From what I have read in other threads in this forum, D captures more kenetic energy than B, and sends it to the battery. The purpose of B is when the downhill is so long that your foot might get tired pressing the brake pedal. Since such a hill would max out the battery, the ICE is used to help control the downhill speed of the car. Also, friction brake comes into play at about 10 mph and less. Therefore, my question is... You go down a long hill in D, and your battery bars max out to the top, and you are traveling, lets say 50 mph, what does the car do with your extra kenetic energy? Does it, 1. start using more of the friction brake? 2. spin up the ICE and use it even though you are still in D? 3. somehow change the excess kenetic energy into heat energy and dump it into the coolant, which then can be used to warm the cabin, or wasted via. the radiator?
The ICE will engage to use up the extra charge. This happened quite a lot on our trip in the Rocky Mountains last year.
Yup, it's pretty dramatic actually. I experienced an 80% SOC twice this past week and was able to watch the ICE spin up to over 3000 RPM and the amerage drop to -1 while braking on a downhill! I tried releasing the brake and reapplying to see if I could get the regenerative brakes to reingage to absolutely no avail. The ECU will go to some pretty extreme measures to prevent any overcharge even at the expense of lower fuel economy.
Going up a big hill in summer, I drive a little slower, and turn off the air conditioning. Going down a big hill, I drive a little faster and crank up the cooling to take advantage of the 'free' electricity. This gets tedious on low rolling hills, but in mountain passes it seems to help the efficiency. (My daughter tried to apply this technique to the music volume, but I don't think it has the same effect. ) I love the regenerative brakes. No more warped discs for me!
I like the idea about wasting excess electricity into the music. The Prius has enough, but it would be nice to add a way to, once the battery is full, to store additional electricity as extra heat in the coolant on less than 45 degree days, and then circulate it through the engine and heater core of the car. I guess somebody, not me, could convert the roof to an airbrake, like on the back of an F15 Eagle. Thanks for the replies. I learned something. -Roy Poston
man i agree 100 %. i have had situations when i am maxed out on a charge, still got a ¼ mile of downhill left, and my CANVIEW says im DRAINING the traction battery!!! man i HATE THAT. whats worse, since doing the 50P update, my max SOC seems to only be 75% now...anyone else notice this??
How can an ICE engine use up any charge? I don't think ICEs can do that. There must be a better explanation.
Tom is correct. It doesn't "waste fuel". It DOES use friction brakes more if you are applying the brake (fully friction brake once you reach what the car considers full allowed charge). It also uses EV further up the accelerator pedal travel. The engine is spun with no fuel injected to "dump" excess charge. Next time you are descending a long hill and are using "B", try applying the accelerator. The engine RPM will -decrease-. Very weird, but correct action.
Perhaps you know better than me, but I thought that the MG1 spun the engine. That work puts a load on the MG1 and the MG1 burned up the excess electrons that way.
That may be a semantic difference. The ICE is configured for maximum drag as a vacuum pump / air compressor, the combination of both MGs spins it, and the wasted heat energy goes out the ICE exhaust.
Exactly what I said. I suppose you explicitly stated that the electric motor uses electricity, but that seems fairly obvious. Tom
Would a shunt to short out the motor allow it to continue the electrical braking? I think diesel-electric locomotives do this on long descents, but I suppose they have miles of cold steel rails they can use to soak up the resulting heat.
^^ The box of air-cooled shunt resistors on the roof would wreak havoc on Prius's otherwise clean aerodynamics, wasting fuel 98% of the time.
The Prius already does this, but rather than using resistors it spins the ICE as an air pump. The net effect is the same. Dynamic brakes on trains use large fans to dissipate the heat, not the rails. Tom
