Optimization of Recompression Reaction for Low-Load Operation of Residual-Effected HCCI 2008-01-0016
In-cylinder pre-processing (or recompression reaction) of direct-injected fuel during the negative valve overlap period of a retention-strategy HCCI engine is investigated for extension of the low-load limit of operation. Experimental studies of three variables (compression ratio, pilot injection timing, and pilot injection amount) were conducted in order to optimize the effects of recompression reaction by changing the sensible and chemical energy environment during recompression. The results from compression ratio variation show that there exist optimum values of equivalence ratio and extent of recompression reaction, which expand the low-load operating region. The pilot injection timing variation demonstrates good controllability of the extent of recompression reaction by effectively changing the in-cylinder residence time of the pilot-injected fuel. Higher extent of recompression reaction results in advanced main combustion timing, which exhibits better stability but slightly reduced efficiency due to enhanced heat transfer. The sensitivity of combustion timing to pilot injection timing increases for less stable operating conditions, indicating that recompression reaction plays an important role in improving engine stability. Variation of the amount of pilot injection (split ratio) can also influence the extent of recompression reaction, but shows limited controllability, depending on in-cylinder conditions. It is also shown that only a small extent of recompression reaction is required to achieve stable operation under conditions where incomplete combustion and high cyclic variation would occur otherwise.