High engine RPM going downhill

That doesn't matter much. It's still a way spend some of the surplus power that the full battery can no longer accept. If "a little more electrical load" didn't slow the car, then regenerative braking wouldn't slow the car.

 

My take:

When the hybrid battery's topped-up, the car ceases generating electricity, uses the regular brakes to hold or reduce speed, aided by the engine's rotational resistance, on each compression stroke?

 

AC is a fairly big electrical load, as numerous folks in warm climates have discovered during car washes.

 

At least one motor (probably both) is still spinning, and there's no reason for the car to cease generating electricity to supply 12V needs, power steering, the water pumps, and AC. You could manually cause it to cease generating electricity by selecting "N", but that wouldn't make much sense on a long, steep downhill.

 

Not true. The fact that the Prius will start turning the engine all of its own when going downhill (when you don't press the throttle or brakepedal) is an indication that it is still generating electricity. Otherwise it will just coast downhill. When you release the accelerator, the Prius always brakes electrically a little bit.
This effect (generating electricity and "burning it off" by cranking the engine) is increased when you press the brake pedal. So I think the Prius still starts braking electrically and only when you press the brake enough it will start using the regular brakes.

 

I agree with the rest of your post, but not this sentence.

I have never felt this. It seems to me that whenever I press the brake and it can't put more regen into the battery, then all the increased braking goes into pad friction. While pressing the brake pedal harder when the ICE is already wound up with engine braking, I've heard the RPM fall many many times, but don't remember it ever rising. This applies with both full and not-full traction batteries.

 

I will remember to keep an eye on this next time I drive in the mountains.

 

The GII does it pretty much the same as the GIII, though the levels are slightly different. Pearl would rev. up descending from Duffy Lake (in places up to 18% grade - fun road). Once down at the bottom the ICE would often shut off and the car would "burn away" the excess charge in the battery, trying to get it back down to about 60%. Pearl S (GIII) does pretty much the same thing. Just at slightly different power levels.

Keep in mind the synergy drive system won't enter this mode until you get the charge display full (all bars green in the GII), and keep generating power after that.

Be aware, the Prius has very low rolling resistance and drive system resistance, so tends to "take off" down hills. This -can- be disconcerting descending mountain passes. In Summer, I have been known to open all side windows in the car to increase the wind resistance (it works) so I don't have to use so much braking power with the friction brakes and "B" mode. There are hills steep enough in B.C. that gravity will overpower "B" mode. The limit seems to be around 6% grades, though the length can make a difference too. It seems to take a minute or two for gravity to overpower the systems. All this with a full battery charge level indicator.

I don't think you could burn up the Prius brakes, even on the GII, descending these passes, but you can get them warm enough to smell. Most passenger cars brakes won't fail unless there is a lot of water in the brake fluid (which is why the fluid should be changed regularly, along with helping to reduce corrosion from the water in the fluid). If you find the pedal going to the floor, you have boiled that water, and had better have the fluid changed ASAP (if you survive ).

 

I'm no mechanic, but how does water get into the brake fluid?

 

Brake fluid is hygroscopic. It absorbs water from the air, or anywhere else it can. Driving in the pouring rain, the brake calipers are soaked in water, and, while the pistons are "sealed" with rubber seals, there are always some leaks. The master cylinder volume is constantly changing, and even though it is "sealed" there is always a possibility of that rubber seal leaking.
Bottom line, the fluid absorbs water. Manufacturers recommend changing the fluid because they know that.

Side note here. Brake fluid is based on glycol, which absorbs water. -Racing- brake fluid can sometimes be silicone based, which does not absorb water and boils at a much higher temperature. If you change your brake fluid -regularly- (as in once every few weeks) you can get away with using this stuff. If you don't the water that DOES get into the fluid pools at the lowest part of the system, the brake cylinders, also the hottest part of the system. Brake failure is guaranteed in this scenario. With glycol based fluid the water tends to be distributed throughout the system, lowering the concentration at the hottest place (especially with anti-lock systems, which pump the fluid around). It takes much longer for the water concentration to reach dangerous levels.

 

I would never have guessed.

 

When car is in cc desending long grades the ICE revs even higher when reaching full charge. I turn cc off. Not sure if this is wise on my part? I just let the speed build and apply brake off and on . I will have to watch and see if the regen indicator is showing after full charge. I believe it is, but not positive on this. Hal

 

Let's say your Cruise Control is On and Engaged (maintaining speed) and you press the Brake Pedal, you disengage the CC, but it is still On (Set goes out, but Speed Dial light remains lit on dash). It makes no difference if you switch it Off or leave it On after it disengages. It is not involved in maintaining speed. But what you need to pay attention to is use the B mode on the transmission when descending long grades to save wear on the friction brakes.

 

Any car will take off if you're going too fast. The slower you go, the lower gear ratio you can use, and the more effective engine braking will be. The Prius doesn't have discrete gears, so assuming the engine is already spinning at maximum RPM, going slower provides a better equivalent gear ratio.

In a regular car, if you're going to fast to get into a low enough gear, the car will accelerate down a steep hill. But if you use the brakes to get it slowed down enough, dropping into a lower gear will allow the speed to be controlled without braking.

 

My dad told me that breaks are less expensive than transmissions to replace. So I. Guess this is why I never go to b mode. I just let her roll and use the brakes and apply a bit of throttle if I want more free wheeling. Hal

 

He's right about that, but I don't think downshifting is necessarily hard on transmissions. Some years back, I began downshifting whenever possible to save brakes. On my Dodge Ram, I shifted to low whenever I wanted decelerate; the computer always shifted properly for engine-braking and kept the engine speed within limits. The Dodge has 250K on it and still going strong. I didn't need a brake-job until 150k. I did the same with My Corolla for several years with no ill-effects. Some operators manuals recommend downshifting instead of riding the brakes.

 

Strange, I've spent thousands on brake jobs over the last couple of decades and $0 on transmission and engine replacements

 

Not so strange when you think about it. Transmissions are made to match torque to traction requirements. Most of them can do that without self-destructing. Brakes are self-destructive in their application; transmissions are not.

 

Tpenny67 you must drive like I do<grin>. Hal

 

The problem with downshifting automatics is not realizing you're going too fast and overheating the fluid.