Effect of direct in-cylinder CO2 injection on HCCI combustion and emission characteristics

Fuel injection during negative valve overlap period was used to realize diesel homogeneous charge compression ignition (HCCI) combustion. In order to control the combustion, CO₂ in-cylinder injection was used to simulate external EGR. Effects of CO₂ injection parameters (injection timing, quantity, pressure) on HCCI combustion and emission characteristics were investigated. Experimental results revealed that CO₂ in-cylinder injection can control the start of combustion and effectively reduce NO_x emission. Either advancing CO₂ injection timing or increasing CO₂ injection quantity can reduce peak cylinder pressure and mean gas temperature, delay the starts of low temperature reaction (LTR) and high temperature reaction (HTR), and lower pressure rise rate; NO_x emission was reduced, while smoke, HC, and CO emissions increased. Since the combustion phase was improved, the indicated thermal efficiency was also improved. Injection pressure determines the amount of disturbance introduced into the cylinder. Generally, with the same injection quantity, higher injection pressure results in higher momentum flux and total momentum. Larger momentum flux and momentum has a stronger disturbance to air-fuel mixture, resulting in a more homogeneous mixture; therefore, larger injection pressure leads to lower NO_x and smoke emissions.