Another explanation of how the eCVT works

Not a bad simplification. It might be worth adding, toward the end, that what's usually called a "differential", like the one that's in the Prius final drive, is designed to split the incoming torque equally to the two outputs. The Prius "power-split device", the planetary gears between the engine and motors, operates on a similar principle, but does not make an equal split of the torque.

The other detail to maybe cover is that the Prius motor-generators are built with permanent magnets. There is no field coil to produce an adjustable magnetic field, so no "field current"; the field is always there and not adjustable. The car only controls how much power it will accept from the generator (or send to it, as a motor), simply by flipping on-off switches in the stator circuits ... really fast.

The difference matters for at least one thing; it's why you shouldn't tow a Prius with the front wheels on the ground. Even if everything is turned off, nothing will stop the permanent-magnet MGs from generating, and even though they will be switched off, above a certain towing speed the generated voltage may damage something.

 

I too have described the “power-split device” as operating a lot like a differential.

I have always thought this video good for explaining the Prius drive system:

A lot of explanations dive immediately in talking about the power-split device. This video is a great complement, because it concentrates much more on the bigger picture of the overall thinking behind the Prius drive system, the power-split planetary gear set just being a means of addressing one critical aspect of it.

 

Willy missed a few points:
The planetary gear set is not a differential. It is a speed & direction control device.

A differential is so named because it allows a speed differential between the driven wheels when cornering. It allows the wheel toward the inside of a curve to rotate more slowly than the wheel on the outside of the curve which must travel a longer distance in the same time. That's it. In a differential's basic form (an open differential) it has nothing to do with torque distribution.

One needs to understand the basic operation of a planetary gear set to understand Toyota's brilliance. The planetary gear set has three elements, the central sun gear, the cluster of planet gears, and the outer ring gear. All mesh with each other. One element is usually held stationary while the others rotate under power. By selecting which element is held stationary the direction of the output is determined compared to the input. By selecting the ratio of the gear teeth of each element to the other elements the speed reduction or increase is determined. A planetary gear set is very compact for its function. (I've worked with 30,000 hp planetary gear sets which were indeed more compact than others of more conventional lay out I've seen.)

As shown in both the Weber video and the animation, the planet cluster is connected to the gasoline engine. The sun gear is connected to motor-generator #1. The ring gear is the power output connected to a separate differential. Toyota's connection of MG1 to the planetary gear set allows varying output speeds by controlling the speed and direction of MG1.

I'm not yet clear on how the engine is connected or disconnected from the planetary transmission. It may have an overrunning clutch, but that needs to be locked out some how for MG1 to crank the engine for starting.

 

Nothing ever connects or disconnects. That's one of the cool parts. The engine is always connected to the planet carrier. The final drive and MG2 are always connected to the ring gear. MG1 is always connected to the sun gear. All of them always obey the speed and torque distribution resulting from the gear tooth counts, exactly, all of the time.

There is a clutch-like "damper" on the back of the engine flywheel, but nothing that ever disengages it; it is able to slip slightly to avoid damage if there are ever high shock loads, but that's all; there's no releasing that happens in normal operation. A Prime has an extra thing on the flywheel that's also called a clutch, but that thing has no effect but to block the engine from rotating backward.

I wouldn't have called the PSD a "differential" either, but it's also not as wrong as you're making out. Each one is a geared device that defines a relationship among the speeds and torques on three shafts.

Well, it can't help having something to do with torque distribution. Torque comes in, and it somehow ends up at both wheels. In fact the rule for a basic open differential is extremely simple: its two side gears have equal tooth counts, and both wheels get equal torque, exactly half of what comes in.

That's also what's behind the complaints about open differentials on, say, ice; if one tire is on a slick spot that can't take 50 foot-pounds without slipping, that means the other wheel, sitting on good pavement, never gets more than that equal torque. Makes it hard to get the car unstuck.

That could come out of a standard chapter on any traditional automatic transmission; they have planetary gear sets and they also have 'bands' that clamp down or release to hold different elements stationary, and clutch packs that can connect or release different elements from the input or output.

None of that goes on in the Prius transaxle. There is a planetary gear set there, but it is just always connected to the same three things, nothing ever clamps down to hold any of it stationary, nothing ever disconnects it from anything, it just goes through its whole life distributing speed and torque of three shafts the way its fixed tooth counts ordain.

That means its life is very, very much like the life of a differential. The one salient difference is that where a differential has equal-sized side gears and makes a 1 : 0.5 : 0.5 distribution of torque, the planetary has a a ring with more teeth than the sun has, so it distributes the torque less evenly (1 : 0.28 : 0.72 in the Prius).

You could build a Prius-equivalent transmission on a bench using a stock open differential, and adjusting your motor specs to account for the different torque distribution. It would otherwise work fine.

Also very fun about the Prius transaxle is, while there is definitely a lot of Toyota's brilliance in it, the idea of doing what they are doing with two motors and a planetary can be followed right back to a patent filed in 1908 by John Godfrey Parry Thomas (if you skim, skim down to lines 30 to 35 on the first text page).