Study On The Effect Of Fuel Components On Combustion And Emissions Performance Under GDI Engine

With the aggravation of environmental pollution and energy crisis,the emissions and fuel economy of internal combustion engines(ICE)have been placed increasingly stringent requirements.Adjusting fuel characteristics is an important way to optimize combustion and emission performance of ICE.Chemical components are the fundamental factors determining the fuel properties.Hence,it is of great importance to understand the laws and mechanisms that fuel components influence engine combustion and emission characteristics for fuel components design and engine performances optimization.Gasoline fuel consists of many chemical components.In order to simplify research,n-paraffins,iso-paraffins,aromatics,olefins and naphthenes are used to build nine surrogate fuels for gasoline.N-heptane,iso-octane,toluene,diisobutylene(DIB)and cyclohexane were selected to form the basic five-component surrogate fuel(CDTRF)for gasoline.On this basis,eight derived surrogate fuels(CDTRFMB and CDTRF-MP,CDTRF-OX and CDTRF-EB,CDTRF-P and CDTRF-H,CDTRF-MH and CDTRF-EH)were obtained by changing the representative hydrocarbons of iso-paraffins,aromatics,olefins and naphthenes,respectively.There is only one component type difference between each derived surrogate fuel and CDTRF,and this component is called the representative constituent of the derived surrogate fuel in this paper.The research octane number of the nine surrogate fuels was maintained at 95 by adjusting the molar ratio of n-heptane and representative constituent.In this paper,the engine speed characteristics and load characteristics of the all test fuels were experimentally carried out on a fourcylinder gasoline direct injection(GDI)engine.The differences in combustion phase,emission characteristics and fuel economy between surrogate fuels and 95# gasoline under various engine speeds and loads conditions are investigated.Under most engine conditions,results show that the combustion phases of the surrogate fuels lag significantly behind that of gasoline,and surrogate fuels exhibit longer flame development durations(FDD).In terms of emission characteristics,the nitrogen oxides(NOx)emissionof gasoline is slightly higher than that of surrogate fuels under all test conditions.Under small and medium loads,total hydrocarbons(THC)emissions,volatile organic compounds(VOCs)emissions and ozone formation potential(OFP)of gasoline are lower than that of surrogate fuels.However,things are on the reverse under high engine load conditions.For all surrogate fuels,VOCs emissions and OFP of CDTRF are comparatively higher under all test conditions.Particulate matter(PM)emissions of surrogate fuels are higher than that of gasoline under medium and small loads.However,things are on the reverse under high engine load conditions.The linear correlation between particulate matter index(PMI)and PM emissions under medium loads is weaker than that of small and large loads,which means that the fuel properties have relatively weaker effect on PM emissions under medium loads.At high loads,PM emissions for high-PMI fuels are approximately three times higher than those for low-PMI fuels.Based on the analyses of fuel reaction kinetics and physicochemical characteristics,it is found that short-chain olefins accelerate the combustion and heat release of fuels under medium speed conditions,while iso-paraffins advance the combustion phase under high speed conditions.Results also show that combustion phase is advanced with the decrease in carbon chain length of iso-paraffins,cycloalkanes and aromatic.Besides,for olefins,the more H atoms and dehydrogenation sites exist in olefins,the shorter the FDD and the more advanced the combustion phase would them be.The long carbon chain components in the fuel are not conducive to the THC emissions.Besides olefins,aromatics and naphthenes are not conducive to NOx emissions and make CO emissions worse.With respect to unregulated emissions,iso-paraffins with shorter carbon chains lead to higher small-molecular emissions(ethylene,acetylene,formaldehyde,etc.)and lower unregulated emissions with C3 and above(propylene,isobutylene,isoprene,etc.).Besides,except for alcohol emissions,other unregulated emissions increase with the increase of the length of olefin carbon chain.This means that more initial reaction paths can not only accelerate the reaction,but also increase the small-molecule emissions.The branching structure of cyclohexane is beneficial for reducing the unregulated emissions.Compared with ethyl side chain,the promoting effect of methyl branch is more obvious.The branching structure of benzene can effectively inhibit the fuel initial reaction and reduce the production of small-molecule unregulated emissions(expect for aromatic emissions).The molecular structure of the representative constituent in the surrogate fuels greatly affects VOCs.Longer carbon chain structure,more double bonds and longer branched chain structure for the ring-structure chemicals all would lead to higher VOCs emission and OFP value.In addition,compared with olefins and iso-paraffins,aromatics and naphthenes are more unfavorable for VOCs emissions.For PM emissions,under small loads,soot precursors are mainly derived from dehydrogenation and debranched reactions of free radicals with benzene ring.More naphthenes and aromatics in the fuel are not conducive to the PM emissions.Therefore,the CDTRF-OX(Representative Constituent: Ortho-xylene)and CDTRF-EB(Representative Constituent:Ethylbenzene)have higher PM emissions.Under heavy load,the double bond equivalent(DBE)and mass ratio of representative constituent exhibit a greater impact on fuel PM emissions.Because gasoline consists more aromatics and naphthenics(≥C9)which have larger DBE and mass.So,gasoline produces more soot precursors such as ethylene and acetylene,resulting in significantly higher PM emissions when compared with surrogate fuels.In addition,under medium and small loads,olefins are more detrimental to PM emissions than other hydrocarbons;under high loads,aromatics are more likely to worsen PM emissions.In terms of fuel economy,under most test conditions,brake thermal efficiency of CDTRF-MB,CDTRF-MP,CDTRF-P and CDTRF-H are higher than that of gasoline.On the contrary,brake thermal efficiency of CDTRF-OX and CDTRF-EB are lower than that of gasoline(except high speed and medium load).