Rearch On The Varaible Valve Timing System Of Opposed Piston Two Stroke Diesel Engine

The opposed piston two stroke(OP2S)engine eliminates complicated valve train and cylinder head,and can significantly reduce the weight to increase power density and thermal efficiency effectively.It receives attention and research as one of the technical ways to cope with the energy crisis and environmental pollution.In this paper,based on OP2 S diesel engine,the simulation analysis method and steady flow test system were used to study the performance of Variable Valve Timing(VVT)system.A one-dimensional simulation model was established to analyze the influence of port height,width,and piston moving pattern on power performance and scavenging performance in OP2 S diesel engine.The indicated mean effective pressure(IMEP)and scavenging efficiency increased firstly and then decreased with the increase of port height,and the trappping ratio decreased continuously.When the exhaust port height was fixed,the lower intake port obtained better dynamic performance at high speed.On the cantrary,the higher exhaust port got better dynamic performance at high speed.However,as the engine speed increased,the max IMEP and the scavenging efficiency correspond to the intake and eshaust height were increased,so the optimization of intake and exhaust port height according to engine speed was necessary.Based on the OP2 S diesel engine,two kinds of VVT system mechanisms were proposed,one based on the layered port structure and one based on the sliding cylinder liner structure.The parameters of IMEP,scavenging efficiency and trapping ratio were selected as the optimization objectives.The intake and exhaust height of different speeds were optimized based on the determination of the additional weighting factor of 5: 4: 1.Based on the layered port structure,it was determined that the single-layer port was used at800～1500rpm,and the height of the intake and exhaust port was 14 mm and 16 mm.The double-layer port was used at 2000～3000rpm,and the height of the intake and exhaust port was 26 mm and 30 mm.At rated speed,the IMEP was 0.74 MPa,and the indicating power was 37.4 k W.The CFD simulation model of OP2 S diesel engine was established to study the influence of radial angle and axial angle of intake port on the scavenging process,and the influence of radial angle of intake port on the scavenging process of the layered port structure.The model was validated by the intelligent port steady flow test.Setting the radial angle and axial angle at intake port could increase the swirl ratio and the tumble ratio,but at the same time,the airflow and the air mass flow were affected,which resulted in the loss of scavenging efficiency.To take into account both scavenging efficiency and airflow,radial angle of the first intake port was setted and comparative analysis of the performance at1000 and 3000 rpm were performed.The radial angle of the first intake port was determined at 5°.At this time,the scavenging efficiency at 1000 rpm decreased,and the swirl ratio increased nearly 3 times.The scavenging efficiency increased at 3000 rpm and the swirl ratio increased nearly double.