Toyota Prius 2010 User Manual page 28

Engine & Motor Drive + Charge
When the small motor is generating more electricity than what is needed by
the large motor at that moment, the surplus is sent to the battery-pack. This
is a benefit often overlooked, an efficiency gain due to running the engine at
an optimal RPM (which also provides an efficiency gain). This is also the
most common method in which hills are climbed, resulting in more stored
electricity available at the top than when the climb began.
Full Power or Gradual Slowing
At times when maximum thrust is required, like when merging onto a busy
highway, the battery-pack will join in. This additional power is used by the
large motor to produce even more thrust to the wheels than is normal
needed. The same activity also occurs when slowing down gradually,
allowing the engine to significantly reduce gas use without the driver
noticing a power distribution change.
Engine Heat
When the hybrid system is cold, it will run the engine to create heat for the
catalytic-converter. This is required to enable the chemical reaction that
cleanses emissions before leaving the tailpipe. Rather than allowing that
power from the engine to be wasted while the vehicle is not moving, it is
used by the small motor to generate electricity to recharge battery-pack.
This same method will also occur when more heat is needed for the heater
to keep you warm.
Stand-By or Gliding
This is the state of the system at rest but still on. You'll see it when power
to the wheels isn't needed; that's both when the vehicle is stopped and when
it is gliding. The engine may be idling or off.
Prius User-Guide (2010-2012) 28 of 33 Last Updated: 6/21/2012