Effects Of Two-Stage Combustion With Various Intake Parameters On Combustion And Emission Characteristics

Much attention has been paid to reduce soot and NO_x emissions simultaneously to meet stricter emission regulations.One of the most promising solutions is to apply new combustion modes such as HCCI,LTC,PCCI.As HCCI still faces many challenges,this paper mainly pays attention to PCCI combustion.Two-stage combustion(TSC)is a typical method to realize PCCI which is composed of twice fuel injection events.The first injected fuel of TSC is to realize HCCI combustion,and the second injected fuel is to meet the need of engine performance.A 3D combustion chamber model was built by Star-CD to investigate the combustion and emission characteristics of TSC at different fuel injection strategies and intake parameters.Firstly,early injection coupled with EGR was investigated.Results showed that premixed heat release is observed when fuel injection timing is earlier than-35 o CA ATDC.Early fuel injection results in lower soot emission but higher NO_x emissions.But when the injecion timing is-55 oCA ATDC,soot increases rapidly because of wall-impingement.Introducing EGR is identified as a control strategy for delaying combustion phasing,which reduces in-cylinder temperature,pressure and NO_x emission.The effect of fuel injection timing,quantity,EGR rate and LIVC on combustion and emission characteristics was also investigated.Results showed that the first combustion process is different from with early injection.Advanced SOI-1(start of first injection timing)has little influence on overall emissions.Retarded SOI-2(start of second injection timing)reduces average temperature and pressure,increases soot and CO emissions,reduces NO_x emission.It was observed that NO_x emission is lower than base engine when SOI-2 is retarded later than-10 o CA ATDC.When EGR is introduced into TSC,NO_x emissions can be reduced by 67% with little impact on soot emission.Temperature and pressure increase with the increased first injection quantity.Increasing first injection quantity can improve fuel and air mixing process,which results in lower soot formation bump but higher NO_x emissions.Later intake valve close timing leads to lower pressure but higher maximum average temperature,as well as lower soot and NO_x emissions.At last,the author gave a contrast analysis of combustion strategies of tradition,pilot injection,early injection and TSC combustion.Results showed that early injection can reduce NO_x and soot emissions,but has bad impact on mechanical and thermal loads.TSC has the potential to reduce soot and NO_x emissions simultaneously.