Yes, exactly.

 

Depending on engine rpm, MG1 may not have to spin 'backwards' at 120kph/75mph.

When MG1 spins 'forward' slower than the engine (down to 0 rpm) it is providing an overdrive gear ratio. IOW, MG1 doesn't have to spin 'backwards' to provide overdrive.

Perhaps a more intuitive way to look at it...
If MG1 and the engine are spinning at the same speed; then, the eCVT output will also be spinning at the same speed... so, 1:1 gear ratio. So, as soon as MG1 is slower than the engine you have overdrive.

I don't have time to check for sure; but, I think at that speed, MG1 would only spin 'backward' if the engine rpm were below something like 2100 rpm.

 

Yes, it would only happen at lower RPMs, basically cruising along. Step on the gas, RPMs rise, MG1 rotation changes, MG2 switches from generator to motor function.

 

Surely I am not the first to discover this; but, I haven't seen anything about it anywhere, so...

Background:
While I was doing some testing, I was wondering how best to perform a quick launch from a stop. Normally, the engine will be off at a stop... not ideal for a launch. Then I thought, I could use charge mode so that the engine would be running... better; but, would that affect the battery output?

Finally I thought, why not try to push the accelerator with the brake applied? The engine will rev to about 4000 rpm for something like 5-10 seconds... release the brake and off you go. (If you don't release the brake before the 5/10 seconds is up, the engine speed will come down but continue running.) FYI: you will almost certainly have wheel slip.

Interestingly, the engine will come on even in EV mode.

 

If you drive agressively and especially with an uphill, the engine could come on to assist. EV mode doesn't means the gas engine won't start. In cold temps, the engine will start even more frequently.

 

Interesting. I have never had the engine come on in EV mode... (except at the max EV speed). I don't normally drive aggressively... only when I'm doing science. Maybe when you request more power than some preset threshold in EV mode and the battery cannot provide it (due to cold, SoC, ???) then HV mode is forced.

Some further info... At max charge the battery pack voltage is ~391 V, at zero EV range it is ~335 V. For one of my 0-60+ tests, I started at 376 V; when the battery reached its max output the voltage had already sagged to 354 V; by the time I reached max EV speed the voltage was only 333; then, quickly recovered to 373 V afterward. This was a relatively high state of charge... 7 of the 8 bars. Maybe if the battery was cold or I started with 6 bars the engine would have come on?

Edit: If you let the car sit idle the engine will start when the battery reaches ~331 V. Maybe ~331 is the minimum allowable voltage no matter the temperature or SoC?

 

On a long downhill descent, even with plenty of room left for regen charging, I've had the engine turn on part way down for it's own reasons.

I wonder if it was to aid in engine braking because I'd done too much regen or it was beyond the regen to hold the car to the cruise control speed.