[Isaac Zachary]

The "power split device" on Toyota hybrids is nothing more than a planetary gear set, which is basically a differential.

You can imagine it like this:

• Take a car and lift one of the drive wheels up off the ground and disconnect the brakes to it. Don't mind that it's now a three wheeled car. We'll just call it "the wheel on the ground" from now on for this mental exercise (although in an actual Toyota hybrid this "wheel" is still inside the transmission and is chained to another differential that goes to two drive wheels.)
• Now connect the engine directly to the differential. For an example, put the manual transmission in high gear and leave it there. You will not be shifting gears nor using the clutch (because in a Toyota hybrid there are no other gears nor a clutch).
• Now put two electric motor/generators on the car, one at each drive wheel. In other words, one on the drive wheel that's still on the ground (this is MG2) and one on the drive wheel that's now lifted up off the ground (this is MG1).

How does this work?

• The engine is off when you first start the vehicle, so both wheels are stopped.
• If you apply momentary power to the MG1 on the wheel that's up off ground it now "pop" starts the engine. The other wheel doesn't turn because you have the brakes on or that wheel is in park.
• But now the running engine that's connected directly to the differential in high gear will be turning the wheel that's up off the ground at a high speed, even at idle. By making the MG1 at that wheel generate some electricity the differential now splits the torque to the two wheels. So the wheel on the ground gets half the torque and the wheel that's off the ground and generating electricity with MG1 is also taking on half the torque.
• You can now gun the engine, but to keep the free wheel from just spinning out of control it now has to ramp up it's generator and take on more torque. The torque will always be split 50:50 between the wheel on the ground and MG1 on the free wheel.
• Now you can use the electricity that's being generated from the MG1 on the spinning wheel up off the ground to do a couple of things.
  • You could use it to charge the HV battery.
  • Or use it to provide MG2 that's on the wheel on the ground some power. (Or both charge the HV battery and power MG2.)
  • Another option would be to power MG2 on the wheel on the ground not only with all the electric power being generated from MG1 but also with whatever electric power the HV battery can provide.
• Let's say you continue to accelerate and maintain a steady engine speed (RPM).
  • Of course The wheel on the ground with MG2 on it will continue to speed up.
  • But the wheel off the ground with MG1 will slow down. (As long as engine RPMs stay the same).
  • This change of speed will continue until you reach highway speeds, at which point both wheels will be spinning forwards at about the same speed.
• But now let's say you want to regen brake.
  • The engine is now left to idle.
  • MG1 on the wheel that's off the ground stops generating electricity, so now that wheel spins freely essentially putting the engine in neutral.
  • And now MG2 on the wheel on the ground starts generating electricity.
• But let's say the engine power is cut and you go into "EV" mode.
  • Besides the engine turning off, MG1 continues to neither power nor generate, therefore leaving the engine in "neutral" so to speak.
  • But since the engine is no longer turning, the freely spinning wheel with MG1 now has to spin backwards. (Have you ever lifted both drive wheels off the ground and turned one of them and watched the other turn backwards with the engine off?)
  • To get out of EV mode while the car is still moving MG1 on the wheel off the ground simply has to start generating electricity again (unless you're going too slow). Since it's spinning backwards this will put torque on the engine to spin it forwards and start it again. Once the engine is up to speed, MG1 on the wheel that's off ground will now be spinning forwards again.