# A Review of Early Injection Strategy in Premixed Combustion Engines
## 2 Injection Pressure
For NOx emissions, the effect of injection pressure was complex; higher injection pressure was better for NOx reduction for more advanced injection timing. [PERSON] et al. [52] studied the effects of different injection pressures (600 and 1000 bar) on the combustion process in a high-speed direct-injection (HSDI) optical diesel engine employing the early injection strategy. They found that increasing the injection pressure resulted in higher NOx emissions because of the leaner air-fuel mixture and higher in-cylinder temperature. Spray development and interaction with the piston with different injection pressures were also investigated. [PERSON] et al. studied [53] the effects of injection pressure on a PCCI engine. The results showed that with increased injection pressure, the emissions of NOx and particulate matter (PM) declined, and the lowest point was at 700 bar. This was due to lower in-cylinder temperature. Figure 4 shows the heat release rate (HRR) variations. The increased injection pressure caused rising HRR and reduced peak in-cylinder pressure due to better air-fuel mixing. Similar results were obtained by [PERSON] [54] using numerical simulation methods; the simulation results showed that increasing the pressure led to increased mixing and decreased fuel-rich regions. [PERSON] et al. [55] showed that there was little effect on ignition delay and soot peak as the injection pressure was changed from 600 to 1400 bar. Although the higher injection pressure benefited fuel atomization and mixing, it also led to more fuel wall-wetting under early injection conditions. [PERSON] et al. [56] investigated the effect of injection pressure on the emissions and performance of a diesel engine using the two-stage injection strategy. The range of injection pressure was 1000 to 1400 bar. Results showed that IMEP and NOx emission increased, while soot decreased with the increased injection pressure because of better atomization. [PERSON] et al. [57] reported the effect of injection pressure in a DI engine running on carbon black-water-diesel emulsion. The nozzle opening was set at 200, 220, and 240 bar. The authors compared engine performance and emission differences at different injection pressures. Results showed that different injection pressures changed the fuel atomizing characteristics; when the injection pressure was 220 bar, CO, HC, and NOx were the least, and smoke had the same characteristics. [PERSON] et al. [58] studied the effect of Calophyllum inophyllum methyl ester on diesel engine performance, emissions, and combustion characteristics at different injection pressures. Their investigations with injection pressures of 200, 220, and 240 bar were carried out to analyze parameters including brake thermal efficiency (BTE), specific fuel consumption, heat release rate, and engine emissions of a direct injection diesel engine fueled with 100% biodiesel. The results showed that brake-specific fuel consumption (BSFC) decreased with increased injection pressure; meanwhile, HC, CO, and smoke opacity decreased, but NOx increased. It was also found that the maximum BTE of Calophyllum inophyllum methylester was obtained at 220 bar injection pressure during part load operation conditions because the droplet size of the fuel decreased. [PERSON] et al. [59] compared the effects of different injection pressures (200, 250, and 300 bar) on single-cylinder compression ignition (CI) engines fueled with a 20% blend of castor biodiesel in diesel. They found that under the same conditions of injection timing, as injection pressure increased, CO, HC, and smoke opacity increased but NOx emissions were significantly reduced. However, the changes of BSFC and BTE were uncertain. [PERSON] et al. [60] investigated the influence of injection pressure (200, 225, and 250 bar) on the performance and emissions of a diesel engine using biodiesel blended with diesel as the fuel. Results showed that as the injection pressure increased, the brake thermal efficiency of the engine gradually increased and BSFC decreased.