3D Simulation Study On Combustion Process Of Medium-speed Diesel Engine Based On The STAR-CD

In this paper, the numerical calculation and analysis to a type of medium speed marine diesel engine are carried out on Star-CD. The main focus is to analyze the influences of different spray cone angles and different nozzle schemes on the quality of the combustion in the cylinder and the emissions generated. We study the influences of different optimization results on the combustion and the pollutant discharge in the diesel engine cylinder. The res ults can provide references for the optimal design of the fuel system. The following conclusions can be obtained:1 First, the calculation model of the three-dimensional combustion chamber is established for one type marine main diesel engine. According to the calculation, the curve of the average pressure in the cylinder is obtained. Then, the simulation results are compared with the experiment data. It can be found that the simulation values are in good agreement with the actually measured values. It improves that the method can be used to simulate the actual working condition of the diesel engine. Meanwhile, it also can provide the basis for the subsequent numerical optimization of the fuel injection system.2 Under the rated conditions, we use the model calculate the working process of the diesel engine combustion chamber under different spray cone angles, the results show that:(1) When the cone angle of the spray is 144°, the oil spray directly to the top o f the piston. It causes local oil accumulation in the fuel combustion chamber, so the oil cannot fully mix with the air in the cylinder. The average temperature and the average pressure in the cylinder decrease, this increase the carbon generation and pollutants.(2) With the increase of the spray angle, when it reach 155°, the fuel and the air mix more evenly, this makes the average pressure and temperature in cylinder increase. The pressure changes from 13.51 MPa with the spray angle 144° up to 14.54 MPa with the spray angle 155°. And the temperature changes from 1385 K to 1551 K accordingly. At the same time, the area formed by the average pressure curve is larger, the power capability of the diesel engine is enhanced. This process generates more NOx and reduces the generation of the carbon smoke.(3) Based on the above consideration, the spray cone angle we choose is 155°. In this situation, the oil beam angle matches best with the combustion chamber. At this time, the combustion in cylinder is more suffic ient, the maximum explosion pressure rises. This improves the power performance and the fuel efficiency of the diesel.3 Under the rated conditions, we use the model calculate the working process of the diesel engine combustion chamber to different nozzle plans, the results show that:(1) For different nozzle schemes, the larger the nozzle hole diameter, the larger the penetrating distance of the fuel oil beam. It makes the fuel steam after the spray atomization can mix with more oxygen, the range of combustible mixture in cylinder becomes larger at the beginning of the combustion, which makes the average pressure and temperature in the cylinder rise.(2) Based the scheme 3, the highest average pressure value and the highest average temperature in the cylinder are higher than those in the other schemes. The generation amount of NOx increases at the same time, the generation of SOOT Ndecreases.(3) According to the comprehensive consideration of the power performance, the fuel economy and reducing emissions o f the marine diesel engine, we choose the optimal scheme in which the nozzle hole number is 8, the nozzle hole diameter is 4.5 mm.