# A Review of Early Injection Strategy in Premixed Combustion Engines
## 6 Summary and Conclusions  
A review was conducted of the literature concerning the effects of early injection strategies on the performance and emissions of HCCI and PCCI engines. The early injection strategies covered in this review are injection pressure, injection timing, injection angle, and blending ratio of alternative fuels. Figure 13: Combustion images at different crank angles under various biodiesel blending ratios [101]. SOI: start of injection. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Author** & **Blending Ratio (\%)** & **Fuel** & **BSFC** & **NOx** & **HC** & **CO** & **Soot** \\ \hline [PERSON] et al. [99] & 20–100 & Biodiesel & na & \(\downarrow\uparrow\) & na & na & \(\uparrow\) \\ [PERSON] et al. [38] & 10–30 & Bioethanol & na & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) & \(\rightarrow\) \\ [PERSON] et al. [55] & 30–70 & Gasoline & na & na & na & na & \(\downarrow\) \\ [PERSON] et al. [100] & 68–84 & Gasoline & na & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) & \(\downarrow\) \\ [PERSON] et al. [82] & 0–15 & n-Butanol & \(\uparrow\) & na & na & \(\downarrow\) & \(\downarrow\) \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Variation of performance and emissions after increasing the blending ratio of alternative fuels. Researchers have carried out many experimental and numerical works to investigate the effects of different injection parameters, and the most important conclusions derived are summarized below:  
* Increasing the injection pressure improved the engine thermal efficiency and decreased soot emissions. This is because a higher injection pressure will result in a better fuel spray, which will make the fuel burn more completely. HC and CO emissions were determined by the paradoxical effect of high injection pressure. They are uncertain. NOx emissions increased slightly with increasing injection pressure due to the higher combustion temperature. Wetting-wall phenomenon caused by higher injection pressure can lead to the opposite results. * Advancing the injection timing of HCCI engines was beneficial in terms of better NOx emissions, but with increased HC and CO emissions. Soot emission depended on the opposite effects of longer premixing time and more serious wall-wetting. In theory, longer premixing time can reduce soot emissions, but wall-wetting can cause incomplete or low-temperature combustion that can increase soot emissions. The engine performance deteriorated with advanced injection timing due to the increased negative work and incomplete combustion. * Advancing the first injection timing of PCCI engines was beneficial in terms of better NOx and soot emissions, but HC and CO emissions increased and engine performance deteriorated. In addition, retarding the second injection timing also reduced NOx emissions. However, soot, HC, and CO emissions increased and engine performance deteriorated due to the shifting of diffusive combustion to later than TDC. From the previously cited literature, wetting-wall phenomenon must be considered when using two-stage early injection strategy, which has an important impact on engine performance and emissions. * Increasing the first injection quantity of PCCI engines increased HC and CO emissions. NOx emissions also increased due to the faster heat release rate and higher combustion temperature caused by a large portion of premixed combustion. Soot emission was determined by the opposite effects of wall-wetting, higher combustion temperature, and lower proportion of diffusive combustion. As far as experimental values go, the first injection quantity should be limited to 40% of the total injection fuel, which will obtain better engine performance and emission characteristics. * Decreasing the injection angle limited or reduced the wall-wetting phenomenon, resulted in decreased HC and CO. However, soot emission was directly affected by the location of spray targeting.