All You Ever Wanted To Know About Continuously Variable Transmissions
Many vehicles today are utilizing a continuously variable transmission (CVT) rather than a geared, automated transmission. Some companies, including Toyota, build entire drive trains on some vehicles around the idea of a CVT.
With improvements in technology and materials, the CVT has become:
- Highly efficient
- Low-cost way to convert engine torque into wheel speed
The Story of CVT
The CVT is believed to have been invented by none other than Leonardo DaVinci, but today's technology is a far cry from what the Master designed back then. The most common CVT used in automotive is called a Variable-diameter pulley (VDP) or Reeves Drive system. This deceptively simple system is a sort of combination of the gear shifting on your bicycle and the spinning top you played with as a kid. It consists of three basic components, all of which are engineered with precision: two V-belt pulleys and a belt that runs between them.
The term "belt" is loosely used here, as it describes not a rubber belt as we would expect, but an extremely pliable type of chain instead. This chain is made of two short-linked chain belts, meaning their links are very close together for maximum flexibility, in between which are suspended very thing, but strong "grips" made of durable and flexible alloys.
The V-shaped pulleys are similar to the child's top you used to spin as a kid. Imagine one on a mirror so you have two of them, with their points always touching. Two sets of those with a belt between makes the CVT. The hourglass-shaped pulleys push together and pull apart to change the diameter of the interior where the chain rides. Since both sides can change diameter at varying ratios, sort of like your bicycle changes sprocket diameter at front and rear, the belt causes the driven pulley to move at a speed ratio that is faster or slower than the crankshaft of the engine.
The Modern CVT
Like a conventional transmission, a CVT requires a lot of lubrication to operate properly and is usually operating in a sealed environment. The chain itself is the most innovative part of a modern VDP, being made to create not only maximum friction, but also to change its shape continually. Metal is used for both durability and because the chain both pushes and pulls when moving along the pulleys whereas a rubber belt can only pull.
The chain actually bends to shape itself to the pulley as the pulley's diameter and angles change wish "shifting" (lateral movement). The bands at the center of the belt are stacked in various sizes, allowing the belt to morph its contact area's shape to maximize friction.
This CVT design allows the engine to run at optimum RPM for the speed and torque output required, putting the onus of the vehicle's movement control on the transmission. In a conventional vehicle, the engine will rev higher and lower to achieve better torque in order to accommodate for the limited gear ratios the transmission it's attached to may offer. With a CVT, the ratios are nearly infinite, so this revving is not required. For many drivers, this can be disconcerting, as the engine makes fewer sounds and sometimes even makes sounds that some may interpret as transmission problems. For this reason, Toyota has opted to program the CVTs in many of its vehicles to somewhat mimic traditional transmission-related rev sounds and "shifting jerks" to make their product more familiar to drivers.
The CVT has many advantages over a traditional automatic transmission. Gear ratios that are nearly infinite and adjustable to the millimeter means far more efficiency out of the vehicle. This also means maximizing engine output for both fuel efficiency and torque/power is possible. Many racing vehicles use this capability to give them better control of power output and for this reason, Formula One racing has banned CVTs from use on the competitive F1 track as it would give unfair advantage to the more well-funded racing teams. In a street car, however, this variability can mean better acceleration as well as better stability and traction control. It can also mean better sport performance and better efficiency, all dependent on what the driver asks of the car.
It should be noted that the CVT used in hybrid vehicles in the Toyota line is called a Power Sharing Transmissions (PST). These work in the same basic way a Reeves Drive CVT does, but have two power inputs (engine and motor), each of which can be varied along with the gear ratio going out to the drive shaft/axle.