High engine RPM going downhill

yes, you're burning off excess electricity that you are generating, going down hill, because the battery is almost full and there's no place to store it. the electricity spins the motor, which spins the engine. the engine turns, but doesn't burn any gas.

 

Normally, you have regenerative braking, where rotational energy is used to charge the battery. Once the battery is 'fully charged'* in the estimation of the computers, then all you would have left is friction brakes. Instead, the engine is used as an air pump, to simulate 'engine braking' in a manual.

You can get this effect even before the battery 'fills' by shifting to B (for engine Braking) which can be helpful for long down hills that will need some engine braking. Since all the energy is wasted in engine braking and friction braking, you only want B when the battery will fill anyway. Most smaller hills you want to save the energy in the battery.

*The computers keep the batter between 40% and 80% charged, so 'fully charged' is relative, not absolute.

 

This is when you want to use B (braking) mode on the tranny to avoid overheating the brakes! Since regen braking is not available (battery full), the friction brakes are being used, and these can overheat on long descents, such as Wolf Creek Pass in SW Colorado!

 

The really funny part of all this is when you are going down the long mountain pass and the car starts to slow, you press on the accelerator to maintain speed, and the engine slows down. Very strange, but normal.

 

And the harder you brake, the louder the engine gets. Very nonintuitive, but normal.

 

Scared the crap out of me first time this happened. Another runaway. Hal

 

I've heard, likely here, that even if you don't select B mode on the shifter, the car will do some B mode, behind the scenes.

Ok, guess Jimbo's already said this.

 

... and perfectly logical. That's equivalent to downshifting a couple of gears to descend part-way down a steep hill with a manual transmission, then shifting back up one gear when the downhill becomes less steep. The Prius does the same thing, except automatically, with less abrupt transitions.

 

Good old Wolf Creek Pass! I haven't driven over that in many years. I've taken many trips over it though when I went to Ft. Lewis College in Durango, CO back in the 80's!

 

I doubt very much you could "overheat" the Prius brakes unless you tried very hard, like the fellow who tried to scam Toyota a few years ago.

"Overheat" means they get so hot they fail. You can certainly get them hot, but I don't think you can make them fail. Prius is a light car with very nice disk brakes on all four wheels (GIII). Even though they are small compared to heavier vehicles, they are larger then necessary. I say this because they are much more capable than the GII brakes, which didn't have overheating problems.

 

I've never felt the brakes on our Prius switch back to friction on a long down hill run, at least as far as I can tell. I do switch B mode on and off, going down a long ski hill we have.

With our previous Honda Civic, there was a most definite transition, when the battery got fully charged and braking switched back to friction-only. There was a very noticeable increase in peddle pressure needed, to keep braking. It didn't take much of a hill either.

 

It's not that you "try" to overheat the brakes. The point is that on a steep, long downhill grades and without regen braking, you will be using your friction brakes alone to slow the car to maintain safe control and speed going downhill unless you select B mode. This was more of an issue with the old drum brakes on cars 50 years ago. Disk brakes are less prone to fade due to heating, but good practice is still to use engine braking (B mode) along with friction brakes when going downhill.

 

I believe the Prius will have rejen braking, even with a full pak. I just watch the hsi guage and never go all the way to the bottom on the scale. Light pumps will keep your speed under control on any hill. If you do this you never actually use the friction part off the braking system. Hal

 

Have you tried descending the Pikes Peak road? There is a reason for the mandatory brake check stop part way down the hill, with a gate where the attendant with an IR thermometer gun checks every vehicle. Those that flunk must take a 30 minute cooling off stop, and I witnessed a Florida vehicle ordered into that holding space. Actually, I smelled it first.

I have to question whether or not you have taken your Prius down enough long hills to really see what happens. When the battery pack on mine is really full (the top SOC bar is lit before it is really full), the HSI bar quits going left altogether, and the engine really winds up. And just try to come down the road mentioned above in traffic without friction brakes. I'd dare you, except I can't take any responsibility for the nearly inevitable consequences.

While the above road is a particularly strong example, there are many more where the same issues apply.

 

I don't really see why regen is not there on full charge as Toyota system allows for surplus energy too bleed off and regen is just another way to get rid of surplus kw. Hal

 

I consider 'regen' to be what goes into the battery, not what goes into the engine to be burned off as compression braking.

I have witnessed, very many times, the battery back topping out followed by the 'CHG' portion of the HSI bar completely vanishing. While it will light up when friction braking is applied via the brake pedal, the light 'synthetic engine drag' regen in D mode and the extra braking regen in B mode are completely gone. And all this while the engine is screaming like a banshee from compression braking.

Note that the battery is not yet full when the top SOC bar first appears. Consider that transition to be at 7.5 bars, rounded up to 8 for display purposes, while the battery keeps accepting charge until it reaches 8.0.

Note also that by 'full', I'm talking about the upper charge limit allowed by the ECU, not the true but inaccessible battery limit revealed by engine monitors such as ScanGauge.

 

That is correct. The engine brake does not work the way a conventional engine brake works. In a conventional car, the engine has a fixed coupling to the driveshaft (through the gearbox) and having the engine rev (by selecting a lower gear) means it slows down the car. This does not work in a Prius or any car with an HSD. What happens is that MG1 drives the ICE (for which it has to use electricity) and at the same time the MG2 is used for regenerative braking.

 

I'm not sure I'd term it "regenerative braking" though, once the battery is filled. That term specifically refers to a system that returns electrical energy to the source (e.g. the electrical grid for a train, or the battery for a HV). It's more akin to dynamic braking, which is used on some trains and buses. (I remember seeing a dashboard light for it on the non-hybrid diesel city buses where I used to live every time the driver pressed the brakes.) Dynamic braking uses generators to convert kinetic energy to electrical energy, then sends that to banks of resistors, which convert the electrical energy to heat.

I don't know if there's a name for a braking system that converts kinetic energy to electrical energy back to kinetic energy (of the engine) to heat, but I wouldn't call that mode "regeneration".