What The Heck Are Continuously Variable Transmissions?
30 Jan 2018
The transmission in your car is the thing used to change gears, but we bet you knew that. Since the 1940s, there have really been just two types of transmissions: standards and automatics. With a standard transmission, you shift the gears yourself and with an automatic, the transmission does it for you. But we bet you knew that too. Today, however, it’s not quite so simple anymore. The transmission family has a new sibling for you to get to know: the Continuously Variable Transmission (CVT).
What’s The Difference?
Conventional automatic transmissions use sets of gears that provide different ratios or speeds. Control units sense speeds and then shift a transmission’s gears automatically to provide the most appropriate ratio for a given driving situation: lowest gears for low speeds, middle gears for acceleration and passing, and higher gears for efficient cruising. This is visually indicated in most vehicles with a selector display (P-R-N-D-L) that shows exactly what gear is selected.
A CVT replaces the complex gears sets common in standard and automatic transmissions with two variable-diameter pulleys with a big metal belt running between them. It’s pretty simple. Here’s how it works. First, one pulley is connected directly to the engine and the other to the drive wheels. During operation, the pulleys move in and out such that a metal drive belt moves higher on one pulley and lower on the other. You can picture this roughly as the way a 10-speed bike works, by routing the chain over smaller or larger gears to change the gear ratios. The gearing effect with a CVT is essentially “continuous”, hence the nomenclature.
What They are Like to Drive:
The controls for a CVT-based car are the same as an automatic-based car: Two pedals (gas and brake) and a P-R-N-D-L-style shift pattern. When driving a car with a CVT, you won’t hear or feel the transmission shift, though. It simply raises and lowers the engine speed as needed, calling up higher engine speeds (or RPMs) for better acceleration and lower RPMs for better fuel economy while cruising. Many people find that driving a CVT-based automobile is a little disconcerting because of the sound it makes. When you push hard on the accelerator, the engine races a bit as it would with a slipping clutch or a failing automatic transmission. This is normal, the CVT is simply adjusting itself to provide optimal power for acceleration.
Engines do not develop constant power at all speeds; they have specific speeds where either torque, horsepower or fuel efficiency are at their optimum levels. Because a CVT can theoretically adjust to any gear ratio, engineers can program it to give the maximum performance during all these situations. For example, when power is needed, or maximum fuel-economy is desired, a CVT can adjust to those conditions more accurately than a standard automatic transmission. Our source at East Hills Subaru of Roslyn, NY verifies this; they tell us that models with Subaru’s Lineartronic CVT installed generally get better gas mileage than the standard shift models do.
The CVT’s biggest problem has been user acceptance. Because the CVT allows the engine to rotate at a wide range of speeds, the noises coming from under the hood can sound odd to drivers accustomed to conventional automatic transmissions. Frankly, the gradual changes in engine note sound like something are “slipping”– signs of trouble with a conventional transmission. Of course, nothing is wrong, it’s just the way a CVT works.
Today’s automakers have gone to great lengths to make the CVT feel more like a conventional transmission. For example, many CVTs are now programmed to simulate the “kick-down” feel of a regular automatic when the pedal is floored, and some CVTs offer a “manual” mode with steering-wheel-mounted paddle shifters.
Because early automotive CVTs were limited as to how much horsepower they could handle, there has been some concern about the long-term reliability of the CVT. However, this has not proven to be a problem. Advanced technology has made the CVT quite robust and reliable. In fact, Nissan has more than a million CVTs in service around the world and says their long-term reliability is comparable to conventional transmissions.
The future of the CVT remains to be seen. In spite of the obvious efficiencies of CVTs, customers can be fickle and may prefer an older technology. We doubt, however, that CVTs will ever head to the scrap bin of automotive technology. They are a good idea and as soon as the public comes to understand them better, the future looks bright.
Featured Image by Pixabay