*3.2. Artificial Inert Species Method*

In this section the so-called artificial inert species method [29–33] is applied to analyze the chemical, thermal, and dilution effects of both ammonia and water injection. The chemical effect is promoted by the chemical reactions. The thermal effect is promoted by the properties, specially the high specific heat capacity of ammonia and water which increases the heat absorption. The dilution effect is promoted by the presence of the additive, which reduces the possibility of reaction between fuel and air.

In the case of water injection, two artificial species were added: inertH2O and inertAIR. The species inertH2O has the same properties as water but does not participate in the chemical reactions. On the other hand, the species inertAIR has the same properties as air but does not participate in the chemical reactions. According to this, the difference between the results using water and inertH2O represents the chemical effect. The dilution effect of water injection is represented by the difference between the results using inertAIR and the base case without water. Finally, the difference between the results using inertH2O and inertAIR leads to the thermal effect of water injection. Figure 10 illustrates the contribution of thermal, dilution, and chemical effects of water injection for a water injection timing of −2.1◦ CA ATDC at a 100% load. As can be seen, the chemical effect is negligible. Water injection reduces NOx by the dilution and, more significantly, thermal effects. The thermal effect is important since water absorbs heat due to its high specific heat capacity. Since water has a higher specific heat capacity than air, the specific heat capacity of the cylinder gases is increased, leading to a reduction in the in-cylinder temperature and thus to the NOx emissions. Figure 10 also illustrates the importance of the dilution effect. The presence of water reduces the interaction between fuel and air and thus deteriorate the combustion process.

**Figure 10.** Chemical, thermal, and dilution effects of water injection on NOx emissions. Water injection timing: −2.1◦ CA ATDC.

In the case of ammonia injection, two artificial species were added: inertNH3 and inertAIR. The species inertNH3 has the same properties as ammonia but does not participate in the chemical reactions. On the other hand, the species inertAIR has the same properties as air but does not participate in the chemical reactions. According to this, the difference of the results using ammonia and inertNH3 represents the chemical effect of ammonia injection. The dilution effect of ammonia injection is represented by the difference between the results using inertAIR and the base case without ammonia. Finally, the difference between the results using inertNH3 and inertAIR leads to the thermal effect of ammonia. Figure 11 illustrates the contribution of thermal, dilution, and chemical effects of ammonia injection for an ammonia injection timing of 58.4◦ CA ATDC at 100% load. As can be seen, the chemical effect is the only one responsible for NOx reduction, while the thermal and dilution effects are negligible.

**Figure 11.** Chemical, thermal, and dilution effects of ammonia injection on NOx emissions. Ammonia injection timing: 58.4◦ CA ATDC.

If ammonia is injected near TDC, particularly at −2.1◦ CA ATDC, the NOx reduction is noticeably lower, Figure 12. The thermal and dilution effects become more important and the chemical effect is reduced.

**Figure 12.** Chemical, thermal, and dilution effects of ammonia injection on NOx emissions. Ammonia injection timing: −2.1◦ CA ATDC.
