Optimization Research Of Cold Start For Aviation Heavy Fuel Direct Injection Spark-ignition Type Engines

The spark ignited piston-type aviation heavy fuel engines have advantages like small size and high power to weight ratio,which make them outstanding in low power-range power unit.Compared to aviation gasoline,aviation heavy fuel(aviation heavy fuel consists of light diesel and aviation kerosene of different kinds,the paper centers on RP-3 aviation kerosene,we call it aviation kerosene for short the following)is safer and easier to store up.Therefore,aviation kerosene piston-type engines are applying in military affairs and universal small-sized aviation field extensively.However,kinematic viscosity of aviation kerosene is big,which causes poor spray atomization quality and difficulty in cold start.This paper took a low-pressure air-assisted direct injection single-cylinder engine as research object and conducted experiments to research the impacts of excess air factor,end of injection timing and ignition advanced angle to combustion characteristics of aviation kerosene direct injection engines.Aiming at the influence factors above,this paper carried out sensitivity analysis based on IMEP.The results show that:Injection in later stage of compress stroke can help preparing adequate amount of mixture for start.As combustion of aviation kerosene is slow,putting off ignition timing will make post combustion intensify.The energy loss it causes is more critical than the increment of compressed negative work caused by advancing ignition,so we should adopt bigger ignition advanced angle.When Excess air factor ranges from 0.65 to 0.7,cyclical variation of peak cylinder pressure descends,incomplete combustion and misfire disappears,both of them are conductive to smooth start of aviation kerosene direct injection engines,thus saturate injection strategy is proposed.Compared to the variety of ignition advanced angle,cold start performance of aviation kerosene piston-type engines are more sensitive to preparation level of mixture.Furthermore,the paper researched the effects of aviation kerosene heated method,compressed air heated method and crank speed on cold-start performance of aviation kerosene piston-type engines.The results show that:For aviation kerosene heated method,appropriate heating temperature can enhance evaporation rate,promote spray atomization quality and reduce activation energy of molecules.The three elements mentioned above are beneficial to accelerate the preparation process of mixture and reaction rate of combustion,which promote cold-start performance of aviation kerosene direct injection engines sufficiently.However,extortionate temperature of aviation increases saturated vapor pressure,the amount of aviation kerosene in gas phase increases,which reduces the amount of fuel injected to cylinder in equal injection pulse width and we need to increase injection pulse width reasonably to promote cold-start performance,but this will bring about risk of air-resistor.Compressed air heated solution can accelerate preparation rate of mixture and shorten start time of aviation kerosene direct injection engines.For increasing heat transfer gradient between fuel droplet and high temperature airflow can promote evaporation rate.However,compressed air heated solution is less effective than aviation kerosene heated solution.Heating consumes energy,so aviation kerosene heated method is preferred.Higher crank speed can promote cold-start performance of aviation kerosene direct injection engines significantly and shorten start time apparently.For higher temperature and pressure can improve evaporation.In the mean time,increasing restriction loss will reduce cyclical air intake,which makes it easier to prepare adequate-concentration mixture for start.