# A Review of Early Injection Strategy in Premixed Combustion Engines
## 2 Injection Pressure  
The injection pressure has a direct effect on the performance and emission formation of HCCI and PCCI engines. On the one hand, higher injection pressure results in shorter injection duration and longer premixing time before the onset combustion. The increased injection pressure leads to better atomization of the fuel, which causes better air-fuel mixing and fewer fuel-rich regions. This results in a higher heat release peak, with a rapid burn rate and a shorter combustion duration, which is beneficial for engine thermal efficiency. In addition, the in-cylinder temperature for higher injection pressure is higher than that for low injection pressure. The better air-fuel mixing and higher in-cylinder temperature benefit the oxidation of soot, CO, and HC. On the other hand, the length of spray penetration increases under higher injection pressure. This can cause serious spray-wall impingement, because both the in-cylinder temperature and pressure are low for the early injection duration. Fuel impingement on the piston head or cylinder wall leads to incomplete fuel vaporization and oxidation, which creates either over-rich or over-lean regions. This conflicts with the advantage of atomization with higher injection pressure mentioned above. This paradoxical effect finally determines the emission levels of soot, CO, and HC. For NOx emissions, the effect of injection pressure is complex. Increased injection pressure raises the in-cylinder temperature, resulting in increased NOx emissions. On the contrary, higher injection pressure means a more homogeneous mixture, which is beneficial for realizing the low temperature HCCI combustion mode and reducing NOx emissions. So a higher injection pressure is better for NOx reduction for more advanced injection timing. [PERSON] [47] studied the effects of five injection pressures (800, 1000, 1200, 1400, and 1600 bar) on the emissions and thermal efficiency of a single-cylinder diesel engine. Results showed that with injection pressure from 800 to 1600 bar, smoke mass was progressively lowered to near zero values and HC and CO emissions were increased. [PERSON] et al. [48] studied the effect of injection pressure on the performance and emission characteristics of a premixed diesel combustion engine. The injection pressure was set from 300 to 1200 bar in increments of 300 bar. The increased injection pressure caused better NOx emissions, while CO and HC increased, especially when the injection timing was close to TDC. The fuel consumption decreased first and then increased with increasing injection pressure. [PERSON] et al. [49] investigated the influence of injection pressure on performance and emissions of a PCCI diesel engine. Two injection pressures, 800 bar and 1400 bar, were studied. The results showed that the higher injection pressure led to higher in-cylinder pressure, a higher heat release peak, and shorter combustion duration due to better atomization. The indicated thermal efficiency and indicated mean effective pressure (IMEP) were also higher with higher injection pressure. Soot and HC emissions decreased, while NOx emissions increased with increased injection pressure due to better atomization and higher in-cylinder temperature. [PERSON] et al. [50] investigated the effect of injection pressure on the combustion and emission characteristics of a direct injection (DI) diesel engine using a two-stage combustion strategy. The injection pressure was changed from 500 bar to 900 bar. Results showed that there were low NOx emissions, even though the injection pressure was increased. In addition, as the injection pressure increased, soot emissions were steadily reduced due to the better mixing between air and fuel. [PERSON] et al. [51] also reported the emission characteristics of bioethanol-blended diesel fuel at early injection with different injection pressures (400 and 1200 bar). Results showed that soot decreased obviously with the increased injection pressure, while HC and CO emissions increased, especially for more advanced injection timing.