Posted on 12/05/2006 7:39:48 AM PST by Red Badger
Engineers at the Virginia Polytechnic Institute and State University have developed a continuously variable transmission based on a cam and a planetary gearset that might increase fuel efficiency 3-5%.
Much work has gone into the field of continuously variable transmissions, which can be categorized as traction, belt, ratcheting and hydrostatic drives. The new cam-based system is a ratcheting drive type. Ratcheting drives take rotational input, convert it to an oscillating motion of varying amplitude, and then rectify this motion through a number of one-way clutches to a rotational output.
One of the distinctive attributes of the new transmission developed by Derek Lahr and Dennis Hong compared to other ratcheting drive transmissions is that it not only produces uniform output given uniform input, but its output can be matched to nearly any periodic waveform.
As a result, the developers believe that the IVT would be ideally suited for application in wind turbines as well as in automotive and other industrial applications. Camcvt2 The cam.
The core of the transmission is the centrally located cam. A number of cam followers are rotably mounted to a carrier plate; each of the followers is fixed to a planet gear. An equal number of sun gears—each meshed to one of the planet gears.—connect to the output shaft.
By stacking an infinite series of profiles along the length of the cam to make a three dimensional camoid, an infinite number of transmission ratios can be selected. By varying the position of the cam followers in relation to the cam, the particular profile they follow can be changed. This affects the magnitude of the follower’s oscillations and therefore the transmission output.
Camcvt4 The transmission. Note the quarter-circle pully (purple) at the mid-right.
Instead of gears with teeth, the transmission uses quarter-circle pullies and kevlar cord—this does away with the friction loss of one gear on another. (See diagram at right.) A flexible nylon ring to move the followers across the 3-D cam to change gears.
The developers have built an initial prototype as a test bed for design concepts such as the shifting mechanism, and to verify the underlying the kinematic equations.
A second prototype is in design, and will be tested for efficiency, durability, stress levels, and overall operability. The second design will also incorporate several improvements to increase the available gear range and torque capacity.
Among those improvements is a major change in cam implementation. The next prototype will have a barrel cam with the cam shape on the inside of a cylinder. The followers thus will be inside the cam, and held against the cam surface by centrifugal force. Sprag clutches are used so energy loss with the ratcheting is nearly zero.
Ping!
Man, I love innovation!
I've often wondered why the automakers don't take a plain manual 5 or 6 speed transmission and incorporate a computer controller to shift the gears automatically. There you would have the best of both worlds. Automatic manual transmission!.......
DAF of Holland made a little car with a cvt back in the early 70s. A few were exported to the States. I think they became a part of Volvo and the project was closed out.
Belt type CVTs have been used on snowmobiles for 30 years pretty successfully. Must be some enormous scaling problems if they haven't figured out how to use in in cars yet.
"rotably"
There's a word I've never seen before. Even Google thinks it's a mispelling of something else. A guy needs a degree of some sort to understand this article.
As for infinitely variable transmissions, would the Buick Dynaflow transmission of the fifties qualify? That sucker never shifted gears, it just whirred on out.
It could have a Manual override to allow you to shift yourself if the computer controller malfunctioned or if you just wanted to do it youself. Automatics need filters and transmission fluid changes, manuals don't. Automatics have torque converters, bands and multiple clutches (plus are very complicated to manufacture) that wear out. A manual has only one clutch and is very simple, comparatively, to manufacture. Manuals typically get better mpg than automatics. It seems to me that this would be the best possible scenario. With computer control you could have maybe even 7 or 8 gears that it could choose from to get the best possible mpg, via speed, rpm and throttle sensors plus mathematic algorithims. The automatic transmission that we know today should have been dumped long ago in favor of a computerized manual...........
My Nissan Maxima has a CVT tranny. Its a big strange to have revs and no shifts, but you get used to it. Then when I hop back into my Tahoe, the shifting seems strange.
The Subaru Justy had a CVT I believe. Somewhere I read that once you get over a certain load they are no longer practical. Maybe this design will change that.
It is interesting though. And not only would they be fuel efficient, but smoothe. Imagine taking off from a light at full throttle and your engine just sits at the peak of the power curve in the rpm as you are pinned to the back of your seat all the way.
Kewl indeed.
"Among those improvements is a major change in cam implementation. The next prototype will have a barrel cam with the cam shape on the inside of a cylinder. The followers thus will be inside the cam, and held against the cam surface by centrifugal force. Sprag clutches are used so energy loss with the ratcheting is nearly zero."
Now THAT will be VERY interesting...nice to see in an era when so much attention is lavished on the latest electronic breakthroughs that good old fashioned mechanics still has room for application of good old fashioned human ingenuity.
I drove a buddy's new VW GTI, which is equipped with the new "direct shift" automatic (Tiptronic). I enjoy a clutch and gearshift, too - but those paddle shifters are mighty entertaining. I noticed no "nanny override" which prevented me from downshifting when speed/rpm was a touch too high - a pleasant surprise. All in all, a very good compromise between stick & clutch and the usual slushbox.
.....but will it ever see the highwyas of America?.........
Didn't Porshe make an automatic clutch a while back?
I find the idea of an automatic manual intriguing. Particularly with the notion of an automatic overide to have manual control reminds me of the old dual gate Hurst Shifters.
I think computer control of a manual would be less to develop than a whole new tramission system.
The Toyota Prius (and the hybrid Camry) use a true CV transmission, which uses a simple set of 3 gears in a sun configuration.
They use a small electric motor on the inside gear, the engine attached to the outside gear, and the drive shaft attached to the middle gearset. If you lock the electric motor, the engine rotating the outside gear gears down to the middle gearset driving the engine at about a 72% ratio.
But if you spin up the electric motor, you can make ANY ratio of rotation between the engine and the drive shaft.
The only limitation is that the electric motor needs to have enough torque to handle it's task, which isn't much because it is geared down from the drive shaft.
Of course, the Prius/Camry have an electric motor directly on the drive shaft, which actually performs speed control. The gas engine can spin at any rotation the computer decides is most efficient, providing torque necessary. And the small motor often spins NOT as a motor, but rather as a generator, capturing power to be used to charge the batteries or run the main motor.
You drive the car, there is absolutely NO shifts at all, and frankly little relationship between your speed and the sound of the gas engine. Acceleration is smooth, and there are NO parts that engage and disengage to get worn out like clutches or gear teeth.
Even reverse is NOT a gear -- just the electric motor spinning in the other direction, with the engine usually idling or turned off (downside -- the engine can provide NO torque to help go backwards, so in the ORIGINAL prius, which had a smaller electric motor, there was a hill in San Fransisco that was so steep you could hardly back out of a parking space if you had to back up the hill).
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