Curious about CVT

[dicer]

I dont' understand why any car maker would use them. On a snow mobile I understand it.
CVT is under constant slippage where as normal automatics have clutches that lock. Constant slippage means wear.

 

[patprimmer]

CVT does not mean constant slippage.

Durable CVTs that are compact and handle high power outputs are not true CVT, they just have so many ratios that changes are non detectable.

Constant slippage does not necessarily mean constant wear, at least not any more than a traditional manual box, for instance, how much wear occurs in a fluid drive like a visco coupler or a torque converter.

The one I have driven work great and now has quite a few miles on it. It is in a Nissan Murano about 3 years old.

Regards
Pat
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[GregLocock]

The more powerful CVTs tend not to rely on slippage as the heat dissiaption becomes a problem. I don't know much about the old Van Dorne variomatic system in practice, but don't remember hearing that drive belt wear was a huge issue.

Cheers

Greg Locock


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[dicer]

Normal automatic transmission clutches are designed to lock after the shift is completed.
In a CVT its is constant motion, sliding up and down the pulleys while being clamped, so there is constant slippage between the belt and the pulley sides. Or if its the toroid type then the wheels are slipping up and down the sides under pressure. There is no positive drive, it is open to slippage radially as well under heavy load conditions, it is not like a cogged timing belt or a gear set, so yes there is constant slippage.

 

[GregLocock]

Yes, there is slip. As I didn't say, tens, hundreds or thousands of thousands of DAFs were sold with Variomatic and belt wear wasn't a huge issue.

Cheers

Greg Locock


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[NormPeterson]

I think it's a perception issue rather than a mechanical one.

You're trying to evaluate an automobile CVT exactly as if it was a conventional automatic with a number of fixed ratios. Your experience with conventional automatics is subconsciously insisting that the disconnects in engine rpm vs road speed and in the rate of engine rpm change vs acceleration has to be coming from "something slipping". Mostly, it's just the ratio automatically varying without there being discrete steps.

There can't be a whole lot of slippage happening, else with current pressures to reduce fuel usage there would be fewer such devices rather than more of them.

It's not that slippage couldn't be made to happen if you try hard enough. But you could reasonably expect some sort of engine torque management or programming to temporarily raise the CVT hydraulic system pressures to be in place to prevent that from actually happening (such as when the drive side pulley is operating at the small diameters).


Norm

 

[patprimmer]

If ever you drove a top gear only automatic, ie a torque converter hooked to a direct drive you would have a CTV over a relatively limited range from total slip to about 90% locked give or take.

I believe the one Nissan currently offers has a steel toothed belt ot chain and two opposing cones with groves cut into the cones so the chain grips them. The chain slides along and grips new sets of groves. It is more a very large number of very close ratios than a true CTV. I am not aware if the clutch mechanism is a torque converter or not

Regards
Pat
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[NormPeterson]

Nissan's version for the 3.5L engines.

http://www.nissan-global.com/EN/TECHNOLOGY/OVERVIEW/cvt.html

Norm

 

[GregLocock]

The EA Falcon's 4 speed auto defaulted to 3rd gear, open TC, as a drive home feature if there was a failure.

It always amused me how many taxis were driving around like this, maybe 10% of the fleet.

Cheers

Greg Locock


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[pontiacjack]

While the DAF vehicles had solid reliability, keep in mind that they were on the order of 1,300 pounds and most were not more than 32 HP.
I always admired the clever design of a separate belt-and-pulleys driving each rear swing axle. Very cost effective, obviating the need for a differential and axle U-joints, yet still offering independent rear suspension!

 

[dicer]

If there is a pinching of the pulleys to force the belt to go to a larger diameter pulley, there is
very high contact pressure at belt and pulley, as well as the torque pressure to or from the belt.
And since the contact area does not even come close to the size of a wet clutch pack, I just can not see how this system can hold up that well. Maybe for underpowered granny cars it maybe fine.

 

[patprimmer]

I don't see it either. My brothers Nissan Murano V6 is a figment of our imagination.

http://en.wikipedia.org/wiki/Nissan_Murano

It seems it has 265 of the granny horse powers. It's 3 years old, done well over 100,000ks and gets used hard, towing loads and going moderate off road.

Regards
Pat
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[JayMaechtlen]

The Prius has a setup that pretty much acts like a CVT, doesn't it?
All the motors and controls involved might make it impractical for anything but a hybrid, I suppose.

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[NormPeterson]

I would think that the mean radius to the "belt" would be a far more important parameter than contact area.

I suppose you could think of a CVT sort of like a conventional automatic transmission band or clutch in the respect that it's not engaged by default. But isn't contact area in a clutch something you use to provide durability when slippage under significant load is completely unavoidable at times and at least one of the surfaces is expected to wear? In Nissan's CVT, contact area might only be a consequence of providing sufficient buckling resistance in the individual links.


Norm

 

[LionelHutz]

I wouldn't call a Murano quick or sporty by any means. They seem to work OK though.

 

[dicer]

The problem is, it is not locked in one particular gear, its in constant motion especially at lower speeds and accelerating and decelerating. Do they have any ability to use the engine to brake like most transmissions do?

 

[patprimmer]

They drive very much like a modern automatic. ie they don't engine brake like a manual, but they do hold a bit.

Regards
Pat
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[MikeyP]

I drove one for a couple of years on a Dodge Caliber. It was a Jatco unit rated to about 250 Nm - I think they make them up to 400 Nm or so. It had a torque converter for launch and no engine braking.

The drivabililty was excellent (I guess that's the whole point of a CVT). Cruising was relaxed because it could go up to a very long overdrive ratio. For acceleration the fundamental control algorithm seemed to work on 3 different engine speeds (this was a 2 litre 4 cyl petrol engine).
For light acceleration it would hold the engine speed at about 1500-2000 rpm, half throttle it would hold it in the 2500-3000 rpm region and WOT would sit at about 4500 rpm. Kick down was pretty fast when needed.

The thing that takes getting used to is the fact that you don't get the speed related auditory cues from the engine. You have to get used to watching the speedo far more than usual to check your speed. I guess it took about 3 weeks to get used to it. Some people really detest the "motorboat" effect during acceleration where the engine sounds at the same note as you accelerate. It didn't really bother me.

In the end I got rid of it because it was too thirsty - I think because of the engine not the transmission.

Some belt CVT designs use a planetary gear arrangment to provide a geared neutral and reverse instead of a torque converter. These have a slight efficiency advantage but the launch is controlled by a wet clutch which limits the torque capacity.

M

--
Dr Michael F Platten