Hi folks,

I was disappointed to see the "Practical Knowlege" in the January Scientific Amercian discussed Constant Velocity Transmissions but only showed the sliding cone - metalized band transmission as the "most common CVT." Granted I have a Prius bias but the sliding cone, CVTs have problems that the planetary gear systems avoid. Most of the recent CVT announcments have been for electrically operated, planetary gear systems. IMHO, Scientific American is doing their readers a disservice.

Bob Wilson

 

Belt driven sliding cone CVT is the future Heh. They both have advantages and disadvantages. It would have been more interesting if they would compare and contrast the various types and maybe throw in a conventional transmision.

 

Bob;
without attempting to excuse Scientific American, perhaps the reason that they mentioned sliding cone as the "most common CVT", is because of the large installed base in small vehicles.... small motorcycles being one example.

Although I own a CVT-equipped Civic and love it, I still believe that planetary-type CVTs are better for larger engines with higher torques. But the cone-band type, being simpler, will hold its own in applications where cost and perhaps size are at premium. Namely, smaller vehicles.

Did the article had some other interesting highlights on CVTs that you could share?

 

Having driven an old Volvo (really an east european car under license! :-() with cone type I can say that it is a horrible experience and I was really hesitant to trust CVTs after that experience. Modern CVTs using planetary gears is a completly different experience and is nicer to drive then normal automatics. Too bad they didn't mention planetary drives

 

My disappointment is I remember the 'sliding cone' article from many, many years ago. Scientific American didn't bring any 'new' knowledge and I felt this was a rehash of earlier articles. But I don't think they addressed the sliding cone belt systems very well.

To work, the 'belt' has to be under significant tension as well as both cones under considerable compression loads. If it starts slipping even a little, the friction heating will wear it out fairly quickly. Then there are friction losses from trying to maintain these high stresses. BTW, CVT failure was a failure mode mentioned in the hybrid fleet reports.

Mechanically, the planetary gear CVT avoids the slippage problem, high axial loads and belt tensioning problems. It has an energy loss in the MG-to-MG energy generation but this seems to be no worse than the 'sliding cone.' However, I suspect there are 'speed / power ranges' where this loss is more significant. This is the type of information needed.

The planetary gear CVT apparently has a long history but it was new to us until we bought our Prius. It looks like many of the recent announced hybrids are going to use variations on the planetary gear CVT (some were adding additional planetary gears, possibly to avoid patent royalties?) These are the future for larger hybrids and Scientific American should be looking forward.

So my complaint is that the article lacked detail ("NO! A Scientific American article lacks details?") and didn't cover both types of CVTs with their strengths and weaknesses. Of course I'd have prefered some efficiency charts but then I do like detail.

Truth be told, the only technical journal I like is the MIT Technology Review. It hits the right balance.

Bob Wilson

 

Someone once told me that Scientific American is like **** for scientists