INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Curious about CVT

Curious about CVT

(OP)
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.

RE: Curious about CVT

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
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules

RE: Curious about CVT

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


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Curious about CVT

(OP)
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.

RE: Curious about CVT

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

RE: Curious about CVT

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
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules

RE: Curious about CVT

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


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Curious about CVT

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!

RE: Curious about CVT

(OP)
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.

RE: Curious about CVT

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

RE: Curious about CVT

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

RE: Curious about CVT

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

RE: Curious about CVT

(OP)
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?

RE: Curious about CVT

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close