Reserch On Ventilation System And Swithing Mechanism Of Valve Timing For Variable Stroke Engine

Variable stroke engines possess large output per litre and high ratio of mass toproduced power, in which the four stroke mode can allow higher load ratio and smallerdisplacement as compared to two stroke mode, thus it is a main working mode in variablestroke engine and has excellent power, fuel economy and emission performances.Therefore, it is meaningful to carry on the theory study and corresponding tests forvariable stroke engine. On the basis of the preliminary study about timing phase andthermodynamic performance, it is concluded that gas exchange type and pressure andvalve timing at the two stroke mode are three most import factors to decide the variablestroke engine performances. In order to further study the related factors, this papercompleted the following research works.The working principle of variable stroke engine was first analyzed according topervious study results as well as structure and working principle of traditional engine. Ithas two working modes, i.e. power mode and economy mode. In power mode, valvetiming principle was established for gas exchange way of top-in and top-exhaust (TITE).Furthermore, the theories of gas exchange both for the four stroke and the two strokeengines were analyzed respectively to prepare for studying gas exchange theory ofvariable stroke engine.Both the thermodynamic mathematic model and simulation model for TITE way ofvariable stroke engine were then established by means of engine thermodynamic theoryand AVL-BOOST software platform. Through setting boundary conditions and initialconditions of simulation model based on test and empirical data in AVL-BOOST software,Campbell chart was achieved as well as valve timing of power mode was optimizedconsidering power performance as optimization goal. The simulation results were alsochecked with a set of experiments.In power mode of variable stroke engine, the simulation models of two gas exchangeways of TITE and TISE (top-in and side-exhaust) were founded separately according tohydromechanics theory and AVL-FIRE software platform, in which the corresponding meshing method was studied. The volume mesh in TISE mode was meshed by combiningfixed mesh and moving mesh. There is a problem in traditional meshing, i.e., thediscontinuous velocity at interface which was solved in the simulation model. Accordingto engine performance results simulated in AVL-BOOST and experimental data, theboundary conditions, initial conditions and ignition advance angle were confirmed foreach gas exchange way, and then the flowing process of working medium in cylinders canbe simulated and observed directly.Through observing velocity vector field, temperature of combustion field andturbulent kinetic energy field on lateral and longitudinal section of gas exchange system,the features of two gas exchange ways were analyzed qualitatively. In addition, test resultswere also used to quantitatively analyze advantages and disadvantages of every gasexchange way besides simulation results. The features were compared with residual burntgas ratio and compression pressure in cylinders, effective power, torque, specific fuelconsumption and the amount of emission. In conclusion, TISE way in power mode canmake the performances of variable stroke engine more excellent than TITE way.Both velocity field and pressure field on longitudinal section and lateral sectionwithin cylinder were analyzed at every critical moment of gas exchange process withdifferent pressure. To eliminate reverse jet and decrease the loss caused by short out in gasexchange, a best gas exchange pressure was gained. On the basis of ADAMS andthermo-mechanical coupling theory, dynamics model of piston-crank mechanism wasestablished, and then the boundary condition including the parameters of temperature andpressure was simulated. Successively, in the case of different gas exchange pressure forvariable stroke engine, both thermal stress field and temperature field were studied aimingat some important parts consisting of piston and rings. The results indicate that most partsof engine can meet the experiment demand.The flexible multibody dynamic model was built for switching mechanism of valvetiming in ADAMS software and model analysis was carried out to various cams. Next,valve lift curve was gained through simulating and was contrasted with the curve achievedby TYCON software. BOOST thermodynamic model was also used to analyze power andtorque difference in two kinds of valve lift curve. The dynamic features of cam profile were then studied with the flexible multibody dynamic simulation. Results show flexiblecam profile is better to meet the dynamic demand of switching mechanism of valve timing.Additionally, cam profiles both for power mode and economy mode were designed in thispaper.The switching mechanism of valve timing for variable stroke engine was developedby referring to the structure and parameters of WH125-6engine. Then, the engine testplatform was set up after determining the spark advance angle, plus width and certainmoments of fuel injection. With the help of engine rig test, the performances of economymode of variable stroke engine, as well as TITE way and TISE way of power mode wereanalyzed besides WH125-6engine. To sum up, the economy mode of variable strokeengine has excellent fuel economy, while TITE way has better power performance thanTISE way in power mode of variable stroke engine.