[quote=Don R;157235][quote=PineywoodsPete;156940]

Please recheck your source. See the diagrams below from Toyota. MG1 and MGR can receive power directly from MG1 through the inverter while completely bypassing the battery.

The thermal to mechanical efficiency of an engine is typically less than 30% but all gasoline engine powered vehicles suffer from this loss so this isn't considered in the hybrid/non-hybrid efficiency comparisons.

Hybrid motor/generator mechanical/electrical conversion efficiency is much better than 70%.

Study the Toyota power split device. The engine, MG1 and MG2 and the front wheels are all mechanically connected by gears in a 4-way planetary gear set.

If any input/output to a planetary gear set is left open (freewheeling), no power can be transmitted between the other input/outputs.

In order for the engine to be able to turn the wheels, MG1 must be able to apply an engine-opposing force to the gear set which in turn generates electricity which in turn is sent to MG2 which in turn contributes positive motive force to the gear set. If MG2 could only supply 45KW of power, the engine could easily overpower MG2 and spin it backward in a partial freewheel mode preventing much power from being sent to the front wheels from the engine.
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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!

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 MPH where MG2 is not active, and which may require only 10 or so ICE HP to sustain, with instateneous 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.

Seems to me the biggest practical result of the dual flow from MG1 is to allow more than double the power in reverse, where the ICE can't directly contribute: 45 KW from the traction battery, and an available direct 50 KW from ICE-driven MG1 to MG2/MGR. Another reason for MG2 to be so large, especially in FWD models. I recall reading that in the early Prius, limited backup power was a problem, and was battery output only, I believe.

Thanks again for your input, Don, in clarifying this complex hybrid system.

Pete