# A Review of Early Injection Strategy in Premixed Combustion Engines
## 4 Injection Angle
[96] investigated the effects of injection angle on HC and CO emissions reduction with the early injection strategy. Two injectors with different injection angles (156\({}^{\circ}\) and 70\({}^{\circ}\)) were used. Results showed that in the combustion using the narrow angle injector, the variation in peak combustion pressure was smaller than the combustion using the conventional injector. In addition, IMEP was increased and ignition delay was shortened. The narrow angle injector induced low and stable HC and CO emissions, which was the weakness in the diesel-bioethanol blend which fueled  
Figure 12: Combustion images of injection angles and injection pressures at different crank angles [52]. combustion. [PERSON] et al. [PERSON] et al. (2019) also investigated the effects of injection angle on DME combustion and exhaust emission characteristics in a common-rail diesel engine. Two narrow spray angle injectors (70\({}^{\circ}\) and 60\({}^{\circ}\)) were examined, and the results were compared with the results of a conventional injection angle (156\({}^{\circ}\)). For the single injection strategy, the combustion pressure and heat release rate of the narrow-angle injectors were increased as compared to the wide-angle injector. IMEP was higher for the narrow-angle injectors. Soot emission decreased slightly using narrow-angle injectors, and NOx emission stayed at a low level with all injection angles using the early injection strategy. HC and CO emissions decreased obviously using narrow-angle injectors due to the decreased wall-wetting fuel. For the multiple-injection strategy, the combustion pressure and heat release rate of the first injection with narrow injection angles were also higher than with the wider injection angle, leading to more thorough combustion. Also the ignition delay between first and second injections was shorter with the narrow injection angle. However, upon the second injection, the combustion pressure and heat release rate were lower with the narrow injection angle. The 70\({}^{\circ}\) injection angle showed the highest IMEP. Low soot and NOx emissions could be obtained with the narrow injection angle with earlier first injection timing. HC and CO emissions with the narrow angle were obviously better due to the reduction of wall-wetting. [PERSON] et al. [PERSON] et al. (2019) investigated the influence of injection angle on the emissions and combustion efficiency of a PCCI diesel engine. The injection angle varied from 100\({}^{\circ}\) to 148\({}^{\circ}\). Results showed that decreasing the injection angle was an efficient method to reduce HC emission. However, NOx emission was highest with the 100\({}^{\circ}\), especially when the injection timing was earlier than 30\({}^{\circ}\) BTDC. In addition, smoke emission was the highest for the 100\({}^{\circ}\), especially when the injection timing approached TDC. Simulation was done by [PERSON] et al. [PERSON] et al. (2019) to investigate combustion and emission characteristics in a small-bore HSDI diesel engine using an injector with different injection angles. Results showed that for wider-angle injection (120\({}^{\circ}\) and 130\({}^{\circ}\)), the spray targeting played an important role in determining the combustion regime. For narrow-angle injection (50\({}^{\circ}\) and 85\({}^{\circ}\)), it was found that the flow patterns were very similar regardless of injection timing. Table 7 shows a summary of the variation of performance and emissions of HCCI and PCCI engines after decreasing the injection angle. In general, decreasing the injection angle will limit or reduce the wall-wetting phenomenon, resulting in decreased HC and CO. However, soot emission is directly affected by the placement of spray targeting. Decreasing the injection angle generally is not good for the control of soot emission, but NOx emission can be suppressed by the rich fuel-air mixture and low combustion temperature.