# Exhaust Gas Characteristics According to the Injection Conditions in Diesel and DME Engines
## 3 Results and Investigations
### Effects of Injection Pressure on Exhaust Gas Emission Characteristics of Diesel and DME Engines
As a result of injecting high-pressure liquid fuel DME, which is a compressible oxygen fuel, the mixing ratio with air in the injection characteristic improved as compared to that with diesel, thereby enhancing the diffusive combustion state due to inject the high-pressure fuel. Figure 5: HC emission characteristics of diesel and DME engines without pilot injection as a function of injection pressure and engine speed. Effects of Injection Pressure and Pilot Injection Period on Exhaust Gas Emission Characteristics of DME Engines  
1. NO\({}_{\rm x}\) emission characteristics of diesel and DME engines depending on the injection pressure and pilot injection period  
Figure 8 shows that the NO\({}_{\rm x}\) emissions of DME as a diesel alternative fuel was more than those of diesel near the top dead center (TDC) at the same diesel injection timing of the experimental engine. Figure 6: CO emission characteristics of diesel and DME engines without pilot injection as a function of injection pressure and engine speed. Figure 7: Smoke characteristics of diesel and DME engines without pilot injection as a function of injection pressure and engine speed. As the injection pressure increased, the amount of NO\({}_{\rm x}\) increased, and pilot injection showed that the diesel could be reduced to 20% (100 ppm) and DME to 10% (50 ppm). In this experiment, based on improving the diesel injection timing as much as possible and the pilot injection about 15% of the total injection amount, it was confirmed that NO\({}_{\rm x}\) could be reduced to about 10-20% of the total emission rate by high-pressure fuel injection and pilot injection. For pilot injection at 6\({}^{\circ}\) before the 14\({}^{\circ}\) BTDC, where combustion occurred, the thermal efficiency decreased with increasing fuel consumption rate; however, it was expected that the NO\({}_{\rm x}\) generated in a high-temperature region would be reduced because the temperature of the combustion chamber decreases. 2. HC emission concentration in diesel and DME engines depending on the injection pressure and pilot injection period  
Figure 9 shows the HC emission with pressure during high-pressure pilot and main injections. HC was reduced by more than 50% in this study as compared to those in other studies (injection pressure 20-35 MPa) [24; 26] for high-pressure pilot injection. HC was generated in the combustion chamber wall or in a state where the air-fuel ratio was rich, and it was generated in a state where the fuel was vaporized and not oxidized. Moreover, DME, as an oxygenated fuel, generated almost no HC, and generated higher fuel density per unit volume. Furthermore, unlike the emission characteristics of NO\({}_{\rm x}\), the emission rate of HC was reduced by the increase of pressure. It could be expected that, as the injection pressure increases, the injection amount in the diesel and DME engines for the injection period is optimized in the laboratory, and incomplete combustion is less likely to occur. 3. CO emission concentration in diesel and DME engines depending on the injection pressure and pilot injection period  
Figure 10 shows the CO emission rate as a function of pressure when performing high-pressure pilot and main injections. CO is generally generated in an incomplete combustion environment because of lack of oxygen in an excess fuel state, which was not a problem because DME is an oxygenated fuel, and compression ignition engines are not a big problem because they operate in a lean air-fuel ratio region. In this study, the CO emission rate was lower than that of the diesel engine. Figure 8: NO\({}_{\rm x}\) emission characteristics of diesel and DME engines with and without pilot injection as a function of injection pressure.