I apologize if this has been covered a thousand times before, but I just don't see how the ICE running up to 4000 RPM helps avoid overcharging the batteries.
Does the CVT work against the engine instead of the generator?
If so, is the fuel flow shut off (I hope so)?
Why not disconnect the generator from the CVT and idle or leave the ICE off?
Seeing the engine rev so high when coasting and/or braking downhill unnerves me a bit.
It seems to me, if the battery doesn't need the power, running the generator into a resistive load (to generate heat for the defroster maybe) would make more sense.

If anyone knows of links to explanations or papers on the subject I'd be grateful.

 

I've not found a definitive source on this, so take it with a grain of salt, but the behavior (especially in "L" mode) appears to fit the following scenario.

1. Going downhill, the traction motor is rigged as a generator to recharge the HV battery. At speeds below 40MPH, the ICE may shutdown if not needed for other things. The motor generator could also be used to generate power, theoretically under this situation.

2. Once the battery is at full charge, you need to either stop providing braking effect (would result in a perceived "surge" by the driver) or find someplace else to put the energy. The method used appears to be to use the power generated by the traction motor to spin the motor generator, and as a result, the ICE. Since the traction motor can put out a lot more current than the motor generator can handle, the braking effect has to be reduced. Since the ICE provides very little compression, it has to be spun at a fairly high RPM to consume any significant amount of energy.

AFAICT, the ICE is consuming no fuel during this activity. There's no load on the ICE and it's already warmed up by definition in this situation, so the wear would be negligible.

As to rigging a resistance element (i.e. Dynamic Brake) that could in theory be done but would add considerable weight and complexity to the vehicle. Dynamic Brakes in diesel-electric locomotives have very large resistance grids along with large blowers to move cooling air through them. There's no good place to put such a setup on a passenger vehicle :-).

 

I had this happen to me, for the first time, on my FEH this weekend. (My wife is the primary driver of the FEH.)

It was *very* disconcerting. I was almost afraid to touch the gas pedal because I was afraid the 4000 rpm would turn into rapid acceleration. But it did not, it was like driving in EV mode.

 

Thanks, I think I follow your analysis.

You're saying the traction motor (in generator mode) is the primary (sole?) "engine braking" provider.
If the battery is charged, the electric power generated by the traction motor is dumped by spinning the engine generator (in motor/start ICE mode).
I'm guessing the mechanical linkage between the engine generator and the ICE requires the ICE to spin.
(Or is this to provide a load for the engine generator in motor mode)?

I don't recall exactly what I did, but I have experienced that "surge" you referred to.
Perhaps it was shifting from L to D to reduce the engine braking and lower the revs.
I don't think I overshot into N.
Or maybe I hit a "sweet" spot with brake pedal pressure.
Anyway, it's only happened twice in 8000 miles.
The dealership just shrugged when I mentioned it to them.

If you haven't seen this, you may find it interesting.
Ford Motor Company – 2005 MY OBD System Operation – Summary for Hybrid Electric Vehicles

Thanks again for your help.

 

Yes, that's basically it. The ICE, generator and traction motor/final drive are all permanently connected via the planetary gear device. There is a "one-way clutch" between the ICE and the transaxle, but it appears to come into play only in limited situations...I've not found documentation on when that is, but during ICE startup seems a likely possibility. So under those conditions the only possible destination for the the energy being dissipated is the ICE.

The most noticable situation for "surge" sensation is descending a long hill below 40 MPH in "D" mode, using the brake pedal to control speed. If the ICE is not running, once the battery charges fully the ICE spin-up will cause a sudden reduction of regenerative braking, causing a brief "surge" sensation between when the regen braking cuts back and the friction brakes apply.

I actually printed out the document you refer to below. It's an excellent resource.

 

That is the situation I was trying to remember.
And a brief "surge" during the switchover does make sense.

 

Well, I had this happen to me the for the first time ever. It's kind of odd. I was coming up the off-ramp from I-696, in L gear, and as I approached the red light, the tachometer went nuts for a few seconds, then as I got to the light, went back to zero. Didn't effect my MPG as far as I could tell.

I quickly flipped over to the Energy display, and the battery was full up to the +, - symbols on the battery image.