Optical Study On Effect Of Premixed Fuel Properties And Fuel Stratification On Dual-Fuel Engine Combustion Process

Low temperature premixed combustion combines the advantages of diesel engine and gasoline engine.It not only can reduce emission of NOx and SOOT,but also has the potential to improve efficiency of engine.The fuel stratification of dual-fuel can effectively improve the ability to regulate the combustion process,which is conducive to broaden the operating range of the dual-fuel engine.However,how to create the proper mixture stratification and the mechanism of mixture stratification controls dual-fuel combustion is still unclear.Based on the optical engine test bench,the experimental study has been carried out to study the combustion process of dual-fuel engine,by using Mie-scattering method,high speed photography technology and image processing technology.The influence of mixture stratification on the development of dual-fuel ignition core and the flame was investigated by creating different mixture atmosphere in cylinder.The mixture atmosphere can be adjusted by changing the fuel equivalent ratio,the types of fuel and direct injection strategy.The HPCCI combustion mode has been realized by implementing proper injection strategy.The exothermic process of combustion was revealed.And the influence factors and control mechanism were investigated.Diesel was selected as the direct injection fuel.Ethanol and gasoline were selected respectively as the premixed low activity fuel.The results show that,when ethanol is used as premixed fuel,the ignition time of diesel delayed and the ignition core decreased.This is mainly due to the strong cooling ability of ethanol and the higher octane value compared with gasoline,which has adverse effect on low-temperature combustion of diesel.The ignition and combustion of the mixture can be effectively promoted by properly raising the inlet temperature.Based on the gasoline/diesel combination,with same inlet temperature and premixed fuel ratio,different stratification conditions in cylinder was created by changing the parameters such as direct injection number,injection timing,main-pilot injection fuel ratio and main-pilot injection interval.For double injection strategy,the results show that different pilot injection timing affects the stratification degree of the mixture.Appropriately improving the activity of the mixture is conducive to the control of the combustion process.The low-temperature heat release of the pre-mixture can promote the combustion of diesel from the last injection or can grow into a new ignition kernel,indicating that the ignition location and timing of dual fuel are affected by low-temperature heat release and fuel-mixture distribution.HPCCI combustion mode has been realized at low load.The combustion process can be divided into three stages:(1)the appearance of weak blue chemiluminescence near-10°CA ATDC generated by the low-temperature heat release of pre-mixed gas.(2)The main injection fuel enters into the cylinder and is ignited rapidly forming the weak source of fire.(3)With the continuous development of fire source,the remaining fuel mixture is ignited and releases a lot of heat,which is the main heat-release stage.The results show that,under a certain pre-mixed atmosphere,the ability of main injection timing to control HPCCI combustion is notable regular.