| The energy scarcity and deterioration of the global environment have become the focus ofglobal attention. Although the world has seen a great development in automobiles of new typeclean fuel, the traditional internal combustion engine fuelled with gasoline and diesel wouldstill be the mainstream of development during a long period time in the future. The dieselengines are widely used in automobile, transportation, agriculture and engineeringmachineryfor its high compression ratio, low losses in thermal isolation and pump gas, powerperformance and economy efficiency. However, the diesel has the characteristics of highviscosity, less volatile and mixture gas difficulty, resulting in exiting local high temperature andover-rich region during traditional diesel engine combustion.The traditional emissions of thediesel engine such as NOxand particulate matter (PM) are relatively higher, restricting thedevelopment of diesel engines.Applications of new technologies reduce the quality of PMemissions and make it ultrafine tended, however, the amount of PM increase and the ingredientsbecome more complex, which will be greater harmful to the environment and human health.Meanwhile, the future emission regulations will restrict not only the quantity of PM but also itsnumber.Therefore, the study of ultrafine particle emission performance in diesel engines alsohas significant meanings.In recent years, new generation combustion theory achieves great development.Researches have proven that higher thermal efficiency and lower exhaust gas emissions can beachieved bythe new mode of premixed combustion. Due to the restriction of fuel volatility inthe process of traditional premixed combustion in diesel engine, advance angle of injectionmust be widenfor achieving higher proportion of premixed combustion. The penalty is high HCand CO emission under low load and narrow operating window of efficient clean combustion.The volatility of gasoline is better compared with diesel, making it easier to achieve morehomogeneous gas mixture, which is helpful to resolve the problems of uneven distribution ofmixture in diesel engine. Therefore, it is how to make full use of advantages of homogeneousgas mixture in gasoline engine and high thermal efficiency and better ignitability in dieselengine to reduce pollutant emissions and achieve new combustion mode of gasoline/dieselunification that has become a hot issue for present internal combustion engine researchers.In this study, a test platform of partially premixed compression ignition (PPCI) based on ahigh-pressure common-rail turbocharged diesel engine was established and the dilutionsampling system for diesel engine exhaustswas self-designed, which Meet the needsof theexperimental study, in order to further study the influence of fuel properties and combustionboundary conditions on the combustion and emissions. Also, we have developed simulation research platform by AVL-Fire, simulated the key combustion boundary and revealed theinfluence mechanism of combustion boundary conditions on activate atmosphere and hotatmosphere in cylinder.The different ignitability, volatility and sulfur content of fossil fuelswere made up according to certain proportion ratio on the basis fuels of straight-run diesel,hydrocracking diesel and kerosene to reveal the influence of the basic fuel characteristicson theparticle number and size distribution. The experiments studied the influences of physical andchemical characteristics of fuelstoultrafine particle emissions. The results turned out that theparticulate emissions of high-pressure common-rail diesel engines are mainly nucleation mode,which occupies72%of the total particles under different conditions, and the ultrafine particlesthat less than100nm take the proportion of over94%. In the working condition of externalcharacteristic, with the increase of speed, nucleation mode particles have a slight increase whilethe amount of accumulation mode particles have a little change. When the engine speedincreases sequentially, the amount of both nucleation mode and accumulation mode particleincreases rapidly, leading to the increase in total particle concentration. With load increases, theamount of nucleation mode and total particlesincrease significantly while the accumulationmode particles change little. One of the important factors that affect the engine particleemission and particle size distributionis fuel sulfur content, and its influence for nucleationmode particle is more obvious compared with accumulation mode particle. With fuel sulfurcontent increases, the concentration of ultrafine and total particles increase obviously. Underhigh load condition, the influence of fuel sulfur content to the number of particles becomesmore obvious. By improving fuel cetane numberproperly, the ignition delay period decreases,the amount of premixed combustion reduces, and the proportion of diffusive combustionincreases, which are useful to reduce the number of nucleation mode particle emission. Thestraight-run diesel（CN66）owes higher fuel sulfur content and cetane number, making it easierto split during combustion, thus resulting in the increase number of nucleation mode and totalparticle. When the cetane number exceeds50, the percentage of DS rises and the proportion ofSOF decreases as the cetane number increases.The fuel’s cetane number which is controlledbetween50and60is proper in practical application. Improving fuel volatility can contribute tothe mixing of fuel and gas, making the gas mixture more homogeneous, which can reduce thenumber and quality emission of particles under high load condition. However, the concentrationof nucleation mode and ultrafine particle increases in light and medium load conditions whilethe concentration of accumulation mode particle changes little.In order to further study the influence of the fuel characteristics and combustion boundaryconditions to combustion and emission, the experimental and simulation study of the influence of injection parameters and EGR to the activate atmosphere and the hot atmosphere in cylinderis conducted, illustratedthe influence mechanism of combustion boundary conditions to fuelatomization, evaporation, and oil and gas mixture in cylinder and further revealed themechanism of combustion boundary conditions on the active atmosphere and the hotatmosphere in cylinder. On this basis, in order to study the influence of the coordinated controlstrategyof fuelcharacteristics and combustion boundary conditionsto combustion and emissions,the gasoline and diesel were blended according to certain proportion for ignitability, volatilityand distillation range between present gasoline and diesel fuel for wide distillation range in thisstudy. Through flexible control of fuel injection parameters and inlet parameters, the controlledmethods and technological approaches for realizing partial premixed compression ignitionmodel of gasoline/diesel fuel blends were explored. The combustion and ultrafine particleemission of premixed combustion engine were also analyzed. The results have shown that whenburning pure diesel fuel, with the main spray time advances, the ignition delay lengthens, whichcontributes to achieve higher proportion of premixed combustion, but it can result in theincrease of NOxemission. Meanwhile, it exits the optimal advance angle of fuel injectiontosootemission and fuel economy. In addition, the advance of main spray time contributes to a slightdecline for all modal particles under low load condition. The influence of changing main spraytime has little effect on the distribution of particles concentration. It can form certain premixedmixture before the main injection fuel burningthrough thepre-injection strategy, which furtherincreases the amount of premixed mixture, and the pre-injection time and ratio have some effecton combustion and emission.The amount of premixed combustion, emission and economyperformance can be managed well by controlling pre-injection time and ratio reasonably. Theslight increase of pre-injection quantity can reduce the number of accumulation mode particleconcentration under low load condition. Increaseappropriately injection pressure can reduceNOx emissions, but the smoke emission increases slightly. At the same time, the over-highinjection pressure can lead to fuel economydeterioration. In the study of particle sizedistribution of particulate emission, it illustrated that with the increase of injection pressure, themodal particle number concentration increase slightly. For gasoline/diesel blended fuel, withthe increase proportion of gasoline, the ignition delay lengthens, the combustion durationreduced accordingly, which is helpful to increase the amount of premixed combustion, andreduce soot emission and improve the constant volume combustion. However, it will lead toincomplete combustion products, low combustion efficiency and thermal efficiencywith toohigh gasoline ratio. Meanwhile, under medium load conditions by using wide distillation rangefuel can significantly reduce the number of nucleation mode and accumulation modeparticleconcentration. The results showed that by burning G30coupled with thecombustion boundary conditions such as fuel injection strategy and EGR, the target of reducingthe number of nucleation mode, accumulation mode and total particles in compression ignitionengines can be achieved. Therefore, in order to ensure the thermal efficiency, reasonableselection of the suitable proportion of mixed gasoline is vital. Through the fuel characteristicsand combustion boundary conditions of cooperative control, it can be found that appropriateselection of the suitable proportion of mixed gasoline and reasonable control of the combustionphase position can ensure relatively high combustion efficiency and fuel economy in a certainrange. At the same time, with the coordinated control strategy of the combustion phase positionand EGR, further improvements on the wide distillation range fuel’s combustion and emissioncharacteristics can be achieved without affecting the thermal efficiency, meanwhile reducingNOxand particulate emissions. |
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