Optical Study On The Flame Development Characteristics Of Partially Premixed Combustion In IC Engine

Novel combustion strategies have the advantages of high thermal efficiency and low pollutant emission but the operation condition is limited because of rough combustion and high pressure rise rate in high load condition caused by highly premixed charge.Compared with homogeneous charge compression ignition(HCCI),the efficient and clean operation condition of partially premixed combustion(PPC)can be extended through fuel stratification.PPC draws great attention in recent researches.Flame development characteristics of PPC are studied in the present research and it is of great importance to uncovering the relation between flame characteristics and combustion heat release and researching the mechanism of combustion control in PPC.Study is conducted through high-speed imaging of natural flame luminosity and combustion analysis based on in-cylinder pressure in an optical engine by using n-heptane as the fuel.First,PPC with different fuel stratification is realized through changing the single injection timing.The relations among the flame development pattern,flame speed,fuel stratification and combustion heat release rate are analyzed.It is found that there is clear flame propagation during flame development process in PPC compared with HCCI.When the fuel becomes more stratified,the ratio of flame propagation becomes larger and the ratio of auto-ignition becomes smaller so the combustion reaction rate becomes lower,the combustion duration becomes longer and the peak heat release rate becomes lower.Flame speeds of HCCI and PPC are in the range from 10m/s to 50m/s.There is a good correlation between the flame speed and the peak heat release rate.The peak heat release rate is high for condition that flame speed is high.So the appearance of clear flame propagation is the essential reason why the operation condition is effectively extended in PPC compared to HCCI.Based on the study on single injection PPC,the flame characteristics of PPC in double injection strategy are analyzed.When changing the fuel ratio of first and second injection with the injection timing fixed,the peak heat release rate changes a little and combustion reaction rate is slightly influenced if the flame development pattern of multi-point auto-ignition is not changed.When the fuel ratio of first and second injection is fixed and the second injection timing is delayed,the proportion of diffusion combustion increases and the peak heat release rate becomes lower.However,the combustion images show obvious signal of soot.Finally,auto-ignition timing is fixed for double injection and single injection PPC through changing the injection parameter and the mechanism why double injection strategy can reduce the noise and pressure rise rate in PPC is studied.It is found that in current boundary condition,flame develops in the way of multi-point auto-ignition for single injection strategy.By changing fuel injection ratio and fuel injection timing simultaneously in double injection PPC,flame can develop in the way of flame propagation.Compared with single injection strategy,flame develops more slowly because of flame propagation and the peak heat release rate and max pressure rise rate reduce significantly in double injection PPC.Combustion reaction rate is effectively controlled and there is no signal of soot in the combustion images.Changing the flame development pattern is an efficient way to control the combustion in PPC.