Continuously Variable Transmission Technology

Transmission and automotive manufacturers constantly seek improved techniques for transmitting power efficiently and smoothly from a vehicle's engine to its wheels. Continuously variable transmissions (CVT) provide precise, independent, and smooth coupling of engine speed and torque output to meet drive wheel requirements. By providing optimal transmission ratios, CVTs match engine performance with road requirements to maximize fuel economy.

SwRI has extensive DC motoring/absorbing dynamometer test stands to evaluate transmissions.

With nearly 50 years of experience in testing, developing, and improving automotive components, Southwest Research Institute (SwRI) supports automotive and transmission manufacturers in developing all types of CVTs, including:

• Belt (steel, fabric, push and pull)
• Traction (toroidal and nutating)
• Epicyclic

Analytical Evaluation

Institute staff members routinely use a variety of advanced analytical techniques to obtain the comprehensive data necessary for effective transmission development. These techniques include:

• Stress, force, kinematic, finite element, and powerpath analyses
• Heat transfer
• Computational fluid dynamics
• Engine-transmission matching

SwRI evaluates continuously variable transmissions using double-ended DC dynamometer test stands with precision torque transducers to obtain efficiency measurements accurate to ±1.4 N·m. Photo courtesy of Van Doorne's Transmissie b.v.

Component Development and Testing

SwRI offers modeling, prototyping, and testing for the following transmission components:

• Start-up clutch
• Torque converter
• Belts
• Toroids
• Clutch pack
• Differential
• Constant velocity joint

Performance and Physical Property Testing

Modern laboratories and equipment, including electric dynamometers, are used to evaluate performance and environmental parameters of transmission components, including:

• Efficiency
• Durability
• Steady-state and transient performance
• High- and low-range temperature operation
• Humidity
• Noise
• Electromagnetic intensity

SwRI tests CVTs in the following ways:

• Transmission only
• Engine and transmission
• Whole vehicle testing, including chassis dynamometer and on-street evaluations

SwRI engineers conduct kinematic and powerpath analyses of complex transmission designs to determine output torque. Photo courtesy of Epilogics, Inc.

Material Development

Because of the extremely high loads imposed on a CVT's internal mechanism, material selection and processing are critical. SwRI engineers use a comprehensive set of material science capabilities:

• Materials evaluation and selection
  • Spring steel
  • Bearing grade and ultraclean steels
  • Composites, including ceramics and polymers
    
    
• Materials processing
  • Ion implantation
  • Flame spray
  • Heat treatment
  • Metal matrix

Tribological Evaluation

In designing any CVT, engineers must consider the lubrication requirements at the torque-carrying interfaces, some of which may experience Hertzian stress up to 3.5 GPa. SwRI scientists evaluate and predict lubrication effects at critical locations, including:

• Pully-sheave contact
• Nutating cone/control point
• Traction drive/roller

Other tribological areas of concern are:

• Non-Newtonian fluid characteristics
• Thermal effects
• Sliding wear
• Surface topology
• Lubricant film deterioration
• Contact fatigue

Fluids Testing

SwRI evaluates fluids for their abilities to support demanding CVT performance and endurance requirements. In addition to testing fluids and additives, the Institute develops specialized tests and test fixtures to evaluate new fluids for properties such as:

• Rolling and sliding friction
• Viscosity
• Oxidation resistance
• Foaming and wear resistance
• Corrosion protection
• Miscibility
• Low-temperature capability
• Extreme-pressure resistance
• Elastomer compatibility

This brochure was published in February 1997. For more information about continuously variable transmission technology, contact Douglas Fussner, Phone (210) 522-3972, Engine, Emissions and Vehicle Research Division, Southwest Research Institute, P.O. Drawer 25810, San Antonio, Texas 78228-0510.

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