# 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  
1. NOx emission characteristics of diesel and DME engines  
Figure 4 shows a comparison of the exhaust gas emission characteristics of DME and diesel engines depending on the injection speed and pressure of the engines at an injection timing of 14\({}^{\circ}\) BTDC and 25% load factor. NOx emissions in DME engines are not significantly different from those in diesel engines; however, the emission characteristics of the fuel injection pressure are different. In particular, NOx emissions are lowest at 2100 rpm in DME engines, unlike those in diesel engines. This suggests that the DME engine used in this study has an unstable combustion state because of the non-optimized fuel injection conditions, such as injection timing and amount according to the injection pressure of the fuel. Thus, the combustion state of the DME fuel, which is an oxygen fuel, is generally improved, thereby increasing the combustion temperature and NOx emission concentration [58; 59]. This is consistent with the emission characteristics of other internal combustion engines. Therefore, it suggests that it is necessary to investigate the possibility of reducing NOx emissions in DME engines using the fuel injection pressure characteristics. 2. HC emission characteristics of diesel and DME engines  
Figure 5 shows the emission characteristics of the unburned HC in DME and diesel engines depending on the engine operating speed and injection pressure with light load and injection timing of 14\({}^{\circ}\) BTDC. At the engine operating speed of 1200-1800 rpm, the HC emissions were slightly lower than the diesel engine emissions. However, at the operating speed of 2100 rpm, the HC emissions of the DME engines were about 2.5 times higher than those of the diesel engines. This indicates that the combustion characteristics of oxygenated fuel appeared in the low-speed range but in the case of 2100 rpm the emission of HC was increased compared with 1200-1800 rpm by incomplete combustion. The excessive injection amount because of the high-pressure injection and the inconsistency of the  
Figure 4: NOx emission characteristics of diesel and DME engines without pilot injection as a function of injection pressure and engine speed. injection timing was shown in the high-speed operation condition. In DME engines, the effect of fuel injection pressure on the HC emission characteristics were not significant at low speeds; however, at high speeds, the higher was the fuel injection pressure, the lower was the HC emission. To optimize the DME engines, it was necessary to optimize the fuel injection pressure and injection amount for the engine operating conditions. 3. CO emission characteristics of diesel and DME engines  
Figure 6 shows the CO emission characteristics of DME and diesel engines depending on the engine operating speed and injection pressure with light load and injection timing of 14\({}^{\circ}\) BTDC. The emission characteristics of CO were similar to those of HC. The CO emission concentration of the DME engine was slightly lower than that of the diesel engine at all operating conditions and not significantly affected by the fuel injection pressure. This was because of the combustion characteristics of the oxygenated fuel of the DME, as described in HC. 4. Smoke emission characteristics of diesel and DME engines depending on the injection pressure and number of revolutions  
Figure 7 shows a comparison of the soot concentration with the injection pressure for operating speed of DME and diesel engines at the engine operating speed of 1800 rpm, light load condition, and injection timing of 14\({}^{\circ}\) BTDC. The exhaust gas emission characteristics of DME engines were not discharged as compared to those of the diesel engine [33; 34; 60].