3.2.1. Cylinder Pressure and Temperature
Figure 2 and
Figure 3 show the curves of changes in cylinder pressure and cylinder temperature during combustion under different diesel energy ratios;
Figure 4 provides a comparison of thermal efficiency and power.
From
Figure 2 and
Figure 3, it is evident that when the diesel energy ratio is 10% and 16%, the cylinder pressure and temperature are relatively low, the cylinder temperature shows a smooth trend, indicating that the cylinder did not reach combustion conditions when the diesel energy ratio was below 16%. When the energy ratio of the diesel engine exceeds 16%, the energy ratio of the diesel engine increases, the maximum pressure and maximum temperature in the cylinder increase, and the combustion in the cylinder starts earlier. This is due to the fact that ammonia has a high energy to ignite; an increase in the diesel energy ratio can make the cylinder reach ignition conditions more quickly, leading to a more rapid increase in cylinder temperature and an easier reaction. However, due to the excessive proportion of ammonia in this engine model, the combustion reaction rate is low, and when the diesel energy ratio gradually decreases, post-combustion phenomena become more severe [
33]. At the same time, it can be observed that the changes in peak cylinder pressure and temperature are not significant during the transition from 34% to 40% diesel energy ratio. This is due to the limited cylinder volume. As a result, the air intake available for fuel combustion is insufficient, resulting in incomplete fuel combustion.
According to
Figure 4, the influence of the diesel energy ratio on engine performance can be observed more intuitively: both power and thermal efficiency show a tendency to increase and then decrease. The power and thermal efficiency of the engine increase with increasing diesel fuel efficiency when the diesel fuel efficiency is less than 22%. Since ammonia is the main fuel during the combustion process, diesel fuel is mainly for the ignition of the higher ignition energy content of ammonia, a diesel fuel energy ratio that is too low cannot effectively ignite the ammonia. When the diesel energy ratio exceeds 22%, both power and thermal efficiency gradually decrease with increasing diesel energy ratio. This indicates that when the diesel energy ratio is higher than 22%, the air content in the cylinder is low, deteriorating the combustion conditions, and the peak combustion performance occurs at a diesel energy ratio of 22%. This further indicates that when the diesel fuel-to-energy ratio is less than 22%, the output power of the engine is significantly affected by the diesel fuel-to-energy ratio, while when the diesel energy ratio exceeds 22%, the influence of the air content becomes more pronounced.
3.2.2. Ammonia Fuel Analysis
The variation of the ammonia fuel mass fraction at different diesel energy ratio conditions is shown in
Figure 5. Ammonia is consumed slowly at first, followed by a rapid rate of consumption. Under conditions where the diesel energy ratio is greater than 22%, the ammonia is almost completely consumed with only a small amount of leakage. The more diesel is injected, the more intense the reaction inside the cylinder. However, under conditions where the diesel energy ratio is 10% and 16%, the total energy of the fuel mixture is low and normal combustion phenomena cannot occur inside the cylinder, resulting in a large amount of unreacted ammonia leakage, thereby causing serious environmental pollution.
Figure 6 shows the overall reaction rate of the ammonia fuel at different diesel energy ratios. Overall, there is no apparent positive portion in the reaction rate, indicating a continuous consumption of ammonia during the combustion process. As the diesel energy ratio increases, the consumption rate of ammonia fuel increases significantly. The increase in the diesel energy ratio accelerates the increase in the cylinder temperature, which leads to an earlier start of the combustion and promotes the progress of the reactions related to the ammonia consumption-related species. This indicates that an increase in diesel quantity results in a more intense reaction of the ammonia fuel.
In order to gain a detailed understanding of the combustion process inside the cylinder, the specific parameters of the main elementary reactions involving the ammonia fuel are presented in
Table 3. Numerical simulations provide the reaction rates of the main elementary reactions in which ammonia is involved during the combustion process in the cylinder of the engine.
Figure 7 shows the curve of the reaction rates of the main elementary reactions involving ammonia during the combustion process at a diesel fuel energy ratio of 28% with respect to the crank angle. A positive reaction rate indicates the generation of ammonia in the elementary reaction, while a negative reaction rate indicates the consumption of ammonia in the elementary reaction.
The study found that the primary elementary reactions mainly involve the consumption of ammonia and a significant consumption of hydroxyl and hydrogen radicals. These radicals promote the combustion process of the fuel and accelerate the release of energy. Among these reactions, R188 and R231 are the primary elementary reactions leading to ammonia consumption. It can be observed that the reaction rates of R188 and R231 both increase with the increase in the diesel energy ratio in combination with
Figure 8. The change in trend for R231 is particularly striking. During the process of increasing the diesel energy ratio from 10% to 40%, the reaction rate of elementary reaction R188 increases from 0.21 mole/cm
3-s to 273.67 mole/cm
3-s, while the reaction rate of elementary reaction R231 increases from 4.22 mole/cm
3-s to 1286.29 mole/cm
3-s. The oxygen in the cylinder mainly provides a large number of OH radicals through consumption reactions, thereby promoting ammonia consumption. With a higher diesel energy ratio, the temperature rise rate inside the cylinder is faster, leading to earlier consumption reactions of ammonia inside the cylinder.
3.2.3. Carbon Oxide Emissions
Figure 9 and
Figure 10 show the simulated curves of the variation of the mass fractions of CO and CO
2 at different diesel energy ratios. It can be seen that as the diesel energy ratio increases, both the production rate and the emissions of CO and CO
2 increase. This is attributed to the increased amount of carbon-based fuel injected. Considering that carbon in combustion originates entirely from diesel, a higher diesel energy ratio leads to higher emissions of carbon oxides. The production of CO exhibits a trend of initially increasing and then decreasing; the production of CO
2 exceeds that of CO. At a diesel energy ratio of 28%, the production of CO
2 is approximately four times that of CO.
Table 4 and
Table 5 present the major elementary reactions involving CO and CO
2, while
Figure 11 and
Figure 12 illustrate the reaction rates of the major elementary reactions involving CO and CO
2 at a 28% diesel energy ratio. Similarly, positive and negative values indicate whether these reactions are generating or consuming CO and CO
2.
Through this study, it was found that during the combustion process, CO is primarily consumed. However, there are important reactions where it is also produced, like CO2, which is primarily produced. The consumption of CO and the generation of CO2 are mainly achieved through the reaction R30: CO + OH <=> CO2 + H, where CO is oxidized to CO2. This clearly explains why the mole fraction curve of CO increases initially and then rapidly decreases with an increase in crankshaft angle and why the production of CO2 is higher than that of CO.
Additionally, it can be observed that in different diesel energy ratio conditions, CO does not completely react to form CO
2; no other particularly intense reactions are evident in the major elementary reactions. The primary reason for this might be the setting of an equivalence ratio of 0.7 in this experiment, indicating that there is insufficient air in the combustion chamber to allow for complete ammonia reaction, thus resulting in overall lower thermal efficiency. The reaction rate of the elementary reaction R30 at different diesel energy ratios is shown in
Figure 13. Under otherwise identical conditions, a higher diesel energy ratio corresponds to a higher reaction rate for R30, indicating a more intense reaction. It should also be noted that lowering the diesel energy ratio significantly reduces carbon dioxide emissions. This is because the combustion of pure ammonia does not produce carbon dioxide emissions.
3.2.4. Nitrogen Oxide Emissions
Nitric oxide (NO) is considered a major precursor to ground-level ozone and atmospheric environmental pollution, while nitrous oxide (N
2O), as a major driver of global climate change, is a significant greenhouse gas with an impact on temperature nearly 300 times greater than carbon dioxide. It takes approximately 120 years for N
2O to completely decompose in the atmosphere [
34,
35].
Figure 14 and
Figure 15, respectively, show the mole fractions of NO and N
2O under different diesel energy ratios. The higher the diesel energy ratio, the faster the rate of NO generation. However, the final emission of NO after combustion exhibits a trend of initially increasing and then decreasing with increasing diesel energy ratio. The maximum NO emission occurs at a diesel energy ratio of 22%. As mentioned earlier, combustion inside the cylinder is not ideal at diesel energy ratios of 10% and 16%. Therefore, it can be understood that under normal combustion conditions inside the cylinder, the higher the diesel energy ratio, the lower the NO emission. When ignition conditions are reached in the cylinder, diesel is ignited first. This generates a large amount of fuel-type NO, causing the NO content to rise sharply. The higher the diesel energy ratio, the faster the rate of NO formation. When the combustion ends, the temperature and pressure inside the cylinder begin to decrease, the remaining ammonia inside the cylinder gradually reduces NO, causing the NO level to decrease. Therefore, it can be observed that under conditions of lower diesel power ratio, the curve of NO generation reaches its peak and then decreases more slowly. However, even with low-temperature combustion, the emission of NO increases at lower diesel energy ratios due to the excessive amount of ammonia in the cylinder. The final emission of N
2O also exhibits a trend of initially decreasing and then increasing. However, in conditions where the diesel energy ratio is 10% and 16%, there is residual N
2O emission in the late stage of combustion, which can be due to incomplete combustion inside the cylinder, leading to too rapid a decrease in cylinder temperature. In conditions where the diesel energy ratio exceeds 28%, although there is a small amount of N
2O emission, its concentration is nearly an order of magnitude lower than that of NO emissions and can be considered negligible.
The major elementary reactions involving NO are shown in
Table 6. The reaction rates of the major elementary reactions involving NO at a diesel fuel energy ratio of 28% are shown in
Figure 16. It can be seen that NO is both produced and consumed during the combustion process. However, the production is predominant. Several reactions influence the NO content, including R286: NO + O(+M) <=> NO
2(+M), R328: OH + NO <=> HONO, R329: H + NO
2 <=> OH + NO and R335: HNO <=> H + NO. Among these, NO generation is primarily caused by R328 and R329. In the early stages of combustion, the forward reaction rate of R328, which tends to produce HONO, is relatively fast under high-temperature conditions as the cylinder temperature increases. In the later stages, when the OH concentration in the cylinder is high, it tends to favor the reverse reaction. In addition, around CA10, R286 and R335 contribute to the consumption of NO. As the combustion continues, ammonia reacts with OH and oxygen atoms through R230 and R231 to form amino NH
2, and then NO is destroyed by NH
2. However, this process typically occurs in the low-temperature range [
36]; hence, the mole fraction of NO exhibits an initial increase, followed by a decreasing trend.
In conventional diesel engines, although combustion in the cylinder can produce N
2O, the amount produced is typically negligible. In ammonia–diesel dual-fuel engines, however, this issue must be considered because N
2O formation is one of the primary concerns associated with ammonia combustion. At low temperatures, NO
2 reacts with ammonia to form N
2O. However, as the cylinder temperature increases under high-temperature conditions, N
2O is reduced back to N
2. Reaction R294 plays a dominant role in the reduction of N
2O to N
2, as shown in
Figure 17.
Table 7 shows the main elementary reactions of N
2O during combustion. Overall, N
2O is primarily consumed during the combustion process. When the diesel energy ratio is less than 28%, the temperature in the cylinder decreases after combustion; at this point, ammonia reacts to produce N
2O again. Therefore, in
Figure 16, it can be seen that when the diesel energy ratio is below 28%, the N
2O content gradually increases in the later stages of combustion; moreover, the lower the diesel energy ratio, the higher the N
2O content.