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Posted by Al Sherman on January 30, 2009, 2:01 am

Not true.  If MG2 is spinning, ICE and/or MG1 must also spin, but they are
not necessarily contributing power.

Two constraints for MG1 are relevant here:
- MG1 runs at a speed to keep the ICE at the desired speed for efficiency
- MG1 must not be allowed to exceed the redline, to avoid damage to itself.

Therefore it is possible to run electrically at high speeds without the ICE
"running", but MG1 must spin the ICE to stay under its speed limit.

Below approximately 42 mph, the ICE need not spin.

Posted by Mr. G on January 31, 2009, 1:53 am

The ICE will spin without generating power at speeds above 42 MPH, such
as when going downhill or decelerating, or in 'B' mode, where it's used
to create drag.  In other situations where the ICE is not needed, the
car will move, but the ICE *does not* spin, as in zero RPM.

Doesn't this contradict your previous statement that when MG2 is
spinning, the ICE must also spin?

Posted by Al Sherman on January 31, 2009, 4:25 am
My previous statement said the ICE OR MG1 OR both must spin whenever MG2 is
rotating.  If BOTH MG1 and ICE are stopped, MG2 (and the car) cannot move.

Posted by David T. Johnson on January 30, 2009, 5:15 pm
 Mr. G wrote:

No.  Please look at the diagram at this link:


The planetary gear system is not a 'differential' system.  The only
thing directly connected to the front wheels is the MG2 rotor.  If you
play with the sliders of the flash simulation that I posted a link to
you can see the mechanical relationship of the MG1 (generator), ICE (gas
engine) and MG2 (electric motor).  The ICE speed can be be zero and the
MG2 and MG1 will continue to turn.  The MG2 speed can be zero (which
means that the vehicle speed would be zero) and the ICE and MG1 can
continue to turn.  The MG1 speed can be zero if the MG2 and the ICE are
turning.  Or the ICE, MG1, and MG2 can all be zero.  The MG2 speed will
ALWAYS be zero if the car is not moving since there is no 'clutch' or
'torque converter' mechanism on the Prius.

No, it's not.  The drive speed is always the MG2 speed (followed by the
chain drive and gear reduction).

MG2 can run backwards but in that case the car is also moving in reverse
and in the Prius, the ICE speed must then be zero when the MG2 is
running in the reverse direction.  You can see this with the flash
simulation (and also when you drive the Prius).  When yo switch the
'shift' lever on the Prius from 'D' to 'R' you are really just changing
the inverter output to run MG2 in the reverse direction and telling the
computer to not start the ICE.

The MG2 speed is set by the Prius driver with the 'accelerator' pedal.
The computer then sets the ICE speed to provide the necessary power
required.  It is not possible for both the ICE and MG1 to not be turning
unless the vehicle is stopped and the MG2 speed is also zero.

Posted by Chas Gill on January 30, 2009, 5:27 pm
<Lots of very intersting stuff snipped

You may be telling the computer not to start the ICE, but in fact the ICE
could already be running (for example after just starting the car).. IME
this happens every time I back out of my drive.  The ICE usually starts
during the manoevre.  Are you implying that it is physically not possible
for the ICE to run when the car is in reverse, 'cos this blatantly ain't the


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