How Does a CVT Work?

A CVT works with an engine in sending power to the wheels.


2020 Subaru Legacy with a CVT
The Subaru Legacy sends power to all four wheels utilizing a CVT.

Every car out there must have a transmission, even the electric cars have a fixed transmission(1), so the first question is what even is a transmission? Basically, a transmission takes the engine’s rotation speed (RPM) and divides it to make the wheels spin slower. An example would be if the engine spins at 2000 rpm in first gear, the wheels will turn at 300 rpm when the car has 15 mph.(2) That engine RPM is reduced by the transmission. If an automatic transmission has 6 gears, the engine speed will be divided consecutively from 1st gear up until 6th gear. Now you’re asking why I’m explaining how a regular transmission works.

CVT, the Gearless Transmission

To understand how a CVT works, you need to understand how a regular transmission works, because a CVT doesn’t have any gears. Or, in fact, it has an infinity of “gears”. In a regular transmission, each gear is made of 1, 2, or 3 pairs of metal cogs which are meshed together (even if it’s a manual or automatic, the concept is the same)(3).

Seeing how you can’t change the cogs’ size; you have a certain number of fixed gears. A CVT works differently. A CVT uses a belt which wraps around two sort of bobbins which can, in fact, change their size(4). By changing their size, they can change how much or how little the engine RPM is divided. CVT comes from a Continuously Variable Transmission, because it can change its gear ratio constantly, within a range of the 1st gear and last gear.

This behavior comes with a few distinct features, such that a transmission can use the engine as efficiently as it can. An engine runs the most efficient around its max torque RPM(5), so it would be in the car’s best interest to run at max torque. However, a regular transmission can’t do that. If you’re too slow in 3rd gear at max torque RPM and you shift gears to get to what speed you need, you won’t be at that RPM anymore.

Below the Redline

A CVT fixes the problem, because it will always stay around the max torque RPM when cruising, regardless of speed. That’s why a car with a CVT stays at a constant RPM and doesn’t shift gears. Your car isn’t broken. That’s how it’s supposed to act. It’s a similar story with accelerating as fast as you can. It won’t go to redline and shift gear. Likewise, it will constantly stay at max power RPM because that’s the fastest way it can accelerate. This is what causes the CVTs to be so efficient. They can constantly use the engine as efficiently as they can. They also drive like an automatic, so no need to shift gears.

Does that hurt the engine? No, you shouldn’t be worried at all. The redline is the point where the engine can start damaging itself, due to heat and stress. If it stays at redline for multiple minutes, you will break the engine. However, a car with a CVT doesn’t have a redline, and the constant RPM you’re hearing is far from that redline. The RPM you’re hearing from the engine is actually engineered to be like that. It’s the point where your engine is the happiest.

Why does my CVT sometimes jolt? Well, as I stated a bit ago, a CVT uses a belt. Even if it’s a metal belt, it’s still a belt. As such, if you use a bit too much force on a belt, as in a brisk acceleration, the belt will stretch a bit. If it stretches a bit, it can come off the bobbins slightly and effectively slip a bit. When the belt settles the RPM also has to settle, which happens with a jolt. This is called the “rubber band effect”. If you barely accelerate and you feel a jolt, the belt is likely used and almost ready to fail.(6)

Artificial Gears

My transmission shifts gears and I have a CVT, is this normal? Yeah, believe it or not it’s normal. Some manufactures create fake gears shifts so people aren’t disturbed that their car doesn’t shift gears. Weird, I know.(7)

CVTs are starting to become increasingly common, due to fuel efficiency and pollution regulations. If the engine can run as efficiently as it can, then it pollutes less. There are many manufacturers who use CVTs for a good while already. Nissan uses CVT on most of their vehicles. Subaru always liked their CVTs. Most hybrid cars use CVTs because they can make them work synchronously with the electric motor. Notably, they are called e-CVTs.(4)

The biggest problem with CVTs is that they aren’t sporty nor beefy. It’s just a belt and two bobbins. They’re flimsy if you think about it, so, they usually have some sort of fault due to them being under-engineered. They are also controlled by the car’s ECU, so if the ECU is programmed poorly, then the CVT will behave weirdly. Knowing this, CVTs are usually recalled and have had plenty of issues, such as Nissan and Subaru.(8)(9) Are they expensive to repair? Probably yes. If that CVT is poorly designed then it probably is, along with no widespread knowledge around them.

Making the Switch…or Not

Lastly, can you switch the CVT for a regular automatic or manual? Probably yes. There are a few select people who do that. Keep in mind that an automatic transmission has a different size and works differently, while also using the car’s ECU differently. Someone who switches them around needs to know how to do exactly that. Also, I wouldn’t recommend it; just sell the car and buy another.


References

1. tue.nl. New transmission system makes electric cars more powerful, more efficient and potentially cheaper. TU/e – Eindhoven University of Technology. [Online] https://www.tue.nl/en/news-and-events/news-overview/07-12-2020-new-transmission-system-makes-electric-cars-more-powerful-more-efficient-and-potentially-cheaper/.

2. Puscasu, Cristian. -. Imgur. [Online] https://imgur.com/a/662WDLi.

3. LearnEngineering. Working Principle of a Manual Transmission. [Online] https://j.gifs.com/Ro822O.gif.

4. Wikipedia. Continuously variable transmission. Wikipedia. [Online] https://en.wikipedia.org/wiki/Continuously_variable_transmission.

5. Mechanics StackExchange. Does Max Power RPM Equal Max FE? Motor Vehicle Maintenance & Repair. [Online] https://mechanics.stackexchange.com/questions/19712/does-max-power-rpm-equal-max-fe.

6. Chandrasekaran, Karthik. Quora. What does rubber band effect means in driving a car with CVT transmission? [Online] https://www.quora.com/What-does-rubber-band-effect-means-in-driving-a-car-with-CVT-transmission.

7. Santos, Joe. Why do CVT Transmissions Simulate Shifts? MotorBiscuit. [Online] https://www.motorbiscuit.com/why-do-cvt-transmissions-simulate-shifts/.

8. Cash Cars Buyer. Nissan CVT Transmission Problems: What You Need to Know. [Online] https://www.cashcarsbuyer.com/nissan-cvt-transmission-problems/.

9. Everything You Need to Know About Subaru CVT Transmission Problems. [Online] https://www.cashcarsbuyer.com/subaru-cvt-transmission-problems/.


See AlsoWhat is the Suzuki Alto?

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Author: Cristian Puscasu
Cristian lives in Romania and is studying to become an automotive engineer. Currently in his third year, he enjoys writing about cars as well as tinkering with them. We are pleased to welcome him as a contributing writer, knowing that he will pursue far greater things in the years to come.

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