Hybrid regen when braking

[Mick F]

I understand how regen works when the car is on "over run" slowing down.  The electric motors are "driven" and therefore turn into generators.

How does the regen increase when the brakes are applied?  Is the braking circuit recognised by something and it increases the generator output?  How does it do it?

Mick.

[Anthony Poli]

I am curious also.
I was wondering if just taking your foot off the gas uses the mg1 the smaller motor to regen and under braking it uses the other motor. Which would explain more regen when actively braking.
Since heat recovery wouldn’t be as effective over short periods of time and would still incur brake wear.


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[Devon Aygo]

Brakes in the Hybrid are fly-by-wire when you press the brake pedal a sensor detects speed of application and force applied and from this dependent on vehicle speed and traction decides what method of braking to apply:

If you brake normally / sensibly then regen is used, the strength of the regen is determined by pedal force applied the greater the force the stronger the electromagnetic field created in the motors making them harder to turn and increasing the regen force

If you brake harder, suddenly or TRC/ABS is triggered the car will brake conventionally with pads/discs again force is determined by the brake pedal sensors which instruct the brake booster pump to apply hydraulic force to the brakes

[PeteB]

1 hour ago, Devon Aygo said:

If you brake harder, suddenly or TRC/ABS is triggered the car will brake conventionally with pads/discs again force is determined by the brake pedal sensors which instruct the brake booster pump to apply hydraulic force to the brakes

I was under the impression that TRC/ABS activation cancels regen braking as you say, but always understood that firmer braking uses the friction brakes to supplement rather than replace regen.  Is that not right?

[Mike J.]

Some more info to consider:

The traction motor supplies regeneration charging and the other electric motor supplies charging when the petrol engine runs.

There is a maximum regeneration figure of around 17kW and this is seen when using cruise control on a steep downhill stretch - given this retardation factor exists solely via the ECU it is probably the maximum value for regen when the brake pedal is used and anything beyond this retardation is via the brakes.

[PeteB]

In fact I believe until the Hybrid System Indicator guage reaches the bottom (or left on linear HSIs) it's all regen braking and harder pressure on the brake pedal after that adds friction braking.

[Bob110023]

Hi. I think on gentle braking it is all regen until the speed gets to under 5 mph then the brake pads etc take over.

[Mick F]

Thanks guys.

This is all very interesting eh?

It seems very mysterious how this all works together for braking.  Our car is the first one ever that it's the rear wheels that get dirtier than the fronts.  "Normal" cars have loads of brake dust on the front wheels and ours never has.  The only dirt on the wheels is road dirt.

This must mean that pads and discs must last far longer than on a "normal" car.

Mick.

[Heidfirst]

3 hours ago, Mick F said:

This must mean that pads and discs must last far longer than on a "normal" car.

It is not unknown for hybrids to need replacement rear discs more often than normal cars due to corrosion as they do so little friction braking that they don't clean surface rust from the rear discs as a normal car would & it gets a firmer hold on the disc. & of course, when you replace discs you should also fit new pads.

[PeteB]

It's all down to amount of use and type of braking.

I recently read a post (somewhere) from a US owner with over 200,000 miles on original discs and pads.

My last Gen 3 Prius had less than a quarter wear at 60k when I sold it, and my current Gen 4 at 37k shows the same very low wear.  I try to plan ahead to get the best regen energy recovery, as well as smooth and comfortable progress.  Since Gen 2, cars have had a Hybrid System Indication which I use to good efect (plus in the Gen 3/4 Prius the HSI is visible in the Head Up Display).

I know another owner whose brakes were fine at 100,000 on a Gen 2 when he sold it (although his was garaged, unlike mine).

My last Gen 1 Prius had minimal wear showing as the car reached 70k, but then for a year I used a company Prius and mine only got driven once very week or two, and then usually for short journeys.  As Scott said, the rust was the killer, and the brakes were very noisy with grinding sounds and all the discs/pads needed replacing within the next 10k miles.  The car was then used daily until I sold it at 9 years old with 163k and the discs/pads were still well under half used.  This car was not garaged either.

From 2007-2011 I was manager of a fleet of Prius London minicabs (about 40 cars when I started and nearly 300 when I left), and some (but my no means all) drivers managed to wear out their brakes from using them enough in slow traffic (earlier generations only used the disc brakes below 7 mph, later versions from 5 mph) and hard enough above that to use the disc brakes as well as the regen.

[FROSTYBALLS]

4 hours ago, Mick F said:

Our car is the first one ever that it's the rear wheels that get dirtier than the fronts.  "Normal" cars have loads of brake dust on the front wheels and ours never has.  

Both of my petrol Auris had the same from new - rear wheels accumulating more brake dust than the fronts.

[Mick F]

Just been checking ...................

No brake dust on our rear wheels, and I can't remember seeing any either.  The dirt is road dirt, not brake dust.

We live in a steep valley, and a hilly area too, and brakes - or should I say breaking - is/are used frequently just getting out and about.  We use the B gear coming down the hills many times too.

Mick.

[PeteB]

Be aware you don't have to use B mode, but if it helps to maintain control then of course that's fine.

Using baking alone recovers more energy unless:

1. the hill is too steep to hold speed with regenerative braking alone, or

2. the HV Battery has 'maxed out ' and won't take any more energy.  Note: this occurs quite a while after the 8th bar on the Battery guage has lit.

In the latter case, I'm told more recent versions of the Hybrid systems (from 2009 Gen 3 Prius onwards) automatically enter something like B mode when the Battery has maxed whether it's selected or not.

[Mick F]

We live on a 25% hill maybe half a mile long to the entrance to our drive. We could brake all the way down, but in a manual normally-engined car, we would be in 2nd gear anyway.  Therefore B is a suitable alternative for a Yaris Hybrid.

Why should we require more energy?  We always arrive home with all eight bars lit anyway, and these days I rarely bother checking as it was always the same .................. a full Battery.

Mick.

[PeteB]

Like I say, 8 bars lit doesn't mean full - at least, not straight away.  Maybe the Yaris is different, but on Prius 1.5 & 1.8 Hybrids I've driven, the very few times I've experienced a truly maxed Battery is after more like 3-4 miles on a 20% gradient.

I never experienced it at all for over 3 years and some 70,000 miles until a trip to Scotland.  Once the Battery maxes out, on hitting level ground it behaves like a true EV as the system tries very hard to make some room for more regenerated power.  

I wish I could find it now, but some years ago someone published a great colour diagram showing the charge levels for each bar - there was some overlap, and the gauge would change at a different point depending on whether the SoC (State of Charge) was increasing or decreasing, and not all bars cover the same range.  IIRC, bars 2 and 8 cover a bigger band than any others.

[TonyHSD]

Hi all, brake by wire are only rear axle brakes on Toyota hybrids, fronts are conventional like any other car. In many cases rear brakes are applied more than the front one, when braking and car decides how to distribute stopping power between regeneration and brake force the latest is first applied to the rear wheels and than to the front so brake forces can be distributed as in a normal vehicle. I personally can sense all this process when is happening during braking and how the car behave. My car has rear brakes both discs and pads worn at higher rate than the front one, I believe others can notice the same and that’s why it’s normal to see more brake dust on the rear wheels. 

Regards 

[HughA]

I think you have that the wrong way round! Regenerative braking can only work on the front as that's where the motor/generator is connected. Rear brakes are conventional only but probably don't operate much during regenerative braking.

[CPN]

Definitely does have it the wrong way around. (the rear brakes only start to operate (while light braking) once the vehicle speed has dropped under 9 mph or alternatively if heavy braking is applied)

[martswain]

I had my Yaris hybrid for 4 years, rear disks replaced due to corrosion after about 10 months.

What accumulates around the rear wheels is rust-dust rather than anything from the pads !

The brake system was checked for balance and the situation improved slightly after a bleed of the system.

As the handbrake actually uses the rear pads, to keep the disks relatively rust-free I used a couple of seconds of the handbrake every now and then when I noticed a build up of rust.

[Anthony Poli]

I agree it is more rust that is the dust.
I do travel regularly on the motorway and these days the braking on the slip road is enough to keep the rust at bay.


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[PeteB]

On 5/13/2019 at 8:56 PM, PeteB said:

...I wish I could find it now, but some years ago someone published a great colour diagram showing the charge levels for each bar - there was some overlap, and the gauge would change at a different point depending on whether the SoC (State of Charge) was increasing or decreasing, and not all bars cover the same range.  IIRC, bars 2 and 8 cover a bigger band than any others.

Found one - this was based on reading from the Gen 2 Prius but the principal is similar for later generations.  (My recollection was wrong, it was bars 6 & 7!).