Prius CVT vs conventional CVT - details

You pretty much answered your own question there, but isn't an engineer supposed to know all this stuff?
The hybrid synergy drive isn't a transmission, its a "power split" device, basically a differential drive. So efficiency (as such) is no different than any differential. It has several planetary gears (latest model) and a gear drive to the front axle. I'm sure it all weighs a good bit, probably more than a belt drive CVT. There's a lot of stuff around online including videos, do some searches.

 

I would suggest that a couple of drawbacks for the HSD are:

• the complexity of computer(s) needed to control it
• the expense and weight of motor/generators that are integral to its function

Some of the advantages might be:

• the low friction losses
• low wear and tear on internals (as evidenced by Toyota's maintenance requirements fluid changes)
• relatively high efficiency for integration for flow of power from ICE and EV as well as for regeneration

 

The Volt gains some efficiency at higher speeds by having more than one gear.

Gen 1 and 2 Prius tended to want to over rev Motor/Generator 1, Toyota added a second planetary gear set in Gen 3 to avoid that.

 

Gen 4 takes it further: has two side-by-side arrays of gears. And a much higher electrtic-only limit. Still can't stomach the styling though.

 

Gen 2

Gen 3

Cone and belt CVT

All the same guy

 

@JimboPalmer, thanks for putting those all together like that. I've only ever seen the gen 2 version before. Definitely going to look at the others asap. Gotta go use my cvt now.

 

I've seen the two Prius videos, but the GM CVT was new. Thanks.

Also, this video does a good job of explaining how the Prius CVT works.

 

Cons;
-Some electrical efficiency ie: if the motor is running and you want the wheels still, the 'splitter' motor needs to spin backwards to compensate
-You don't get any of the torque multiplication of a large cog driving a small cog like a conventional CVT. For this you need to analyse like a single gear drive.

The pro's vastly outway the con's. Just think about the complexity of the PHEV IONIQ. A normal engine, driving through a clutched flywheel electric motor, directed through a DSG, and made in Korea...ouch

 

The electric motor's peak torque from a standstill compensates for lack of torque multiplication in an ICE-powered CVT.

 

That doesn't really change things - it was just a different way to achieve the gear reduction. In high-end Lexus versions they do have a two speed selectable gear ratio but not the Prius.

Over the evolution of the various generations Toyota has gradually increased the rated speed of the motor generators to make them smaller. I see that MG2 is now rated up to 17,000RPM. Up from 13,500 RPM on gen3.

They have gone away from the planetary reduction as used in gen3 and gone to a parallel shaft reduction gear. In the gen2 it used a chain drive!

kevin

 

Hi Kubosz, welcome to PriusChat!

Prius CVT disadvantages vs belt CVT:

• Maximum torque when driving in reverse is less than when driving forwards. (could easily be fixed by adding a reverse gear just like on a belt CVT, but Toyota chose to keep it simple)

• The Prius CVT is more suited to hybrid vehicles than to ICE vehicles. The Prius CVT is so deeply integrated in the hybrid system that it is almost just a byproduct of electrification. That is an advantage for a hybrid vehicle, but, in my estimation, in a non-hybrid vehicle it would be more complicated and expensive than a belt CVT due to the peripheral electronics. (would be interesting to calculate if the efficiency gains outweigh the manufacturing costs for a non-hybrid)

Here are some of my favorite studies of the Prius drivetrain. The first one even has a direct comparison of simulated efficiency with the Honda belt CVT:

A fundamental case study on the Prius and IMA drivetrain concepts
Publication: A fundamental case study on the Prius and IMA drivetrain concepts

Evaluation of the 2010 Toyota Prius Hybrid Synergy Drive System
http://info.ornl.gov/sites/publications/files/pub26762.pdf

Comparative Assessment of Hybrid Vehicle Power Split Transmissions
http://john2211.nl/Hybride_links_files/Miller_W04.pdf

 

Fred! You did a wonderful job on those research papers. Have checked and downloaded, waiting for a day to study everything.

Please just in case you've other stuff, I'd be happy to read em.

Thanks for the links again.

 

Hi Dxta, I do have a few more, but I need to sift through my files a bit. Here's another from Uncle Sam to start with:

EVALUATION OF THE 2007 TOYOTA CAMRY HYBRID SYNERGY DRIVE SYSTEM
https://www.osti.gov/scitech/servlets/purl/928684 ​

 

Followed the link, but couldn't download it.

Would try again.

Thanks!

 

The link loads a pdf.

 

As a former Gen 2 owner, I liked this chart of 2004 Prius M/G2 HP and Torque.

 

John Kelly at Weber University in Utah is the resident expert in my opinion on all the Prius CVT gearboxes.

He also has worked on every type of CVT out there.

They buy new cars and dissect then and see what makes them tick. His videos are addictive if you have ever watched one.

 

OK, thanks Sir! Checking it again.