Prius-Harley guy #1577

As for me, I never had a problem understanding that power-split unit, 
once that I realized that it was a variation of something that I had 
come to understand many years ago.  Perhaps my explanation may help 
those who are still puzzled by Toyota's planetary arrangement,
what 
it does, and how... something to keep in mind during a visit to 
Graham's site.

I suggest that if you understand what an automobile's
differential 
does and how it works, then you might more easily understand what the 
planetary cluster in the Prius does and how it works.  In fact, they 
operate in the same way.

First, a quick (or wordy, you tell me) refresher on the differential 
gear-set (that's right, the mechanism that allows wheels to turn
at different speeds when we go round a corner):  a differential gear-
set is a `box' with three rotating shafts; the vectored
action of one shaft directly relates to the SUM of the vectored 
actions of the remaining shafts.  (By vectored, I mean that the 
direction of rotation is important.)  This gear-set offers an 
infinite number of wheel speed differences by placing a 
differentiating unit (pinion gears and their carrier) between the two 
wheel shafts.  The remaining interface shaft is connected directly to 
the differentiating unit.  Bevel gears let us orient the shafts as 
needed.

<<  Skip this paragraph, if you know how a differential gear-set 
works. >>  During normal straight-line driving, both wheel shafts 
rotate equally in a given direction.  To do this, the entire 
differentiating unit rotates, yet the little pinion gears within it 
are not rotating on their little shafts.  When the automobile is 
rounding a bend, one wheel shaft must rotate slightly faster than the 
other.  To do this, the little pinion gears must rotate slowly on 
their little shafts within the differentiating unit, which itself is 
rotating.  Finally, if you were to jack both driven wheels off of the 
ground, and prevent the transmission shaft from turning (motor off 
with automatic transmission in park or manual transmission in gear), 
you would find that if you rotate one wheel in one direction, then 
the other wheel rotates in the opposite direction.  To accomplish 
this, the little pinion gears are spinning within the stationary 
differentiating unit.

So, imagine that a differential gear-set is cut open and made of 
clay, in order that you can stretch, twist, and reconfigure 
components somehow, but without affecting the relationship they have 
among themselves.  Conceptually, create a Prius power-split device 
by:  1, fashioning a Prius ring gear from one of the beveled axle 
gears (give it more teeth however), 2, mold the other beveled axle 
gear into a sun gear, and 3, convert the pinion carrier into a planet 
carrier by cutting the pinion shaft in half, and by locating the 
pinion gears (planets) around the new sun gear and within the new 
ring gear.  Since there's room, add two more planet gears!

Voila!

In short, a planetary gear-set IS one kind of differential gear-set, 
a kind devoid of bevel gears.  Moreover, a planetary gear-set 
presents more useful proportional relationships among elements 
through careful selection of ring-to-pinion gear ratio and pinion-to-
sun gear ratio.

It works the other way, too.  Some readers may find that if they 
imagine a Prius with a differential for its transmission, the 
workings are a snap:  put MG2 (drive motor) on one axle, put MG1 (ICE 
generator) on the other axle, and attach the ICE to the propeller-
shaft yoke.  Despite what MG2 might be doing with the tires, even if 
you are going backward in reverse, the ICE and MG1 together will take 
care of other business at hand (charge a battery, run a compressor, 
stay warm, whatever).