| Intake system, fuel injection system and structure of combustion chamber are composed of the three elements of combustion system during the diesel engine working process. The reasonable match between them is of great significance to improve the overall performance of the diesel engine. Small bore vehicle diesel engine with four-valve can not only increase the cross-sectional area of the intake and improve volumetric efficiency, but also can effectively reduce the rate of fuel consumption and emission. But the intake flow of two adjacent intake ports of four-valve high-speed DI diesel engine can interfere with each other, thus affecting the flow of intake system and formation of vortex. Meanwhile, the shape of combustion chamber plays an important role to the squeeze flow and inverse squeeze flow, and other flow characteristics. For this reason, for a four-valve high-speed DI diesel engine, influencing factors of in-cylinder flow characteristics in the intake and compression process were studied by using the combining methods of three-dimensional modeling, numerical simulation and steady state test.Firstly, the flow characteristics of helical port, tangential port and combined port were investigated by using steady state experiment and the numerical simulation method. Analysis method of flow interference about combination of tow intake ports was proposed. And the effect of intake flow on in-cylinder flow field was analyzed. Based on the numerical simulation method, the regularity of in-cylinder flow was explored further. The results indicate that the simulation result and test result of flow coefficient about four combined ports are coincident, to compare with single intake port, in-cylinder charging is increased by using double ports, and vortex ratio is also improved, there is some mutual interference between two ports, and this interference effect has little influence on flow coefficient, but great influence on in-cylinder vortex movement, in-cylinder flow shows some regularity.Secondly, the three-dimensional transient of intake-compression process of four-valve high-speed DI diesel engine was simulated with AVL FIRE software, the average pressure, the average mass temperature, the average mass flow and other parameters of in-cylinder gas were obtained. And the velocity vector distribution of in-cylinder gas of different crankshaft angle in the intake stroke and the compress stroke was analyzed, and also the development process and the rule of in-cylinder vortex were described. The results indicate that due to the guiding role of the valve, many small eddies are formed near the cylinder wall, and two symmetrical tumble are produced in the ω combustion chamber, the symmetrical vortex intensity in the combustion chamber weaken with the increasing of crank angle, the in-cylinder large scale tumble break into many small scale vortex during the compression stroke, squish movement between the surface of the piston and the cylinder head is formed at the end of compression stroke, further enhance the intensity of turbulence in the combustion chamber, in the compression process, a regular rotation movement is produced in the cylinder, the in-cylinder flow characteristics presents compound flow of the squeeze flow movement and the compression vortex.In addition, the influence of the shape of combustion chamber on the in-cylinder flow was researched. Through NX UG of the three-dimensional modeling software, four different combustion chambers were designed, the effects of the main structural parameters of combustion chamber on in-cylinder speed characteristics, and squish movement, inverse squish movement and the distribution of turbulent kinetic energy were analyzed. The analysis results show that the influence of the shape of combustion chamber on the in-cylinder flow during the intake stroke near BDC and the start phase of compression stroke is very small, the size of squish area has very important influence on the squish intensity, to reduce the diameter of the throat of the combustion chamber and to increase the squish area can generate stronger squish movement at the end of compression, and promote the burning rate, the shape of convexity at the bottom of the combustion chamber is one of the key factors for the formation and development of inverse squish, the taper-shape convexity with45°slope has more obvious diversion effect for gas, and can keep the squish intensity at a higher level about10℃A, to reduce the diameter of the throat of the reentrant combustion chamber appropriately is advantageous to enhance the turbulent kinetic energy and improve the activity of flow movement. |
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