The Effect Of VNT And EGR On The Performance Of BED Fuel Diesel Engines

Today's diesel engines feature higher specific power,lower fuel consumption and emissions with resources exhausted and emission regulations getting more stringent.The focus of the diesel emission control is NOx and SOOT.EGR is one of the internal cleaning methods to reduce NOx emission and is widely used.However,EGR could lead to increase of specific fuel consumption and SOOT emission.Also,exhaust may fail to enter intake manifold because intake pressure is greater than exhaust pressure.VNT is an effective approach to cope with insufficient torque,response lag at low speed and degradation of performance at high altitude in terms of ordinary turbocharger.Meanwhile,intake and exhaust pressure difference could be changed by VNT,broadening range of EGR.BED could contribute to resources reservation and reducing emissions.Therefore,it is of great significance for high power and low emission of diesels to investigate effect of VNT and EGR on engines using BED under different atmospheric pressure.A high pressure common rail diesel engine is set as research object.Under pressure of 80kPa and 100kPa,effect of VNT and EGR on power,economy performance,combustion and emission characteristics of diesel engine using burning diesel and B15E5 was investigated by test.Based on tests,effect of VNT and EGR on combustion process of BED engine under different pressure was obtained using FIRE software.Results show that:(1)when valve opening of EGR remains the same,pressure difference of engine decreases with increase of valve opening.When valve opening of VNT increase to a certain extent,pressure difference of the engine gradually levels off.When pressure is at 100kPa,and valve opening of EGR is 100%,largest drop of pressure difference under max torque condition is 68%as opening of VNT increases from 22%to 28%.When opening of VNT remains constant,pressure difference decreases with the increase of valve opening of EGR.When EGR rate remains the same,amount of air intake increases with the decrease of opening of VNT.When pressure is at 100kPa,EGR rate is 10%,largest rise of air intake is 8.76%under max torque condition as opening of VNT decreases from 28%to 22%.Decrease of pressure could lead to drop of pressure difference and air intake.Pressure difference and compensation of compressor to intake flow are improved by reducing opening of VNT,making sure that there is a high EGR rate under different conditions while EGR is applied.(2)When the engine is running under high load condition,power and economy performance of the engine could deteriorate owing to appliance of EGR.It could recover power and economy performance of the engine to reduce the opening of VNT.When EGR rate is at a constant level,max combustion pressure and heat release rate decrease with the increase of opening of VNT or with the decrease of pressure.NOx emission decreases by 1.4%in average and SOOT increase by 27.5%as pressure drops from 100kPa to 80kPa,while there is difference in CO emissions with different conditions.When pressure is 80kPa,and EGR rate increases from 0%to 24%,NOx emission decreases by 57.9%.SOOT and CO emission increases by 139%and 42%,respectively.When EGR rate remains constant,SOOT and CO emission drops while NOx emission goes up with the decrease of opening of VNT.(3)The engine using B15E5 fuel has low output torque and specific fuel consumption.When pressure is at 100kPa,and speed at 2200r/min,torque of B15E5 engine decreases by 4.6%in average,while there is an increase of specific fuel consumption by 3.7%.Peak cylinder pressure and HRR both drop.Start timing of combustion is advanced.Rate of combustion speed up and combustion duration shortens.CO and SOOT emission decrease by 21.9%and 32%,respectively.NOx increase by 5%.It contributes to the reduction of NOx emission of B15E5 to increase EGR rate.When pressure decrease from 100kPa to 80kPa,performance of engines using B15E5 vary slightly compared with engines with diesel.(4)Simulation of combustion process with a constant EGR rate was carried out by modifying opening of the VNT at a speed of 2200r/min under 25%and 75%load condition.Results show that when engine runs in 25%load condition,in-cylinder temperature and broken oil droplets of the same crankangle are almost the same and there is circular rotation in spray tip with an increase of SOOT emission as pressure decrease from 100kPa to 80kPa.In 75%load condition,in-cylinder high temperature region of same CA extends.Distribution area of oil droplets decreases.Circumferential excursion movement of oil front disappears,broken performance of droplets.Distribution area of NOx shrinks while that of SOOT expands dramatically.Simulation of combustion process of different fuels with a constant EGR rate was carried out by modifying opening of the VNT at a speed of 2200r/min and pressure of 80kPa under 75%load condition.Results show that engines using B15E5 have similar temperature distribution region of same CA with those using diesel.However,its averaged temperature is lower than engines using diesel.Droplets diameter of B15E5 is larger than that of diesel.Therefore,B15E5 features a poor atomization effect.Distribution area of NOx is larger than its counterpart,while SOOT and its peak value is the opposite.