Investigation on Premixed Charge Compression Ignition Combustion Control Using Multi Pulse Ultrahigh Pressure Injection

Compression ignition (CI) engines provide higher thermal efficiency compared to other internal combustion engines although large amounts of NO_x and soot are produced during combustion. NO_x and soot emissions can be reduced by using Premixed Charge Compression Ignition (PCCI) combustion. However, the problems of PCCI combustion include limited operating range, unstable start of combustion and an increase in combustion noise. The multi-pulse ultrahigh pressure injection allows fuel to be injected near TDC, improving mixture formation and enhancing the possibility to extend the operating range of PCCI combustion. The objective of this paper is to control and extend the operating range of PCCI combustion using multi-pulse ultrahigh pressure injection. This has not been studied before. Combustion characteristics were investigated using apparent rate of heat release analysis, heat balance analysis, exhaust emission measurement and soot concentration measurement. The experiments were carried out on a single cylinder engine at 0.7MPa gross IMEP at 2,000 rpm. The injection pressures were varied at 200, 250, 300 and 350MPa to investigate the effects of injection pressure and shortened fuel injection timing. The injection pulses were varied at 2 pilot injections followed by 2, 3 and 4 pulses of the main injection. The main injection timings were varied from -14 to 4.5 TDC to compensate for combustion duration which was prolonged by the multi-pulse injection. The results showed that increasing main injection pulse resulted in increased thermal efficiency and smoke. It also resulted in decreased NO_x. In the case of 2 pulses main injection, the heat release rate at 1st pulse is PCCI. For 3 and 4 pulses main injection, 1st, 2nd pulse are PCCI followed by diffusive combustion. Increasing injection pressure resulted in decreased NO_x and smoke. A combination of increasing injection pressure combined with injection pulse resulted in increased thermal efficiency, while simultaneously decreasing NO_x and smoke.