High engine RPM going downhill

The term regenerative braking has always been used for the Prius to describe the fact that you brake by using the MG2. And it is regenerative braking: the power generated by MG2 goes into the battery which is then used by MG1 to cranck the enginge. Sure, you won't accumulate more energy (that's being prevented by this process), but you still genereate power by braking.

 

I've always thought the correct term was compression braking, but I'm not sure that any term means much. The question seems to be is there any engine braking happening once the battery is fully charged and isn't the answer Yes?

 

The term "regenerative braking" means you are generating power from your kinetic energy, braking the car somewhat. It does not mean or imply actually USING that power to recharge a battery or heat resistors. It does not address what the energy is used for, though obviously it has to be dumped into some kind of "load". In the Prius it can be used to recharge the battery or to spin the engine. Or both.

 

Good points David.

 

The battery is a one port device, so that excess power is not actually going through the battery. It skips the battery and goes direct to MG1.

Here is a conflicting opinion, at wikipedia: Regenerative brake.
"A regenerative brake is an energy recovery mechanism which slows down a vehicle or object by converting its kinetic energy into another form, which can be either used immediately or stored until needed."

When the energy is used to heat resistors, it is called Dynamic braking.

 

Yes. The wheels are mechanically driving the engine, as well as the motors.

Agree that the old "compression braking" term is dubious. It's partly true, but maybe more of the power to spin the engine is going to friction within the engine than to compression---depending on details of the moment, of course.

 

Compression braking refers to diesel engines, where the valve timing is changed so that the exhaust valve opens at the peak of compression, thus preventing the compressed air from pushing back on the piston. It's that sudden release of compressed air that makes the loud rumble you hear from a truck's compression brake, and dissipates energy in the engine. (It does work compressing the gas, but doesn't recover that work during the expansion stroke.)

The Prius (like engine braking in other gas engines) doesn't do this; the compressed air stays in the cylinder, pushing back on the piston. Instead, energy is lost by friction in the engine and trying to pump a vacuum in the intake, since the throttle is closed.

I think the best term for this is probably just "engine braking", and it just so happens that it's using an eCVT as the means to make the "gearing" appropriate for engine braking.

 

That is 'compression-release' braking: Compression release engine brake - Wikipedia, the free encyclopedia.

When I hear 'compression braking', it most often refers to the pumping-loss variety of dynamic braking, which is the same as vacuum pumping with the throttle mostly closed that you mentioned.

Agreed.

 

How does the gen 2 get rid of extra kw's. I have never heard my gen 2. spine the the engine to rid surplus kw's. hal

 

It should do it exactly the same way. Have you ever driven longer stretches downhill and observed the battery indicator to be at maximum?

 

They mean the same thing. "Compression-release" is more accurate and descriptive, but a lot of people leave the "-release" off because it's unwieldy. Eg in this section, Wikipedia reverts to calling it a compression brake a couple times. Most ordinances that prohibit or limit them refer to them alternately as "Compression brakes" and "Jake brakes". (Try googling "compression brake", you'll see them used pretty interchangeably.)

 

Try searching Wikipedia for 'compression braking', and you'll also find numerous articles where it used interchangeably with engine braking in traditional gasoline engines, eg. in this section.

 

I have a gen 2 and a gen3. I drive all mountain conditions with many grades 6 degree slope and for 1000 two 2000' drop. My gen 2 has never gone into b mode or sounds like it is in b mode. Hal

 

The car won't go into B mode by itself, you must intentionally shift it there. While it will fill the battery and then wind up engine RPM in D mode, it does so to a much smaller degree.

A 6 degree slope is a 10.5% grade. While certainly present on some streets and minor roads, it is highly unusual for a major highway for that much distance and elevation change. Is this on a major highway? If so, where?

 

"Excess energy." Woo-hoo, there's got to be a better way to burn it off instead of compressing air with the engine. Maybe drag-flaps?

 

Sorry, I ment to say 6- 10 percent slope. Hal

 

Thanks, I'll be sure to fix those.

 

Max AC would help significantly, weather permitting.

 

I doubt the AC would make much difference as it is not a belt-driven compressor, simply a little more electrical load...

 

Compressed-air tanks?