Investigation And Numerical Simulation Of Effect Of Boundary Condition On Diesel Engine Combustion Process Under Transient Operating Conditions

With the development of automobile industry, automobile tenure increasesdeeply. With people's increased caring about automobile exhaust pollution problem,automobile exhaust regulations are stricter than before in America, Japan, and Europe and soon. Automobile exhaust regulations also have been issued in China, emphatically on NOx andparticulate limits to diesel engines. The performance of automotive diesel engines undertransient operating conditions is much different and some pollutant ingredient emissions andnoise are higher than that under steady operating conditions. What's more, the phenomenon ismore obviously seen in turbo-charged diesel engines. Considering the real operating operationsof the vehicle engines and in order to restrict the pollutant emissions, transient test cycles fordiesel engines have been applied in US(EPA transient cycles) and EU(European transient testcycle). The combustion process, the emissions behavior and the performance improvement ofautomotive diesel engine under transient operating conditions have been a very importantresearch field.Establishment of control and measure system for transient operatingcondtionsA repeatable transient condition control system has been realized by using a MCS51single chip processor (SCP) as core control element and a step motor as real time actuator. Twokinds of typical transient operating conditions at constant speed and constant torque can berealized by a conventional eddy current dynamometer combined with the developed transientcondition control system. After developing the communication function of eddy currentdynamometer, air flow-meter, fuel flow-meter smoke meter the measured data can be taken inanalog signal for after treatment. After designing data acquisition program a high frequencyA/D (Analog/Digital) converter board labeled as PCL 818HG can deal with and convert theanalog signals mentioned above to digital signals and feed to computer. So, the performance ofa diesel engine under transient operating conditions can be measured by this A/D convert boardsystem.The DS9100 system is used to measure the in-cylinder pressure cycle to cycle. Using thetrigger pulse generated by SCP the synchronization of DS9100 and the developed transientcondition control and measurement system can be realized. The normalized heat releases andin-cylinder combustion temperature are calculated from in-cylinder pressure measured byDS9100. Combustion start and end points, heat release rate, combustion duration andcombustion temperature etc. are calculated by DS9100. Thus, the combustion process can beanalyzed in-detail by the system. The shortcoming of being disabling to measure particulateemission under transient operating conditions has been made up by using a special bagsampling system. This system can measure the average particulate emission of transientoperating conditions under the precondition that the sampling ratio is constant.It is obvious that developed transient operating conditions control and measurement system canrealize the control of diesel engine typical transient operating modes. The major real-timeparameters such as combustion process parameters, power output, fuel consumption, air inlet,air-fuel ratio, smoke opacity etc. can be measured. At the same time, the average particulateemission can be measured too. Therefore, this control and measuring system has provided thefundament hardware for the researching of turbo-charged DI diesel engine behavior undertransient operating conditions.Establishment of combustion and emission simulation for diesel engineEstablishing of air intake, spraying, combustiom and emission model, this can be used tosimulate combustion process of disesl engine. By changing the combustion boundaryconditions, the effect of that on disesl combustion performance can be simulated. Geometrymodel and good mesh quality of CA6DE1-21K disesl engine was got. By experimentalverification, this model can be used to simulate disesl engine combustion and emission process.The combustion process of 1000r/min, 75%load perating condition of CA6DE1-21Kdisesl engine was simulated. The changing history of fuel concertation, velocity, temperature,NO concertration and Soot concertration spatial distribution with crank angle were got bysimulation. The simulation results show that:To this operating condition, the permeability of fuel is 0.833, this value isvery perfect for improve the combustion and emission performance ofdiesel engine.After leading fuel reached the combustion chamber wall, most of fuel ismoving along the wall and enters the combustion chamber, at the sametime, a strong swirl is generated by which in the combustion chamber.Little of fuel is moving up and enters the small region between the plattop of piston and lower surface of cylinder head, and this fraction of fuelhas obviously effect on NO emission.The analysis of NO mass concentration spatial distribution shows that:most of NO is generated around of flame, and more than 60% of NO iscoming from the small region between the plat top of piston and lowersurface of cylinder head.The analysis of Soot mass concentration spatial distribution shows that:the generation of Soot is starting from the time of fuel injected intocombustion chamber;the Soot and fuel mass concentration spatialdistribution are very similar;about 95% of Soot are oxygenated beforeexhausting.The analysis of temperature spatial distribution shows that: although themost combustion temperature being more than 3000K and the averagecombustion temperature being more than 2000K, the temperature ofregion closed to cylinder wall is lower than 1000K during the wholecombustion process. The above result is got with the assumption that thewall is adiabatic, and if consider the wall cooling, the temperature of thisregion will be much lower.Investigation and simulation of constant speed increasing torque operatingconditionsIn this study, combustion process of CA6DE1-21Kdiesel engine undertransient operating conditions were investigated and simulated. The results showthat: the air intake and fuel consumption are increasing with the rise of torquerising rate, but the air-fuel ration falls down and this trend is very obvious underlow speed operating conditions. The analysis of 1000r/min, 75%load operatingcondition shows that: the torque changing rate have little effect on combustionstarting point and premixed combustion heat release maximum;with the increasingof torque rising rate, diffusion combustion heat release maximum increased and hasback moving trend, and the center of gravity of heat release has back moving trendand combustion duration extended.Emission results show that: PM and soot emission are increasing with thetorque rising rate, especially under the low speed conditions, and the maximumaugmentation is more than four times under 1000r/min operating conditions. Bysimulation concluded that: there are no big differences during the first combustionstage between soot emissions, but the peak of soot emission curve increase with thetorque rising rate. It is the main factor leading to soot emission increasing that thefuel injected is increasing with the torque rising rate, and the increased fuel isinjected in the high temperature and lean oxygen region. The peak of soot emissioncurve of 5 second increasing torque operating condition(5SITOC)is more 35.4%than 15SITOC, and the soot emission of 5SITOC is more 150.5% than 15SITOCbefore the exhaust clack opening. The torque rising rate has little effect on NOemissions. The decreasing of oxygen mass concentration leads to NO emissionreducing under 5SITOC. Most of NO is coming from the small region between theplat top of piston and lower surface of cylinder head.Investigation and simulation of effect of air intake temperature on combustionprocessThe effect of air intake temperature on combustion process were investigated andsimulated. Results show that: air-fuel ratio increases with the reducing of air intaketemperature, especially under low speed operating conditions. The effect of airintake temperature was simulated under 1000r/min, 75% load operating conditions.Results show that: air intake temperature has little effect on combustion startingpoint and premix heat release peak, and the center of gravity of heat release ratecurve has back moving trend with the increasing of air intake temperature.Emission results show that: air intake temperature has obviously effect on sootemissions, especially under low speed operating conditions. The simulation resultsshow that: oxygen mass concentration increasing is the main factor leading to thesoot emission reducing under lower air intake temperature operating conditions.Before exhaust clack opening, the soot emission under 35℃ air intake temperatureoperating condition is less 22% than 55℃ air intake temperature operatingcondition.Investigation and simulation of effect of fuel supply advance angle oncombustion processThe effect of fuel supply advance angle on combustion process were investigatedand simulated. Results show that: fuel supply advance angle has obviously effecton combustion process. The effect of fuel supply advance angle was simulatedunder 1000r/min, 75% load operating conditions. Results show that: withincreasing of fuel supply advance angle, combustion delay period prolonged,combustion duration shortened and the center of heat release rate curve advanced.The maximum combustion pressure under 14oCA BTDC fuel supply operatingcondition is more 25% than 6oCA BTDC fuel supply operating condition.Emission results show that: fuel supply advance angle has obviously effect on sootemissions, especially under low speed operating conditions. The simulation resultsshow that: average combustion temperature increasing is the main factor leading tothe soot emission reducing and NO emission increasing under large fuel supplyadvance angle operating conditions. The maximum average combustiontemperature under 14oCA BTDC fuel supply operating condition is more 130Kthan 6oCA BTDC fuel supply operating condition. Before exhaust clack opening,the soot emission under 14oCA BTDC fuel supply operating condition is less88.7% than 6oCA BTDC fuel supply operating condition. The peak of NOemission curve under 14oCA BTDC fuel supply operating condition is more 20.4%than 6oCA BTDC fuel supply operating condition. In addition, the increasing offuel fraction entering the small region between the plat top of piston and lowersurface of cylinder head is another factor leading to NO emission increasing underlarge fuel supply advance angle operating conditions.