# A Review of Early Injection Strategy in Premixed Combustion Engines
## 4 Injection Angle  
Wall-wetting caused by the early injection strategy directly influenced the performance and emissions of the HCCI diesel engine. Limiting the injection angle has been proved to be a useful approach to reduce the wall-wetting phenomenon. For the strategy with early injection timing, with a decreased injection angle, the distance between the nozzle and the cylinder wall increased relatively and more fuel was atomized before reaching the cylinder, leading to reduced wall-wetting fuel mass. For the strategy with middle injection timing, the spray with narrow injection angle was confined in the piston bowl, and the spray targeting the piston bowl varied with different injection angles. The magnitude and direction of the spray rotation in the bowl were directly affected by the injection angle, as shown in Figure 11. This difference further impacted the fuel-air mixing in the piston bowl and finally impacted combustion and emissions. As mentioned in Section 3.1, the impingement target is an important factor influencing emissions and is commonly determined by the injection timing, injection angle, and piston structure. Many researchers have shown that when the spray impinges at the bowl lip bottom edge, the secondary atomization process is enhanced, which benefits the fuel-air mixing. For the second injection of the two-stage injection strategy, using a narrow injection angle against the fuel-air mixing was proved, because the spray targeting moved to the inside part of the piston bowl, where the air motion was weak. Furthermore, the fuel film deposition of the narrow injection angle was stronger, resulting in high soot generation and incomplete combustion. [PERSON] and [PERSON] [62] investigated the effects of a narrow fuel spray angle on improved exhaust emissions in a HCCI diesel engine with an early injection strategy. Two injector nozzles with different spray cone angles (156\({}^{\circ}\) and 60\({}^{\circ}\)) were used in the study. Results showed that in contrast to the conventional injector, the ISFC indicated a modest decrease when the injection timing was advanced to 50-60\({}^{\circ}\) BTDC in the case of a narrow angle injector. In addition, using dual-injection strategy with narrow angle fuel injection made it possible to reduce CO emissions, maintaining high thermal efficiency and low NOx emissions. [PERSON] et al. [52] investigated the effects of injection angles on the combustion process using multiple injection strategies in an HSDI optical diesel engine. Two injector tips with different injection angles, 70\({}^{\circ}\) and 150\({}^{\circ}\), were used. The results showed that after the first injection, the maximum cylinder pressure and heat release rate were slightly higher for 70\({}^{\circ}\) than 150\({}^{\circ}\). However, after the second injection, the cylinder pressure peaks and heat release rate were lower for 70\({}^{\circ}\) than 150\({}^{\circ}\). The combustion images for different injection angles and pressures at different crank angles are shown in Figure 12. Nonluminous flame was seen for the first injection of 150\({}^{\circ}\), while two types of flame, nonluminous and luminous film combustion flame, could be seen for 70\({}^{\circ}\). Ignition occurred near the spray tip in the vicinity of the bowl wall for the 150\({}^{\circ}\) tip, but it was near the injector tip in the central region of the bowl for the 70\({}^{\circ}\) tip. More soot luminosity was observed with the 70\({}^{\circ}\) tip due to fuel film combustion. On the other hand, the fuel film combustion led to lower NOx emissions due to its rich mixture. [PERSON] et al. [63] investigated the effects of injection angle on the