I may have to eat some words I've posted in the past. And this may not apply to the pre-'09 FEH's.

I live in a place where it's common to descend grades long and steep enough that the hybrid battery gets charged to capacity. Then I have to rely on the mechanical brakes to keep the speed in check.

When I first got the FEH, I did what I was used to doing on non-hybrids, which was to gear down to use engine braking to keep the speed under control on these descents. Using the brakes and having the brake lights lit up was a sign of less competent driving.

After a year or so I figured out that using engine braking wasted energy by making the engine run at higher rpm's. So if I wanted to maximize the amount of energy going back to the hybrid battery, it would be better to use the brakes (which causes the engine rpm's to speed up just a bit).

But then I learned from this forum that when you brake with the FEH, even if the braking is light enough that "all" the energy is being recaptured by regeneration, the rear brakes are also applied lightly in the interests of stability. That made perfect sense, although it was a shame to be wasting that energy.

So then the question came up: which mode wastes the least energy, downshifting to raise the engine rpm's, or braking and having the rear mechanical brakes applied? The former wastes energy by running the engine faster while coasting; the latter wastes energy by application of the rear mechanical brakes.

As I said I regularly come down descents where I fill up the hybrid battery charge before I get to the bottom. I've done these hill so often I know exactly how far down I can get before the battery is full. I've been doing these descents by downshifting because I thought this would waste energy via engine braking and therefore delay the point where I had to rely on the mechanical brakes. And so reduce wear on the brakes. (On lesser descents, I'd just use the brakes.)

So I tried the descents using the brakes instead of downshifting. The result was clear that it took longer to fill up the hybrid battery to capacity by using the brakes to keep the speed in check, than by downshifting to do the same.

Which means that under exactly the same conditions, downshifting provides more charge faster to the hybrid battery than using the brakes. So on less substantial descents, I've gone back to downshifting to get the most charge possible out of the descents and reduce wear on the rear brakes. For the big descents which fill up the battery anyway, I don't think it makes much difference since the brakes end up being used either way.

Now I'll be watching if the downshifting approach results in a noticeable mileage gain.

 

Mine is an '07, but I have found it better to use the engine breaking on long downhills. Even though the combustion engine starts up, my scan gauge very quickly shows 9999 MPG, same as in EV mode. Although I'm not a car guy, I'm told it goes into a "closed loop" state where even though the RPMs are high, that no fuel is being consumed. I didn't initially believe this, but my overall MPG seems to bear it out.

 

This is correct. The software engages the engine to use its compression to do engine breaking without actually burning fuel, though I believe this only kicks in once the hybrid battery has reached a certain charge threshold.

 

Yes. I think the hybrid owner's manual even states that no gas is being burned if the gas pedal is not depressed. Even if it's not, it will run the engine and burn gas if something needs to be heated up.