Research On The Improvement Of Thermal Efficiency At Low Load With Variable Valve Lift Technology

The variable valve lift technology plays an important role in decreasing engine pumping loss and will be the main technology for gasoline engines.The pumping losses is not only be considered when selecting the valve lift curve,but also need to consider the application range of the vehicle driving cycles a nd turbulent kinetic energy in the cylinder.In this paper,the research of VVL technology on improving thermal efficiency at low load is studied by the method of simulation and test.The miller camshaft will reduce the engine fuel consumption rate at low load,and increase thermal efficiency.The simulation results showed that there is negative correlation between intake valve lift event and pumping losses.There is positive correlation between exhaust valve lift event and pumping losses.The pumping losses will be reduced with increasing of the internal EGR rate.The test results showed that the ignition delay angle is so longer that advance ignition angle in order to make the MFB50 % corresponding angle within 6-8 °.Although the pumping losses is reduced,the combustion duration is too long to make constant volume lower,so the fuel consumption is not be reduced.In this paper,the quasi-dimensional combustion model will be calibrated according to test data,and the intake duct design target of turbulent kinetic energy for the demand of stable combustion will be obtained on the basis of quasi-dimensional combustion model.And the new intake duct will be validated on the test bench.A intake airflow guide will be increased at the outside of intake seat,that will not only obviously increase tumble ratio at the low valve lift,and have little effect on the flow coefficient at high valve lift.The air flow will form significant roll flow down along the cylinder wall because of the intake duct,and the large eddy will be remained when the piston go upward.The simulation results showed that the distribution of turbulent kinetic energy in cylinder is evenly,and the turbulent kinetic energy around spark plug is increased significantly.And the test results have a n agreement with simulation results.