Continuously Variable Transmissions

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More and more vehicles are featuring a continuously variable transmission (CVT) as a means of maximizing fuel economy and minimizing vehicle drivetrain weight and complexity. Many automakers, such as Nissan, Subaru, and Toyota, have embraced the CVT and put considerable research and engineering into furthering its development. Most of Nissan’s cars and crossovers, for example, now feature a CVT as standard equipment while many Subarus and some small Toyota vehicles also have the CVT as standard or an option. Nearly all hybrid-electric vehicles use a CVT as well.
The CVT’s strengths are in its relatively simple nature of operation, smaller size, and lighter weight over a traditional geared transmission. A CVT also improves fuel economy by allowing the engine to run at its optimal RPM rate.

CVTs also have inherent weaknesses, however, not the least of which is their relative low torque delivery capability. For this reason, the CVT is not currently well-suited to large vehicle applications or high-torque uses such as heavy towing. A CVT is less efficient in terms of power transfer than is a geared transmission, but generally makes up for this in efficiency gains in engine operation and weight savings.

VDP or Reeves Drive

The most common type of CVT used in automotive is the variable-diameter pulley, also called a Reeves drive. To visualize the inner workings of this transmission, simply imagine two hourglass-shaped pulleys sitting parallel to one another with a belt running between them. The “gear” ratios are made by sucking in or pushing out the ends of the hourglass to make the center larger or smaller in diameter. As one of the pulleys shrinks in size, the other expands, keeping the belt between them taught.

If you’ll imagine that one pulley is the “drive” pulley, receiving power from the engine, while the other is the “driving” pulley, sending rotation to the wheels, you can visualize how the shrinking and growing of one pulley versus the other changes speed ratios between the drive and driving pulleys via the belt between them.
This simple operation is either hydraulically or electronically controlled. Electronics are more common and allow variability to improve performance and, in some cases, mimic the “shifting” patterns of a standard transmission. The latter is felt in the current-generation Toyota Corolla, for example, and, to a lesser extent, in the 2015 Suburu Legacy.

The secret to the VDP is in the V-belt. Until recently, the metallurgy required to build a belt capable of transferring the power of an automobile engine reliably and without losses to slippage was not available cheaply enough to make a CVT like this possible. Advances in metals and the lower costs of manufacture with high-strength steels changed that, allowing the CVT to begin to thrive.

The V-belt between the pulleys must be very stiff in axial (side-to-side) direction so that the belt can “push” itself and the pulley it is driving. The belt between pulleys on most Reeves drive CVTs is actually a chain with many, many links and a conical shape to its sides. Although technically a chain, this V-drive chain has the appearance of a belt and is usually referred to as one. Lubrication, usually transmission fluid or motor oil of a light weight, is also used to keep the metal belt from scraping the pulley surfaces while keeping the “grip” necessary to allow the belt to turn them.

The easiest way to envision the makeup of the belt in a CVT is to imagine it as having snake-like scales that vary in size, each sliding over one another so that the largest surface area possible is reaching the variable sized sides of the pulleys. Everything is held together by a thin central band of chain-like links, each being very small, giving the belt a fluid motion.

Automotive CVT Systems in Use Today

Nearly all continuously variable transmissions being used in automotive today are various forms of electronically-controlled belt/chain-driven transmissions based on the Reeves design. Some are made in-house while others are licensed from supply partners such as Van Doorne, Bosch, ZF Friedrichshafen, and others.

Virtually every automotive manufacturer is using a CVT in one or more vehicles today. Notables include:

The Toyota Hybrid Synergy Drive system for hybrid gasoline-electric cars, which include the Prius, Camry Hybrid, and in various Lexus vehicles. This is a Power Split Transmission which combines a CVT with a power splitting device that allows the gasoline engine to send some of its power to a generator for use by the electric-drive system. Gear ratios are determined by this power split.

Nissan’s Xtronic CVT is now in use in the majority of the company’s models, including the first mass-produced CVT for larger-displacement engines in the six-cylinder configuration and in midsize crossover/SUVs with all-wheel drive. The CVT in the Maxima, Murano, and other V6 vehicles is considered the first production-level “3.5L Class” belt CVT and has won the company many awards.

BMW has a belt-driven CVT manufactured by Friedrichshafen used in the MINI line of vehicles and that is now being introduced in other BMW-branded cars.

General Motors utilizes the VTi, an electronically-controlled CVT originally introduced in the now-defunct Saturn brand with more modern variations now being used in some smaller GM vehicles globally.

Ford’s Chain-Driven CVT was designed in conjunction with Friedrichschafen, but was discontinued when a new design, a belt-driven CVT, was introduced for the Focus and C-MAX vehicles. Some European Ford models also contain this CVT.

The Future of CVT

The continuously variable transmission has earned a place in the automotive landscape and will likely become more and more dominant as torque capabilities improve. The majority of small cars on the market now employ a CVT and with automakers like Nissan, Subaru, and Toyota embracing it across their model lineups. Others are likely to follow as fuel efficiency gains and manufacturing costs for multi-gear transmissions begin to weigh in, especially as vehicles powered by four-cylinder gasoline engines, hybrid powertrains, etc. dominate the market.

By | 2018-02-13T20:51:15+00:00 May 18th, 2017|Car Tech|0 Comments

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