# A Review of Early Injection Strategy in Premixed Combustion Engines
## 3 Injection Timing
### Two-Stage Early Injection
#### 3.2.1 First Injection Timing  
With the advance of first injection timing, the combustion mode transitions from conventional diffusion combustion to premixed HCCI combustion. Earlier first injection timing causes a more homogeneous in-cylinder mixture at the time of ignition since enough time is available to attain a large part of the premixed mixture, and the combustion process is divided into two stages, as seen in Figure 7. The first stage is HCCI combustion mode, which contains low-temperature reaction (LTR) and high-temperature reaction (HTR); the second stage begins just after the start of the second injection with the diffusive combustion because of the short ignition delay. With the advance of the first injection, the interval between the first and second stage combustion is first prolonged and then shortened. This is because with a too-early first injection, the fuel might inject into clearance and squish regions where the temperature is relatively low, and the mixture in these regions is over-lean and hard to be oxidized, resulting in a longer ignition delay. Based on the different combustion processes mentioned above, the effects of first injection timing on engine performance and emissions are discussed. Variations of BSFC of first injection timing mainly depend on the weight of the heat release process. When the first injection timing is closer to TDC, BSFC is improved. With advanced first injection timing, the combustion event shifts to the earlier side, which increases negative work, and wall-wetting becomes more serious, which causes incomplete combustion. For NOx emissions, advancing the first injection timing induces better fuel-air mixing and reduces the local over-rich phenomenon, resulting in lower combustion temperature, which decreases NOx emissions. However, the intense and fast combustion caused by premixed combustion leads to increased NOx. For soot emission, better fuel-air mixing due to earlier first injection restrains soot generation and relatively higher in-cylinder temperature in the expansion stroke because the second injection promotes the oxidation of soot emission. So the soot emission generally decreases with the advance of first injection timing. HC and CO emissions commonly increase with the advance of first injection timing because there is more fuel in crevices and wall-wetting, resulting in incomplete combustion near the cylinder wall. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Author** & **Injection Timing \(\mathbb{C}\) BTDC** & **Fuel** & **BSFC** & **NOx** & **HC** & **CO** & **Soot** \\ \hline [PERSON] et al. [(61)] & 33–24 & Diesel & na & na & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) \\ [PERSON] et al. [(49)] & 40–20 & Diesel & \(\uparrow\) & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) \\ [PERSON] and [PERSON] [(62)] & 70–20 & Diesel & \(\uparrow\) & \(\downarrow\) & na & na & na \\ [PERSON] et al. [(67)] & 80–40 & Diesel & na & \(\downarrow\) & na & na & \(\uparrow\) \\ [PERSON] et al. [(63)] & 180–20 & Diesel & \(\uparrow\) & na & na & na & \(\downarrow\) \\ [PERSON] et al. [(64)] & 180–20 & Diesel & \(\uparrow\) & \(\downarrow\) & na & na & \(\downarrow\) \\ [PERSON] et al. [(65)] & 180–20 & Diesel & na & \(\downarrow\) & \(\uparrow\) & na & \(\downarrow\) \\ [PERSON] et al. [(66)] & 200–50 & Diesel & \(\uparrow\) & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) & \(\downarrow\) \\ [PERSON] et al. [(51)] & 40–20 & Bioethanol blends & \(\uparrow\) & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) \\ [PERSON] et al. [(68)] & 40–20 & DME & \(\uparrow\) & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) & \(\rightarrow\) \\ [PERSON] et al. [(69)] & 40–20 & Gasoline & \(\uparrow\) & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) & na \\ [PERSON] and [PERSON] [(70)] & 26–20 & Diesel & \(\uparrow\) & \(\downarrow\) & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Variation of performance and emissions after advancing the early injection timing (single).