Improving Thermal Efficiency With Exhaust Gas And Air Combined Dilution In A Low-temperature Combustion Gasoline Engine

High-dilution low-temperature combustion of gasoline engine, represented byhomogeneous charge compression ignition (HCCI) combustion, also known ascontrolled auto-ignition (CAI), is attracting more and more attention for its capabilityof improving fuel consumption and NOx emission simultaneously. However,optimized combustion process and the improved in-cylinder charge acquirementcannot be satisfied at the same time by using traditional approaches. Thus, in thepresented research, the coordinated optimization of heating effect and dilution effectof the in-cylinder charge was carried out with the consideration of both the requiredthermo-dilution state and in-cylinder charge acquirement. The fuel economy ofhigh-dilution low-temperature combustion was further improved and the relatedimplementation strategy throughout the whole operating range was proposed.The research was carried out both experimentally and numerically based on asingle-cylinder gasoline HCCI principle prototype engine. First, at the target ofimproving HCCI/CAI combustion, the combination of the intake preheating strategyand residual gas trapping strategy was investigated. Second, the positive valve overlapstrategy was applied to achieving hybrid combustion based on exhaust gas rebreathing.Then, a multi-mode heat release model of SI, CAI and SI-CAI hybrid combustion isestablished to enable the engine cycle simulation of high-dilution low-temperaturecombustion. Finally, a full-load operation strategy based on waste heat recovery andpositive valve overlap was proposed to achieve the coordinated optimization of therequired thermo-dilution state and charge acquirement.The optimization method employing both intake preheating strategy and residualgas trapping strategy was proposed to overcome the restriction of fuel economyimprovement in CAI combustion. The influence of this combination strategy oncombustion and charge acquirement was investigated. By coordinating the heatingeffect and dilution effect of the air and residual gas, the contradiction between highdilution ratio and low thermal supply were solved at a relatively low cost of chargeacquirement, realizing8-12%fuel consumption improvement and expanding the lowoperation limit to0.8bar.The effect of exhaust gas rebreathing implemented by positive valve overlap onhybrid combustion and gas exchange was analyzed. A related optimization strategy, inwhich the positive valve overlap is formed mainly by advancing intake valve timingand subordinately by retarding exhaust valve timing, was proposed to coordinatein-cylinder state and charge acquirement. Compared with hybrid combustionemploying negative valve overlap strategy, up to9%improvement of fuel consumption can be achieved with the optimization of in-cylinder chargeimplementation.A heat release model for high-dilution low-temperature combustion wasestablished based on the general characteristics of flame propagation and multi-siteauto-ignition. By analyzing the ideal demand of high-dilution low-temperaturecombustion, a full load operation strategy based on waste heat recovery and positivevalve overlap was proposed to improve both the combustion process and chargeacquirement process. Compared to the low temperature combustion implemented bynegative valve overlap strategy and the traditional SI combustion,6.2%and21%offuel economy improvement can be achieved respectively.