Continuously Variable Transmissions

Does anyboy own a car with one of these, or have any knowledge or experience with these? I know a little about these devices. Unlike a traditional manual transmission with a choice of perhaps 5 speeds, or an automatic with 3 or 4 speeds, a CVT tranny offers basically an unlimited number of gear ratios (within a certain limit, of course). This allows an engine to turn at its optimum speed for given road conditions. I know there are a number of different schemes of CVT: friction, hydraulic, electric, etc.

I've driven a Toyota Prius with one and it seemed very smooth. But, I have questions about its strength. I mean, have you ever seen a race car with a CVT? I think I also remember about Suzuki using one in a little 3 cylinder micro commuter a few years ago; I believe the tranny suffered a lot of breakage problems even with that puny motor.

How are they in terms of performance? If you put three identical Honda Civics in a 1/4 mile drag race, one with a CVT, one with a manual, and one with an automatic, how would the CVT fare? Last place?

The main reason I'm asking is with gas prices raping us every time we need to fill up, I'm seriously comsidering getting one of the Hybrids with a CVT.

Bruise

This is a side answer about the CVT. Next door to were I worked just out of high school (almost 30 years ago), there was a company that swept parking lots. One of the mid-sized riding sweepers they used had a CVT--belt type.

Most people are used to a gas pedal to go faster or slower--a side effect is that with a gas pedal, the more you push, the more power you would get.

With these sweepers, the engine was run at a constant speed with a governor, and the "gas" pedal was actually connected to the CVT. When a new driver first drove the sweeper, all seemed well. Push the pedal a little--you go slow. Push the pedal hard--you go fast.

The problem came when the new guys would be in a parking garage going up the ramps. They would be trucking along, and start to slow down going up the ramps. And, they would push on the "gas" to pick up speed and add power. But in this case, they were actually changing the CVT into a higher gear--thereby slowing down more and lugging the engine and eventually, they could burn the belt out.

What they had to learn was to let up on the "gas" pedal. This shifted the CVT into a lower gear, which did slow the sweeper down, but it then climbed the ramp without problems.

The Prius must be an interesting drive. I wonder what its "gas" pedal is connected too (obviously a computer)... But I would guess that the "gas" pedal is more of a speed device and the computer handles the mixture of throttle, CVT ratio and electric energy management to obtain a fixed speed. The other option would be for the computer to supply a "fixed" torque or "fixed" power mode.

A quick test would be to set a speed/gas pedal setting on a flat stretch of road and hold it as the hybrid goes up a hill... Does the car slow down or speed up?

Anyway, here is an interesting site on how the Honda and Toyota Hybrids work:

How do Hybrid Cars Work?

-Bill

I believe you are thinking about the Subaru Justy, it had one of the early CVTs, not sure about reliability. I did see one on the road recently.

The beginning of the CVT, the "DAF" Variomatic, later followed up by the push belt cvt.

from what I've read of CVTs, they are usually implemented as belts driven by (and driving) a pair of cones that effectively form a variable diameter pulley. A simple idea, but difficult to implement.

The typical limitation is the amount of power that can be transmitted before the belt starts to slip. As such, CVTs are usually only used on vehicles with low power engines.

The advantage of CVTs is that they allow the load to be matched to the power source, which permits the use of a smaller engine. A good example of this principle is the gearing on bicycles. A bike can be built with a single speed, but the cyclist will have a terrible time going up hills or going fast. With lots of speeds (i.e. the 27 speeds that are common on bikes now), the cyclist can get up any hill, and can go 40 or 50mph under the right conditions.

Steve K.

CVTs are improving. Nissan figured out a way to make the belt out of durable links or something (I don't completely understand it) so that it could take a lot more power before slipping than previous CVTs. Thus you have the Nissan Murano and it's Infinity brother.

The Murano offers excellent performance in part because of it's CVT. Basically it gives you as close to linear power as you can get from an ICE. 0-90 with no lag as the transmission shifts gears, that is, the power keeps flowing even as the gear ratios change.

Audi is pushing this technology hard.
http://www.histomobile.com/histomob/tech/2/86.htm

The newer CVTs have a linked belt that gets pushed, not pulled. Much stronger and more efficient. Recently bought a '93 Fiat Panda with CVT and it's still going strong. More acceleration than I'll ever need (and it only has a 1.1 litre engine). Gets 38 real-life MPG (not remarkable for such a small and light car, but then it's 10 years old).

The Panda also has a rather clever clutch, with an electro-magnetic flywheel and iron filings providing the link to the gearbox. It can be a bit jerky at times, but again, this is probably old age creeping in (or just Italian temperament?). Can't help thinking a centrifugal clutch would be just as good, and simpler.

CVTs have one very special feature : an infinite number of reverse gears, giving rise to some pretty spectacular wrong-way racing on Dutch television (maybe B@rt has some pics of that too?)