Search Results

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.

Studies have been conducted to assess the potential of variable valve actuation to initiate homogeneous charge compression ignition (HCCI) through reinduction of exhaust from the previous combustion cycle. As opposed to strategies which induce HCCI through use of either intake or exhaust throttling, use of exhaust reinduction incurs no pumping penalty, making it particularly attractive as a method for achieving efficient, light-load combustion. Using a fully flexible electrohydraulic valve actuation system, tests were conducted on a single-cylinder research engine using three strategies: late exhaust valve closing, late intake valve opening (used in conjunction with the exhaust valve being left open throughout the intake stroke), and a combination of the two. Results show that IMEP values from ∼30-55% of unthrottled SI combustion output could be obtained by varying the valve timings used to implement reinduction.

Residual-effected HCCI is investigated using a single-cylinder research engine equipped with fully-flexible variable valve actuation. Dilution limits are explored with various valve profiles in order to gain insight into the best way to use exhaust residual to achieve and control HCCI. The tests repeatedly point out the importance of delayed combustion phasing to reduce thermal losses and maximize efficiency. Combustion phasing is not significantly affected by charge in-cylinder residence time, but is strongly influenced by both the level of exhaust residual and by valve strategies that aim to affect homogeneity. Further dilution with air shows little promise for reaching lower loads, but does suggest that operation near the lean limit can maximize efficiency while minimizing NO and CO emissions.