Experimental Study On The Effects Of Concentration And Reactivity Stratification On Performance Of An Engine

Because Chemical kinetics plays a leading role in HCCI combustion,this advanced combustion mode is difficult to operate in a large operating range.However,the SCCI combustion combines the advantages of HCCI and DICI(Direct-Injection Compression Ignition),which could manage the preparation of charge mixture in time and spatial scale.It is a significant way to achieve high efficiency and low emission at full load.This paper studied the combustion and emission characteristics in the concentration stratification and reactivity stratification respectively.Firstly,the effect of direct injection timing,premixed degree,EGR rate and other factors on the combustion and emission characteristics of PRF30 and PRF70 were studied systematically.The results showed that when Rp was decreased or in-cylinder direct injection was delayed,the ignition timing of PRF70 fuel was delayed,and the peak of premixed heat release is higher than diffusion combustion.For PRF30 fuel,ignition timing is less affected by the change of in-cylinder direct injection in the high premixed ratio.when Rp is 50%,the peak of diffusion combustion heat release rate was higher than premixed combustion.Reducing Rp could reduce the maximum pressure rise rate significantly,but the NO_x emissions became higher.EGR could reduce NO_x emissions and increase indicated thermal efficiency.Lower emission of THC and CO could be achieved by avoiding medium premixed degree(Rp=50%).Secondly,theeffectsoftwokindofreactivitystratification(iso-Octane/n-Heptane and n-Heptane/iso-Octane dual fuel mode)on combustion and emissions characteristics were studied.For iso-Octane/n-Heptane dual fuel combustion,the phase of diffusion combustion were affected by Rp significantly.This kind of reactivity stratification combustion have the potential to operate in larger load conditions,because the MPRR(Maximum Pressure Rise Rate)became lower,the combustion process were soft and the combustion stability were improved.For n-Heptane/iso-octane dual-fuel combustion,the phase boundary between the premixed and diffusion combustion were ambiguous.Due to the staged combustion mode,heat release rate were soft in high premixed degree instead.However,compared with single fuel,the reactivity stratified combustion don't have the advantage of larger load expansion,because the MPRR is high relatively.CO and THC were reduced,and some advantages were shown in emissions reducing.The above discipline of concentration stratification and reactivity stratification of the mixture in the cylinder were a supplement to the combustion theory of the internal combustion engine,and had scientific significance and engineering application value for achieving the high efficiency and clean combustion in internal combustion engine.