Design And Research Of Natural Gas Engine Bushing Pistons

With the CNG(Compressed Natural Gas)engine continuously developed toward the direction of high speed and high-power,as well as the national requirements of the emission characteristics of the engine have been improved,thus making the working environment of the piston is getting worse and worse,and the load on it is getting larger and larger.While the piston does non-uniform reciprocating linear motion in the cylinder,it is receiving load from the combustion pressure inner bowl,reciprocating inertia force,side pressure,friction,etc.,so the piston pin hole is affected by the joint action of the periodic impact load and heat load.The strong combustion pressure inner bowl and reciprocating inertia force the piston bears make the piston pin hole caused stress concentration.In addition,the high temperature environment has a significant effect on the mechanical properties of the piston material,which reduces the strength of the material,and gradually leads to its fatigue cracking,thus all of this will result to immeasurable economic losses.Through making researches on references both at home and abroad,it is discovered that the failure of the piston pin hole is a common model that causes the failure of the piston.Therefore,in this research,the NQ140BN5 engine of Geely Sichuan Commercial Vehicle Co.,Ltd.will be taken as a research object,and the relevant structures of the piston pin hole will be designed and analyzed.The main research contents are as follows:(1)Aiming at models of failure of the piston pin hole,a new structure of arc surface bush piston pin hole is designed on the basis of the original piston of NQ140BN5 engine,and the assembling conditions of the arc bush piston are discussed,and the equations of motion,contact equations,velocity equations and force equations of the arc bush are established.(2)Three-dimensional geometrical model of the new structure piston is established in this research with the help of the software of SolidWorks,then the finite element model is imported to the ABAQUS software.The thermal analysis boundary conditions have been obtained by further using the empirical formulas and semi-empirical formulas.And the piston temperature field and the thermal stress field have been solved by simulating.Finally it is found that comparing with the original piston,the distribution state of the new structure piston is only 0.36%and 7.2%respectively.(3)Comparing with the original piston stress distribution state,the stress distribution state of the piston under different load conditions of mechanical load and thermo-mechanical coupled load is analyzed by the finite element method,which finds that the Mises stress and contact stress value at the piston pin hole can be effectively reduced by the new piston-bush structure.And the stress distribution state of the new piston-bush structure under different combustion pressure inner bowl has been discussed,which finds that with the increase of the combustion pressure inner bowl,the growth rate of Mises stress and contact stress in the new piston-bush structure is significantly lower than that of the original piston.(4)The crank connection mechanism of the new piston-bush structure has been analyzed by the rigid flexible coupled dynamic method,which obtains the magnitude of the force of piston pin hole before and after improvement when the speed of the engine reaches at 2800 r/min,and the simulation results show that the interaction force of the new piston pin hole is smaller than that of the original one.In addition,the kinematic characteristics of the new structure piston are analyzed,which finds that the movement rule of the new piston pin hole is similar to that of the original.(5)The formulas of the arc length of the core-type curved bush and the heart-shaped arc bush,as well as the centroid trajectory equation for the core-shaped bush are established.The effects of different bush arc sizes and different bush materials on the stress distribution state of the piston pin hole are analyzed.