Evaluation and Comparative Study of ValveTrain Layouts with Different Rocker Ratio

摘要

The Valve Train system is an integral part of any engine and the impact of its design is very crucial, particularly in high speed engines. Maintaining the required valve timing throught the engine operating speed and longer component life are the two important parameters which drive current valvetrain designs. An engine ValveTrain system designed for a valve lift of 7mm is to be modified for an increased valve lift of 8mm. A study was conducted to understand which design parameters are to be changed/modified to make this possible. For this study, the valvetrain of an air-cooled motorcycle engine is taken up. The valvetrain arrangement was an Over Head Camshaft (OHC) design with a Roller-Follower. A 1D commercially available numerical code was used to simulate the kinematics and dynamics of the system. The effect of the addition of stiffer springs to the base valvetrain layout to counter the decrease in its dynamic stability because of the larger cam (which was provided to produce the required 8mm valvelift) is studied. Also the outcome of increasing the Rocker Arm Ratio (RAR) and how it alters the dynamic behavior of the valve train was understood. RAR is the ratio of the length of the valve side of the rocker arm to the pivot and the length of the follower-roller to the rocker pivot. Increasing the RAR is an effective way of increasing the valvelift of the engine (hence it's breathing capacity) with the same cam profile. Also because of the increased RAR, undesirable valve dynamic phenomenon such as valve float would occur later in the rpm range, hence aiding in the dynamic stability of high speed engine valvetrain. But by increasing the RAR, the forces and Hertz stress generated at the cam/follower interface will also increase. In this paper Valvetrain layouts with three different rocker ratios i.e. 1.2, 1.5 and 1.8 were studied. Also two spring designs were evaluated on these rocker designs. How the change of RAR and the spring stiffness affects the valve dynamic phenomenon like valve bounce, valve float and how it alters the valve train forces and stresses were understood. From this study an optimized rocker ratio with suitable spring design is suggested for the considered engine.