Effects of Single and Double Post Injections on Diesel PCCI Combustion

In this study, single and double post injections were applied to diesel premixed charge compression ignition (PCCI) combustion to overcome the drawbacks those are high level of hydrocarbons (HC) and carbon monoxide (CO) emissions in a single-cylinder direct-injection diesel engine. The operating conditions including engine speed and total injection quantity were 1200 rpm and 12 mg/cycle, which are the representative of low engine speed and low load. The main injection timing of diesel PCCI combustion was set to 28 crank angle degree before top dead center (CAD BTDC). This main injection timing showed 32% lower level of nitric oxides (NOx) level and 8 CAD longer ignition delay than those of conventional diesel combustion. However, the levels of HC and CO were 2.7 and 3 times higher than those of conventional diesel combustion due to over-lean mixture and wall wetting of fuel. The efficiency of PCCI combustion was also 13% lower than that of conventional combustion under single injection and no EGR conditions due to the advanced combustion phase. With fixing of the main injection timing, single and double post injections were applied with exhaust gas recirculation (EGR). The quantity of single post injection was varied from 10 % to 30 % of the total injected fuel. For double post injections, the quantity of each post injection was set to 10 % or 15 % of the total injected fuel with various post injection timings. The applying of single and double post injections resulted in a significant reduction in HC and CO compared with main injection only because the combustion by post injection helped to combust unburned species completely, and double post injection was more effective to reduce these unburned species due to shortened spray penetration distance by split of post injection. However, the level of NOx emissions was increased because the combustion by post injection occurred under high ambient pressure and temperature. For the reduction of increased NOx emissions, the post injection timing was required to be retarded more than 30 CAD after start of main injection (ASOI). Under various EGR rates, the application of single and double post injections was effective to improve the trade-off relationship between NOx and HC, CO emissions. Thus, it was possible that low NOx level with lowered level of HC and CO emissions was maintained compared to the combustion with main injection only.