# A Review of Early Injection Strategy in Premixed Combustion Engines
## 3 Injection Timing
### Two-Stage Early Injection
#### 3.2.1 First Injection Timing
However, it was maintained at a similar value in a range from 40\({}^{\circ}\) to 70\({}^{\circ}\) BTDC of injection timing. NOx emissions were continuously decreased when first injection timing was advanced, and there was no particular effect on soot emission. CO and HC emissions were increased when injection timing was advanced due to the wall-wetting phenomenon. [PERSON] and [PERSON] [80] investigated the combustion and emission characteristics of direct-injection compression ignition engines by means of two-stage early fuel injection. The first injection timing was varied from 70\({}^{\circ}\) to 110\({}^{\circ}\) BTDC. Results showed that the NOx concentration was very low, less than 100 ppm, at an earlier first injection timing of 70\({}^{\circ}\) BTDC. NOx and soot emissions decreased as the first injection timing was advanced, but ISFC increased. In addition, the chemical species of formaldehyde (HCHO) was discussed; with the advance of first injection timing, HCHO emission increased. This was mainly because the cool flame with HCHO was quenched at the wall region with low in-cylinder temperature and not all the HCHO transitioned to hot flame for early first injection timing. [PERSON] and [PERSON] [81] investigated the combustion and emission characteristics of a single-cylinder PCCI engine using two-stage diesel fuel. The first injection timing was varied from 50\({}^{\circ}\) to 250\({}^{\circ}\) BTDC and the second injection timing was fixed at 20\({}^{\circ}\) BTDC. Results showed that extremely advanced timing of 200\({}^{\circ}\) and 250\({}^{\circ}\) BTDC showed higher IMEP values than retarded first injection timing. The highest IMEP with 200\({}^{\circ}\) BTDC was mainly because of the highly increased peak rate of heat release and longer ignition delay and combustion duration, which led to the main heat release stage being closer to TDC. [PERSON] et al. [68] investigated the effects of a two-stage injection strategy on DME combustion and exhaust emission characteristics. The first injection timing was changed from 35\({}^{\circ}\) to 15\({}^{\circ}\) BTDC, and the second injection timing was fixed at 5\({}^{\circ}\) BTDC. NOx emissions were gradually reduced according to advanced first injection timing. Soot emission was similar for all first injection timings. The concentrations of HC and CO emissions were linearly and simultaneously increased as the first injection timing advanced. [PERSON] et al. [82] experimentally investigated the effect of injection intervals on the performance and emissions of two-stage injection HD diesel fueled by n-butanol/diesel. The second injection timing was set at 5\({}^{\circ}\) BTDC. Soot emission decreased, while CO emission increased with the extended injection interval. In addition, the first-stage heat release became inconspicuous when the injection interval was longer than 24 CAD. [PERSON] et al. [83] investigated the effects of first injection timing on combustion, performance, and emission characteristics in a DI-diesel engine fueled with diesel from direct coal liquefaction under second injection timing at 1\({}^{\circ}\) ATDC. The first injection timing was changed from 45.5\({}^{\circ}\) to 8.5\({}^{\circ}\) BTDC. Results showed that advancing the first injection timing advanced the start of first-stage combustion and increased the peak in-cylinder pressure, and the first-stage combustion became inconspicuous when the injection interval was longer than 25 CAD. NOx and soot emissions deceased with the advance of first injection timing. However, BSFC rose due to the earlier first injection. [PERSON] et al. [84] investigated the effects of first injection timing on combustion and emissions under high EGR rates in a diesel engine fueled by blends of two fuels (gasoline and n-butanol). The first injection timing was changed from 65\({}^{\circ}\) to 25\({}^{\circ}\) BTDC, and the main injection timing was kept constant at 14\({}^{\circ}\) BTDC.