Photo courtesy Subaru France
Sold between 1989 and 1993, the Justy never attracted the attention of American drivers. So what's different about newer CVT-based cars -- cars like the Saturn Vue, the Audi A4 and A6, the Nissan Murano and the Honda Insight? The best way to answer that question is to take one of these cars for a "test drive." The animation below, which compares the acceleration of a car with a CVT to one without, gives you a good feel for the experience.
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 (100 km/hr) 25-percent faster than the same car with the same engine and a manual transmission [ref]. That's because the CVT converts every point on the engine's operating curve to a corresponding point on its own operating curve.
If you look at the power output curve for the car without a CVT, you can see this to be true. Notice that the tachometer in this situation shows the engine revving up and down with each gear change, which is recorded as a spike in the power output curve (and which the driver feels as a jolt).
CVTs are equally efficient on hills. There is no "gear hunting," because the CVT cycles steplessly down to a gear ratio appropriate for the driving conditions. A conventional automatic transmission shifts back and forth trying to find the right gear, which is far less efficient.
With all of their advantages, CVTs do have some shortcomings. In the United States, 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.
For more information on continuously variable transmissions and related topics, check out the links on the next page.
Related HowStuffWorks Articles
- How Automatic Transmissions Work
- How Gears Work
- How Gear Ratios Work
- How Manual Transmissions Work
- How Torque Converters Work
More Great Links
- InsightCentral.net: CVT Transmission
- Automotive Engineering International Online: Audi takes CVT from 15th century to 21st century
- Edmunds.com: CVT Enters the Mainstream
- MSN Autos: Engineering New Transmissions
- Nissan USA: Xtronic CVT
- Birch, Stuart. 2000. Audi takes CVT from 15th century to 21st century. Automotive Engineering International Online. January.
- Carney, Dan. 2002. Small SUVs face off. Popular Science. February 18.
- Cars.com Glossary. Continuously variable transmission (CVT).
- CVT: Continuously Variable Transmission.
- Encyclopedia Britannica 2005, s.v. "automatic transmission." CD-ROM, 2005.
- Gizmology.net. Notes on Continuously Variable Transmissions.
- Job, Ann. Engineering new transmissions. MSN Autos. Accessed March 20, 2005.
- InsightCentral.net. CVT Transmission.
- Kluger, Michael A. 2000. Gearing up for CVTs. Technology Today. Summer issue.
- Lienert, Dan. 2003. Cool. Where can I get one? Popular Science. January 16.
- Lienert, Dan. 2003. Putting power to pavement. Popular Science. August 13.
- Memmer, Scott. CVT enters the mainstream. Edmunds.com. Accessed March 20, 2005.
- Nissan USA. Xtronic CVT.