An Experimental Investigation On Combustion Process And Emission Of RCCI Combustion Mode

RCCI?reactivity controlled compression ignition?has great potential for optimizing combustion processes,increasing thermal efficiency and reducing NOx and soot emissions.Diesel injection timing?SOI?,premixed ratio?RP?and low reactivity fuels with different chemical prosperties and physical prosperties can largely affect the reactivity stratification and equivalence ratio in the cylinder,which has a significant impact on combustion and emissions.Therefore,the author presents an experimental study on the combustion,emission and cycle-by-cycle variation characteristics of RCCI in which in-cylinder direct injection of diesel combined with port injection of gasline,ethanol and iso-butanol,respectively,are used with a six-cylinder engine.The specific conclusions are as follows:With the advance of the SOI,the ignition timing advance,combustion duration increases,the highest instantaneous heat release rate and pressure rise rate as well as BTE first increases then decreases for all three low reactivity fuels.The effect of RP on combustion is different under different cycle energy.At low cycle energy,the highest instantaneous heat release rate decrease,combustion duration and BTE increases for all three low reactivity fuels.But the conclusion is the opposite under high cycle energy.The latent heat of vaporization,the cetane number and heat value of port injected low reactivity fuels play important roles in the process of RCCI combustion.Compared with gasoline,ethanol or iso-butanol has a later ignition time and LTHR?low temperature heat release?stage is weaker,but the shorter combustion duration and higher instantaneous heat release rate make the engine knock easily under high cycle energy,especially for ethanol.With the advance of the SOI,CO emissions first decrease then increase,NOx emissions first increase then decrease,HC emissions decrease.Both of the amount and the surface area of nucleation and accumulation mode are decrease.With the RP increase,HC emissions increase,the total amount of particle decrease,but the amount,surface area and CMD of accumulation mode will increase.NOx and CO emissions follows different trend for different cycle energy.CO emissions increase and NOx emissions decrease at low cycle energy.But CO emissions decrease and NOx emissions increase at high cycle energy.Compared with gasoline,ethanol or iso-butanol has a higher BTE and can significantly reduce the amount of particulate matter,the amount and the surface area as well as CMD are lower.But it will increase the number and surface area of aggregated particles,especially for ethanol at high cycle energy and high RP condition.With the advance of SOI,the P_max,AHRR_max,?dp/d??_max and IMEP of RCCI mode decrease first and then increase,the corresponding phase is concentrated first and then dispersed.With the RP increase,the COV of P_max,AHRR_max,?dp/d??_max,and IMEP of the RCCI mode gradually increase,the corresponding phase dispersed.Diesel injection timing and premixed ratio have a greater influence on AHRR_max,?dp/d??_max and IMEP compared to that on P_max.When SOI was-24°ATDC and RP was 40%,AHRR_max,?dp/d??_max and IMEP have the lowest COV and the combustion process is the most stable.Low reactivity fuels properties have an important influence on the cycle-by-cycle variation of RCCI.When SOI was-24°ATDC and premixed ratio was 40%,the cyclic variation of P_max,AHRR_max,?dp/d??_max and IMEP of ethanol and iso-butanol RCCI mode were lower than that of gasoline RCCI mode,and cycle-by-cycle of ethanol RCCI mode was the lowest and combustion process was the most stable.