# A Review of Early Injection Strategy in Premixed Combustion Engines
## 1 Introduction
### Conception of HCCI and PCCI Combustion  
Figure 1 shows an illustrative \(\phi\)-T diagram with conventional, HCCI, and PCCI combustion [11]; in the figure, \(\phi\) means equivalence ratio and T means temperature.  
The conventional diesel combustion process can be classified into four major phases: ignition delay, premixed combustion, mixing controlled combustion, and the late burning phase [12]. During the premixed combustion phase, poly aromatic hydrocarbons, the precursors of soot, are quickly formed in the hot (1600-2000 K), fuel-rich combustion regions. Soot formation follows, filling the entire downstream jet cross-section. Near the peak heat release rate of premixed combustion, a diffusion flame forms in the periphery of the fuel-rich, high-temperature downstream regions of the jet. NOx emission forms in the hot (1800-2000 K) and near-stoichiometric mixtures in the periphery of the jet near the diffusion flame. So the conventional combustion phase regime encompasses both NOx and soot islands.  
HCCI combustion was first proposed by [PERSON] et al. [13] and [PERSON] et al. [14]. The main characteristic of HCCI is a (more or less homogeneous) premixed air-fuel mixture that undergoes auto-ignition as a result of compression. The auto-ignition allows the combustion of a very lean mixture, which helps to eliminate the fuel-rich region, resulting in low soot emission. The combustion temperature is significantly lower than that of conventional diesel combustion, which is beneficial for the reduction of NOx emissions. However, a major difficulty in HCCI is to get a homogeneous admixture of air and fuel. This is especially true for diesel engines, because the lower volatility of diesel fuels makes it more difficult to obtain a homogeneous mixture compared to gasoline fuels. Besides, the high cetane number of conventional diesel fuel results in large rates of pressure rise and difficulties in combustion phasing control [15; 16; 17; 18; 19].  
PCCI combustion has been described as a middle path between conventional and HCCI combustion modes [20; 21; 22; 23]. In HCCI combustion, there is the challenge of combustion phasing control and homogeneous mixture preparation. To overcome these problems, for PCCI combustion, only part of the fuel undergoes the HCCI type of clean combustion, while the remainder undergoes conventional combustion. Since the remaining fuel undergoes conventional combustion, the combustion phasing is still controlled by the injection timing. Also, only partial fuel is used to prepare the homogeneous mixture, so the mixture preparation for PCCI combustion is simpler than for HCCI combustion. The part of the premixed fuel results in peak equivalence ratios staying below the soot formation threshold. Further, high levels of EGR are often used to decrease the oxygen concentration and lower peak flame temperatures, resulting in movement of the NOx island.  
A comparison of the key characteristics of conventional diesel, HCCI, and PCCI combustion are presented in Table 1.  
Figure 1: \(\phi\)-T diagram of conventional combustion, homogeneous charge compression ignition (HCCI) combustion, and premixed charge compression ignition (PCCI) combustion [11]. \(\phi\), equivalence ratio; T, temperature.