Continuously Variable Split-Power Transmissions with Several Modes
Doctoral thesis, 1996
A continuously variable transmission (CVT) of split-power design consists of a variator and an arrangement of planetary gears. A black box model describing such transmissions is presented. The model can be used for synthesis as well as loss-free analysis of split-power CVT concepts in general. The planetary gear arrangement may have an arbitrary number of shafts. It is described by a sufficient number of basic speed ratios. By using several modes, both a large overall speed ratio range and low variator power ratios can be obtained. This is necessary for optimal engine use and high transmission efficiency. In each mode, specific components are active in the transmission. The black box model includes two modes, but CVT concepts with an arbitrary number of modes can be described by combining matching two-mode CVTs. General conditions for performing mode shifts are presented.
A general relationship between overall speed ratio and variator speed ratio has been derived, as well as a general relationship for variator power ratios of loss-free transmissions. Different variator types are categorized by their ratio coverage. Possible combinations of variator power ratio, overall ratio coverage and variator ratio coverage are mapped and presented graphically for different configurations. Suitable combinations of these quantities can be chosen. Design parameters included will then indicate how the CVT should be designed.
On the basis of the black box model, two split-power CVT concepts for automotive use are proposed. Each of the transmissions, having a geared neutral, is an infinitely variable transmission (IVT). They do not need such devices as starting clutches or hydrodynamic torque converters. They are designed for use in front wheel drive cars with transversely mounted engines. The main components of the first IVT are a traction ball drive variator, a simple epicyclic gear train with three shafts and some gear stages. The transmission can be run in three modes, including a separate mode for reverse. The second IVT consists of a hydrostatic variator, two simple epicyclic gear trains with three shafts and a few gear stages. It can be run in two modes. Reverse drive is included in one of the modes. Losses in gear meshes and variators are estimated. Hereby, constant component efficiencies are used, as well as efficiencies calculated taking load and speed dependence of the losses into consideration.
continuously variable transmission
speed ratio range
infinitely variable transmission
epicyclic gear train