The Continuously Variable Transmission, or CVT, is by no means new – and has been around since 1958.
Lately, however, the CVT is proving popular and is being used by more manufacturers in more models.
This includes Subaru, who uses CVT transmissions in all their automatic vehicles.
We spoke to Subaru to find out why they like the CVT.
How it works
A CVT transmission works with two pulleys with varying diameters that are connected to each other via a flexible running chain.
The one pulley connects to the wheels, while the other goes to the engine.
As more power is needed throughout the rev range, the width of the pulleys will change in opposite directions – thus providing an infinite ratio of “gears”.
According to Subaru, the adaptive control system in their Lineartronic CVT “automatically shifts to the most optimal gear to suit any driving style and road condition”.
At the other end of the transmission ring stands the “traditional” automatic gearbox – also known as a hydraulic planetary automatic transmission.
This unit has a set amount of gear ratios, usually in a “planetary” configuration. The smaller gears are arranged around the larger “sun” gear, and are held in a ring called a “planet carrier”.
Sensors will tell the gearbox when it is time to change gears, and a hydraulic system will make this happen.
In order to change the gears, the transmission must first be decoupled from the engine via a torque converter – which is similar to stepping on a clutch in a manual vehicle.
Three fan-shaped components inside the torque converter shift all the mechanical parts to where they need to be to engage the next gear, and then reconnect the transmission to the engine to continue doing its job at a lower or higher ratio.
Neither of these transmissions require any input from the driver to change gears, but the Lineartronic CVT (pictured below) in Subaru’s new XV will now allow human gear changes for a more engaging experience – a feature that was generally only available with a traditional automatic box.
Benefits of a CVT
The main benefit of using a CVT, according to Subaru, is “for instant, seamless response to changing conditions”.
The infinite gear ratios assure that any ratio is the optimal one for that moment, whereas this is not always applicable in a normal automatic.
Since a normal automatic box has a set gear ratio, the driver may need to intervene and shift to a higher or lower gear for better throttle response.
“Another advantage of a CVT is that it allows the engine to operate longer at a lower speed than conventional automatic transmission systems,” explains Subaru.
This leads to better fuel economy.
“CVTs also deliver a smoother ride, as it eliminates shocks when changing gears and allows the engine to run quieter.”
Drawbacks of a CVT
The main argument against CVTs is their tendency to lose power, or “hang”, at higher speeds.
This is a large point of discussion for car enthusiasts, and Subaru has been through a lot of effort to minimise the “hanging” occurrence.
“Many drivers have complained about the CVT’s propensity to hang at a high rpm, causing the engine to rev wildly under acceleration,” said Subaru South Africa’s technical department.
“Subaru’s CVT has an expanded gear ratio coverage and lowered overall weight to improve acceleration and fuel efficiency. This reduces the hanging effect.”
Additionally, with improving technologies and increasing market share for this gearbox type, potential costs differences for construction and repairs may fall away.