Study On The Control Mechanism And Optimization Of Heat Release Process Under High-dilution And Heavy-load Conditions Of Gasoline Engine

Spark-assisted compression-ignition combustion(SI-CAI)can effectively improve the fuel economy of gasoline engines while improving the ability to control dilution combustion.Therefore,it has become a research focus in the field of internal combustion engines in recent years.However,problems such as knocking or overlimiting explosion of gasoline engines under heavy load still limit the operating range of high-diluted hybrid combustion gasoline engines.Therefore,from the perspective of overall combustion heat release process control,based on the regulation of mixture’s activity distribution and the thermal state of the cylinder in the high-dilution hybrid combustion under heavy load conditions,the control mechanism for the early ignitionflame propagation process and the later auto-ignition combustion process was established to optimize the heat release process and thermal efficiency of the engine under heavy load.First of all,by analyzing the shape of the heat release rate under heavy-load,it is found that the heat release shape of “double-peak” is an effective heat release shape considering both thermal efficiency and mechanical strength.At the same time,the fact is confirmed that the heat release of the first peak is a key factor affecting the final performance of the overall heat release process.Secondly,based on different heat release processes achieved by adjusting the states of the mixture in the cylinder as well as fuel activity distribution,the survey found that the DME late injection time can effectively regulate the shape of heat release rate.DME becomes more concentrated when the late injection time delays.Moreover,the ignition delay time of DME would shorten and the heat release rate increase on account of the rise of the temperature in the cylinder.The increase in the rate of heat release makes the first peak of the heat release rate earlier,and the shape of the heat release rate transitions from "three stages" to "double peaks";The late injection time mainly affects the first peak heat release by controlling the heat release during the flame propagation phase in the hybrid combustion.By adjusting the DME ratio of early and late injection,the activity distribution in the cylinder can be changed to increase the peak height of the first heat release peak.What’s more,the shape of the heat release rate approaches a “trapezoid”.Finally,the later auto-ignition heat release process is mainly controlled by the thermal atmosphere in the cylinder.The increase in the amount of heat release from the first peak rises temperature in the cylinder,which advances the time of auto-ignition combustion and shortens the duration of auto-ignition combustion.In order to further optimize the heat release process,a method of changing the internal residual exhaust gas rate and the effective compression ratio is proposed to regulate the thermal state and dilution state in the cylinder,which can control the pressure rise rate while maintaining the overall heat release rate.The indicated thermal efficiency can be further increased to 41.2%,which is 20.2% higher than the indicated thermal efficiency of34.326% of the conventional PFI combustion.