Simulation Study On The Charge Mixing State Of CAI Based On Delayed Exhaust Valve Closing

With the fast development of industry, the vehicle population increases rapidly. The human society is facing many critical problems such as energy shortage and environmental pollution. So many countries reenact gradually and strictly emission regulation to protect the environment. In 1990's, a new combustion theory for internal combustion engine was formed to enhance the fuel utilization and to reduce harmful emissions. Science then many works were done on the theory named homogeneous charge combustion ignition (HCCI). The studies showed that HCCI can enhance thermal efficiency and reduce emission at the same time. While, HCCI can't work at high load conditions or low load conditions, because of knocking or misfire. Previous studies showed that EGR is a practical method to realize CAI combustion on automobile engine through gradually mature variable valve timing technology.Numerical simulation has important theory value and practical value to fuel and air mixing and burning process of internal combustion engine. It is because that HCCI combustion is mainly controlled by chemical reaction kinetics, a single cylinder visual engine geometric model was set up by using a three-dimensional CAD software named CATIA. The mixture state in cylinder was calculated with CFD software and Chemical Kinetics Reaction Mechanism. In this paper volume mesh is divided with FAME tool (the Flexible Automatic Meshing Environment) in fire software. The dynamic meshes between 360℃A-710℃A were set up by using FEP (FAME Engine Plus). And the accuracy of mesh model was proved. Finally by setting initial ground and boundary conditions, selecting apposite calculation method and convergence condition, the detail distribution of velocity, concentration, temperature and turbulent momentum field of mixture in the cylinder was simulated. At the same time, HC,CO and NOx emissions were predicated accurately. The software can be also using to study the influence of combustion chamber on internal combustion engine thermal efficiency and emissions.The gas flow rates and turbulence kinetic energy was reduced in the cylinder under the condition of exhaust valve late closing strategy, but the changing trend of them is roughly the same. Because of inlet valve late opening and exhaust valve late closing in the test, there was difference in pressure between cylinder and exhaust port which leads to the exhaust gas being breathed into the cylinder with the piston going downward. When the inlet valve opened gradually, fresh air and oil flowing into cylinder, the fluid flow can be divided into considerable blend stage and weak flow stage (inlet valve and exhaust valve closing stage). At the considerable blend stage (intake charge blend stage), the flow rate and turbulence kinetic energy are both high, while they are reduced at the weak flow stage, for the inlet valve and exhaust valve both closing. With the piston going upward the mixture in the cylinder flowed into combustion chamber because of squeeze effect. The temperature was 1070 K at 10°CA BTDC, reaching the ignition point of gasoline. It can be considered that HCCI was achieved.When the rotational speed changed, the properties of working fluids changed at the same time. With the increasing of rotational speed, the mixture flowed intensely, the turbulence kinetic energy enhanced, the concentration of oil reduced at the same time. It is because that with the increase of rotational speed, the increased resistance of intake gas resulted in the decline of intake efficiency. Thus the fresh working fluids reduced, EGR rise, the temperature in cylinder increased.