Effects Of Key Initial Boundaries On The Combustion Of Diesel Engine And Their Optimization Strategies

The internal combustion engine has experienced a long-term development with improving optimizations on the structure and combustion technologies since its appearance a hundred years ago. Conveniences have been brought to human life, but the emissions of cars are being tightened all over the world as a result of severe problems from ICE cars, especially of consumption and environment pollutions.Up to now, the diesel engine is the highest efficiency in the power plant area. And it is used for car power plants more and more widely. The difficult of traditional diesel emission control is the NOx-PM trade-off relationship that seems to be irreconcilable.So researchers at home and abroad returned to focus on the combustion progress, some combustion concepts more being focused on were developed: 1. Low temperature lean combustion such as PCCI, PREDIC; 2. Low temperature combustion with the mixture concentration nearΦ=1, such as MK, LTC; 3. Rich and low temperature combustion such as Smokeless Rich Diesel Combustion; 4. Two-stage or multi-stage combustion such as MULDIC, UNIBUS, i.e. high temperature low O2 diffusion combustion after premixed lean combustion.It can be seen that EGR was used to control combustion temperature in all of the low temperature combustion modes and the mixing of O2 and fuel was also reinforcing to accelerate the evaporation of fuel and to reduce the increase of HC and CO caused by a lot of EGR. So EGR and modulation of fuel injection parameters (including injection pressure, injection phasing, distribution of injection quantity, etc) are two key initial boundaries to control diesel engine emissions.To expressly study the effects of these two key initial boundaries on diesel performance and emissions, especially to reveal their microcosmic mechanism, a lot of numerical simulations and experiments were carried out on an electronic high pressure common rail diesel engine, CA6DL2-E3, in this paper. The main work and conclusions were summarized:1. Experiment and simulation beds were set up.Simulation bed was set up by commercial CFD code, STAR-CD. Firstly, according to 6DL2-35E3 diesel geometric parameters, geometric figure was drawn through Pro/E code. Then the part of combustion bowl was input to PRO-AM to divide static meshes, and the other was input to ES-ICE to divide moving meshes, and moreover they were combined to the computation meshes. Initial boundary conditions were configured in PRO-STAR. Computation was carried out by STAR. Then post-processing of the results was done in PRO-STAR. A lot of repetitionary works was done to validate rationality. The final result indicated that the bed is suitable to simulate the combustion of diesel engine.Improvements of computation meshes were performed with further studies. The CMC-Ogrid template(Cylinder mesh centering-O-grid Template)which was created by myself has cylinder meshes at the central part and polar coordinates system meshes at exterior part. The CMC-Ogrid template was used to cut the geometric figure to form combustion bowl meshes. It can avoid the faultage phenomena caused by Couple, special linkage format of STAR-CD, at central part of linkage and produce high quality surface meshes as well. So cost of calculation time has been shortened and the stability of calculation has been improved as well as the characteristic, easy-meshing, of Match piston method is maintained. Also with the amelioration of valve seat ring meshes, actual stroke of piston has been promised and flow distortion around valve seat ring has been effectively improved. The couple of cavitations model, primary and secondary droplet break-up models were used to incarnate the three main spray ingredients (cavitations disturbance, turbulence disturbance, aerodynamic disturbance). Turbulent diffusion model and spray-impingement model were also used to solve the movement history of droplets, which led to higher accuracy of numerical simulations.The low pressure EGR test system that exhaust gas was educed just before turbine and was returned to intake system just before supercharger was founded. To reach the 3% accuracy of EGR rate, EGR rate was correctly adjusted by the combination of EGR valve and manual valve control. Furthermore, using INCA5.4 online calibration system to control exoteric ECU, an open control calibration experiment system of injection figure was founded. This bed on which EGR and injection parameters can be adjusted singly or together, is based on an electric high injection pressure six-cylinder diesel engine which satisfies with the EuropeanⅢregulation.2. The combustion process of prototype engine was simulated and investigated, and the post-processing methods of microcosmic parameters were improved.The combustion process of prototype engine (speed of 1650r/min, load of 50% and single injection MAP) was simulated and analyzed. Two key sectional figures of microcosmic field were obtained to display the process of in-cylinder flow, spray, and combustion: to catch the information of spray area that is more concerned by us and to obtain its movement on swirl orientation, the spray cone section formed through eight injection hole center lines was adopted. A vertical section cut through a pair of injection hole center lines was used to display the movement along with the combustion chamber. The method can show more information with fewer figures.A set of post-processing method based cells for quantitatively analysis of microcosmic parameters was proposed. The cells were filtered based state properties to obtain a key set of cells. Then parameters statistics was carried out based on it. The in-cylinder temperature field has been divided into high, middle and low temperature regions, based on which some statistic quantities can be calculated such as the size of region and the Soot quantity in it. The flow field based on the burning area (T>1100K), the fuel vapor area (Φ>0.1, eliminated nonessential disturbance) can be regarded as key sets of cells, then the volume and mass average kinetic energy can be calculated. The microcosmic parameters concerned with concentration field such as the expectation and departure of equivalence ratio can be calculated based on the same key sets of cells. Furthermore, based on mass fraction and statistics results, every cell in the key set can be distributed on a chart to display the in-cylinder concentration distribution. Based on state properties such as temperature, equivalence ratio, status cloud distributed onΦ-T chart was realized, which can be used to open out the in-cylinder state variation process, and to forecast emissions and so on.The microcosmic parameters have a measurement as a whole through the statistics method. The biggest merit of the post-processing statistics method is to take out the key set of cells firstly, and then calculate the statistics value, which makes the results closer to the reality. It is helpful to display microcosmic mechanism. The post-processing statistics method of microcosmic parameter has laid the foundation for the following work.3. The effects of EGR on diesel combustion and emissions were simulated and investigated.Based on the simulation bed, the effects of different EGR rates, i.e. 2.5%, 5%, 10%, 12% separately were simulated and analyzed. The operation point was 1650 r/min and 50% load (prototype single injection MAP). Correspondingly investigations were carried out and the measured EGR rate were 0.69%, 1.79%, 5%, 7.5%, 10.3% and 12.3% separately.The results of simulations and investigations were the same. NO emissions substantially decrease by increasing the EGR rate, but Soot emissions increase remarkably as well. As the EGR rate increases, the NO reduction degree of simulations has a better agreement with that of experiments. So it is quite credible to qualitatively analyze the NO emission by numerical simulations.The study of the EGR mechanism indicated that in the phase of most NO appearance, the average temperature changes little. So the average temperature is insufficient to explain the EGR mechanism of NO reduction exactly.In this paper the in-cylinder temperature field is divided into different regions to study the effects of EGR. The region whose temperature is higher than 2200K is defined as high in-cylinder temperature region. And it has been discovered: high in-cylinder temperature region changes at the trend that it firstly increases and then drops with the increase of crank angle. And the phase of high temperature region occurrence corresponds well to that of most NO appearance. The region of NO appearance in the cylinder has also a good agreement with high temperature region. And NO and high temperature region substantially reduce with the increase of EGR rate. It is more reasonable to explain the effects of EGR on NO reduction by the changes of high temperature region.Low temperature region decreases at first, then increases with the increase of crank angle. As the EGR rate increases, low temperature region decreases and high temperature region decreases as well, in that in-cylinder temperature goes with the trend to the average value.The experiment results indicated that As the EGR rate increases, Soot emissions and fuel consumption increase directly. There is an abrupt increasing point of smoke value at about 5% EGR rate. And that point of fuel consumption value is at about 10% EGR rate. So EGR rate should be controlled under 5% or 10% based on different optimization principles.4. The effects of injection pressure on diesel combustion and emissions were simulated and investigated.Experiments were carried out under the torque identity mode and the operation point is 1650r/min and 50% load (injection timing -0.54°CA). The study based on the single injection MAP increased the rail pressure with about 10% of prototype engine rail pressure as a step. During the process of experiments the peak pressure in-cylinder was real time monitored and when it exceeded 140bar, the rail pressure increases was stopped. The operation points of rail pressure of 84.8, 104.6 and 124.8MPa separately have been simulated. The initial and boundary conditions were according to experiments.Both results of simulations and experiments indicated that with the increase of injection pressure, peak of heat release rate increases, Soot emissions reduce substantially while NO emissions increase greatly. According to microcosmic statistics results, spray velocity increases versus common rail pressure, SMD decreases, the quality of fuel evaporation improves, fuel evaporation rate greatly increases, premixed combustion and high temperature region increases.5. The couple effects of EGR and injection pressure were investigated.With the rail pressure maintained at 100MPa under the operation point of 1650r/min and 50% load, the coupling experimental study was carried out at different EGR rate. The results indicated that at light load, the heat release rate appears a single peak and the majority of fuel has carried on the premix combustion. With the increase of load, the premix and the diffusion exothermic stage become clear gradually, and the premix exothermic peak value becomes lower. With the increase of EGR rate, the initial cylinder pressure, the compression pressure peak value and the combustion pressure peak value can reduce. The ignition delay also increases; at light load, unimodal heat release rate peak value increases, and at middle and high load, the premix combustion peak value increases and the diffusion combustion peak value reduces.Under the same load, as EGR rate increases, NOx emissions reduce substantially; although the smoke emission increases monotonously, there is an abrupt increasing point at about 10% EGR rate. Taken it as the optimization control point, based on the trade-off relation of the income and the loss the EGR rate suitable window under different operating points can be determined.In the experiment it was also discovered that when the load is lower than an certain extent, after using EGR, the smoke emissions decrease as well as NOx, but at this time, the engine is quite sensitive and instable, which is similar to the low temperature combustion mode. But its heat release rate peak value is very high, and it increases along with EGR rate increase. At low-speed and light load operation, the heat release rate peak value may reach as high as 400J/°CA, so it can't be applied directly.6. The effects of multiple injection on diesel combustion and emissions were simulated and investigated.The effects of pre-injection were simulated under the operation points of 1330 r/min, 50% load and 1650r/min, 25% load. The peak of pressure raise rate decreases substantially when pre-injection is used. And it decreases versus the dwell of pre-injection and main injection until the dwell decreases to some extent where the peak of the pressure raise rate is nearly steady.Simulation study on post injection under the operation point of 1330r/min, 50% load, as well as the experiments under the operation of 1650r/min, 25% load indicated that Soot in-cylinder history appears to two peaks from one with the increase of post injection fuel quantity. Soot emissions reduce greatly because of the reduction of Soot emission peak and the increase of later oxidation of Soot caused by post injection. Only when post injection quantity exceeds certain fuel mass, turbulence kinetic energy increases greatly, and the disturbance comes into effect remarkably. The more of the post injection quantity, the fiercer disturbance is, and the better for oxidation of Soot. When post injection quantity reaches 25% of total fuel mass, the Soot emissions reduce to the lowest value, about 37.6%. Given the heat efficiency, it is reasonable that the post injection quantity is 20% more or less.The simulation results indicated that post injection fuel injected to the O2 rich area, burns sufficiently and results in remarkable reduction of peak Soot value. So it can be understand as a regionalized combustion at time order. Taken the combustion management into consideration, diesel fuel is injected into the cylinder for several times, resulting in time schedule combustion. The post injection fuel is separated from the tail of main fuel mass. But compared with non-post-injection, the post injection fuel vaporization happens at the O2 rich area, and then burn here which means high using rate of O2 and sufficient combustion, so the time schedule combustion can be regarded as a regionalized combustion at time order. The eddy flow at vertical section induces O2 movement to cone angle area of injection spray to form O2 rich area during the interval of main injection and post injection to realize the regionalized combustion. To get the optimal regionalized combustion, the dwell of main injection and post injection should be as small as possible under the premise that the supply of O2 is sufficient.In general, under the optimal regionalized combustion condition, the region of the high temperature is the biggest one, and the ratio between mass of Soot in high temperature area and total in-cylinder soot mass is the highest one, so its oxidization rate is the fastest.