The Impact Of Fuel Compositions On Primary And Secondary Particulate Formation Of Gasoline Direct Injection Engines

Gasoline direct injection(GDI)engines have developed rapidly in recent years,driven by the stringent legislation requirements of vehicle fuel efficiency,but face the great challenge of particulate emissions.The fuel compositions in Chinese market are different from that in Europe and United States.It is known that GDI particulate emissions can be affected by fuel properties,while the influencing mechanisms of fuel compositions on primary and secondary particulate formation of GDI engines have not been revealed well.This work was conducted from three aspects to investigate the impact of fuel compositions and its associated mechanisms as follows.Firstly,the impact of different fuel compositions on primary particulate,polycyclic aromatic hydrocarbon(PAH)and volatile organic compound(VOC)emissions of GDI engines was investigated through engine bench tests.Based on one dimension premixed laminar flow,CHEMKIN was applied to analyze the chemical kinetics on combustion process of GDI engine using Chinese gasoline surrogates consisting of 5 components and PAH formation model consisting of 106 components and 395 elemental reactions.The computational simulation and experiment results agree well with each other.It not only reveals that the fuel components impacting on PAH formation in the descending order of toluene,diisobutylene,isooctane,n-heptane,ethanol,but also provides the insights regarding the influencing mechanism of fuel compositions on particulate emissions of GDI engines.Higher aromatics content in gasoline results in promoting the reaction pathway of toluene decomposition into benzene(A1)and relatively weakening that of hydrocarbon decomposition into small molecular hydrocarbons following by addition reactions to A1.Secondly,the secondary particulate testing system was set up combining the vehicular chassis dynamometer emission measurement and smog simulation chamber testing,in order to investigate the impact of aromatics content in gasoline on the primary and secondary particulate emission characteristics.This work was carried out for both port fuel injection(PFI)engine and GDI engine running under Beijing city driving cycle.It was demonstrated that higher aromatics content in gasoline not only resulted in higher primary particulate and VOC emissions from both PFI and GDI engines,but also resulted in elevated secondary organic aerosol(SOA)formation from tailpipe emissions.The SOA formation mechanism of the PFI engine is mainly related to A1 aromatic VOC,but that of the GDI engine is mainly associated with semi volatile organic compound(SVOC)such as PAH.Finally,the systematic approach was conducted to investigate the impact of gasoline detergent additives and injector deposits on particulate emissions of GDI engines,through the comprehensive studies using optical spray characterization,injector deposit morphology analysis and vehicle durability emission tests.The mechanism on how the injector deposits impact on GDI engine particulate emissions was elaborated.Injector deposits resulted in higher droplet velocity at the early stage of injection,longer spray penetration,larger liquid fuel droplets when needle valve was closed,eventually leading to significantly increased particulate emissions due to possible wall-impingement,large liquid fuel droplets and fuel absorbed by porous deposits.