*3.1. NOx Reduction*

First of all, it is necessary to determine the appropriate quantity of ammonia and water. Regarding ammonia, its main disadvantage is the non-reacted ammonia slip in the exhaust gas. Ammonia is highly toxic, and thus it is important to maintain an un-reacted ammonia slip to the exhaust that is as low as possible. Figure 6 represents the NOx reduction as well as the ammonia slip in the exhaust gas against the ammonia to fuel ratio, Equation (7), at 100% load. In this figure, the ammonia injection took place 58.4◦ CA ATDC. This value was chosen because it provides the maximum NOx reduction, as will be shown below. The e ffect on CO, HC, and SFC remained practically negligible, so these are not represented in this figure. As can be seen, NOx reduction improves with the ammonia to fuel ratio, with a tendency to level o ff around 4%. Ammonia to fuel ratios higher than 3% provide a few additional NOx reductions with a considerable increment of un-reacted ammonia emitted to the atmosphere, and NOx reduction drops again for higher ratios since ammonia itself oxidizes to NO. For this reason, an ammonia to fuel ratio of 4% was employed in the remainder of the present work.

$$\text{Annmonia to fuel ratio } (\%) = \frac{\text{mass of ammonia}}{\text{mass of fuel}} \ 100\tag{7}$$

**Figure 6.** NOx reduction and ammonia slip against the ammonia to fuel ratio. Ammonia injection timing: 58.4◦ CA ATDC.

As indicated previously, the present work focuses on water injection through a dual nozzle with separated needles for water and fuel. Using this DWI system, typical water to fuel ratios, Equation (8), in practical applications are within the range 40%–70% [20]. Figure 7 shows the NOx reduction, as well as the e ffect on SFC, CO, and HC for water to fuel ratios from 0 to 100% at 100% load. As can be seen in this figure, the water to fuel ratio improves NOx reduction, but increments both consumption and emissions of CO and HC. For this reason and taking into account usual practical applications, a water to fuel ratio of 70% was employed in the remainder of the present work.

$$\text{Water to fuel ratio } (\%) = \frac{\text{mass of water}}{\text{mass of fuel}} 100 \tag{8}$$

**Figure 7.** NOx reduction against the water to fuel ratio. Water injection timing: −2.1◦ CA ATDC.

Figure 8 shows the effect of the injection timing on NOx reduction. Ammonia and water were compared using 4% ammonia to fuel ratio and 70% water to fuel ratio. As can be seen in this figure, using water injection a maximum 57.1% NOx reduction was obtained at −2.1◦ CA ATDC. On the other hand, if ammonia is injected around TDC, then the NOx reduction is considerably smaller than when using a water injection. Nevertheless, at 58.4◦ CA ATDC, NOx reduction reaches 78.1% using ammonia. As mentioned in the introduction, NOx reduction using ammonia is very sensitive to the temperature. Injected near TDC, ammonia is not efficient due to the excessive in-cylinder temperatures. Nevertheless, at 58.4◦ CA ATDC, the in-cylinder temperatures reduce to the optimal values required for NOx reduction using ammonia. Instead of 100% loads, at lower loads the in-cylinder temperatures are lower too and thus the optimum injection time takes place before 58.4◦ CA ATDC.

**Figure 8.** NOx reduction against the injection timing using ammonia and water. Ammonia to fuel ratio: 4%, water to fuel ratio: 70%.

Figure 9 shows the maximum temperature for the base case without a water or ammonia injection, with a 4% ammonia to fuel ratio and with a 70% water to fuel ratio at 100% load. In these simulations, both ammonia and water were injected at −2.1◦ CA ATDC. As can be seen, water promotes a reduction in the combustion temperatures. The maximum temperature is lowered 93.2 ◦C if 70% water is injected at −2.1◦ CA ATDC. On the other hand, ammonia increases the maximum temperature 8.4 ◦C if this is injected at −2.1◦ CA ATDC. This explains the effect on CO, HC, and SFC. As indicated above, ammonia has a negligible effect on these parameters and water increases them. Water reduces the combustion temperature due to the increment in the specific heat capacity of the cylinder gases (water has higher specific heat capacity than air) and lowers the concentration of oxygen, which reduces the availability of oxygen for the NOx forming reactions. The main effect is a reduction in NOx emissions due to the lower temperatures, but water injection also promotes incomplete combustion and thus increases both CO and HC emissions as well as SFC. SFC is increased due to the lower pressures, which promotes lower power. On the other hand, when injected near TDC, ammonia acts as a fuel and slightly increases the combustion temperature with a negligible effect on CO, HC and SFC. In the next section, the chemical effect of water and ammonia will be analyzed too using the artificial inert species method.

**Figure 9.** Maximum temperature without water nor ammonia injection; with a 4% ammonia to fuel ratio and an injection timing of −2.1◦ CA ATDC; water to fuel ratio: 70% and injection timing: −2.1◦ CA ATDC.
