| The stringent emission regulations and energy shortage put forward strict requirements for high efficient and clean combustion of internal combustion engines.Gasoline compression ignition(GCI),which is a new low temperature combustion concept,can obtain high efficiency and low emissions simultaneously without many modifications of standard diesel engines.There have been many investigations focusing GCI combustion,indicating that intake conditions and control strategies are the key factors to achieve high efficiency,however,little research has been carried out to synergistically optimize fuel activity and combustion technologies in a wide load range to achieve high efficient and clean combustion.In this paper,a multi-cylinder diesel engine was used and the fourth cylinder was modified to a test cylinder.The intake conditions,injection timing,exhaust gas recirculation(EGR),mixture preparation strategies and fuel octane number were coordinated in order to achieve high efficiency and low emissions in a wide load range.Fuel reactivity is represented by octane number in this paper,n-heptane and isooctane were used to build four primary reference fuels(PRF)with the research octane number(RON)of 90,80,70 and 60,denoted as PRF90,PRF80,PRF70 and PRF60,respectively.The engine performance and emissions of the four PRFs were tested under different conditions,and the results were compared with those of actual low-RON gasolines.In this paper,the effect of intake pressure on combustion and emissions characters of GCI combustion was first investigated under direct injection mode.On this basis,the effect of intake temperature on combustion and emissions of PRF90 were investigated in a wide load range,and the intake temperature and fuel octane number were further coordinated to optimized GCI engine performance and emissions.The results showed that the increase in intake pressure enhanced the in-cylinder pressure at top dead center,thus improving the fuel economy and reducing CO,THC and unregulated gas(alkane,olefin and aldehyde)emissions.The increase in intake temperature also enhanced the air amount in the cylinder,decreasing the maximum mean gas temperature,which ultimately decreased the NO_x emissions.Under low loads,the GCI performance can be improved by heating the intake charge properly,thus improving the fuel economy and decreasing CO,THC and unregulated gas emissions,but increasing the NO_x emissions.However,the fuel economy would decrease if the intake temperature is too high.In addition,the influence of intake temperature on GCI combustion and emissions decreased with the decrease in fuel octane number.Therefore,it could be summarized that high efficient GCI combustion in a wide load range could be achieved by increasing the intake temperature properly under low loads and using supercharge under all loads.However,it is not enough to only adjust intake conditions to solve the problem of high NO_x emissions and combustion noise under medium and high loads.In order to explore the adjusting range of injection timing for different active fuels under each load and achieve high efficient,stable and clean combustion,the effects of injection timing and EGR on GCI combustion and emissions were further studied,with the maximum in-cylinder pressure rising rate and NO_x emissions as targets.The results showed that the injection timing adjustment ranges of different active fuels under different loads,as well as the injection timing corresponding to the high efficient and stable operation point were different.The results also showed that adopting EGR increased the specific heat capacity of the air-fuel mixture and reduced the oxygen content in the cylinder,which ultimately reduced the NO_x emissions of all the fuels.Moreover,the effects of EGR on different fuels were also different,PRF90 with higher octane number was more sensitive to EGR,the combustion parameters and emissions of which changed obviously with the increase in EGR rate.Based on the above investigations,it can be concluded that high efficient,clean and stable GCI combustion can be achieved in a wide load range under the direct injection strategy through coordinating fuel octane number,intake conditions,injection timing and EGR rate.Considering the importance of mixture preparation strategy,this paper further optimized the GCI combustion and emissions by introducing port injection.Firstly,the combustion and emission characters of the four fuels was investigated under port injection.Then the engine performance of direct injection strategy and double injection which consisted of port and direct injection,were also compared.The results showed that compared to direct injection,port injection possessed lower indicated thermal efficiency,which was lower than 40%under most test condition,however,port injection had obvious advantages in NO_x emissions.The NO_x emissions under port injection were lower than 10 ppm under all test conditions,and even lower than 1 ppm under partial loads.Under high load conditions,double injection mode possessed lower in-cylinder pressure rising rate but also lower indicated thermal efficiency than direct injection.It is worth noting that introducing port injection would cause more fuel squeezed into the boundary layer during the compression stroke,this part of fuel was released during the expansion stroke,which resulted in insufficient combustion.Therefore,port injection and double injection produced more CO,THC and unregulated gas emissions such as alkanes,olefins and aldehydes.In addition,the GCI engine performance of different active fuels under different injection modes were also significantly different.Under port injection,the operating range of PRF90 with higher octane number was 4~5 bar,while those of PRF70 and PRF60 were 1~4 bar.Under double injection,PRF90 with higher octane number only presented a two-stage heat release,and had lower peak heat release rate of the first-stage than other fuels.High efficient,stable and clean GCI combustion has been achieved in a wide load range based on PRFs,and PRF70 and PRF80 presented excellent engine performance and low emissions.To deeply understand the GCI combustion,the combustion and emission characters of low-RON gasolines with the octane number of around 80 and 70were investigated under different test conditions,and the results were compared to those of PRF80 and PRF70.The results showed that under direct injection mode,the low-RON gasolines had better fuel economy,because they possessed more components with lighter weight and higher adiabatic flame temperature,which contributes to more sufficient evaporation and higher heat release rate.Under port injection,the operating range and control strategy were different for low-RON gasolines and PRFs,G70 could only be burned when the load was 3~4 bar,and G80 could only be burned in the range of 4~5 bar when the intake temperature is 50 ~oC.Under port injection and double injection,G70 and PRFs presented obvious low temperature heat release,while G80 did not.In terms of emissions,the low-RON gasolines produced more CO,THC and unregulated gas emissions under low loads as well as port injection mode,while presented the opposite under medium and high loads due to their high combustion rate.Based on the investigations in this paper,high efficient and clean GCI combustion could be achieved in a wide load range through synergistically adjusting the intake conditions,injection timing,mixture preparation strategy and fuel octane number,and it is worth noting that the fuels with the RON around 70 presented the best combustion and emissions performance,especially G70 possessed the indicated thermal efficiency over 50%under direct injection.In terms of combustion characteristics,the maximum in-cylinder pressure rising rate could be controlled below 10 bar/~oCA under most test conditions.In terms of emissions,the NO_x emissions were lower than 500 ppm through using proper EGR rate,especially the NO_x emissions under port injection were nearly zero without supercharge and EGR. |
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