Design Improvements on a Vee Belt CVT and Application to a New In-line CVT Concept 2005-01-3459
Researchers at the UC Davis HEV Center have developed a new design of a continuously variable transmission for use in all vehicle sizes. They have implemented this design by modifying a conventional pulley-type CVT both significantly increasing the efficiency and allowing for implementation in hybrid powertrains. The CVT is a pulley-type transmission in which a chain or belt is used to transmit torque from the input shaft to the output drive shaft. The hydraulically controlled pulleys allow for an infinite number of transmission ratios in a specified range, eliminating the need for discrete shifts. With drivability characteristics better than that of an automatic transmission and higher efficiency, the CVT is a considerable alternative.
Modifications have been developed at UCDavis and Gear Chain Industries for conventional CVTs that increase the efficiency. The modifications include the use of a Gear Chain Industry (GCI) chain in place of the Van Doorne Transmissie (VDT/Bosch) belt and a Servo-Hydraulic control system in place of the engine driven pump. The chain consists of pins tapered to the pulley surface, mated to an involute pin to allow transition from linear to rotary motion without friction. The transmission of torque from the primary to the secondary drive pulley is also increased. A second improvement is the servo hydraulic control system, consisting of two servo pumps that control clamping pressure and ratio. Replacing the conventional engine driven pump and control valves this allows the transmission to be controlled independently by a microprocessor. Since the hydraulic pressure is developed on-demand, there are minimal losses and energy needed is greatly reduced. With these modifications, the efficiency is increased to 95%, near that of manual transmissions. In addition, the application of a servo hydraulic control system is advantageous for hybrid powertrains in which all-electric modes can be used. These modifications have been applied to both past and current research hybrids at the UC Davis HEV Center and extensive dynamometer testing has been performed to compare the modified and standard transmissions.
Researchers at UC Davis have also developed a CVT for implementation in large vehicles and hybrids (300 hp and up) called an Inline CVT. The inline CVT consists of two sets of driving pulleys equipped with GCI Chains. When put in series, these driving pulleys and chains allow for greater range of ratio response without sacrificing size. The size of the CVT is considerably smaller than an automatic transmission of equal power, comparable to a manual transmission of equal power and is more durable and more efficient than both. The prototype developed has a capacity of 500Nm and 220kW, however the inline concept can be scaled up for larger vehicles up to class 8 trucks, with a 2000NM and 500kW capacity. Where noise is a consideration, the inline is much quieter than the comparable manual and automatic transmissions. Designed for volume production, the inline CVT minimizes the number of parts required for manufacture, a total of 12 parts including bearings is all that is required.
Both the Inline CVT, and modifications developed for the smaller duty CVTs have been designed for mass production and suppliers for all components are available.