Some say you can't teach an old dog new tricks. But the innovative continuously variable transmission (CVT), which Leonardo da Vinci conceptualized more than 500 years ago is now replacing planetary automatic transmissions in some automobiles. Since the first Toroidal CVT patent was filed in 1886, the technology has been refined and improved. Today, several car manufacturers, including General Motors, Audi, Honda, Hyundai and Nissan, are designing their drivetrains around CVTs.
Cars with CVTs, sometimes called shiftless transmissions, have been common in Europe for years. But it's taken a while for the technology to gain a foothold in the U.S.. The first production automobile to offer a CVT in the United States was the Subaru Justy in 1989. This was followed by the Saturn Vue in 2002 and Ford in 2004.
How Does a CVT (Continuously Variable Transmission)Work?
Instead of a normal transmission that has gears that are different sizes to produce different ratios, a CVT essentially has an unlimited number of ratios. The most common type of CVT operates on an ingenious pulley system that allows an infinite variability between highest and lowest gears with no discrete steps or shifts. It contains two 20 degree cone-shaped objects with a V-belt wrapped around them.
One input cone brings in power from the engine and one output cone takes power out of the transmission and to the drive axle(s). The belt travels around both cones and is actually what transmits power from the engine to the wheels. The cones slide back and forth so that the belt is traveling in low gear on the larger part of the engine side cone and the smaller part of the axle side cone, and then in high gear, smaller part on engine side, larger part of axle side. The different diameters of the different parts of the cones are what provide the different ratios. However, the movement of the cones is so seamless, that the RPMs remain constant to keep the engine in its power band so that you get the best performance possible.
A belt-driven design offers approximately 88% efficiency, which, while lower than that of a manual transmission, can be offset by lower production cost and by enabling the engine to run at its most efficient speed for a range of output speeds. When power is more important than economy, the ratio of the CVT can be changed to allow the engine to turn at the RPM at which it produces greatest power. This is typically higher than the RPM that achieves peak efficiency. In low-mass low-torque applications (such as motor scooters) a belt-driven CVT also offers ease of use and mechanical simplicity.
CVTs should be distinguished from power-sharing transmissions (PSTs), as used in newer hybrid cars, such as the Toyota Prius, Highlander and Camry, the Nissan Altima and newer-model Ford Escape Hybrid SUVs. CVT technology uses only one input from a prime mover and delivers variable output speeds and torque, whereas PST technology uses two prime mover inputs and varies the ratio of their contributions to output speed and power. These transmissions are fundamentally different.
Automakers are using CVTs for several reasons. A CVT can get maximum power out of a small engine for quicker and more responsive acceleration. That’s why you’ll most often find CVTs used in vehicles having four-cylinder engines. Hyundai, for example, is offering its version of a CVT, called an Intelligent Variable Transmission (IVT) with its 2.0L engines for the 2020 models.
The introduction of new materials makes CVTs even more reliable and efficient. One of the most important advances has been the design and development of metal belts to connect the pulleys. These flexible belts are composed of several (typically nine or 12) thin bands of steel that hold together high-strength, bow-tie-shaped pieces of metal. Metal belts don't slip and are highly durable, enabling CVTs to handle more engine torque. They are also quieter than rubber-belt-driven CVTs.
Other Versions of CVTs
The Toroidal CVT system replaces the belts and pulleys with discs and power rollers. Although such a system seems drastically different, all of the components are analogous to a belt-and-pulley system and lead to the same results- a continuously variable transmission.
A second type of CVT is a Hydrostatic transmission. Unlike the pulley-and-V-belt CVT and the Toroidal CVT that are frictional CVTs, it uses variable-displacement pumps to vary the fluid flow into hydrostatic motors. In this type of transmission, the rotational motion of the engine operates a hydrostatic pump on the driving side. The pump converts rotational motion into fluid flow. Then, with a hydrostatic motor located on the driven side, the fluid flow is converted back into rotational motion.
When you step on the gas pedal of a car with a continuously variable transmission, you notice the difference immediately. The engine revs up toward the rpms at which it produces the most power, and then it stays there. But the car doesn't react immediately. Then, a moment later, the transmission kicks in, accelerating the car slowly, steadily and without any shifts. In theory, a car with a CVT should reach 60 mph 25% faster than the same car with the same engine and a manual transmission. That's because the CVT converts every point on the engine's operating curve to a corresponding point on its own operating curve.
Advantages of CVTs
·Eliminates gear hunting as a car decelerates, especially going up a hill.
·Constant, step-less and smooth acceleration from a complete stop to cruising speed.
·Improved fuel efficiency (keeps car in optimum power range).
·Responds better to changing conditions, such as changes in throttle and speed.
·Better acceleration due to less power loss than a typical automatic transmission.
·Better control over emissions due to increased control of the engine’s speed range.
·Replaces inefficient fluid torque converters and can incorporate versions of automated mechanical clutches.
With all of their advantages, CVTs do have some shortcomings. In the U.S., they are still trying to overcome an image problem. The Subaru Justy, for example, was known as a gutless micro-car. Traditionally, belt-drive CVTs were limited in the amount of torque they could handle and were larger and heavier than their automatic and manual counterparts. Technological advances have put CVTs in the realm of their competition- the Nissan Murano's CVT can handle its 3.5-liter, 245-horsepower V6 engine- but first impressions are hard to overcome.