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. Comparison of exhaust reinduction and exhaust retention (negative valve overlap) strategies shows that, at the same load, reinduction strategies have significantly higher efficiency and reduced NO emissions, though they suffer from higher HC emissions. Reinduction strategies also show some ability to recover from a misfire while retention strategies do not. The data show that the best combination of load range, efficiency, and emissions may be achieved using a reinduction strategy with variable intake lift instead of variable valve timing.
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