# A Review of Early Injection Strategy in Premixed Combustion Engines
## 3 Injection Timing
### Single Early Injection
However, NOx emissions were reduced rapidly when the injection timing was advanced in the early injection region. This was due to the lower combustion temperature caused by lean combustion. [PERSON] and [PERSON] [70] investigated the effect of injection timing on a direct injection (DI) diesel engine fueled with a synthetic fuel blend. Different injection timings were chosen from 20\({}^{\circ}\) to 26\({}^{\circ}\) BTDC to compare the effect on the performance and emissions of a DI engine. The results showed that the earlier the injection timing, the higher the maximum combustion temperature in the cylinder, which caused higher NOx emission; lower BSFC; and lower CO, HC, and soot emissions. [PERSON] et al. [71] studied reduction of NOx emissions from diesel engines using split injection. In that paper, different injection timings were chosen; with delayed injection, the temperature of the cylinder got higher, NOx emission increased, and soot decreased. [PERSON] et al. [59] studied the influence of injection timing on the performance and emissions of a single-cylinder CI engine fueled with a 20% blend of castor biodiesel in diesel. Results showed that different injection timings had different effects on the engine. [PERSON] et al. [72] also found that with earlier injection timing, soot decreased and NO increased. [PERSON] et al. [60] reported the effect of injection timing (24\({}^{\circ}\), 27\({}^{\circ}\), and 30\({}^{\circ}\) BTDC) on performance and smoke emissions of a CI engine. The results showed that with an injection timing of 27\({}^{\circ}\) BTDC, BTE was the highest and BSFC was the lowest. As for smoke opacity, the lowest value appeared at 30\({}^{\circ}\) BTDC due to advancement of injection timing, with extended ignition and decreased charge temperature and pressure. In research by [PERSON] et al. [73], according to changes of injection timing in a single-injection combustion engine, it was found that NOx tended to increase with advanced injection timing, and HC and CO emissions were the lowest at an injection timing of 15\({}^{\circ}\) BTDC, then increased again because of combustion instability. Meanwhile, with injection timing of 15\({}^{\circ}\) BTDC, IMEP and brake mean effective pressure (BMEP) were highest, which is considered the most favorable combustion condition. [PERSON] et al. [74] reported on the combined influence of injection timing (21\({}^{\circ}\), 23\({}^{\circ}\), 25\({}^{\circ}\) BTDC) and EGR on combustion, performance, and emissions of a DI diesel engine fueled with neat waste plastic oil. The results showed that at all EGR rates, when the injection timing was 21 crank angle degree (CAD) BTDC to 25 CAD BTDC, NOx emissions increased because of the higher combustion temperature and CO, HC, and smoke density decreased due to the higher oxygen content. BSFC decreased because there was ample time for fuel-air mixing. BTE was higher when the injection timing was earlier at all EGR rates. Table 3 shows a summary of the variation of performance and emissions of the HCCI engine after advancing the early injection timing. In general, advancing the injection timing results in better NOx emissions but worse HC and CO emissions. Soot emission depends on the opposite effects mentioned  
above. Engine performance deteriorates with advanced injection timing due to the increased negative work and incomplete combustion.