# 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 mass was varied from 2 to 10 mg/cycle. The first and second injection timings were fixed at 50\({}^{\circ}\) and \(-5^{\circ}\) BTDC, respectively. When the first injection quantity was increased, the peak pressure increased and the heat release of the first stage increased gradually and the ignition delay decreased. NOx first decreased then increased as the first injection mass increased. The lowest NOx emission was obtained when the first and second injection quantities were the same, and soot emission increased rapidly as the first injection quantity increased. [PERSON] et al. [79] investigated the effects of the first injection rate in a PCCI engine. The fuel mass portion of the first injection was varied from 10% to 50% of the total fuel mass with the first and second injection timing fixed at 60\({}^{\circ}\) and \(-5^{\circ}\) BTDC. The results showed that both the fuel mass part of the first injection and the cylinder peak pressure increased, which resulted in a shorter ignition delay for the second combustion, causing rapid combustion before TDC and increased BSFC. PM emission increased as the first injection rate increased because of the shorter ignition delay time before the second combustion occurred and the wall-wetting issue. On the contrary, NOx emissions decreased because of the lower rate of heat release (ROHR) peaks of the second combustion when the first injection quantity was increased. As shown in Figure 10, the first injection portion of 20% seemed to cause a brighter flame than that of 30%, meaning a higher heat release rate and higher peak combustion temperature. At the first injection portion of 30%, neither the rapid combustion flame nor the diffusion flame appeared in the combustion chamber owing to the uniform mixture distribution. When the first injection portion was 40% and 50%, a diffusion flame generating soot appeared due to the short ignition delay and mixing time. [PERSON] et al. [90] investigated the effects of fuel quantity during first injection on the emissions of a DI diesel engine under moderate and heavy EGR. Under moderate EGR (\(\sim\)30%), the quantity of the first injection was changed from 1 to 5 mg/cycle. The first injection timing was changed from 100\({}^{\circ}\) to 10\({}^{\circ}\) BTDC and second injection timing was fixed at TDC. The gas temperature in the cylinder was reduced when the first injection quantity increased, resulting in lower NOx emissions. However, because of the early timing of the first injection, most of the first injection fuel was not included in the main combustion process. As a result, HC emissions increased as the first injection quantity increased, and BSFC deteriorated. Soot emissions were also higher as the first injection quantity increased, and an early and small first injection was helpful to reduce the soot emission. Under heavy EGR (\(\sim\)60%), the amount of the first injection ranged from 1 to 10 mg/cycle. The first injection timing was changed from 45\({}^{\circ}\) to 25\({}^{\circ}\) BTDC and the second injection timing was fixed at 15\({}^{\circ}\) BTDC. CO, HC, and PM emissions decreased as the amount of the pilot injection increased. A large pilot injection closer to the main injection was recommended. The contrary results between moderate and heavy EGR were due to the differences in ignition delay and air-fuel mixture conditions. [PERSON] et al. [83] investigated the effects of first injection quantity in a DI-diesel engine fueled with diesel from direct coal liquefaction. The first injection quantity was varied from 2 to 6 mm\({}^{3}\)/cycle, timing was changed from 44.5\({}^{\circ}\) to 7.5\({}^{\circ}\) BTDC, and the second injection timing was fixed at \(-1^{\circ}\) BTDC. Results showed that with increased first injection quantity, the maximum heat release rate of first-stage combustion and the peak in-cylinder pressure rose; moreover, the maximum heat release rate of the second stage was reduced. These changes shortened the ignition delay of second-stage combustion.