Several others have posted fine links which describe the Prius'
"eCVT"... thanks to all.
Let me add my own nutshell description, using a familiar effect many
folks might have witnessed for themselves, that might illuminate how a
simple planetary unit can act as a CVT:
Have you ever seen a conventional car, say a '63 Dodge or some other
stone-simple vehicle, up on a lift with its engine running? Or
attempting to accelerate on an icy patch? The engine spins the
driveshaft, and if traction is equal at both rear wheels, then both
spin identically. But if one has more "grip" than the other (either
because it's on a comparatively dry spot, or because the mechanic is
holding onto one wheel with the car on a lift) then the wheel with the
lesser grip will do all the spinning.
That's how a conventional differential channels power from the engine.
That's also how the Prius' power-split device (aka "electronic CVT")
works. Kinda/sorta: Imagine the gas engine in place of one wheel and
an electric motor on the other. The driveshaft, then, becomes the
output (instead of the input, as in the '63 Dodge). You can now run
the gas engine at whatever speed you want and still get the desired
output-shaft rotation rate by varying the speed (and direction) of the
electric motor, either contributing power or taking it away depending
on the circumstances. This means you can use an Atkinson/Miller-cycle
design for your gas engine (or a direct-injection diesel) and take
advantage of its higher thermodynamic efficiency while accommodating
the peaky powerbands characteristic of these designs. It also means
you can dispense with an alternator, since the motor can act as a
generator. It also means you can eliminate the starter motor, since
the motor can spin the gas engine if you hold the output shaft steady.
And, by putting another motor on the output shaft, it ALSO means you
can drive the car electrically when you want, or capture kinetic energy
through regenerative braking. Being able to drive the car electrically
means no reverse mechanism is needed either.
See how a simple differential lets you eliminate all that stuff you
used to need? Transmission, clutch or torque converter, alternator,
starter... and so on. Each elimination means a whole cascade of weight
savings and loss reduction.
It's flippin' brilliant, and my real point is that it's a radical
simplification of conventional automotive design, contrary to the
picture painted by idiot journalists who wouldn't know a differential
from a Dove Bar.
You can also see why I rebel at calling it a "CVT". There's no
friction belt, for one thing, which is what other CVTs use. (All
right, so Audi uses a compression belt, so sue me.) I don't trust 'em.
They wear out, they're touchy about torque, they're stratospherically
expensive to repair, and they're lossy. The only reason you're seeing
them in other cars is as a kludge to allow their engines to run at peak
efficiency, which luckily more than offsets the inefficiency of the
friction belt. Talk about a technology demonstration! They're
practically a science-fair project, IMHO. The Prius power-split device
makes way more sense and is way simpler. Mark my words: there's a good
likelihood that belt-type CVTs will be a dirty word in five years or
so, when the first wave of disgruntled owners sees their transmissions
disintegrate at 80,000 miles. Do the Prius a favor and help me stamp
out "CVT" as a description of its technology!
I appreciate the analogy but have a concern that some may be misled into
thinking the Prius gears have differential efficiency. The planetary
gears are ever so much more efficient yet we've had a couple of skeptics
in "Prius Techical Stuff" try to use an SAE paper on differential
efficiency to 'prove' the inefficiency of the PDS transmission. We
simply pointed them to the DoE paper showing ~1.5-.97 kWh.
You might want to emphasize that internally, the PDS uses a planetary
gear system that has much less drag than an ordinary differential. Then
there is the ~3 to 1 gear ratio between MG1 and the ICE. It isn't that
the analogy is wrong but it can mask subtlties. The skeptics will still
ignore these subitles but at least you've tried.
I'll take E-CVT but that is about as far as I can go.