Visualization Research Of The Mixture Distribution In The Cylinder And Combustion Characteristics Under Different Intake And Fuel Injection Strategies

Homogeneous charge compression ignition(HCCI)combustion is an effective way to improve the efficiencies of gasoline engines at part loads.However,the auto-ignition timing and combustion phasing of HCCI combustion is controlled by the in-cylinder temperature and mixture distribution.The realization of HCCI combustion also depends on the spark ignition at highly diluted conditions due to the existence of exhaust gas.Therefore,the effects of intake valve timing/lift and injection strategy on the distribution of the in-cylinder mixture and the combustion characteristics were investigated via planar laser induced fluorescence and high speed flame images on an optical gasoline engine.Results show that:With delayed intake valve opening(IVO)timing at a fixed intake valve maximum lift and a fixed exhaust timing/lift,the average temperature of the mixture in the cylinder first increases and then decreases,but the inhomogeneity of mixture temperature and concentration in the cylinder increases monotonously.when the intake valve maximum lift increases at a given intake valve opening timing,the average temperature of the mixture in the cylinder increases first and then decreases,while the inhomogeneity of mixture concentration in the cylinder decreases.The average in-cylinder temperature of the mixture in the case of direct injection is lower than that in the case of port fuel injection,and it decreases with the delay of direct injection timing.Flame propagation speed is slowed down at highly diluted conditions.The start of flame propagation and the speed of heat release are influenced by residual gas fraction and the flow motion in the cylinder,and the residual gas fraction is found to have more influence on the flame development and the combustion processes in the cylinder.At fixed valve timings and lifts,the flame propagation speed and heat release rate at the early stage of the combustion are higher when direct injection is applied than when port fuel injection is used.However,an opposite trend is observed after the middle stage of the combustion when the combustion with port fuel injection displays a faster flame propagation and heat release.Furthermore,the flame propagation speed and heat release rate of n-butanol are faster than that of iso-octane.Compared with iso-octane,the brightness of n-butanol flame is more uniform.