Posted to Technical Theory Forum on 9/26/2010
1 Reply
Friends,
I have hopefully learned to be careful about making dogmatic
statements about complicated cars, like the Prius hybrid.
But I was sitting in a brake class a couple of weeks ago and
the instructor told the group that the Prius B mode
increased regeneration to a higher level to add vehicle
braking. I kept my mouth shut even though this ran counter
to what I had observed previously, mostly because the
information is not terribly significant when it comes to
fixing a car. But, I decided to see if I could prove it.
Now, I think I have.
What I did was flog my car up a fairly good grade to get the
state of charge down to a level where I knew that the THS
would be trying its best to recover to its comfortable
levels of 50-63% SOC. I was able to punch the SOC down to
36% which is not easy to do without going way over the speed
limit (I think I hit 79 mph going up hill in order to
accomplish it.) I then did a U-turn and went back down the
hill. I recorded a ten minute recording on the Techstream
and then zoomed in on a flagged area where I had shifted
into "B" mode on a pretty steep downhill. [2004 Toyota
Prius, Engine/Propulsion Scan Data] As you can see, the
battery SOC had already recovered up to 52% just by
coasting. I think I saw as high as 60 amps of regen without
applying the brake, which I presume was being supplied
primarily by MG2, though since I didn't capture revolution
and torque on the two motors I can't say for sure. But,
beyond any doubt, the B mode actually is drawing massive
amps (82) out of the battery pack in the process of reving
up the ICE rpm to accomplish the B mode braking. It is
definitely counter-indicated for fuel economy. As you can
see here [2004 Toyota Prius, Engine/Propulsion Scan
Data] after I have shifted back into drive (at the flag;
there is a bit of a delay before it actually happens)we get
a pretty good regen, once again only by coasting.
So, it is my conclusion that the B mode is using the
electric motors to transfer more load back through the power
splitter (planetary gear set) to the gas motor in order to
slow the car down. It kind of makes sense if you realize
that the battery pack and its life are really important to
Toyota. Force feeding it high rates of amps for the purpose
of slowing the car on a long hill is a bad idea. I tried to
duplicate this test on Saturday, but I waited too long going
down the hill and I heard my battery fan kick on high blow.
Even though battery SOC was only at 65% (on a cool battery
pack I've reached just over 81% on a long down grade) the
hybrid system shut down regen entirely, no doubt to protect
the battery, so I couldn't prove anything with the B mode.
It is dangerous to make dogmatic statements about the Toyota
hybrid system and how it behaves. I did not know till
yesterday that it may not be possible to push SOC to the max
on a long hill if battery temp went too high. I just never
tried it under ambient conditions of 105 degrees. So, we go
on learning...
Phil from California
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