"Thanks for the links and more explicit diagrams than Toyota offers, Don, clarifying the dual power flow from MG1. But I can't imagine that in any situation the ICE and MG2 would be engaged in a 45 KW or more power struggle - at that level, a cumulative 90 KW/120 HP before any net motive effect. Possibly you could explain this in more detail. I thought the idea was Synergy, not Mortal Combat!"

MG1 and MG2 can be considered an electric torque converter. MG2 has copious torque at low RPMs where the engine is weak on torque. Thus under heavy acceleration, the engine is allowed to spin up in RPMs quickly, generating power in MG1 which immediately converts into high levels of low RPM torque from MG2 to get the vehicle moving quickly and more efficiently than a conventional hydraulic torque converter. In this mode, the engine is trading its horsepower for MG2's torque.

MG1 and MG2 also play the role of continuously variable transmission as they directly control the engine RPM for optimal efficiency at all speeds and power demands without the need for shifting gears and clutches found in a conventional transmission.

"Diagram #2 suggests that MG2 is active whenever the ICE is driving the wheels, through MG1 generation, rather than only when we see the instrument icon showing MG2 activity. Is this because the MG1 input prevents freewheeling when MG2 is not in power mode? Surely this wouldn't require much energy input for a steady state speed of, say 45 miles per hour where MG2 is not active, and which may require only 10 or so ICE HP to sustain, with instantaneous FE readout of 35-40 MPG. Where you do see a huge drop in FE is when the traction battery is depleted and MG1 is re-charging actively, where FE will fall to half that under the same conditions."

Keep in mind that whenever the vehicle is being powered forward under engine power, both MG1 and MG2 are always energized and double-converting power. Neither motor can take a break while the engine is providing power, as the gearset will then freewheel and power will cease flowing to the wheels.

When the vehicle is cruising at speed under light engine power, a different more efficient operating mode engages that is not described in the diagrams I provided. Rather than MG1 generating power and MG2 immediately supplying it back to the planetary gear set, the reverse occurs. MG2 generates power and MG1 immediately supplies it back as mechanical torque. Since MG2 has an extra gear reduction of 2.478 to 1 in its connection with the engine than MG1, it needs to generate less power to oppose the engine torque during steady state cruising. This reduces the amount of double power conversion taking place (mechanical -> electrical -> mechanical) for higher efficiency operation at a steady cruise.