Three-dimensional Simulation Study On Influence Of Diesel Fully-Variable-Valve Parameters On Flow Field In Cylinder

Fully variable valve timing technology can improve the engine's charge efficiency,torque and power,also can reduce and control nitrogen oxide emissions,for the engine energy saving is of great significance.Through the one-dimensional simulation of the influence of fully variable valve timing on engine performance,the influence law of fully variable valve timing parameters on performance parameters can be obtained.However,due to the lack of details of the in-cylinder flow,cannot get the reason of diesel engine performance improvement.In this paper,three-dimensional computational fluid dynamics analysis of fully variable valve timing parameters on the diesel cylinder flow field.This article uses SolidWorks software to establish the exact model of diesel air-valvecylinder,basing on the cylinder structure,intake system and exhaust system of medium-high speed diesel engine.According to the Fluent dynamic mesh technology,a cylinder meshing strategy was developed.ICEM CFD software was used to update the high-quality and fastmoving dynamic layer mesh to meet the dynamic mesh updating of the piston and the valve movement.The Fluent fluid calculation software was used to simulate the intake,compression,spray and combustion processes of the diesel engine.The pressure and temperature in the cylinder were simulated and compared with the experimental values to verify the accuracy of the selected spray and combustion model.On this basis,the in-cylinder flow performance of the engine intake and compression process under fixed air distribution parameters is simulated and studied.The eddy current,squish flow,and turbulent flow characteristics in the cylinder during the intake and compression process are affected by the changes of the piston and valve movement.The simulation results show that large-scale eddy currents appear in the compression process,and the tumble flow formed during the intake process is first enhanced and then weakened during the compression process.Finally,all crushers develop into squish flow and turbulent flowFinally,based on the variable gas distribution scheme that changes the opening phase of the intake valve,exhaust valve,and maximum lift,the CFD simulation of in-cylinder flow performance under variable gas distribution parameters is carried out.The results show that opening the intake valve earlier increases the swirl ratio and squish flow rate without reducing the charge.On the basis of avoiding the backflow of the intake air,the intake valve may be closed later in order to increase the eddy current ratio and the squish flow rate at the end of the compression process;appropriately reducing the maximum lift increases the swirl ratio and increases the turbulent kinetic energy.According to the variation of the flow in the cylinder with different gas distribution parameters,the effects of cylinders flow field structure on the performance of the diesel engine were analyzed.It is shown that the optimization of the gas distribution parameters can increase the intensity of turbulent flow,squish flow and turbulence in the cylinder to promote oil and gas mixture,and then increase the power.The above simulation results reveal the development and variation of the flow characteristics in the cylinder under different gas distribution parameters,and provide a theoretical basis for the optimization and control of fully variable valve parameters in medium and high-speed diesel engines.