Optimization Of Combustion Process For Partially Premixed Diesel Engine With Different Injection Strategies

PCCI(Premixed Charge Compression Ignition)is a type of low temperature combustion strategies which has the potential for simultaneously reducing NO and soot emissions from direct injection diesel engines.The high level of EGR is utilized to reduce overall combustion temperature and to lengthen the ignition delay and thus more homogeneous mixture is obtained through advanced direct injection and port fuel injection.The goal of the paper is to explore the influence of EGR rate,injection timing,pilot injection proportion and post injection characteristics on PCCI combustion and emission process from the macroscopic and microscopic aspects on a common rail diesel engine,meanwhile,the computational fluid dynamics software AVL Fire is selected as a main computational tool to simulate the cases.The main research contents and results are as follows:(1)It is indicated that EGR has an obvious inhibitory effect on NO emissions rather than soot emissions.When the EGR rate changes from 34% to 44%,NO emissions are decreased about 80%,while the soot emissions change a little.The dilution effect of exhaust gas is more apparent with 44% EGR rate,local fuel rich regions are reduced,combustion temperatures are decreased and the ignition delay are prolonged by EGR.It is beneficial to achieve premixed charge combustion because more and more fuel is mixed with air in ignition delay.(2)Distributions of equivalence ratio,temperature,NO and soot in typical crank angles are chosen to explore the NO and soot formation process with different EGR cases.The main time of NO generation is middle and later stages of combustion,at the same time,the inverse squish in the combustion chamber makes a lot of NO generated in the clearance between the piston and cylinder head,and there is a better consistency between the high temperature and NO concentrated area.When the EGR rates change from 34%,39% to 44%,the distribution of area and concentration of NO gradually decrease,NO generation is at the minimum level with EGR rate equal to 44% in different crank shaft angle.Soot formation area is related to the in cylinder atmosphere at each combustionstage.Correspondingly,there is a small amount of soot formed in the combustion chamber wall before the injection fuel spray at injection phase.Then the soot is mainly generated in fuel rich region of combustion chamber along with the burning.The combustion chamber volume is increased while the piston continues downward,and then total amount of soot is reduced because partial amount of soot is oxidized in cylinder,while the generate area spreads to the clearance volume.(3)It is indicated from the adjustable injection timing cases that the position of cylinder pressure and heat release rate are delayed,as well as the second peak of cylinder pressure and heat release rate peak value are decreased gradually when both pilot and main injection timings are retarded as a whole.There is an obvious low temperature trend in the combustion chamber.NO reduced as the injection timing delayed,and then a better trade-off relationship with soot is achieved at the main injection timing of-3°CA ATDC.(4)It is indicated from the adjustable pilot injection proportion cases that the peak values of in-cylinder pressure,heat release rate and temperature are raised along with the increase in the second pilot injection quantity.This is because the heat released by the second pilot injection fuel and the main injection fuel is overlapped,NO and soot emissions then increased.However,it is beneficial for low temperature combustion and emissions when the quantity of the first pilot injection is less.When the pilot injection proportion is 1:1,the combustion and emission characteristics of diesel engine are compromised well.(5)The post injection quantity and the interval between main and post injection are two key parameters of post injection strategy based on post injection characteristic researches.The post injection has a certain influence on the middle stage of combustion.There is a heat release peak after the main peak of heat release rate,and the position of peak of later heat release rate is delayed in line with the increase of the interval between main and post injection timing.Moreover,the post injection is an effective measure to reduce soot emissions while it has little influence on NO emissions.The fuel with air is mixed well in later stage of combustion which is promoted by air motion in-cylinder.Meanwhile,it is conducive to the second oxidation process of soot assisted by higher bulk gas temperature in-cylinder which is caused by the post injection fuel burned.Optimize for the interval between main and post injection timing and post injection quantity,the relative optimal soot and NO emissions are acquired at 20°CA injection interval and 10% post injection quantity case.