Experimental Investigation Of Particulate Matter And Optical Diagnostic In Gasoline Direct Injection Engine

Energy and the environment are two hot topics that have attracted much attention in contemporary society,thus the goal of optimization and control of internal combustion engines is to be efficient and clean.Although the sales of new energy vehicles are rising,gasoline vehicles will still dominate the light vehicle market over the next few decades.At present,the most promising technologies for low-carbonization of gasoline engines are highly boosted,downsized and in-cylinder gasoline direct injection.Compared with port fuel injection(PFI),gasoline direct injection(GDI)engines have certain advantages in reducing carbon emissions and improving fuel economy,however they are known to emit more particulate matter(PM)emissions.In order to achieve the goal of efficient and clean for GDI engines,it is urgent to control the emission of particulates from the source of in-cylinder combustion.In view of this,this paper conducted a series of researches on engine tests and optical diagnostic tests from dimensions of technical application and theoretical study,in order to clarify the influencing factors,control methods and forming sources of particles from gasoline direct injection engines.In order to clarify the optimization direction of the fuel injection parameters corresponding to the difference of engine operating conditions,this paper firstly based on the engine bench test,according to the actual operating conditions of tested GDI engine,studied the influence of injection timing and the injection pressure on combustion and particulate emissions under multi-speeds and multi-loads conditions.It was found that under 2000r/min conditions,the peak of cylinder pressure and heat release rate first increased and then decreased with retarding the injection timing,while they continuously increased under lower and higher speed conditions.When increasing engine load,the fraction of large particles was enlarged,the minimum value of particle number concentration increased first and then decreased.While when increasing engine speed,the peak of accumulation mode particles moved to smaller size,the minimum value of particle number concentration gradually increased.For all cases,with retarding the injection timing,number concentrations of total particles,nucleation mode particles and accumulation mode particles decreased to the lowest and then increased,and the corresponding injection timing of the lowest value advanced as the engine speed and load were increased.Injection pressure showed relatively strong influence on combustion process at low load condition,the in-cylinder pressure and heat release rate increased first and then decreased with elevating fuel pressure.While at mid to high speed and load conditions,the combustion process varied very little.At all cases,injection pressure did not sensitively change the shape of particulate number-size distribution.The total particles concentration continuously dropped at low speed and mid speed-high load conditions as injection pressure was elevated,while it showed a rise trend when further increasing injection pressure at mid speed-low load and high-speed conditions.For the particles mass concentration distribution,the change in the first peak was related to changes in peak of nucleation mode and accumulation mode number concentration and which is dominant between the two.The second peak was relatively sensitive to the number of particles due to the large size of the particles in its range,resulting in random changes.Generally,as the load increased,the third peak gradually disappeared,and as the speed increased,the third peak gradually raised.The effects of spark advance,injection pressure,excess air ratio and exhaust gas recirculation(EGR)on the combustion and particulate matter emissions of GDI engine were investigated through five typical operating conditions.Based on this,analyses of signal-to-noise ratio and variation of control parameters were conducted by using Taguchi method.The multi-parameter collaborative optimization strategy based on output target orientation was determined under stoichiometric and lean-burn combustion conditions respectively,the quantitative sensitivity levels of tested parameters for the specific output performance,especially the fuel economy and particles emission,were presented.The results showed that Retarding ignition timing,rising injection pressure,increasing excess air ratio and inducing proper hot EGR would reduce number concentration of nucleation and accumulation mode particulate,and the later three approaches could improve brake specific fuel consumption(BSFC)to different degrees at the same time.Retarding ignition timing and adding EGR increased the fraction of nucleation mode particles,while rising injection pressure and increasing excess air ratio reduced the fraction of nucleation mode particles.At stoichiometric combustion condition,the lowest particle number(PN)was expected to obtained at 17°CA BTDC spark timing,7.0 MPa injection pressure,60°CA ATDC injection timing and 20% cooled EGR.While at lean-burn combustion mode,they were 17°CA BTDC spark timing,7.0 MPa injection pressure,90°CA ATDC injection timing and 20% cooled EGR.The factor of EGR mode affected NOX emissions and BSFC much more important than other factors at both combustion mode conditions.Injection timing showed the highest contribution to the variation of PN emissions,especially at lean-burn combustion mode(the percentage contribution is 96%).EGR mode and spark advance affected PN emissions for 13% and 10% contribution respectively at stoichiometric combustion mode.In order to deepen the mechanism of dilution combustion and provide basis and direction for the optimal control of diluents,this paper deeply discussed the air dilution and exhaust gas dilution of GDI engine.Influence of diluents on combustion and particulate matter emissions were investigated using three elemental gases(and Ar).It was found that CO2 dilution affected the combustion process most,followed by N2 and then Ar.For CO2 and N2,with increasing the addition ratio,the flame development and combustion duration were extended,the combustion center was retarded and the cycle-by-cycle combustion variation was deteriorated.With the increase of addition ratio,the BSFC decreased first and then increased for N2,while it continuously raised for CO2 and continuously decreased for Ar.Under the same addition ratio,Ar dilution showed the best BSFC,followed by N2 and then CO2.For THC emission,CO2 dilution showed an effective decrease mainly due to its high exhaust gas temperature,but N2 and Ar dilution showed a slight increase.All gases can suppress NOx emission,CO2 dilution emitted the lowest level of NOx emission at the same addition ratio,followed by N2 and then Ar.Each dilution gas showed similar trend in total,nucleation mode and accumulation mode particles.As increasing the addition ratio from 0 to 10%,CO2 addition kept decreasing in particle number concentration,while Ar and N2 dilutions kept increasing.When further added N2,the particles also decreased.Under each dilution condition,the particle mass concentration distribution showed a bimodal distribution,and the first peak change was consistent with peak trends of the nuclear mode and accumulation mode particles.As the addition ratio increased,the relationship between the second peaks corresponding to the three gas dilution conditions did not change,but their absolute values increased.For GDI engines,the best BSFC-NOX relationship could be obtained when using Ar dilution.That is when the NOx emission is reduced to the same level,Ar dilution got the lowest BSFC,followed by N2,and CO2 is the highest.Ar showed insufficient effect in suppressing particles number concentration,CO2 dilution could greatly reduce the number of particles in the exhaust gas while ensuring a little economical sacrifice.However,under large addition ratio condition,the combustion process was deteriorated more,and particulate matter emission raised.In order to clarify the main sources of soot within combustion chamber of GDI engines,and to provide theoretical support and verification for the ‘parameter input-exhaust output' way of particles control.This paper established a quantitative measurement platform of soot based on laser induced incandescence(LII)on a direct injection spark ignition(DISI)optical engine,to study soot distribution and development on specific plane in the cylinder.Besides,other optical diagnostic techniques(high-speed photography,OH*-chemiluminescence and OH-PLIF)were combined in order to observe distribution and evolution information of soot and hydroxyl(OH)within combustion process in a fully spatial-fully temporal range.The results showed that,from the spatial point of view,the in-cylinder soot was first produced near the edge of the combustion chamber,and as time passes,the soot cloud gradually diffuses toward the center of the combustion chamber.Pool fire on the piston surface moved down together with piston,and sooting flame rose upward during this process,resulting in the highest average soot volume fraction measured on the 10 mm plane during 50-70 ° CA ATDC.In the same observation plane,45.5mg/cycle condition presented the highest average soot volume fraction,and soot was concentrated on the side close to the intake valve and injector.From the temporal point of view,the premixed flame propagated rapidly from the vicinity of the spark plug in the center of the cylinder to the periphery on the process of ignition to about 10 °CA ATDC,within which the OH* fluorescence signal gradually increased but no obvious soot was generated.At 15 °CA ATDC,fuel film on the top of piston and partially rich mixture on the edge of chamber were ignited,and soot luminosity signal began to appear.At 30 °CA ATDC,the in-cylinder soot luminosity and OH* fluorescence reached the maximum at the same time,indicating that the stage of soot rapid generation is also the most favorable process for soot oxidation.By combining the results on the three planes,it could be presumed that the in-cylinder soot reaches a maximum at around 30-40 ° CA ATDC.From 40-70 °CA ATDC,sooting flame became large and scattered,spatial distribution of soot became more uniform,the average volume fraction of soot became smaller,the soot luminosity and OH* fluorescent signal both became weaker but OH continued to oxidize soot.After 70°CA ATDC,the OH* fluorescence basically disappeared,but some sooting flames were still burning.At this time,the new formed soot and the remaining soot were slowly oxidized by oxygen,and moved randomly with the in-cylinder air flow until it was emitted out of the cylinder.The soot luminosity signal and the OH* fluorescent signal of the combustion chamber showed the same trend in the range of 0-70°CA ATDC,the soot-PLII signal and OH-PLIF signal showed the same trend in the range of 40-70°CA ATDC on the 10 mm plane.Based on the results of the used optical diagnostic techniques,it can be concluded that pool fire and locally rich mixture combustion are the main sources of soot for the tested DISI optical engine.During normal injection,the pool fire continued until about 50 °CA ATDC,and the sooting flame caused by the partially rich mixture continued until about 70 °CA ATDC,while for advanced injection timing,pool fire and sooting flame would last to the exhaust stroke.