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Toyota Prius' Power Split Device (eahart.com)
105 points by desdiv on Feb 15, 2015 | hide | past | web | favorite | 32 comments

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:

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

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


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.

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...

You're right that the ICE can essentially directly drive the wheels. Take a look at the model, and think of the outer gear (connected to MG2) directly powering the wheels. Then try setting the ICE to near max, and MG2 to about the same level, so the grey line is horizontal. That should simplify the visualization. See how the ICE pulling the planetary gears around directly pulls the large gear as well? Once you grok that, you should be able to see that even if you vary the MG2 slider, the ICE is still providing torque to that outer gear.

I didn't watch the video and cannot properly explain it, but there is direct mechanical connection from ICE to wheels. AFAIK both MG1 and MG2 can work as motors and generators and spin in either direction. If you apply proper counter-torque by one of the electric motors (for planetary ring), the torque effectively flows from ICE to wheels. I'm Prius driver myself, it's a nice car for us who cannot yet drive full electric vehicle for a reason or another.

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


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.

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).

> 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.

What is the benefit in using this scheme opposed to a gearbox? Is most efficient to use two electric motors (powered by the combustion engine through a generator) or use gears in a gearbox, but without the power penalty of the generator?

Is this more efficient than a gasoline drive system only if the energy to drive the electric motors come from an external source (the power grid, though the batteries)?

Well, a simple reason is a CVT can keep the ICE in a very narrow RPM band where it's operating at maximum efficiency.

Agreed. But there are many others CVT schemes without electric motors.

And most of those will have problems with high torque motors.

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.

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...).

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.

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.


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.

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

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.

yes, the statement itself is inaccurate, but in the mechanism, there are 3 separate gears which can be adjusted to make a continuously variable ratio.

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?

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)

The Model S has a Single speed fixed gear with 9.73:1 reduction ratio

Source: http://www.teslamotors.com/support/model-s-specifications

Tesla uses a single gear, which limits the top speed of the car to only ~ 120 mph. Germans scoff at that, but it works well for everyone else.

The torque curve of an electric motor is linear with speed inversely. Tesla never changes gears. There is a fixed reducer between the motor and the wheels.

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).

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.

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


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.

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

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


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