Effects of carbon dioxide in-cylinder injection on premixed charge compression ignition combustion

In this study, two fuel injections were used to realize premixed charge compression ignition combustion. The first was during the negative valve overlap period; the second was just before the compression top dead centre. First, the effects of the second fuel injection timing are discussed, and then carbon dioxide (CO2) in-cylinder injection parameters, including injection timing, injection quantity, and injection pressure, are discussed. In addition, CO2 in-cylinder and intake pipe injection are also compared in the study of injection quantity. The results revealed that, for CO2 in-cylinder injection, either advancing CO2 injection timing or increasing cyclic injection quantity can greatly reduce the emission of nitrogen oxides (NOx) with little impairment of the thermal efficiency. With increase in the CO2 injection quantity, intake pipe injection has a similar effect to external exhaust gas recirculation, such as NOx emission reduction accompanied by smoke increase; while CO2 in-cylinder injection has a weaker effect in reducing NOx emission than intake-pipe injection does, it has little effect on smoke. Generally, with the same CO2 injection quantity, higher injection pressure results in higher momentum flux and total momentum. Since larger momentum flux and momentum have a stronger disturbance on the air-fuel mixture, resulting in a more homogeneous mixture, a larger injection pressure leads to lower NOx emission.