What's The Deal With "CVT" Transmissions?

I just test drove a Honda CIVIC HX, which has a most unusual automatic transmission. It is a “CVT”-that is, the engine drives the wheels through a belt, and the pulleys driving the belt change their diameter, thus changing the gear ratios. This is a really strange sensation-the engine stays pretty much at the same revolution level, while the car speeds up. I did a little research, and found that the first such transmissions were invented by a Dutch company (DAF) over 40 years ago. And, SATURN plans to use such a tranny in their new SUV (the “VUE”). These trannys are supposed to offer the highest gas mileage, and are said to be reliable too. My question is, since these CVTs have been around so long, why haven’t they been popular? Do the belts wear out?
And (Id like to hear from some Honda drivers) is the CIVIC HX a good car?

CVT’s do indeed increase gas milage. There’s no wasted time between shifting gears, because as soon as engine torque can be utilized more efficiently, it is. Automatics need a bit of excess time for the hydraulics of their clutches to activate. Manuals are better, as the human brain can judge more efficiently when a shift is required.

Of course, this CVT technology is more expensive, which is probably why it hasn’t caught on. Also, many people (like myself) like the control of shifting manually.

When I bicycled more, I read about CVT’s for bikes. That’d sure beat juggling around with 15 gear ratios (actually 13). But the price is still more than a standard derailer (sp?).

CVTs have been around almost as long as the automobile, but never caught on, primarily because of problems with belt life. Presumably the current crop have some new approach to belt design that makes the manufacturer feel better about offering a CVT.

The best feature is that the engine runs close to optimum RPM, efficiency-wise, all the time.

Incidentally, apropos of AWB’s comment, manual transmissions are more efficient than automatics because there are no losses in the torque converter and because there is no need to pump hydraulic fluid to actuate the gear-change mechanism.

The Subaru Justy had an optional tranny like that back in the late 80s I believe, never caught on.

The concept is very straight forward, and has been (still is) used in industrial applications for years. As a matter of a fact, I just spec’d a set of 30HP variable sheaves with ratios of 4.75 (either a speed up or reduction). Since there is basically infinite ratios in between, as you noted, you get a extremely smooth transition from a start to cruising speed. Since you have a set centre distance between the two sheaves (or pulleys), both of them vary diameters which gives you your varying ratio.

I understand the type of belts used in the automotive applications are metalic, whereas the industrial ones are rubber. Either way, concept is the same and it is extrememly sound and proven.

Are you going to buy the car?

The main reason you haven’t seen very many of them around is that they (uptil recently) could only handle a very low power engine. That’s why they were used in micro cars and other econboxes like the Justy.

I think it had to do with the slippage on the belts under heavy load. The new GM CVT (it’ll be used in the Saturn first) I believe uses rollers instead of belts.

I’ve read somewhere that Nissan are producing a model with a cvt transmission but it will give the driver six fixed ratios. Is this disadvantageous or the best of both worlds?

Hmmm. I can’t think of any advantage of fixing the ratios myself.

There are basically two different kinds of mechanical CVTs;
belt with variable pulley ratios, and friction drives, composed of cones and/or toroids (power and torque are transmitted at the tangent of the toroids, through a film of oil). As stated above, the main drawback is the limited amount of torque and horsepower the CVT can handle.

A more common form of transmission is the electric transmission. An engine is hooked up to a generator which powers motors at each wheel. Not only does this arraingement produce CVT action at high torque/load yields, but with an electric storage unit (battery, capacitor, or
magnetic flywheel) one can recover the energy otherwise lost during braking.

I love mine. It’s 4 1/2 years old and the only problems were with the power windows, which both fell off their rails and got stuck. But that was fixed under warranty.

The CVT is pretty quiet, which is good because there’s a fair amount of cabin noise at highway speeds. I do get excellent gas mileage–about 30 mpg in town with snow tires on. As you probably noticed on your test drive, the HX isn’t doesn’t have a ton of kick when you stomp the gas. I don’t know if that’s directly related to the CVT or if it applies to all Civics generally, because I only test drove the HX.

I’ve only seen one other HX on the road ever. If you’re looking for a used car, they seem to be cheaper than ones with normal transmissions.

Lacking kick is attributed to the car in general, not the transmission. As is the window problem and the noise. It’s a Civic, afterall.