Research On Combustion Mechanism And Emission Control Of Mixing Flows In The Diesel Engine

With the increase of vehicle population and the stringency of emission regulation, one keyproblem in energy and environmental fields is to decrease emission from internal combustionengines (ICE). Diesel engine, as power device with strong dynamic, high thermal efficiency, lowfuel consumption, has been widely used. However, it also has higher NO_x and particles (carbonsmoke) emissions and which restricted by “NO_X－particles” tradeoff curve. In order to improve theemissions characteristic of diesel engine, this paper studies the burning mechanism of mixingcombustion by the way of the burning visualization experiment and the value simulation in theZS195diesel engine, explores the purpose of reducing NO_x and particulate by the internalpurification to crack “NO_x–particles” emission problems of diesel engine.An optical engine modified from production engine with observation window provided on thecylinder-head has been developed beyond the actual situation and the advantages and disadvantagesof structure type of visualization system. Before applying the way of three-primary-color fortemperature measure, calibrating the optical measurement system for temperature. The results showthat the temperature values of calculation and measurement are very similar, and which is credible inthe project. The ignition and combustion process of diesel engine are analyzed by the use ofthree-primary-color in the optical engine. The results show that the temperature distribution analyzedby three-primary-color consistent with which obtained by other methods, so the methods ofthree-primary-color is an effective way to measure and analyze the ignition, combustion andflame-propagation process in cylinder of ICE. In the meantime, the research result of the fuelignition and combustion process in diesel engine taken by the methods of combining the temperaturefields with indicator diagram is corresponded with that of real engine.The combustion characteristics of intake air blended with hydrogen, EGR, hydrogen and EGRhas been studied in the optical diesel engine. The experimental results show that the ignition timeshowing delay of the trend and the combustion duration reduction because of incomplete combustionas EGR rate increasing when EGR technique is adopted. EGR system with increasing of EGR ratewill result in peak value of the cylinder pressure and heat release rate decreasing, the curve ofcylinder pressure and heat release rate postponing which will lead to negative effects on powerperformance and fuel economy of the diesel engine. In addition, EGR system with increasing ofEGR rate will also result in maximum temperature and average temperature of cylinder reducing andoxygen concentrations decreasing which will lead to beneficial effects on NO_Xemission reducing.When the intake air mixes with hydrogen, the ignition time appears ahead of trend as the hydrogenratio increasing; the combustion duration shorten because of the rapidity of combustion speeds upand the peak of both the cylinder pressure and heat release rate increase and becomes ahead of timewhich will lead to beneficial effects on power performance and fuel economy of the diesel engine. Inaddition, cylinder maximum temperature and average temperature increase accordingly as thehydrogen ratio increasing. When the intake air blends with hydrogen and EGR and the EGR rate isconstant, the ignition time appears ahead of trend as the hydrogen ratio increasing; the combustionduration shortens with small EGR rate and extends with big EGR rate, the peak of both the cylinder pressure and heat release rate increase and becomes ahead of time and cylinder maximumtemperature and average temperature increase accordingly as the hydrogen ratio increasing. Inaddition, under not producing a significant impact on engine power and economy and decreasing inoxygen concentrations, the intake air mixing with hydrogen and EGR can improve combustionwhich is advantageous in reducing the harmful emission of diesel engine.In simulated exhaust diesel reforming gas (SEDRG) conditions, when the EGR rate is constant,there is fire timing advance with increasing of hydrogen ratio and more effect on ignition in H2thanCO. Hydrogen has more effect on combustion of mixed gas from the flame development picture.Due to mixed gas of SEDRG larger and total intake gas and spray oil keep not variable, there is theexcessive air coefficient small, mixed gas concentrated and oxygen volume number declined whichwill led to fuel not full burning and combustion duration shorten with burning end early. Asincreasing of hydrogen ratio, combustion gradually improved and combustion duration time beganextended. When the EGR rate is constant, the peak of both the cylinder pressure and heat release rateincrease and becomes ahead of time as the hydrogen ratio increasing; while cylinder maximumtemperature and average temperature increase accordingly.Three-dimensional-combustion model has been developed using FLUENT software. Thecylinder pressure of the different programs is simulated in the ZS195diesel engine under calibrationconditions. The modeling results are good agreement with the measured data obtained from aparallel experimental investigation, so the combustion model of establishment is feasible. The flowfield, temperature, oxygen concentrations and harmful emission are analyzed and the effects ofdifferent programs in SEDGR condition on work process of diesel are contrasted. The velocityvector was influenced largely by the fuel injection quantities at the begging of fuel injection. Thecylinder oxygen concentration drops with increasing of mixed gases volume. When the EGR rate isconstant, cylinder maximum temperature increases with increasing of hydrogen ratio. When thehydrogen ratio is constant, cylinder maximum temperature decreases with increasing of EGR rate.The effect of CO admission is little on combustion temperature which is agreed with experimentalresults.The forecast analysis is studied on the diesel harmful emission of different programs withthree-dimensional-combustion model in SEDGR condition. The value of both NO_x and sootemission of program A are large. The results of NO_x emission decreasing are the same and sootemission are better and better from program D to program C and then program B among program B,program C and program D with5percent EGR. In the program E, program F and program G with15percent EGR, the soot emission of three programs especially program F are all decreased largely andNO_x emission decreasing are better and better from program E to program F and then program G.Therefore, the SDEGR technique can reduce NO_x and soot emission and provide an importanttechnical approach for resolving “NO_x–particles” emission problems of diesel engine.