# Exhaust Gas Characteristics According to the Injection Conditions in Diesel and DME Engines
## 3 Results and Investigations
### Effects of Pilot Injection Rate on Exhaust Gas According to Injection Time in DME Engines  
In this study, the emission characteristics of HC, NO\({}_{\rm x}\), CO, and carbon dioxide (CO\({}_{2}\)) were analyzed for the engine performance tests. DME showed unleaded combustion and no special combustion method was required for exhaust gas reduction. However, NO\({}_{\rm x}\) and HC vary depending on the operating conditions and fuel characteristics; thus, the reduction rate was investigated for each pilot injection rate, injection timing, needle lift, and injection timing in this experiment. Figure 10: CO emission characteristics of diesel and DME engines with and without pilot injection as a function of injection pressure. Figure 9: HC emission characteristics of diesel and DME engines with and without pilot injection as a function of injection pressure. NO\({}_{x}\) emission characteristics of DME engines depending on the injection pressure and pilot injection period  
Figure 11 shows the effect of pilot injection period on NO\({}_{x}\) emissions with injection timing; other studies [35; 39; 46] have shown that the NO\({}_{x}\) emission rate of DME as a diesel substitute fuel decreases as it approaches the TDC. Therefore, in this experiment, the pilot injection rate was examined from 0% to 30% of the total injection amount with the injection pressure of 50 MPa and the injection timing at the lowest was 10\({}^{\circ}\) BTDC and at the highest was 22\({}^{\circ}\) BTDC. As a result, NO\({}_{x}\) decreased as the pilot injection rate increased. This was because the pilot injection before the main injections slowed the diffusion combustion because of the injection period delay. Furthermore, it reduced the combustion chamber temperature and average effective pressure because the injection delay decreased, thereby reducing the emission of NO\({}_{x}\) generated at high temperatures. NO\({}_{x}\) was mostly composed of nitrogen monoxide (NO) and nitrogen dioxide (NO\({}_{2}\)), which were produced by the combination of nitrogen molecules in the air and oxygen at high temperatures. NO\({}_{x}\) in the compression ignition engine were divided into thermal NO and fuel NO, oxidized by the nitrogen contained in the fuel, most of which was thermal NO. The generation mechanism was based on the Zeldovich mechanism and had a mechanism (N\({}_{2}\) + O \(\rightarrow\) NO + N) in which NO molecules dissociated during combustion in an excess air atmosphere to produce NO. The generation of NO by this dissociation was a strong endothermic reaction, which exponentially increased at high temperatures and was discharged during the expansion stroke. That is, the concentration of NO was greatly dominated by the combustion temperature, and the reaction time and oxygen concentration were also important factors. 2. HC emission characteristics of DME engines depending on the injection timing and pilot injection rate  
Figure 12 shows the effect of pilot injection rate on HC generation with injection timing. The comparison in Figure 12 shows that NO\({}_{x}\) and HC were opposite to each other. Furthermore, the pilot injection quantity increased and, as the TDC was approached, the HC emission amount increased [47; 49]. This was because, when injecting more than 10% of the pilot injection, the injection period was delayed, and the amount of HC emission increased because of unstable combustion. Figure 11: NO\({}_{x}\) emission characteristics of DME engines with the pilot injection duration rate as a function of injection timing. Moreover, HC were generated by unstable combustion of fuel, generated because of lack of oxygen in a state where the combustion chamber wall surface or air-fuel ratio was rich, and discharged without being oxidized. It also occurred when the fuel was evaporated during fueling or when the fuel was burning unsafely, similar to CO.