Planetary

The discussion at the following website may be helpful:

http://home.earthlink.net/~graham1/MyToyot...PriusFrames.htm

At that url follow "Understanding the Prius" -> "The Power Split Device"

 

Please try following link for animation...
http://homepage.mac.com/inachan/prius/planet_e.html

You can move the sliders for each rev.

 

That shows how the planetary gears turn under different situations, but it doesn't explain how the two MGs and the ICE and Wheels are connected to it, or how the different relationships are controlled.

For example, the ICE can turn the wheels, or it can drive one MG to generate electricity which turns the other MG to move the wheels. How does the computer make that happen? The ICE can drive the wheels, or it can drive the wheels and simultaneously charge the battery. What changes to switch from one condition to the other? Etc.

 

MG1 is tied to the sun gear. It's virtually always a generator except when used as a motor to start the engine.

As an aside, here's how MG1 acts as a starter motor: For engine start, the computer tells the battery to supply processed battery power to the generator (MG1) stators, thus speeding up the generator/sun gear. The ring gear with its low speed load can't follow the increased sun gear speed so the planet carrier (engine shaft) has to start turning, making up the difference between sun gear's and ring gear's speed. By the way, while MG1 has been used as a starter motor (turning the engine), the computer has also commanded both spark and fuel for the engine.

MG2, when NOT in regeneration mode, is always acting as a motor (at least at those times when the computer allows power to be provided to it). MG2 receives processed power from sometimes the battery, sometimes the generator (MG1), and sometimes both. MG2, by the way, is tied directly to the final drive through fixed reduction gearing. MG2 also ties directly to the ring gear of the planetary drive.

So:

. We step on the accelerator from stop, supplying processed power from the battery to the motor to the final drive and the car starts moving, then speeding up.

. When we reach that point where the benefits of the motor's low RPM torque aren't going to gain us that much more, the engine is started and it starts helping the motor, to the extent it can, drive that ring gear, thus speeding up the car.

. Although the engine (tied to the planet carrier), can increase RPM, the ring gear can't follow the engine RPM because the car's speed is still too low to follow without step down gearing (we don't have that). So, the engine's energy that doesn't transfer to the ring gear, transfers to the sun gear, and thus to the generator (MG1), increasing its speed, thus helping the battery to supply more power to the motor (MG2).

. With more power to the motor (MG2) the motor works harder, working with the engine to speed up the car (remember, both are driving the ring gear).

. With that additional help of the generator (MG1) to the motor (MG2), the further increased speed of the car allows higher ring gear RPM, thus helping to bring the ring gear's speed up to where ultimately its RPM will become a best possible match for the engine's high RPM maximum torque capability.

. And so it goes. The motor helps the engine which helps the motor which helps the engine (ad infinitum).

This cooperative effort between the power split device's sun gear, ring gear, planetary drive, MG1, MG2, and the gasoline engine makes up Prius' definition of a continuously variable transmission (CVT).

Hope this helps,
Ken

 

Thank you Ken. This is the first time I've seen it explained like this, and I think that gives me a pretty clear idea of what's going on. It is totally cool.

 

Excellent summary Ken. Now I know why I dropped calculus and became an economics major. Anyone for marginal propensity to consume? :mrgreen:

 

Aha! I knew I'd seen a great 3D animation for how the Power Split drive works!

It's over at howthingswork.com -- part of their "How Hybrid Cars work" (great article!). The Driving The Prius page has animation, but the whole article is full of great diagrams and animations. The page before it has one of the best diagrams of the planetary gears I've seen.

 

i wrote Graham a note just now... great site, but i think his primary diagram is misleading.... here was my guess....

the ICE and MG1 and the sun gear in the center are on one shaft and always turn at the same rpm.
MG2 is connected to the "carrier" or outer ring, and can turn in either direction.
the planet set is where the power is taken off for the chain and gear drives to the wheels.

the color-coding of his diagram isn't quite consistent, and has no input (power in) or output (power out) connection that i can tell or understand from his text.

it's getting clearer, though!

 

DING!!! You nailed it dead on. What I need to get a clear understanding of the system is a diagram of WHERE they are connected to the PSD, and how the power gets transmitted to the wheels. All of the pictures I've seen, only show me how the planetary system behaves within the gearset. Unfortunately for me, I don't have the mechanical prowess to connect in my head, where the power goes once it's gotten into the PSD, and how it's routed out.

 

I've had the same problem -- I need to see clear pictures with the diagrams. However, given all the resources and information posted on this page, I think I've (finally) figured it out:

• Look at the Power Split Device diagram at HowStuffWorks.com, along with the summary description posted there. That's the best diagram I've seen of "which gear is connected to what motor/engine".
• Supplement this with the Power Split Device diagram and section at the Toyota Hybrid System THS II site (half page down).
• Advance to the next page of the HowStuffWorks.com site. (This also has the 3D animation.)
• Read Ken's excellent description above.
• Start up the THS II Planetary Gear simulation, and play with it while you re-read Ken's description.
  • 
    I'm sure I'm still only 80% of the way there (I love the "heretical mode" Rick mentions!), but I think I've grokked it.
    
    Actually, I think it's the opposite of Rube Goldberg -- brilliant but simple, and can do a multitude of things simultaneously. (Unless there's a gerbil wheel in there somewhere...)

 

That 3D animation finally fit the missing piece of the puzzle for me. Along with the picture of the chain drive part of the transaxle connected to MG2 the way it is, it explains the way that the whole system gets the works transferred to actually moving the car.

I think that the 3d animation is new. I'd been on there a few times looking for an explination, and it wasn't on there. Thanx guys.

 

There's no magic here:

The fact that the sun gear will attempt to slow down as MG2 has to work harder is simply because: As the load on a generator increases, it takes more work to turn its rotor. This consequence just adds to the 'help me, help you' effect of the planetary system:

 While the car was initially coming up to speed and after the engine cut in, the engine, while doing what it could to help turn the final drive connected ring gear (helping the motor, MG2 which is also connected to that same ring gear), most of the engine’s RPM was being transferred to the more lightly loaded sun gear (before the sun gear’s generator, MG1, got ever increasingly loaded down in trying to help the ever increasing in speed, motor, MG2).

 Now, with the generator connected sun gear presenting ever more load, even more of what the engine has to offer gets transferred from the engine’s planet carrier over to the ring gear (more evenly distributing the work that the engine is doing between sun gear and ring gear).

 With more engine help transferred to the ring gear, that eases the load a bit on the motor, MG2, (connected to that same ring gear), which in turn now presents less load to the generator, MG1 (connected to the sun gear).

 This, of course, again allows a little more of the engine’s efforts, via the planet carrier, to transfer back to the now less loaded sun gear.

And the beat goes on. The internal combustion engine work emphasis will repeatedly be transferred back and forth from planet carrier to sun gear, then to ring gear, then to sun gear, ... It’s the nature of a planetary gear system. The fact that the Prius engineers showed such genius in taking advantage of this planetary characteristic, then tying it in so cleverly with the engine, motor/generators, and final drive, making it all work as a continuously variable transmission (CVT), is, in my eyes, the marvelous heart of what our magnificent Prius is all about.

 

A follow-up question:

So, when you're in stealth mode (EV button on, so to speak), you're having to power not only MG2 but MG1 from the battery, correct? I.e., MG2 is moving the wheels, and MG1 is run (as a motor) just fast enough so that the sun gear keeps the planet carrier stationary.

Or is it the other way around for MG1 -- the planet carrier is kept stationary, and the sun gear is turned by MG2's ring gear via the planetary gears, and MG1 is a generator in this case? (I.e., MG2 is powered from the battery, and MG1 generates current as a by-product, thanks to the planetary gears turning the sun gears.)

Talk about your chicken-or-the-egg problems!

 

Let's see now, if I understand correctly, in EV mode we're simply driving the motor, MG2, with current from the battery.

If I was a Prius computer, in order to make the battery last for the longest possible time driving in EV mode, I’d want no more load on the motor than necessary. Here’s what I’d do:

1) I would remove all load from the generator and allow it to free run (leaving the generator connected sun gear to present as little load as possible on the ring gear via the plantet gears.

2) I wouldn’t care whether the planet carrier (tied to the engine) was turning or not. I wouldn’t expect it to.

Of course one could leave the generator in a mode where it supposedly helps provide power to the motor, but there’s no such thing as a free lunch. The generator would simply be demanding more energy (due to load placed on the sun, planets, ring, and motor) than it would be providing to supposedly help the battery propel the Prius via the motor.

Why complicate things?

Ken