# A Review of Early Injection Strategy in Premixed Combustion Engines
## 3 Injection Timing
### Single Early Injection
The results showed that the fuel injection timing influenced the start timing of heat release, further affecting the temperature distribution before and after combustion. The overall lean fuel mixture made by the extremely early injection restrained the increased in-cylinder temperature, resulting in low NOx emissions. However, the spray wall impingement caused by early injection timing increased the unburned fuel emission, such as HC emission. [PERSON] et al. (2016) investigated the effect of injection timing on premixing and combustion in a single-cylinder diesel engine. The injection timing was varied from 50\({}^{\circ}\) to 200\({}^{\circ}\) BTDC. The results showed that the early injection produced negative work. This negative work decreased as the injection timing was advanced by 70\({}^{\circ}\) BTDC due to the extended ignition delay period and retarded combustion phasing, and this caused increased IMEP and thermal efficiency. However, at more advanced injection timing, IMEP and thermal efficiency started to decrease because of the decreased combustion temperature. Decreasing trends in NOx emissions with advancing injection timing were seen. Smoke emission decreased at advanced injection timing due to the lean mixture and low flame temperature. In contrast to smoke and NOx emissions, HC and CO emissions increased at advanced injection timing. Figure 6 shows an image of the spray formation process with different injecting timings. [PERSON] et al. (2017) investigated spray and combustion in an optical diesel engine using a narrow-angle injector with the early injection strategy, with timing set at 40\({}^{\circ}\), 60\({}^{\circ}\), and 80\({}^{\circ}\) BTDC. Results showed that for all early injection timings, the heat release rates showed a typical two-stage heat release pattern. The earliest rapid pressure increase was seen with injection timing of 40\({}^{\circ}\) BTDC, and the latest was seen with 80\({}^{\circ}\) BTDC. Figure 5: Air–fuel distribution at various injection timings [63]. Reprinted with permission from Energy & Fuels. Copyright 2019 American Chemical Society. After Start Of Injection (ASOI). Figure 6: Spray development in a constant-volume vessel: injection quantity = 11.5 mm\({}^{3}\); injection pressure = 120 MPa [66]. Reprinted with permission from Energy & Fuels. Copyright 2019 American Chemical Society. [PERSON] et al. [51] reported the emission characteristics of bioethanol-blended diesel fuel under early injection conditions. The results showed that retarding injection timing led to a decrease in ignition delay and an increase in net IMEP. NOx was reduced through advanced injection timing by the ethanol blending. This was because the injected ethanol fuel absorbed heat in the combustion chamber during the long ignition delay. Soot emission increased slightly and HC emission increased extremely with an advance in injection timing for ethanol blending due to the spray impingement. CO emission also increased because of the advanced injection timing due to the low ambient pressure, temperature, and incomplete combustion. [PERSON] et al. [68] investigated the effect of injection timing on DME combustion and exhaust emission characteristics. Results showed an overall decreasing trend of IMEP with advanced injection timing due to the early ignition. DME combustion indicated a low filter smoke number, and decreased with the advance of injection timing. NOx emission decreased rapidly with the advance of injection timing due to the lean premixed charge and the higher latent heat of DME evaporation. HC and CO emissions increased rapidly at more advanced injection timing due to the interaction of spray and piston head. [PERSON] et al. [69] investigated the effect of early injection timing on combustion characteristics in a direct injection compression ignition engine fueled with gasoline. They found that IMEP decreased monotonically as the injection timing advanced due to the increased negative work. With early injection timing, HC and CO emissions increased compared with late injection timing.