Study On An Innovative Axial Shift Valvetrain System(ASVS)for Variable-stroke Engine And Dynamic Characteristics

Variable stroke technology could be applied to special vehicles as it can improve the low-speed torque and specific power of an internal combustion engine.The valve operating frequency,timing,and lift are quite different in 2/4 stroke mode,and this technology requires completing to switch mode between cycles.To date,there is no practical valvetrain system to meet the above requirements.Therefore,the variable valve actuation(VVA)system is indispensable to the variable-stroke engine(VSE)and is of great significance to meet the power demand of the special vehicles.In this paper,a practical axial shift valvetrain system(ASVS)for the variable-stroke engine with a diameter engine bore of 115 mm and two overhead camshafts was designed.ASVS consists of a camshaft and a camshaft sleeve connected by a spline.The camshaft sleeve is arranged with 2/4-stroke cam blades side by side,and different cam blades can be switched by axial movement of the camshaft sleeve,at the same time,changing different working modes.The two challenges of ASVS are:(1)designing a two-stroke cam profile for high valve lift in short valve opening duration;and(2)designing a switching profile with large displacement in the short common-base-circle duration.Therefore,the research can be listed as follows:According to the objectives of the valve motion parameters,the cam profiles were fitted by polynomial function and optimized by reducing the first to third derivatives of the cam lift.A rigid dynamic simulation model based on ADAMS was created to analyze the dynamic valve characteristics.The simulation results show that,in the 2/4-stroke mode,the valve lift is over 7 mm in 80?Ca A and 8 mm in 140?Ca A,and the fullness coefficient is more than 0.5and 0.6,respectively.Under the target engine speed of 3000 r/min,the valves run stably and the seating speed is lower than 0.3 m/s.Moreover,the contact force is in the allowable range.The dynamic shifting characteristics of the ASVS were studied by establishing the mathematical model of the switching process.To reduce the contact force between the metal pin and the camshaft sleeve,two ideas of designing the switching profiles in a short duration were put forward,and two specific switching profiles were designed in the same duration.Besides,the more suitable switching profile was selected by comparing the curvature,pressure angle,and dynamic results.The results show that the reduction of the first and the second derivatives of the switching profile has an obvious effect on reducing the contact force between the metal pin and the camshaft sleeve.The switching profiles designed in this paper have high dynamic performance.It can realize a changeover smoothly in one revolution of the crankshaft under the target engine speed of 3000 r/min.