Simulation Study On Combustion Process Of Diesel Engine For Passenger Car

With the crisis of energy and the rise of fuel cost,in order to reduce the environmental contamination of automobile exhaust matter in the world, advancing thermal efficiency and reducing vehicle exhaust matter become focus recently. The diesel engine for passenger car is widely used in the world. And now the target is lessen exhaust gas, improving economical performance and reducing noise pollution. Different from diesel engine for high-duty truck, diesel engine for passenger car is designed and manufactured with less weight that makes vehicle more power, more economical and more comfortable.Because it's unique advantages, diesel engine vehicles will become mainstream in the future. In the course of studying passenger cars of diesel engine, the demands of power, comfort, driving, economic function, and engine operating conditions become even harsher. For this reason, researchers and producers of diesel engines has been faced with severe challenges, many new technologies are successfully developed and applied to diesel engine, such as the direct injection diesel system, electronically controlled fuel injection system, high-pressure fuel injection technology. And the multi-inlet valve design, variable swirl technology, turbocharger and cooling supercharger, the cooling EGR, cold start preheating system, exhaust after-treatment technology, the design of light-weight diesel engine technology.To satisfy the strict emission law the main method is to optimize the diesel engine combustion process. With the complexity of diesel engine combustion, simulation soft wares are used in the process of analysis.In this paper,ωchamber is built with three-dimension-geometry-maker CATIA. A simplified geometry is used to reduce the efforts for mesh generation. The research method used in this thesis is simulation with FIRE order which is developed by Austrian AVL Company. In this thesis intake stroke, details like intake ports and valve are neglected, only the compression stroke, combustion strokeand expansion stroke of direct-injection diesel engine for passenger car has been simulated. Because of relatively complex structure, combustion chamber is most meshed with hexahedron unstructured grid by automatic mesh generating order of FIRE. The cylinder mesh at different crank angle is generated by moving grid maker in this thesis. Because of movement characteristic of the piston, the cylinder is meshed with hexahedron grid which can short the calculation time more. As the piston moves up and grids of cylinder are compressed, grid of the cylinder field is meshed by the Re-dimension order of FIRE. The parameter in the model was adjusted to ensure the indicated diagram resulting from the simulation accorded with the data from the testing.The influence of the fuel injection advance angle and the effects of combustion chamber geometry on the performance of the diesel engine were studied in the thesis. The exhaust gas recycle and the intake temperature on the NOx and particulate in the diesel engine emissions.The fuel injection advance angle is an important parameter to control the ignition delay period. In this paper, the effects of the fuel injection advance angle of 5oCA, 8oCA, 10oCA, 14oCA were studied. Temperature field, concentration field are simulated, and it is concluded that:1) With the increase of the fuel injection advance angle, the squeeze flow of spray weaken, the impact points of the fuel injection will lean to combustion chamber top wall, the points are the lowest when the fuel injection advance angle is 5oCA.2) With the increase of the fuel injection advance angle, more mixture are formed in the ignition delay period. it will lead to initial rate of exothermic combustion increasing, resulting in the increase of NOx emissions, But The maximum concentration of late fuel combustion shows a downward trend, and the less regional distribution conducive to inhibit the formation of soot.3) With the increase of the fuel injection advance angle, the maximum temperature at injection time is reducing, the maximum temperature of cylinder is increasing.4) With the increase of the fuel injection advance angle, when the crank angle is at 385°CA, the scope of NOx distribution increases. The soot distribution is trend to regressive.The shape of chamber plays an important role in combustion process. The mixture formation and the process of combustion of direct-injection diesel engine for passenger car are effected by the shape of combustion chamber. The shape of chamber convex influence air flow in side the cylinder and emission performance of a diesel engine for passenger car greatly. Three different diesel combustion chambers, one is pyramidal type, one is central projection type, the other is flat type, were selected and calculated by means of multi-dimensional modeling. By analyzing the process of spray and combustion, it is known that:1) The shape of combustor central convex influences the fuel spray and split when injecting. The flow lead by geometry shape of A-type combustion chamber is in favor of oil droplet split and split secondly, and is be propitious to the atomization process, it makes the droplet became smaller, and more premixed gas is formed.2) The central convex shape of combustion chamber impacts gas flow characteristics greatly in compression and expansion strokes. When the pistons close to the top dead point, the mixed gas is squeezed to the cylinder combustion chamber, it improves the velocity of flow at combustion chamber throat port. For A-type combustion chamber, the intensity of squeezing flow is greater, the duration of the eddy current is longer, the distribution of the turbulent kinetic energy is reasonable, it helps to improve the quality of fuel and air mixture near the combustion chamber wall and makes the performance of the engine much better. For the B-type chamber, as the intensity of air flow is weaker, the film formed at the chamber wall evaporated slowly, and the air involved in mixing reduced, so it is adverse to the mix of fuel and air, the temperature and the gas concentration of local combustion tend to too strong, and the combustion becomes deteriorated, causing a huge of Soot and NOx. For C-type combustion chamber, the volume of the central convex is larger, and the rate of reverse squeeze flow is higher, it helps unburned fuel and intermediate products of the combustion chamber enter the upper to find fresh air, itguarantee the fuel combusted more completely and timely and can reduce exhaust matter of smoke,yet because of partial local temperature excess the other ,it lead to the increase of NOx.