A continuously variable transmission (CVT) is a type of automatic gearbox that has gained popularity among many car manufacturers, especially in Asia, in recent years. It doesn’t work like a conventional automatic gearbox, so in this article we’ll explain what a CVT is, how it works, and its pros and cons.
A CVT gearbox is unlike a conventional automatic gearbox because it doesn’t use six or seven different-sized cogs, or ‘steps’, when the car changes gear. Instead, a CVT uses a belt and conical-shaped gears, so shifts are smoother and stepless.
It’s a strange, but simple, concept. The belt is an elastic drive band that continuously varies the ratio between engine and wheel speed, ensuring that it stays permanently in the correct power band.
The idea behind the CVT has been around for centuries, with Leonardo DaVinci credited with designing the first CVT in 1490, and patents filed as far back as the 1880s (by a couple of chaps called by Daimler and Benz).
The first production car to use it, however, was the 1958 DAF 600, built in The Netherlands. A quirk of the DAF system was that the cars it was fitted in didn’t have a reverse gear: instead, the CVT box simply worked in reverse, which meant that theoretically, the cars had a top speed that was achievable going both backwards and forwards.
Since then, it has been traditionally applied in small-engined little cars, but its popularity, especially in Japan, means that manufacturers worked out how to use the system in larger, more powerful vehicles.
That means you’ll often find a CVT in Toyota and Lexus models, especially hybrid models, and cars from Honda and Nissan. South Korean manufacturers Hyundai and Kia also used to fit CVT gearboxes in many of its automatic models, but they have increasingly switched to dual-clutch transmissions (DCT) in recent years.The Multitronic gearbox in Audi models is a version of a CVT ‘box too.
However, with 87% mechanical efficiency, CVTs are not the most efficient technology on the market. With stricter emission legislation and more efficient transmissions, such as DCT, now on the market, the days of the CVT may be numbered.
How does a CVT transmission work?
A CVT gearbox has a straightforward arrangement of two conical pulleys, with a V-shaped drive belt connecting them. One of the pulleys is connected to the engine, with the other connected to the wheels. When you press the accelerator, the engine turns over faster and the connected cone moves accordingly. The other cone adjusts to how the engine cone is responding to the amount of engine power, the drive belt maintaining the same tension. This determines how the car accelerates.
As the two cones move independently, there are no set gear ratios, essentially making a CVT a single-speed unit with an infinite number of gear ratios available. Its operation also explains why cars fitted with a CVT have a high-revving character, because the engine revs aren’t related to wheel speed.
The latest CVT-equipped cars have lots of soundproofing to ensure improved refinement and the technology has progressed so that the modern CVT gearbox is better than it used to be, both in terms of driving enjoyment and efficiency.
Indeed, Nissan has developed a CVT gearbox called Xtronic which features a number of ‘steps’ in its power delivery, which is designed to make it feel more like a conventional gearbox by ‘shifting’ through the gears.
Elsewhere, Toyota’s Hybrid Synergy Drive (HSD), which is used in hybrid models such as the Prius, uses a form of CVT gearbox as part of the hybrid system. This has a split power delivery so that the car’s engine is either driving the wheels or sending power to the battery.
CVT transmission pros
- Better fuel economy. Modern hybrids are fitted with CVTs for fuel efficiency, while many ICE models with a CVT offer decent fuel economy figures.
- Easier uphill driving. A conventional transmission tends to work harder to find the right gear ratio for the drive, while a CVT finds the exact ratio needed almost instantly, allowing the engine to seamlessly provide power without hard shifting.
- Smoother driving and acceleration. Because there are no ‘steps’, a CVT eliminates the need to make sudden downshifts when trying to get additional power for overtaking.
CVT transmission cons
- More costly than traditional gearboxes. The repair and replacement costs for a CVT can be more expensive (in thousands of pounds) because CVT systems don’t tend to last as long as traditional systems. They can fail after just 100,000 miles, half of the expected life of a conventional gearbox.
- It’s not designed for high-performance vehicles. CVTs can’t handle the increased horsepower of a high-performance engine, so you won’t find them in performance cars.
- Noisy. The noise that a driver notices, while lower than it used to be, is much greater than with a conventional auto ‘box.
What CVT problems to look out for
If you have a car with a CVT gearbox, it’s worth monitoring its performance, especially if the mileage is close to six figures. There are a number of tell-tale signs that the CVT in your car might not be running optimally.
- Slow shifting. You don’t have to shift a CVT from first to second gear, but you do have to shift between park, drive and reverse. If this change takes more than a second or so, you could have a faulty CVT.
- Strange sounds. CVTs are louder than their conventional counterparts, but humming and whining sounds aren’t normal. Excessive noise when accelerating is a tell-tale sign.
- Slipping. When accelerating, your car should experience continuous, seamless acceleration. If it slips or temporarily loses power, the CVT is faulty.
- Jerky shifting. When a CVT gearbox is shifting, it shouldn’t cause the vehicle to jerk.
- Inconsistent rpm. If you notice any fluctuation in the engine’s revs when driving at a constant speed, there could be a problem with the CVT.
- Dirty transmission fluid. When getting your car serviced, if the transmission fluid is dirty, this could indicate an issue with the CVT.
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