# A Review of Early Injection Strategy in Premixed Combustion Engines
## 3 Injection Timing
### Two-Stage Early Injection
#### 3.2.1 First Injection Timing
[PERSON] and [PERSON] [75] computationally investigated the operating impact of first injection timing on the performance and emissions of a heavy-duty diesel engine. The first injection timing was swept from 30\({}^{\circ}\) to 15\({}^{\circ}\) BTDC, and the second injection was set at 9\({}^{\circ}\) BTDC. The results showed that more advanced first injection timing produced a more homogeneous in-cylinder mixture to attain a large part of the premixed mixture and higher heat release rate, resulting in increased NOx emissions. Moreover, for earlier injection timing, some of the spray missed the piston bowl. This phenomenon will cause the increase of BSFC. [PERSON] et al. [76] investigated the effect of first injection timing on a modern V6 common rail direct-injection diesel engine. The first injection timing was changed from 30\({}^{\circ}\) to 21\({}^{\circ}\) BTDC, and the second injection timing was fixed at 1.4\({}^{\circ}\) after top dead center (ATDC). Results showed that earlier first injection timing affected the intermediate ignition delay, leading to a complete combustion process. As a result, the early injection timing produced higher in-cylinder pressures, causing higher temperatures and NOx emissions but lower soot emission. [PERSON] et al. [77] investigated the sensitivity of NOx and soot emissions to early injection in PCCI diesel engines. The first injection timing was changed from 34\({}^{\circ}\) to 26\({}^{\circ}\) BTDC, and the main injection timing was kept constant at 18\({}^{\circ}\) BTDC. Soot emission generally increased as the early injection timing was brought closer to TDC, and NOx emissions remained significantly lower than conventional diesel engines for all the first injection timing. In addition, BMEP decreased with the early injection timing. [PERSON] et al. [78] investigated the optimization of a split injection strategy for light-vehicle diesel low-temperature combustion. First injection timing ranged from 10\({}^{\circ}\) to 35\({}^{\circ}\) BTDC, and the EGR rate and second injection timing were set at 50% and TDC, respectively. The results showed that as first injection timing was retarded, the typical NOx-soot trade-off was obtained, NOx emissions initially decreased but increased below 15\({}^{\circ}\) BTDC, and soot emissions increased sharply. On the contrary, BSFC decreased with the retarded first injection timing. [PERSON] et al. [79] investigated the effect of a two-stage injection strategy on the combustion and flame characteristics of a PCCI engine. The first injection timing was changed from 70\({}^{\circ}\) to 45\({}^{\circ}\) BTDC, and the second injection timing was kept constant at 5\({}^{\circ}\) ATDC. The results showed that as the first injection was close to TDC, soot and NOx emissions increased, and BSFC also increased. The best performance occurred when the first injection timing was set at 60\({}^{\circ}\) BTDC. [PERSON] et al. [50] investigated  
Figure 7: Representative heat release characteristics of PCCI combustion. L/HTC: low-/high-temperature combustion. the effect of a two-stage combustion strategy on combustion and emission characteristics. The first injection timing was varied from 70\({}^{\circ}\) to 20\({}^{\circ}\) BTDC, and second injection timing was fixed at 5\({}^{\circ}\) ATDC. Results showed that the combustion pressure of the first stage increased up to the injection timing of 40\({}^{\circ}\) BTDC and decreased after that. In addition, the ignition delay became longer when the first injection timing varied from 20\({}^{\circ}\) to 40\({}^{\circ}\) BTDC, but extended after the injection timing of 40\({}^{\circ}\) BTDC. This is because the initial combustion of the first injection timing between 20\({}^{\circ}\) and 40\({}^{\circ}\) BTDC was rapidly generated as premixed combustion. However, the air-fuel mixture of too-early injection of fuel was ignited under low ambient temperature, and the ignition delay and combustion duration were prolonged. ISFC was increased up to 40\({}^{\circ}\) BTDC by the advanced injection timing.