Study Of The Effect And The Control Mechanism Of The Fuel Activity Distribution To High-Dilute Hybrid Combustion In A Gasoline Engine

Spark Ignition-Controlled Auto Ignition(SI-CAI)hybrid combustion,also called Spark Assisted Compression Ignition(SACI)has shown great potential to achieve highefficiency and low-emission engine performance.However,the instability of SI-CAI hybrid combustion limits its wide application in practice.The main reason is that the gasoline is very challenging to auto-ignition as gasoline-air mixture.The temperature and dilute degree are hard to control so that it is difficult for the combustion and heat release control of gasoline high dilute mixture.In order to solve the instability combustion and optimize the whole combustion process in gasoline engines,two combustion control methods were mentioned in this research by adjusting the fuel chemical characteristics.The first method is called micro flame ignition,in which a small amount of high activity fuel is injected into cylinder to build fuel activity stratification and form micro flame kernel.The second one is called reform hybrid combustion.The fuel mixture is reformed in cylinder during hybrid thermal cycles.In this research,different combustion control methods were studied by simulation combined with experiment to achieve the effect of two methods to the combustion process.The micro flame ignition strategy adjusts the fuel chemical characteristics by adding other fuel into cylinder.In this research,the reaction path and control mechanism of hybrid combustion were investigated for two kinds of micro flame ignition strategy: the congregate micro flame ignition strategy and the discrete micro flame ignition strategy.The discrete micro flame enhances the absolute flame speed of flame propagation,while the congregate micro flame improves the relative flame speed of micro flame kernel and spark ignition.With the reasonable combination of two kinds of MFI strategy,the initial ignition and flame propagation process can be stabilized,which can make the subsequent auto-ignition process more controllable.The gasoline reform hybrid combustion bases on hybrid thermal cycles adjusted fuel chemical characteristics through the fuel reforming process of gasoline.In this research,the dilute degree and temperature of fuel reform were studied for the feasibility of gasoline reform hybrid combustion.The effect of reaction path and products to the activity and flame speed of reform mixture was investigated.Simulation results indicated that although the fuel reform process can enhance the laminar flame speed at high diluted condition,the energy loss lead to lower in-cylinder temperature,which limited the speed of flame propagation.As a result,the consumption of energy and the gain of flame speed in fuel reforming process should be balanced during the practice of fuel reform hybrid combustion.The influence of reforming fuel ratio to hybrid combustion process was further studied.At last,the two ways of combustion control were compared under similar engine operating conditions to prove the effectiveness of gasoline fuel chemical characteristics adjustment to the heat release control of hybrid combustion.The similarities and differences of the heat release and flame propagation control mechanism of the two kinds of hybrid combustion were concluded at the same time.