Research On The Influence Of Two-stage Pilot Injection Parameters On Combustion And Emission Characteristics In A PCCI Diesel Engine

Due to increasingly stringent emission regulations,the requirements for diesel aftertreatment devices are getting stricter.Compared to complex and expensive aftertreatment devices,low temperature premixed combustion technology can fundamentally promote combustion process,improve energy efficiency and suppress emission generation.PCCI combustion is a typical low temperature combustion method,which core control concept is to reduce NO and soot generation by lowering the in-cylinder combustion temperature and improving diesel and gas mixture as much as possible.The test prototype was a reformed single-cylinder common-rail diesel engine equipped with EGR technology and multi-stage injection strategy to achieve PCCI combustion mode.The CFD software AVL Fire was selected to explore effects of two-stage pilot injection timing,pilot injection proportion,EGR rate on PCCI combustion and emission characteristics under multi-stage injection strategy.Meanwhile,strategies of optimized pilot injection timing coupled pilot injection proportion and pilot injection proportion were discussed,which provided some references for clean combustion control of diesel engine.The specific research contents and results are as follows:?1?Based on the pilot-pilot-main injection strategy,the processes of PCCI combustion and emission generation were simulated with advancing or delaying the SOI-P1?start of 1st pilot injection?by 5°CA.With the delay of SOI-P1,in-cylinder temperature and equivalence ratio distributions show that more pilot injection fuel is involved in the initial heat release while the vaporization and evaporation of the main spray fuel are accelerated.Simultaneously,main peaks of cylinder pressure,instantaneous heat release rate and temperature in the cylinder are increased and the corresponding phases are also advanced.When SOI-P1 is retarded,the main combustion process is advanced and the main combustion duration is shortened.With the delay of SOI-P1,NO formation is more serious and in good agreement with the trend of temperature peak,while soot generation is increased slightly,which has a good consistency with the fuel-air equivalent ratio distribution.?2?Based on the pilot-pilot-main injection strategy,the processes of PCCI combustion and emission generation were simulated with advancing or delaying the SOI-P2?start of 2nd pilot injection?by 5°CA.The simulation results of combustion process show that the main peak values of the cylinder pressure,instantaneous heat release rate and in-cylinder temperature are increased with the advance of SOI-P2,and the phases are advanced.Meanwhile,the phase of the exothermic peak resulted by the pilot injection fuel combustion is advanced,indicating that SOI-P2 affects the ignition timing.The amount of NO is increased with the advance of SOI-P2 and mainly distributed in the upper part of the combustion chamber with higher temperature and bottom of the pit with thinner mixture.Under different injection strategies,the amount of soot has a little variation and whole generation process of soot is advanced with the advance of SOI-P2.?3?The two-stage pilot injection proportion and timing were discussed when Case2 was taken as research object.It is shown that when Q₁?first pilot injection quantity?was 4:1 than Q₂?second pilot injection quantity?and original injection scheme remained unchanged,the formation of NO is effectively suppressed but the main peaks of cylinder pressure,heat release rate and combustion temperature drop and the rate of pressure rise is lower,which indicates that the combustion process is deteriorated.From the previous study,it is found that SOI-P2 can accelerate the main combustion exotherm.Hence,the scheme of advancing SOI-P2 is adopted to replace Case2.On the basis of Q₁ increased and Q₂ decreased,the pressure rise rate peak is increased with the advancement of SOI-P2,which suggests that combustion exothermic process is improved.But NO emissions have little difference compared with Case2.It can be seen that under the multi-stage injection strategy,on the basis of increasing Q₁ and decreasing Q₂,it is possible to keep NO and soot formation at a lower level while improving the in-cylinder combustion with an appropriately advanced SOI-P2 injection scheme.?4?The original scheme of Case3 with high EGR was taken as research object to explore effects of decreased EGR rate and optimized pilot injection timing strategies on the combustion process.It is indicated that decreased EGR rate is beneficial to the combustion exotherm process.Main peaks of cylinder pressure,pressure rise rate,heat release rate and combustion temperature are increased,but the NO production is significantly increased.According to the previous study,advanced SOI-P1 can reduce the in-cylinder combustion temperature.Hence,the scheme of advanced SOI-P1 is adopted to replace Case3.With reduced EGR rate and advanced SOI-P1,the simulation results show that the increased NO caused by the decline of EGR rate is effectively alleviated and the deterioration of combustion process caused by excessive EGR rate can also be improved.The above research results show that the diesel engine PCCI combustion can be flexibly controlled by coordinated adjustment with pilot injection timing and EGR rate.