# Exhaust Gas Characteristics According to the Injection Conditions in Diesel and DME Engines
## 4 Conclusions  
Based on the results of this study on diesel and DME engines with common rail conditions applied, we examined the engine performance changes and emission characteristics of exhaust gases with varying injection pressure and injection rate. The emission characteristics of NO\({}_{\mathrm{x}}\), HC, and CO were investigated for different injection pressures of the pilot injection. The exhaust gas emission characteristics for the pilot injection period and needle lift were investigated as follows:  
1. For DME engines, the effect of fuel injection pressure on HC emissions was not significant at low speeds; however, at high speeds, higher fuel injection pressures caused lower HC emissions. To optimize the DME engine, it is necessary to optimize the fuel injection pressure and injection amount depending on the engine operating conditions.
2. The emission characteristics of CO were similar to those of HC. The CO emission concentration of DME engines was slightly lower than that of diesel engines. The fuel injection pressure did not have a significant effect at all operating conditions. This was because the combustion characteristics of the oxygenate fuel of DME described in HC. As the liquid fuel DME, which is a compressible oxygen fuel, was injected at high pressure, the mixing ratio with air was improved in the injection characteristic as compared to that with diesel. Further, it was expected to enhance into a diffusive combustion state.
3. The HC emission can be extremely low, as it was generated by oxygen deficiency in a state where the combustion chamber wall surface or the air-fuel ratio was rich, and because DME was generated in a state where the fuel was not vaporized and the oxidized fuel was an oxygen fuel. Moreover, as the injection pressure increased, the emission rate of NO\({}_{\mathrm{x}}\) reduced. This was because the injection rate in diesel and DME engines was optimized according to the injection period, and incomplete combustion is not expected.
4. The NO\({}_{\mathrm{x}}\) emission was reduced at the injection pressure of 50 MPa, with pilot injection rate increasing from 0% to 30% of the total injected amount, and the minimum and maximum injection timing being 10\({}^{\circ}\) and 22\({}^{\circ}\) BTDC, respectively. The NO\({}_{\mathrm{x}}\) emission decreased with the increasing pilot injection rate because the pilot injection before the main injection delays the rapid diffusion combustion. This was because the injection period delay lowered the combustion chamber temperature; and it was considered that NO\({}_{\mathrm{x}}\) emissions generated at a high temperature were reduced because the average effective pressure caused by the injection delay decreased.
5. It can be seen from this experiment that NO\({}_{\mathrm{x}}\) and HC were opposite to each other. Further, as the pilot injection amount increased, or as it got closer to TDC, the HC emission amount increased. When pilot injection was applied in this study, it was expected that the injection period would be delayed if more than 10% was applied, so that the HC emissions would increase because of unstable combustion.
6. The increase in injection timing was caused by the delay in injection period, occurring with the temperature rise in the combustion chamber because of diffusive combustion. The decrease of 0.14 mm and 0.36 mm of needle lift at 22\({}^{\circ}\) BTDC was caused by incomplete combustion because of the longer supply period of the characteristics of common rail for supplying the required fuel when injected with the required injection period at 0.14 mm.  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] analyzed the data; [PERSON] contributed reagents/materials/analysis tools; [PERSON] wrote the paper.  
This study was supported by the development of key fusion technology of the industry-academia research cooperation cluster support project.  
The authors declare no conflict of interest.