Date of Award
Doctor of Philosophy (PhD)
Over the last few decades, a lot of research effort has increasingly been directed towards developing vehicle transmissions that accomplish the government imperatives of increased vehicle efficiency and lower exhaust (greenhouse gaseous) emissions. These exhaust emissions can be regulated or lowered by increasing the fuel economy of a vehicle, and CVTs (continuously variable transmissions), indubitably, play a crucial role in this plan to improve the fuel economy. A continuously variable transmission is an emerging automotive transmission technology that offers a continuum of gear ratios between desired limits. A CVT offers numerous advantages over the conventional multi-step gear transmissions such as lower fuel consumption, higher vehicle efficiency, and smooth power flow from the engine to the wheels. However, the potential advantages of a CVT have not been realized completely in a mass production vehicle.
Since the belt- and the chain- type CVTs are the most commonly used CVTs, the present research outlines detailed dynamic models that give profound insight into their system characteristics, which could be exploited later to meet the goals of greater fuel economy and vehicle efficiency. The present research reports a detailed continuous one-dimensional transient-dynamic model of a metal V-belt CVT that accurately captures the various transient dynamic interactions occurring in the system, whereas the dynamics characterized by the discrete structure (that cause polygonal excitations) of the chain are captured by developing a detailed planar multibody model of a chain CVT. Moreover, belt and chain CVTs are friction-limited drives as they rely on the friction characteristic of the contacting components in order to successfully transmit torque. Since the friction characteristic of a contact zone between two sliding surfaces may vary in accordance with the operating and loading conditions, various mathematical models of friction are introduced to understand the influence of friction characteristic on the performance of a belt/chain CVT drive. A CVT, like any other dynamical system, is susceptible to clearance formation due to continual operation, wear, and fatigue. So, a mathematical model of clearance is embedded into the CVT model (i.e. the chain CVT model) to study the influence of clearance on its dynamic performance. A considerable effort is dedicated towards comprehensive modeling of the slip dynamics and inertial coupling among band pack, belt element and the pulleys, and studying their influence on the performance of a metal V-belt CVT system. Simple trigonometric functions are also introduced in these models to capture the effects of pulley flexibility on the thrust ratio and slip behavior of a CVT system. The results discuss the influence of band pack slip, friction characteristic, clearance, and pulley flexibility on the dynamic performance, axial force requirements, and torque transmitting capacity of a belt/chain CVT drive. The results also illustrate that for certain friction (contact) conditions and clearance parameters, it is plausible for a CVT system to exhibit quasi-periodic or chaotic behavior, which severely lowers its torque carrying capacity and dynamic performance. It is also observed that the slip behavior of a CVT system is affected not only by the torque loading conditions, but also by the inertial coupling and the nonlinearities existing between the various components.
Srivastava, Nilabh, "Modeling and Simulation of Friction-limited Continuously Variable Transmissions" (2006). All Dissertations. 29.