The Synergistic Effect Of Oxygenated Fuel And EGR Stratification On Engine Emissions Generation

With the increasing energy demand and increasingly serious environmental pollution problems,energy-saving and emission-reduction has become a major challenge of the modern internal combustion engine industry which intends to maintain substantial development.As a clean alternative fuel with great development potential,oxygenated fuel has been concerned and studied by relevant scholars because of its good physical and chemical properties.Due to the diffusion combustion method of compression ignition engine,it is inevitable to form high temperature oxygen enriched zone and high temperature anoxic zone in cylinder.In order to achieve the objectives of efficient and clean combustion of diesel engine,the"trade-off"relationship between NO_x and soot has become one of the problems to be solved.This study combined the characteristics of oxygenated fuel and the characteristics of stratified EGR,from the synergetic effect of fuel oxygen and environmental oxygen,the distribution of components in cylinder was controlled rationally,aiming to improve the fuel combustion efficiency and improve the"trade-off"relationship between NO_x and soot emissions.Under the same total calorific value of the cycle,the blend fuel direct-injection mode and the dual-injection mode of n-butanol/diesel both have significant influence on the combustion and emissions of the engine.In terms of combustion,both injection modes can increase the amount of premixed combustion volume,the peak value of pressure rise rate and heat release rate increased.With the increase of n-butanol ratio,the ignition delay period of the blend fuel direct-injection mode was gradually lengthened,the heat release center and combustion phase were relatively backward,and the indicating thermal efficiency was reduced.The ignition delay period of the dual-injection mode was less influenced by the ratio of n-butanol,the heat release center was gradually advanced with the increase of premixed combustion ratio,and the indicated thermal efficiency was increased gradually.In terms of emissions,both injection modes were conducive to improving the uniformity of the mixture in the cylinder.With the increase of n-butanol ratio,the NO_x emission was increased,the soot emission was decreased obviously,the HC emission was deteriorated and the CO emission was improved.The soot and CO emissions of the dual-injection mode were superior to the blend fuel direct-injection mode,but the NO_x and HC emissions were relatively poor.In addition,EGR Technology can effectively solve the problems of high pressure rise rate and NO_x emission deterioration of n-butanol.Comparing the two injection modes,the combustion characteristics of dual-injection mode were less affected by EGR,and the comprehensive effect of combustion and emission under the effect of EGR was better than that of blend fuel direct-injection mode.The evolution of the original emissions of the two injection modes with n-butanol was similar.In the early period of combustion,the soot emission was mainly concentrated near the piston pit and the cylinder wall,and gradually diffused to the cylinder center in the middle and later period of combustion,the NO_x emission was mainly distributed in the appropriate equivalence ratio and high temperature area around the soot,the CO emission generation area and mixture concentration area were roughly same,the HC emission was mainly appeared in the slit area between the piston and cylinder head in the later period of combustion.Compared with the blend fuel direct-injection mode,the pollutant generation time of the dual-injection mode was earlier,the NO_x emission concentration area was larger,and the oxidation of soot and CO emissions were relatively faster.The EGR introduction mode of spiral airway with introducing plate and tangential airway with introducing plate can form the exhaust gas distribution of lower concentration in middle area and lower concentration in peripheral area.The combustion and emission characteristics of n-butanol under the homogeneous EGR mode and the stratified EGR mode were compared.The results showed that the combustion characteristics of n-butanol under homogeneous EGR mode and stratified EGR mode were slightly different.The ignition delay period of the two injection modes was slightly prolonged under the spiral airway EGR stratification strategy,the indicated thermal efficiency of the dual-injection mode was slightly increased,and the indicated thermal efficiency of the blend fuel direct-injection mode was slightly reduced due to the delay of combustion phase.In terms of emissions,compared with the case of homogeneous EGR mode,the soot emission under the spiral EGR stratification strategy was significantly reduced,the NO_x emission was increased,the HC and CO emissions were reduced.The synergistic effect of n-butanol and the spiral EGR stratification strategy can improve the"trade-off"relationship between NO_x and soot emissions.The emission law of the tangential airway EGR stratification strategy was opposite to that of the spiral airway EGR stratification strategy,which had no effect on the"trade-off"relationship between NO_x and soot emissions.The boundary conditions have great influence on the emission characteristics of n-butanol coupled with stratified EGR.Later injection time and higher intake pressure would reduce the emission difference of n-butanol under the homogeneous EGR mode and the spiral EGR stratification mode,the improvement effect on the"trade-off"relationship between NO_x and soot emissions would be reduced.