Optical Study On Effects Of Mixture Stratification And Fuel Properties On Dual Fuel Combustion

Dual fuel combustion mode not only can achieve low soot and NO_x emissions at the same time,but also further improve the thermal efficiency,revealing the potential of high efficient and clean operation condition in engine heavy load.However,the recognition of fuel concentration stratification and combustion properties is still very limited.Optical technology is a critical research means of engine mixture combustion in cylinder,beneficial to further study of the combustion process in cylinder.The main content of the paper is about carrying out the study of dual fuel combustion effect on which the mixture stratification and properties have by using this technology.This research is conducted on an optical engine.Dual-fuel combustion method is adopted.Stratification of mixture is controlled by changing the time of direct injection.Fuel properties are studied by changing the kinds of in-cylinder injection fuel and port injection fuel.Techniques of high-speed video,spectrum acquisition and laser induced fluorescence are used to reveal the effects of fuel stratification degree and characteristics of fuel to dual fuel's ignition,combustion and flame development course.The study shows that:From the perspective of mixture stratification degree:The natural luminosity imaging results indicated that,with retarding n-heptane injection timing,combustion regions moved from the whole combustion chamber to the edge of the combustion chamber,and the natural flame luminosity got stronger and more stratified,showing more signs of soot radiation.The flame emission spectra from RCCI combustion under different mixture conditions showed band spectra of OH,CH,CH₂O and C₂,as well as continuous spectra of CO oxidation and soot radiation.These band spectra became more distinct and appeared successively with the retarding of direct injection timing.OH measurement results show that when the direct injection time is-25°CA ATDC,differences between OH formation courses of different direct injection fuels in dual fuel burning are small.When direct injection time is delayed to-10°CA ATDC,the differences become larger.For the same kind of direct injection fuel,distribution of OH concentration when direct injection time is-25°CA ATDC is more uniform than-10°CA ATDC.From the perspective of fuel properties:?1?direct injection fuel characteristics.The relationship of penetration and the spray cone angle both are n-hexadecane>PODE>n-heptane.The relationship of the injection duration is PODE>n-heptane>n-hexadecane.Theorderofluminosityintensityis n-hexadecane>n-heptane>PODE.In the case of n-hexadecane injected,the color of flame changes from original blue to bright yellow rapidly.Under the condition of direct injection fuel is PODE,the flame is mainly blue but partly bright yellow.When n-heptane is injected,the color of flame is mainly blue before CA50 and is bright white after CA50.?2?From the perspective of intake port injection fuel characteristics.The working conditions are studied in which in-cylinder injection fuel is fixed as n-heptane and intake port injection fuels are isooctane and PRF70 respectively.Results show that,in terms of fuel's activity stratification,mixture overall activity of PRF70's case is higher than isooctane's case.And mixture activity gradient is smaller,making its heat release rate lower.The study shows that the changes of in-cylinder direct injection timing and the fuel properties of dual fuel both influence the distribution of mixture concentration and activity,leading to different flame development characteristics.The study has certain guiding significance to the dual fuel engine combustion law based on mixture concentration stratification and fuel activity stratification.