
Toyota Prius' Power Split Device - desdiv
http://eahart.com/prius/psd/
======
spditner
I had to stare at this for few minutes before I figured out how the ICE part
drove the planetary gears. They're mounted on a disc, so that the system in
profile is:

    
    
            ___           
           |   |____ 
           |MG2|____|
       ___ |___| ||||        ___
      |   |      |   \______|   |
      |MG1|/////////> ______|ICE|
      |___|      |___/      |___|
                 ||||
             ___            
            |   |__________ 
            |MG2|__________|
       ___  |___|  ___  ||||    ___
      |   |       |__ \________|   |
      |MG1|/////>  __> ________|ICE|
      |___|       |___/  __    |___|
                        ||||

------
SigmundA
This video allowed me to understand it better, it truly is a elegant design:

[https://www.youtube.com/watch?v=ZmHpSyTsfm0](https://www.youtube.com/watch?v=ZmHpSyTsfm0)

~~~
fernly
I enjoyed that video but it also confused me a bit. It is useful in that it
clearly makes the point that what's different about the PSD is that, unlike
the planetary gear system in the Ford Model T and many others, at no time is
any of the three components ever locked or fixed. The sun-gear, planetary
gears, and ring gear are always free to rotate, and do.

What was not clear to me was the power flow from the ICE to the wheels. It
seemed as if he was saying that the only connection from the ICE to the wheels
was indirect, by turning MG1 to generate current that could drive MG2 which
drives the wheels. But I don't think that's the case.

I know that if I floor the pedal entering the freeway, the ICE spools up to
something like 5000rpm and holds there while the car accelerates. So clearly
there is a flexible connection. But is all of that mighty horsepower being
transmitted by first being turned into AC in MG1, which flows to MG2, which
turns it back into torque? Because that would be rather inefficient I would
think.

~~~
reacocard
MG1 can provide some resistance against the ICE's motion, causing part of the
ICE's power to drive the ring gear instead. So in some cases, you have MG1
acting as generator (thus resisting the ICE) and driving MG2 electrically, AND
you have mechanical contribution by the ICE as well.

There's a detailed explanation of all the various modes the gears interact in
here:
[http://prius.ecrostech.com/original/Understanding/WhatsGoing...](http://prius.ecrostech.com/original/Understanding/WhatsGoingOnAsIDrive.htm)

------
zackmorris
Here is a video demonstrating a (possibly less elegant) electrically
controlled CVT called the D-Drive:

[https://www.youtube.com/watch?v=F6zE__J0YIU](https://www.youtube.com/watch?v=F6zE__J0YIU)

The concept is straightforward if you visualize mechanical advantage. A high
torque at low RPM is the same power output as a low torque at high RPM. If you
picture an asymmetric differential with primary power going into it where one
wheel is geared higher than the other, then a weaker motor on one of the
wheels could control the torque and RPM of the other wheel.

So we probably could have been using electrically assisted CVTs decades ago
where a small fraction of the power could have gone into a generator/motor
pair at about 90% conversion efficiency to control the output RPM. I can't
help but feeling that most of the gains in things like 6 speed automatic
transmissions to increase fuel economy have largely been a waste of time. We
could have gotten rid of traditional transmissions and gone with a design like
the Prius that are simpler, more efficient and more durable. By now economies
of scale probably would have made them less expensive as well.

I’m actually a little curious why this hasn’t happened with supercars.

~~~
choppaface
Supercars generate 5x-10x more torque, which would likely break this sort of
transmission. Most automatic transmissions have a similar planetary gear
design but use clutches to control power delivery (from the single motor).
These clutches will burn up under lots of torque.

Furthermore, it would be hard to implement a way of shifting the CVT to a
predictable ratio and holding it there while in a tight turn. (Holding the
ratio might not be hard, but the driver will probably want a _predictable_
ratio before going into the turn).

~~~
rodgerd
> Supercars generate 5x-10x more torque, which would likely break this sort of
> transmission

Perhaps, perhaps not. Williams had a working CVT in the FW14C F1 car, which
was pre-emptively banned at the behest of their competitors. It probably set
back CVT research by a couple of decades, and marked the end of F1 as a tech
testbed.

Currently it's worth noting the Toyota are running the Lexus 450h in racing,
and the Lexus LS600h runs a 327HP/520Nm V8 though a CVT mated to an electric
motor running over 200 HP.

------
riobard
This is truly genius engineering. Initially you would think hybrid vehicles
must be more complex and less reliable than conventional vehicles because they
have another system to fail, right? Turns out Toyota figured out a way to make
hybrid vehicles structurally simpler, more reliable and energy efficient.
Really well done.

~~~
ak217
I believe the Prius is definitely more complex than a typical ICE car. It has
many, many other systems to support the hybrid drive in addition to the power
split device, including many that are genuine innovations too. It's a
testament to how good that team's engineering is that the Prius is generally
considered reliable (unlike the piece of crap that the Toyota ETCS ECU was
until the recent crackdown - [http://betterembsw.blogspot.com/2014/09/a-case-
study-of-toyo...](http://betterembsw.blogspot.com/2014/09/a-case-study-of-
toyota-unintended.html)).

------
wstrange
We have owned a Prius for 6 years now. Not sexy, but an incredibly utilitarian
car.

The power split device is indeed a brilliant piece of engineering.

------
jdietrich
Planetary gearing has been used in bicycles for over a century. A planetary
mechanism built inside the rear freehub provides clean and reliable gearing,
albeit at a slight efficiency loss versus derailleur gearing. These gearhubs
are extremely popular in many parts of Europe, where utility cycling is the
norm.

The greatest example of hub gearing is the Rohloff Speedhub, which features
fourteen ratios with an overall range of 526%. It is a true mechanical marvel,
and several examples have done over 100,000km without major servicing.

[http://en.wikipedia.org/wiki/Rohloff_Speedhub](http://en.wikipedia.org/wiki/Rohloff_Speedhub)

~~~
bri3d
Automobile automatic transmissions (with the exception of Honda transmissions)
have also used planetary gearing since their introduction in the 1930s. The
Prius is innovative because of its design utilizing MG2 to alter the output
speed continuously, not because of its use of planetary gears.

At any rate the Rohloff hub is really cool, and I'm glad the technology has
finally trickled down into popular, attainably priced planetary gear hubs like
the Shimano Nexus as well.

------
lsaferite
Isn't this any oxymoron? "It acts as a continuously variable transmission
(CVT) but with a fixed gear ratio"

~~~
ronancremin
Confusing but not an oxymoron. From the point of view of the ICE the
transmission is CVT because the road speed can vary (thanks to the electric
motors) whilst keeping the ICE running in the same speed band.

------
motoboi
On a fully electric car, as Tesla, they use a standard transmission? Or there
is something inherent to electric motors that need a different technology?

I mean: electric motors can rotate at very high speeds. Do it even need a
gearset? Or can it just rotate from 0 RPM to say, 15k RPM? Or there is a
torque curve or efficiency issues involved?

~~~
ChuckMcM
Sort of. Unlike gas motors which get part of their mechanical advantage from
the angular momentum of the flywheel, electric motors have a torque that is
strictly proportional to the current through the coils.

AC motors (like the ones Tesla use), work by pushing a sinusoidal wave around
the "outside" of the stator, while a similar sinusoidal wave (at a retarded
phase) is run inside the stator. This creates a magnetic field which is
"ahead" of the stator's field (well in forward mode) which "pulls" it toward
the field. The torque curve is "constant" and the speed is limited only by how
well you can modulate that voltage. (and of course the mechanical
construction)

------
pkulak
That power-split device is genius. But, I'm a fan of the latest trend where
you put enough batteries in to supply the power needed to completely propel
the car by electric motor. That way, the gas engine only runs a generator at
low speed (series hybrid), then clutches in directly to the wheels to avoid
conversion losses at freeway speed. You still have no "gears" (so it feels
like driving a CVT), but the design is far simpler and even more efficient.
The Mitsubishi PHEV does this, as well as the new Accord Plugin (I think).

------
ableal
Took me a bit to understand that the four "planet" gears are connected by a
ring (faint shadow in the animation, better shown on mouse-over), which is
where the combustion engine pumps power.

------
Shivetya
Here is a breakdown of the Volt 2.0 drivetrain, ignore the thread title, the
diagrams and description are good

[http://gm-volt.com/forum/showthread.php?158881-New-Voltec-ap...](http://gm-
volt.com/forum/showthread.php?158881-New-Voltec-appears-to-be-an-engineering-
masterpiece)

------
madengr
Very cool. I always thought motor and ICE were series connected, such that the
motor would have to push back against the ICE to move the car. I suppose the
still are though when with this gear setup.

------
lsaferite
Is this the same gearing setup used by the Chevy Volt?

~~~
desdiv
They're very similar, yes. Here's a detailed article comparing and contrasting
the two:

[http://www.motortrend.com/features/editorial/1010_unbolting_...](http://www.motortrend.com/features/editorial/1010_unbolting_the_chevy_volt_to_see_how_it_ticks/)

