Performance Investigations Of Half Toroidal Continuously Variable Transmissions (CVTs)

As Environmental Protection Agencies of developed countries enact new regulations for automotive fuel economy and emissions, the continuously variable transmissions (CVTs) emerge to be one of the key technologies for improving fuel efficiency of automobiles with internal combustion engines (ICE). Commonly used CVTs, such as different kinds of belt type CVTs are not new to the automotive world, but their torque capability has been limited in the past. New developments in material, transmission oil, manufacturing technique as well as the control have led to more robust and rapid reacting toroidal type CVTs, which allows them to be used in more powerful automobiles.The work presented in this dissertation deals with a theoretical and experimental investigation of the performance of half toroidal CVT. The study aims at providing a design method for such mechanical transmissions. Moreover, the shape of contact areas between the rolling elements is studied, which is proved to be ellipses. These contacts are lying under the elastohydrodynamic lubrication regime, where the shearing of the fluid is subjected to high contact stress and transmits the power. A study of the Hertzian contact parameters is carried out at different system parameters. The study provides some observations of the relationships between the maximum Hertzian stresses and transmission ratios in the form of graphs. It showed that the material property (Modulus of elasticity), operating conditions (input torque, maximum traction coefficient and etc.) and geometrical parameters (aspect cavity ratio, curvature ratio and half cone angle of power roller) play important roles in such a system. These graphs provide adequate information for designer to know the maximum Hertzian stress that can be usefully applied to the traction force analysis between contact surfaces under the specific condition and duty.Mathematical models for estimating the torque capacity, power loss due to spin and geometric slippage, and contact efficiency of the elliptical contacts of the half toroidal CVT (coaxial and parallel axle) are established and analyzed in details. These models are expressed in the form of integrals, which can be readily evaluated by numerical scheme. The analysis is based on the kinematic relationships and the expressions of Hertzian parameters within the contact area. The contact efficiency calculation method of the half toroidal CVT is developed for the proper spin points under the effect ofsystem parameters. Numerical results are presented in graphical forms for some values of parameters (the applied contact load, the curvature ratio of the rolling elements and the speed ratios), which all contribute to the performance of the system. These graphical forms can help the designer to select the proper system parameters to minimize the undesirable spin effect.An experimental test rig is developed and provided with a new mechanism to change the speed ratio. A test program is conducted to evaluate the actual performance of the designed traction drive for different speed ratios, input torques and rotational speeds under the effect of different transmission fluids.A measurement setup is integrated to the experimental unit with appropriate instruments to measure the rotational speeds and torques under different operating conditions.The experimental results show that the drive efficiency depends mainly on different operating conditions such as input torque, input speed, speed ratio and transmission fluid.The results show that traction fluid Ub4 presents higher drive efficiency and good behaviour as compared with traction fluid Ub3 and continuously variable transmission fluid CVTF, therefore can be recommended for the half toroidal CVT transmission.