# A Review of Early Injection Strategy in Premixed Combustion Engines
## 3 Injection Timing
### Two-Stage Early Injection
#### 3.2.3 First Injection Quantity  
The first injection quality is in direct relation to the proportion of HCCI combustion and diffusive combustion. Increasing the first injection quantity caused more fuel and air to be mixed before second injection, which is beneficial for auto-ignition of first injection fuel, and more heat was released before second-stage combustion. So the ignition timing of the mixture was advanced, resulting in increased BSFC. Moreover, increasing the first injection quantity shortened the ignition delay and raised the combustion temperature of the second-stage combustion, which promoted the formation of NOx emissions. However, NOx emissions could be limited with the use of heavy EGR. Besides, a large first injection quantity might cause wall-wetting and more fuel to enter the region near the cylinder wall, which would aggravate the incomplete combustion, leading to more CO and HC emissions. For soot emissions, on the one hand, the wall-wetting caused by the first injection promoted its generation. On the other hand, the higher temperature and smaller proportion of diffusive combustion also decreased soot emission, but usually it was increased with increased first injection quantity. [PERSON] et al. [77] investigated the effects of first injection quantity on NOx and soot emissions in PCCI diesel engines. The first injection quantity was assessed by considering between 20% and 60% of the total fuel mass. The first injection timing was fixed at 34\({}^{\circ}\) BTDC, and second injection timing was changed from 26\({}^{\circ}\) to 8\({}^{\circ}\) BTDC. Results showed that increasing the first injection quantity resulted in decreased engine BEMP and increased BSFC. This was because with the increased first injection quantity, the combustion phasing advanced and less fuel was burned with proper combustion phasing. NOx emissions decreased as the first injection quantity increased. Soot emissions changed slightly with the change of first injection quantity. [PERSON] et al. [84] investigated the effects of first injection quantity on combustion and emissions under a high EGR rate in a diesel engine. First injection quantity ratios were chosen from 10-20%. The first and second injection timings were set at 44\({}^{\circ}\) and 14\({}^{\circ}\) BTDC, respectively. Results showed that increasing the first injection quantity led to increased cylinder pressure and advanced due to more fuel being mixed with air, which resulted in better combustion performance. Increasing the first injection quantity also increased NOx, HC, and CO emissions. There \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Author** & **First Injection Timing** & **Second Injection Timing** & **Fuel** & **BSFC** & **NOx** & **HC** & **CO** & **Soot** \\  & **(\(\,\)**PTDC**)** & **(\(\,\)**PTDC**)** & **(\(\,\)**PTDC**)** & & & & & & \\ \hline [PERSON] et al. [89] & 240 & 30-15 & [PERSON] & na & \(\downarrow\) & \(\uparrow\) & na & na \\ [PERSON] et al. [77] & 34 & 26-8 & [PERSON] & \(\downarrow\) & \(\downarrow\) & na & na & \(\uparrow\) \\ [PERSON] and [PERSON] [1] & 200 & 20-TC & [PERSON] & \(\uparrow\) & na & na & na & na \\ [PERSON] et al. [21] & 56 & 18-5 & [PERSON] & \(\uparrow\) & na & na & na & \(\uparrow\) \\ [PERSON] et al. [79] & 60 & 5 to \(-75\) & [PERSON] & \(\rightarrow\) & \(\downarrow\) & na & na & \(\uparrow\) \\ [PERSON] and [PERSON] [62] & 60 & TDC to \(-20\) & [PERSON] & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) & na \\ [PERSON] et al. [86] & 80 & 20-TC & DME & na & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) & na \\ [PERSON] et al. [82] & 42 & 10 to \(-25\) & n-butanol & \(\uparrow\) & na & na & \(\uparrow\) & \(\downarrow\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Variation of performance and emissions after retarding the second injection timing (two-stage). was no obvious difference in smoke emissions when the first injection rate was changed. [PERSON] et al. [50] investigated the effects of a two-stage combustion strategy on combustion and emission characteristics.