Maximizing Efficiency with a Continuously Variable Transmission

[kandelabr]

I wonder what's the best efficiency that's possible to achieve with a continously variable transmission. I've googled a lot and can't find anything useful (with useful i mean an approximate value in [%]).
thanks.

 

[Bob S]

The following website has a good discussion of the various types of continuusly variable transmissions (CVTs)
http://en.wikipedia.org/wiki/Continuously_variable_transmission
None of the CVTs have efficiencies as high as manual transmissions: about 96% in all but the 1:1 gear which is about 98%. Manufactirers of CVTs will not disclose the efficiencies of their designs; it is proprietary.

Note that manual transmissions in standard automobiles do not require water cooling. If a particular CVT requires water cooling, that is proof that it is less efficient than a manual transmission.

 

[kandelabr]

yes, I've seen this page. I've read quite a lot of stuff regarding cvt's, but never found anything like efficiency.

are you sure it's 96 to 98%?
i was told in school a pair of gearwheels has 99% efficiency; either i was told wrong or efficiency of standard transmission should also be around 98%. there's still some friction in those things.

cars with cvt's should have 5-10% lower fuel consumption, but they either have the same or even higher than cars with standard gearboxes. i don't think that this 2% difference would make such an impact on fuel consumption.

anyway, i don't know where, on well-made pair of gears, 2% could be lost... :)

 

[Bob S]

I found a measurement (see attachment) that shows that a 5 speed manual in passenger cars has an efficiency of about 96% in gears 1,2,3,and 5, and an efficiency of about 98% in 4th (1:1) gear. Recall that the input and output shafts are co-aligned, so that in gears 1,2,3, and 5 there are two gear-to-gear contacts. As in other transmissions, the efficiencies depend on both RPM and torque.

 

[nucleus]

This article gives a loss of 8% (page 11). There is more than one type of CVT and some systems gain overall better efficiency by allowing the ICE to run at a more efficient RPM and get overall lower emissions.
http://www.torotrak.com/Resources/Torotrak/IQPC_2008.pdf

 

[kandelabr]

thanks a lot for this data, means a lot to me, really.

 

[Bob S]

This article gives a loss of 8% (page 11). There is more than one type of CVT and some systems gain overall better efficiency by allowing the ICE to run at a more efficient RPM and get overall lower emissions.
http://www.torotrak.com/Resources/Torotrak/IQPC_2008.pdf

For engine brake power output less than the power at the power corresponding to the maximum efficiency ("sweet spot" usually about 35% of redline and 80% of maximum torque), the maximum efficiency for a given power output is usually about 50% to 80% of maximum torque and a lower RPM than the "sweet spot". Cruising at city speeds usually requires about 10 kW or less. and should not require running above about 30% of redline. IF a CVT HAS WATER COOLING, then it is less efficient than a manual transmission (typ 96% to 98%).
Review this paper for the physics and eficiency of automobile engines
http://sitemaker.umich.edu/mhross/files/fueleff_physicsautossanders.pdf

 

[xs400]

http://www.zeroshift.com/pdf/Seamless%20AMT%20Offers%20Efficient%20Alternative%20To%20CVT.pdf
http://www.zeroshift.com/pdf/Seamless%20AMT%20Offers%20Efficient%20Alternative%20To%20CVT.pdf"

Manual - 97%
Automatic - 86%
CVT belt - 88%
CVT torroidal - 93%

 

[kandelabr]

yes! that's what i was looking for, thanks!

 

[Bob S]

Where is a comparison to the efficiency of the planetary CVT used in the Prius. The Prius CVT is unique in that it allows both an electric motor and a gasoline engine to simultaneously drive the vehicle in forward direction (but only electric motor in reverse).
Bob S