Simulation On The Combustion And Emissions Of A Diesel Engine With Exhaust Gas Recirculation

As a higher thermal efficiency power machine, diesel engine has been applied abroad to traffic transport and used as agriculture machine and engineering machine, which has gained marked society benefits. It is well known that diesel engine brings advantage to people, at the same time, also brings huge press to environment for its NOx and PM emissions. With the stricter of emission control regulations and the pursuing of higher thermal efficiency, large numbers of investigation are carrying out on diesel engine structure factors, fuel injection law, the shape of combustion chamber, Exhaust Gas Recirculation and so on. To reduce emissions in diesel engine cylinder, only seeing about the general parameter to research the performance of diesel engine is not enough. Combustion, air flow in cylinder and mix of fuel and air must be investigated thoroughly. In-depth study on the effect of relative factors on the combustion process is imperative under current situation. The researchers considere that one effectual technique for controlling emissions of oxides of nitrogen (NOx), used in both spark-ignition and compression-ignition engines, is exhaust gas recirculation (EGR).In order to exert the effect of EGR on reducing NOx,the mechanism of action should be investagated. The simulation is an effectual way to get the microcosmic data ,and its precision become higher and higher. So, In this dissertation the effects of EGR on characteristics combustion perfformance and emissions were investigated by The software STAR-CD in CA6DL2-35E3 high-pressure common rail diesel engine.Firstly,established the model of simulation. the former model was ameliorated through using the self-edition CMC-O-grid template to generate the mesh of combustion chamber.Compared with the former model ,the new model has boundary layer and the mesh in the valve retainer was improved. Under the new model, it was reduced that computation were stoped because of the divarication , instability and negative density in some cells,and Considerable time was economized because of the longer time step for period. The three different conditions were used to validate the model through the test data at 1650r/min, 50%load in the CA6DL2-35E3 diesel engine(The model was validated through the he test data under the three different conditions at 1650r/min, 50%load in the CA6DL2-35E3 diesel engine).the three different conditions was showed below: 80Mpa press of the common rail, without EGR, 120Mpa press of the common rail, EGR rate 2.5%,120Mpa press of the common rail, EGR rate 5.0%. The verification results showed that the model and the method of compution were accepted to simulate the combustion of diesel engine.Simulate the spray , atomization, catching fire and the process of combustion under different EGR rates. Combined action of the common rail pressure improved and the EGR rates changed was investigated on the change of combustion and emission . The effect of EGR rates on combustion and emission was analysed via the experimental datas in macro aspect and the distributation of microcosmic field(temperature, the concentration field).1. Under the 80Mpa of the common rail, 1650r/min, 50%load , the simulation was finished with EGR rates 2.5%,5.0%,10%,20%,respectively. Compared the results of samulation with the ones without EGR in the same conditions(the invariable speed,the same oil consumption,the definite pressure of common rail).The conclusions were showed below.a. The indicated thermal efficiency decreased with the increase of EGR rate.b. The effect of EGR on reducing NOx was obvious. When EGR rates were increased ,the decrease extent of NOx from simulation was consilient with the one from experiment. So ,the qualitative investigation to NOx was properly believable through the simulation.c. With the increase of EGR rates ,the change of the mean combustion temperature was unconspicuous in the initial stage of Combustion.But in the late stage of Combustion,the temperature increased properly ,and the downtrend of it became slower.We can see that the combustion shifted backwards with the EGR rates increased. The mechanism of EGR reducing the NOx was not explained by the mean temperature in cylinder,because the change of the mean temperature was not obvious in the production area of main NOx.d. The change trend of high temperature area was discovered: increase in the beginning and decrease after some time. With EGR rates increasing, the appearance of high temperature area was later , and the end of it was earlier . The duration of high temperature was basically accordant with the period of NOx production .NOx and high temperature area decreased with EGR rates increased . The low temperature area increased after decrease for a period of time.The low temperature area was cut down with ERG rates increased . the temperature trends to uniform in cylinder with EGR rates increased . It results from the decrease of high and low temperature area and the tiny change of average temperature . The location of high temperature was basically accordant with the location of NOx production .e. With the EGR rate increases, the peak of SOOT is enlarged, the rate of SOOT formation is increased, the rate of oxidization at the later period of the combustion is slower, so the amount of discharged is augmented. Heavy concentration region of SOOT appeared at the heavy concentration region of fuel vapor. The region of SOOT in cylinder is inosculated with fuel vapor. With the EGR rate increases, heavy concentration region at clearance space and the bottom of the combustion chamber is augmented.2. Improve the pressure of the common rail to solve the disadvantage of EGR under keeping the fuel consumption unchanged . In the condition of 1650r/min, 50%load, 120Mpa pressure of the common rail ,the simulations were finished with EGR rate 2.5% and 5.0%, respectively . Compared the results with the ones of relevant condition under 80 Mpa common rail pressure to research the effect of EGR on performance and emissions when the pressure of the common rail(fuel injection rate)changed .Typically,there are double peaks to indicated diagram after the pressure of the common rail is increased . After the pressure of the common rail was increased to 120MPa, the time of combustion end was advanced so much and the indicated thermal efficiency is elevated a little. After the pressure of the common rail is increased, the time of NO formation was advanced, the quantity of NO is increased, the appearance time of high temperature region was also advanced , and volume of the cell sets in high temperature region was augmented . And also after the pressure of the common rail is increased, the production of SOOT was cut down, but the rate of oxidization at the later period of the combustion is slower too, and the amount of discharge was lower than the condition relevant with 80 Mpa pressure of the common rail .