Strong engine braking in D

<div class='quotetop'>QUOTE(Tideland Prius @ Jun 7 2007, 01:17 AM) [snapback]457189[/snapback]</div>

I've seen the engine rev up to around 3,000RPM when the battery bars are up to the top. And I also noticed I can disable the engine revs by shifting to 'N' when going down a long hill. I've always wondered why the car has to rev the engine up so high when the battery is full.

But this isn't engine braking, just revving up. I didn't notice any stronger braking when going down the hill.

Dave

 

<div class='quotetop'>QUOTE(DaveLeePrius @ Jun 6 2007, 11:32 PM) [snapback]457193[/snapback]</div>

Wouldn't it be to protect the battery from getting too much charge by using excess energy to rev the engine??? Since the battery doesn't charge in neutral, that's probably why it doesn't rev.

 

Sorry, double post.

 

If the car was not also strongly decellerating, then it was not engine braking.

Methinks you're perhaps obsessing about eight bars.

 

My bet is your HV battery is too hot to receive the regen current.
Prius had to consume some energy by such strong engine braking on downhill.

Ken@Japan

 

Tideland, do you have equipment to monitor battery temperatures? I am interested in Ken's suggestion.

In my 2001 Prius I have seen max HV battery temperatures up to 50 oC. When they get above 45 (and if I notice) I use the air conditioner in flow-through mode. This will cool the battery about 1 oC per 5 minutes.

However, at these hot battery times, I have not felt the high engine rpm. Perhaps that went unnoticed. Or, perhaps the NHW20 temperatures are more tightly managed?

 

I drove through mountains once and used B going down the long downhills. I switched to B before the battery icon was at 8 bars because I knew the hill was too long for re-gen braking all the way.

I heard the revving as soon as I put the lever in B. This is what I expected to happen. All perfectly normal.

I think Ken's explanation is correct: if your HV battery was too hot the computer might have denied it enough current to charge all the way. I know I cannot go into EV (which would require more output current) if the battery is over about 105 degrees F.

 

It just does that. It's normal. See the B-mode writeup.
.
Basically, when the battery approaches full, the hybrid system
begins doing what it would do if you switched to "B" while at a
more normal SOC. Then, if it's in that mild-engine-braking state
and you *then* switch to B, it *really* howls. The amount of
engine braking seems to have two regions, not just one.
.
And remember, there's delay and hysteresis in the SOC indication on
the MFD, so it might have been at eight virtual bars but just hadn't
updated quite yet. I see that quite a lot.
.
_H*

 

Shame on you, with 9,137 posts you should've known the car's functioning normally.

 

<div class='quotetop'>QUOTE(mcbrunnhilde @ Jun 7 2007, 12:18 AM) [snapback]457197[/snapback]</div>

Well, revving the engine up high uses gas, and it also wears the engine out. I don't believe in this concept of the battery getting too much charge either, especially as people say the bars don't represent 0-100% charge state.

I wish the Scan Guage II could show me the traction battery state of charge, instead of just the aux battery.

Dave

 

<div class='quotetop'>QUOTE(wiiprii @ Jun 7 2007, 10:32 AM) [snapback]457445[/snapback]</div>

The bars don't represent 100% state of charge, but I think the computers don't WANT the battery to go above the 80% SOC that the bars represent. Hence the engine rev when all the bars are there.

 

A few notes on this topic.

The Scangauge will not show traction battery temperature, so don't purchase it hoping it will. It -will- show RPM. Traction battery temp and SOC are CAN messages. The Canview would be required or some other system more advanced than the Scangauge.

Prius will turn the engine with no fuel to dump energy, usually in "B", but I suspect it will do so in "D" when the SOC is high as well. It will do it when you are "coasting", to simulate engine drag (in this case it would be real engine drag) and to slow the car (when in "B"). The latter would result in higher RPM than the former. I don't think this would result in much extra engine wear. It's the higher pressures and temperatures of combustion when the engine is burning fuel that really wears the parts.

I do like the explanation of a hot traction battery forcing the system to avoid taking or dumping charge to it to allow it to cool off. Have any Canview guys seen this?

 

<div class='quotetop'>QUOTE(Tideland Prius @ Jun 8 2007, 02:09 AM) [snapback]457427[/snapback]</div>

The HV battery becames hot very quickly when the charge/discharge current is high enough.

Ken@Japan

 

<div class='quotetop'>QUOTE(Tideland Prius @ Jun 8 2007, 04:02 PM) [snapback]457900[/snapback]</div>

You've never mentioned about fans. Maybe, your fan control circuit became bad.

Ken@Japan

 

<div class='quotetop'>QUOTE(wiiprii @ Jun 7 2007, 10:32 AM) [snapback]457445[/snapback]</div>

Because the Prius does not have actual gears, like a conventional car, it simulates the condition of being in low gear with no gas sent to the engine. This is like the jake brakes on an 18-wheeler, or like putting a regular car in low gear going down a hill: The engine compression is used to slow the car as it goes down a steep hill. This causes the engine to spin fast, and you hear it rev, but no gas is being sent to it.

The "normal" way to slow the car is to use MG2 as a generator. But since the car does not have a resistive heater to dissipate the energy produced in this way, the car needs some other way to slow down when the battery cannot accept any more charge. Ordinarily, this "other" way is the friction brakes. But on a long downhill the friction brakes become overheated, so engine braking is used instead. This causes a revving sound, whether the car is a Prius or a conventional car. In an 18-wheeler, it causes so much noise that small towns on the highway often have signs asking truckers not to use engine braking in town.

As mcbrunnhilde correctly points out, the computer will not allow the battery to get to 100%, because that would shorten the life of the battery. The icon shows the portion of the actual battery capacity that the car actually uses.

I will offer one other (unlikely, but possible) explanation for the OP's experience:

Perhaps, due to one of those occasional computer "burps" the car briefly thought the battery was full, or overheated, when it was not.