Another regenerating braking question

Thanks for the reminder. I didn't want to go searching for the data. That's one of the good things about PC: you wait five minutes and someone will tell you.

Tom

 

Ken,

Thanks for the charts. I'm trying to figure out what to make of the first. Should that be considered an efficiency hit for regeneration? A capacity limit on regeneration at that temp? Or both?

For example, would the efficiency of the conversion be about the same with the regenerative braking power capped at the given rate (with excess braking power demand being taken up by friction braking) or would it be lost in some other fashion rather than being taken off by the friction braking.

If at -10 C (14 F) at the battery I've only got about 1/3rd the regenerative braking power and EV accelerating power I had at 15 C (59 F) then I should drive accordingly. If the lower temp is more efficient at the reduced power, then I'm happy to let it remain cool, but if I need to heat it up to get in an efficient range then I will be more willing to do early transfers to heat the battery.

Or might the battery be relatively efficient at the conversion in the cold but only supply/charge at a low output/demand?

Related to this is all the charging the vehicle seems to be intent on doing when cold. It's busy trying to warm the ICE and at the same time charging the battery well into green SOC at every opportunity when it is particularly cold. Is it actually converting most of this shaft power into charge or is it being wasted and/or turned into heat in the battery? I'm trying to figure out which part will do what when a given shaft power is applied (regen or ICE) or a shaft load is demanded (me stepping on the accelerator.)

 

Shawn,

It shows the output (discharge) power.
The battery can supply more power when it is hot, because the chemical reaction on battery is more active when it is hot, ie. low internal resistance.
Therefore, the charge (regeneration) power also can be increased when it is hot.

When it is cold, the internal resistance is increased, then the efficiency becomes lower.

I believe the hotter temp is more efficient.

I think Prius is designed for trying to charge more when the battery is cold, and it is trying to warm up the battery too.

Ken@Japan

 

Thanks, Ken.

I'm going to try to recap this and hope you or someone else will jump in if I misstate this:
1. The charge/discharge power is greatly reduced when cold.
2. The internal resistance of the battery is greater in the cold state so that efficiency of transfers to/from the battery is reduced.
3. The increased resistance will result in more heating of the battery...which then moves it to a more efficient state with a greater power available.

So the lesson I am taking from this is that cold weather is not only increasing fuel consumption because of the need to keep the ICE warm, but it is also resulting in substantial heating losses to the battery during initial warm up of the battery.

I guess this explains why the vehicle seems so slow to build charge during cold weather despite being able to feel quite a bit of load being shifted to charging. This is something that has puzzled me during my first winter with the car. I originally thought it was mainly an issue with SOC dropping a bar or even two when the battery became cold overnight, but I was curious that it was taking so long to charge since I knew the charge was good when I last parked it.

 

Hello all:

Just a newbie here having a problem finding what I'm looking for.

Basically I was wondering if there is already a resource posted anywhere on PC that adequately describes the entire braking system and how it functions.

I understand about the main braking coming from the regenerative system. But today someone asked a question that got me thinking. He was wondering about how the rear wheels are braked. I know that they are only friction, but then I got to thinking "what keeps that rear end in line on slippery surfaces??"

During non-panic braking, does the VSC monitor the relative wheel speeds and apply only the rear friction brakes (when the MGs are braking the front end) to keep the rear end in line? This would seem to be important in snow, on ice, etc. And even on dry surfaces, it would seem to be advantageous to apply some small amount of rear braking.

For those that have had to have brakes replaced: have the rear worn out first, since much of the front braking is regenerative? Or do front and rear wear out evenly?

Thanks!

 

In a nutshell, here is how the braking system works:

1) Regenerative braking is through the front wheels only. If any loss of traction is detected, regenerative braking is disabled and full ABS friction brakes are used. Friction brakes are also used in a panic stop, which is detected by rapid brake pedal actuation.

2) Friction braking is used to supplement regenerative braking if regenerative action is insufficient. The friction brakes, when running normally, act on all four wheels.

3) Engine braking is used when the battery reaches a high charge limit, or if B mode is enabled.

4) Regenerative braking operates down to about 8 mph, below which friction brakes take over.

5) During normal operation, the master cylinder pushes hydraulic fluid against a spring loaded "stroke simulator". The stroke simulator makes the brake pedal feel normal. A pressure sensor on the hydraulic line tells the brake controller how much braking to use. The controller decides what combination of friction, engine, and regenerative braking to use. For friction braking, pressure is supplied by an electrically powered brake accumulator pump.

6) In the event of failure of all powered components of the braking system, hydraulic valving fails into a configuration where the master cylinder supplies manual hydraulic pressure to the front wheel brakes. The rear wheel brakes can be manually actuated by the parking brake lever.

Tom

 

Thanks for that explanation, Tom. It was a big help. That "stroke simulator" is really clever. I wonder what Honda and other hybrid manufacturers use for this if they employee regenerative braking?

I'm a little confused as to why only the front hydraulic brakes would activate in a power failure, depending on the user to manually activate the parking brake. Why wouldn't the entire hydraulic/friction system engage for all four wheels?

Is there a paper or explanation somewhere on PC that documents all of the details of the braking system?

And again, for anyone that has replaced their friction brakes: were all four wheels equally worn (did you replace the front and the back simultaneously)? And how were the front rotors? Did they need to be replaced??

 

I have attached a PDF that shows how the Prius brakes work. The reason that only the front brakes activate with a total power failure is that the emergency system can only run two brakes. With only two brakes, you want to use the front ones for best result. Keep in mind that this is a backup system only used in emergencies.

As for friction brake wear, the front brakes wear faster, just as with any other car. Typically the front brakes are good for better than 100,000 miles, while the rear brakes will last the life of the car. Most of the wear is from rust. It is really more an issue of rusting out than wearing away. Because of this the rotors are almost always replaced.

Tom

 

Followings are additional resources...
http://www.autoshop101.com/forms/Hybrid05.pdf
Prius tech training

Japanese Prius users are collecting the brake pad/shoe wear data.
We see approx 5.5mm/100kmiles front pad wear which translates the front pad lasts approx 180kmiles. The rear shoe wear is less than the front, and it lasts more than 180kmiles.
We never heard anyone replaced the front rotors.

Ken@Japan

 

I would expect that the rear brakes are on the same hydraulic system, thus all four wheel's brakes would be actuated in a failure of any regenerative component.

The system probably does use a manner of bias so that braking force is properly distributed so as not to cause further instability.

The rear brakes, though, would likely be connected to the parking brake, however with the brake's method of operation, I would NOT recommend using it to try to slow the vehicle, since it would lock in place and not give you any degree of control like a hand lever parking brake would.


(this is all based on my knowledge of traditional vehicles including complete braking system rebuilds on a couple old Porsches. The Prius may employ variations on the above assumptions, but the basic concepts would be the same)

 

Have you seen the PDF shown by Tom?
On the page-2, the power failure situation is shown.
The fail-safe fluid pressure, red line, goes to only front brakes.

Ken@Japan

 

If regen fails, the car will brake entirely using electrically-actuated friction braking, the same as when you brake sharply.

The solenoid valves kick in only when the 12V system fails completely, the car goes utterly electrically dead; in this case the hydraulic pressure is only routed to the front cylinders. I suppose they could have added an extra pair of solenoids to open links to the rear brakes as well, but that would add extra complexity for what should hopefully be a very rare occurrence. It would require the DC/DC converter and the auxiliary battery to fail.

The friction brake system is effectively overdesigned for normal brake use, but it has to be robust enough for hard sharp braking in emergencies.

 

One thing I did notice.. if travelling at speed, say 80mph, and you brake heavily (not panic braking) the ScanGauge II shows the amps at 6,000 odd... clearly incorrect.
but I wonder what it's trying to read?

 

Speaking of regen capability of the brakes... Does braking really regenerate the battery more than coasting down hill??