Strange engine braking behavior, Prius 2009

When my '09 was doing almost exactly this my hybrid battery was actually going south at the time I didn't know it but quickly got up to speed. And that's the only thing I can think of that would have been making that happen and there were other things going on too but that's not for this discussion. As soon as the hybrid battery was replaced a whole bunch of problems went away but I do remember this region breaking business kind of just not working at a pretty slow speed 10 miles an hour or something along those lines and like it just clicked off I don't think it does that now I'll check it in the morning I rarely use my friction brakes until I'm at the stop sign and I have to clamp on the rotors to not roll forward or backwards that's about the only time I'm clamping with the hydraulics where I'm driving I have nothing but time.

 

idk much about engine braking, but isn't 6-12mph the area where regen cuts out and full friction braking takes place?

 

I have to ask....why do you use the B function so much?

 

"This issue does not bother me too much but I'd rather not give up the use of the engine break."

Times two.

Slowing a car down requires doing "something" with the kinetic energy (mass x velocity). Normal hydraulic brakes mechanically turn it into heat via friction. Engine braking does "the same thing" in a different manner.

Regenerative braking converts kinetic to electrical energy (with some heat due to efficiency losses), but you get to reuse that instead of it all being "wasted".

Normally, a Prius has a low level regen braking going on when you coast. This mimics the feel of a "normal" car with slight engine braking effect when the throttle is closed.

Stepping on the brake pedal tells the ecu how fast you want to slow the car. The ecu decides how much regen vs friction braking to use. (hint - it's a bunch more regen than "coasting").

Now on a long braking event - say going down a mountain - regen will "fill" the hybrid battery to its limit. That leaves only the friction brakes, which can overheat with constant application (the Prius is kinda heavy for its size).

So Toyota added B mode, which shifts more of the rotational kinetic energy from the wheels into forcing the engine to spin (engine braking). So less regen, and less friction (at the wheels). It's slowing the car down, but it's wasting energy.

So, using B mode all the time to slow down is going to reduce MPG vs normal (light pedal apply) braking that uses lots of regen.

As for "is this normal"? No idea since I never have any reason to use B mode.

Posted via the PriusChat mobile app.

 

The main time I use B mode is when descending from mountain passes when there is very long downward grade. For instance, the Grapevine after Lebec on I5 northbound out of LA. That is a 2600 ft. descent in 10 miles (roughly, which takes about 14 minutes). For this descent the headlights are on, the cabin fan is blowing at max, and the radio and defrosters are on. All of those are to provide a destination for the power coming out of the regen braking other than the HV battery, which unfortunately tends to be nearly full just as I pass Lebec, and so hasn't got much space left to hold more energy.

Sometimes I also use B mode when descending a shorter but very steep grade, in that case usually without all the other electrical loads added.

I don't recall if I ever worked out the energy/power for the grapevine descent. Let's see, E = mgh, mass of a Prius with one passenger is ~3000 lb so about 1360 kg, g is 9.8 m/s, 2600 ft is around 800m, which gives E = 1360 * 9.8 * 800 = 10662400 Joules = 10.7 MJ. (A stick of dynamite is around 1MJ, but of course that is all released in a fraction of a second, not 14 minutes.) Mean power would be 10662400/(60*14) = 12.7 kW. Some, maybe half(?), will be lost to air and rolling resistance, and the remainder is roughly the same as the power from 6 100 W bulbs.