# A Review of Early Injection Strategy in Premixed Combustion Engines
## 1 Introduction
### Early Injection Strategy  
The preparation of a homogeneous mixture is important for both HCCI and PCCI combustion. In order to allow enough time for fuel to mix with the air before combustion, the early injection strategy, by which the fuel is injected in an early stage of the compression stroke, has been applied widely in HCCI and PCCI diesel engines. The start of early injection is typically 20-200\({}^{\circ}\) before top dead center(BTDC). Based on the characteristic of HCCI and PCCI combustion, the early injection strategy can be classified as single injection and two-stage injection, as seen in Figure 2. For two-stage injection, the first injection is also called the pilot injection, and the second injection is also called the main injection. Based on the injection timing, the early injection strategy can be divided into three patterns, as seen in Figure 3: The injection closest to TDC is defined as late; that farthest from TDC is defined as early; and the one in between is defined as middle [24]. The demarcation points of these three patterns in this paper are defined as 60\({}^{\circ}\), 40\({}^{\circ}\), and 20\({}^{\circ}\) BTDC, respectively.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Conventional Combustion** & **HCCI Combustion** & **PCCI Combustion** \\ \hline Injection strategy & injection close to Top Dead & Early injection & Early injection + TDC injection \\ \hline Combustion mode & Diffusion & Premixed & Premixed + diffusion \\ \hline Ignition & Auto-ignition & Auto-ignition & Auto-ignition \\  & (controlled by injection timing) & (controlled by chemical kinetics) & (controlled by injection timing) \\ \hline Combustion temperature & Partially high & Relatively low & Relatively low \\ \hline NOx & High NOx emissions due to high combustion temperature & Low NOx emissions due to low combustion temperature & Low NOx emissions due to low temperature and exhaust gas recycling (EGR) dilution \\ \hline \multirow{2}{*}{Soot} & High soot emissions due to low diffusion combustion mode & Low soot emissions due to lean homogeneous charge & Low soot emissions due to lean homogeneous charge \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of key characteristics of conventional diesel, HCCI, and PCCI combustion.  
Figure 3: Early injection strategy divided by injection timing.  
Figure 2: Single and two-stage early injection strategy.  
Most of these early injection strategies have a certain unavoidable influence on the fuel spray formation inside the combustion chamber, further affecting the combustion and emissions of HCCI and PCCI engines. In addition, using the early injection strategy will cause a wall-wetting problem. Because of the lower temperature of gas and the density in the cylinder during the early injection period, the fuel spray will impinge on the cylinder wall or piston head due to the slow fuel vaporization rate and longer liquid penetration length. Wall-wetting mainly leads to (1) low combustion efficiency, (2) excessive soot/carbon monoxide (CO)/hydrocarbon (HC) emissions, and (3) (local) oil dilution [25; 26]. Many methods, including limiting the injection angle, have been proposed to limit or reduce wall-wetting.