 15a,b, it can be observed that as the ammonia injection timing is delayed, the area of high temperature (deep-red) decreases. This phenomenon may be attributed to the early mixing of ammonia with air before reaching the top dead center, leading to the commencement of the combustion process before the piston reaches the top dead center, resulting in rapid heat accumulation and a localized temperature rise. In contrast, in conditions after the top dead center, the combustion process occurs later, and the piston has already begun to descend, resulting in a decrease in the area of high temperature.
Considering the significant differences in the combustion characteristics of ammonia compared to traditional fossil fuels, and the disparities in spatial scale and operational status between low-speed and medium/high-speed marine engines, this section continues the investigation on the impact of different ammonia injection timings on the combustion and emission characteristics of low-speed engines based on
Section 3.1. The aim is to determine the optimal injection timings.
Under the conditions of a 100% load, a 95% ammonia energy substitution rate, and an injection pressure of 700 bar, with diesel injection advanced by 0.5 °CA, simulations were conducted at ammonia injection timings of 2.5, 2.0, 1.5, 1 °CA BTDC, 0, 0.5, and 1 °CA ATDC.
Figure 16 illustrates the variations in major emissions of the engine under different ammonia injection timings. It can be observed from the graph that with the delay in the ammonia injection time, there is an overall reduction in NO
X emissions. Particularly, under these conditions and before reaching the top dead center (TDC), the NO
X emissions decrease with the delay in the ammonia injection time. This reduction may be attributed to the synchronization of the ammonia injection time with the high-temperature zone in the combustion chamber near the TDC, which helps in reducing the area of the local high-temperature and effectively inhibits NO
X formation.
The trends in N2O and CO2 emissions are similar to NOX, showing a decrease overall under conditions before reaching TDC and stabilizing after reaching TDC. This phenomenon is due to the smaller area of the local high-temperature zone before reaching TDC, which reduces the generation of N2O and CO2. After reaching TDC, the dissipation of heat in the combustion chamber causes a decrease in cylinder temperature compared to before, slowing down the generation of N2O and CO2, and resulting in minor changes in emissions.
The emission of unburnt NH3 shows a gradual increasing trend, mainly because the reduction in mixing time between ammonia and air leads to decreased uniformity of the mixture, reducing combustion efficiency, and subsequently increasing the emission of unburnt NH3.
Figure 17 depicts the distribution of NO
X in the cylinder under different ammonia injection timings before and after reaching TDC. It can be observed from the graph that with the delay in the ammonia injection time, the area of the NO
X distribution region gradually decreases, especially the reduction in the area of the high concentration region is more significant. This phenomenon indicates that delaying the ammonia injection time contributes to reducing NO
X generation. The possible reason is that delaying the ammonia injection time reduces the mixing time between ammonia and fuel, resulting in a decrease in the average temperature in the cylinder during the combustion process, and thereby reducing NO
X generation. The distribution of NO
X after reaching TDC shows significant differences compared to before reaching TDC, although there is not much change in the distribution area, which corroborates the analysis in
Figure 16.
3.3. Effect of Diesel/Ammonia Injection Strategy on Combustion and Emission Characteristics of Low-Speed Engines
Figure 18 illustrates the impacts of different injection strategies on the in-cylinder mean pressure and heat release rate. As depicted in the figures, compared to the D-A (diesel-ammonia) mode, the A-D-A (ammonia-diesel-ammonia) mode significantly increases the initial rate of rise in the in-cylinder pressure, with an earlier and more pronounced peak phase shift. This is primarily due to the pre-injection of ammonia, which evaporates within the cylinder and rapidly forms a combustible mixture, effectively accelerating the combustion rate. Additionally, an increase in the pre-injection proportion further elevates the peak values of the in-cylinder pressure. Moreover, the peak phase of the heat release rate advances and significantly heightens, with the heat release curve reflecting characteristics of premixed combustion.
Figure 19 presents a comparison of the CA10, CA50, and combustion duration under different injection strategies. It can be observed that, compared to the D-A mode, the A-D-A mode significantly advances the CA10 and CA50, and also substantially shortens the combustion duration. This is primarily due to the addition of pre-injected ammonia, which initiates combustion earlier and accelerates the reaction rate. Additionally, increasing the pre-injection ammonia ratio from 10% to 30% further advances the CA10 and CA50, and continues to shorten the combustion duration. This is mainly attributed to the increased proportion of pre-injected ammonia, which enhances the fraction of premixed combustion and consequently speeds up the combustion rate.
Figure 20 depicts the comparison of the mean effective pressure, the indicated thermal efficiency, and the indicated specific fuel consumption under various injection strategies. The results demonstrate that the A-D-A mode exhibits higher mean effective pressure and indicated thermal efficiency compared to the baseline engine, while also achieving lower indicated specific fuel consumption. Furthermore, an increase in the pre-injection ammonia ratio further enhances these effects, underscoring the significant role of the A-D-A mode in improving the combustion performance of dual-fuel low-speed engines.
Figure 21 presents a comparison of the in-cylinder average temperature distribution under different injection strategies. It is evident that in the A-D-A mode, the CA10 occurs earlier, and compared to the D-A mode, the temperature distribution at CA50 is more uniform with a larger area of high-temperature regions. Additionally, as the pre-injection ammonia ratio increases, the area of the high-temperature regions within the cylinder also expands. This indicates the effectiveness of the A-D-A mode in achieving a more favorable temperature distribution during combustion.
Figure 22 illustrates the comparative emissions of NO
X, N
2O, CO
2 and unburnt NH
3 under various injection strategies. It can be observed that in the A-D-A mode, the emissions of NO
X and CO
2 are significantly elevated, while N
2O emissions decrease and unburnt NH
3 is substantially reduced. Despite the significant increase in NO
X and CO
2 emissions, their levels remain substantially lower than those of the baseline engine.
Figure 23 shows a comparison of the in-cylinder NO
X distribution under different injection strategies. The contour plots indicate that the A-D-A mode leads to an increase in NO
X production and an expansion of its distribution area compared to the D-A mode. Additionally, an increase in the pre-injection ammonia ratio further enhances the in-cylinder distribution of NO
X, highlighting the influence of pre-injected ammonia on NO
X formation.