Woodrow

I hope this is an appropriate forum for this question!

I've been reading about regenerative braking, because there seems to be a few different ideas about how it works. Based on the majority of the articles I read and the videos I watched, (one notable exception was a Toyota mechanic, but I think he confused himself) I've decided that regen braking has fairly little to do with the standard friction brakes. What seems to happen is that one of the electric motors(MG2?) is reversed, turning it into a generator. The friction (resistance?) to the drive-train caused by what is now a generator, ends up slowing the vehicle. I would welcome comments on this part if I'm off base.

What I don't understand, is that when I'm "coasting", the indicator in my dash tells me the batteries are being charged. However when I actually touch the brakes, it indicates much more charging is occurring. I don't understand why, unless it's in the electronics. My idea is that when you touch the brakes, the electronics tell the motor/generator to significantly increase the electricity production, thereby increasing the drag to the drive-line and aiding the friction brakes. This give the appearance that the brakes themselves actually create electricity, but I can't see how that might happen. The aforementioned Toyota mechanic said that it recaptures the heat energy created by the brakes, and converts it back to electricity, but I don't believe him.

Is anyone able to help me get this straight in my head?

Droid13

You are pretty much bang on. The MG2 (and MG3 for AWD) becomes a generator to charge the battery and the resulting drag slows the vehicle down (hence the "G" in MG2). I had a good chuckle about the comment about it using the heat from the normal brakes, Ha, NO, the mechanical brakes play no role whatsoever in producing electricity.

While coasting the regenerative braking is active as you state. The RX450H is actually very aggressive at regenerative braking while coasting. I traded in a Hybrid Camry and it was no where near as aggressive, first time driving the RX in comparison felt like I had my foot on the brake. If you slightly hold your foot on the gas pedal while coasting, you can actually eliminate the regenerative braking while coasting (called "gliding" by hybriteers) but still not providing forward drive.

And as you step on the brake pedal, the amount of regenerative braking increases (which will be assisted by the mechanical brakes anywhere from 0% to 100% depending on the circumstances. Those circumstances are how fast you are stopping, if the ABS is activating due to slippery conditions, etc. This has been an area of contention on Toyota Hybrids, sometimes during regen braking if ABS activates during regen, regen is reduced mechanical braking is activated instead, and sometimes there is a split second delay between the two which can be unnerving to some drivers because it feels like the car lurches forward (when in fact momentarily it is not braking).

So, as you can see, the gas pedal and the brake pedal are not gas pedals and brake pedals. What they do, all depends. The gas pedal can ease off regen braking and allow it to "truly" coast/glide, and the brake pedal will control both regen and mechanical braking.

kitlz

Thanks for the explanation Andy. On one of the straight hills I have to drive with a 25 MPH speed limit. I barely touch the brakes or gas pedal for close to a mile. The 450h only gains minimal speed. When I approach a plateau-like area of the road with railroad tracks, I brake lightly and release. The speed climbs more dramatically. It is a bit of a steeper but I notice a difference in the way it feels, almost like releasing the parking brake after starting to drive. Not quite that strong but you get the idea.

NateJG

You mean that our cars don't have infrared thermal sensors in the rotors to capture the heat of friction and send it as an electric signal to charge the battery (If someone now patents that idea I'll slit my wrists)?!?

mcomer

Nice thread, you both have the hybrid activities well understood and stated.

Woodrow

Thank you, and I really appreciate the help! I'm often amazed at the technical expertise available on this forum!

vlad_a

Here's something fun to try. When going slowly, (less than 10MPH), start braking. Put the RXh in N. Electric motors will disengage from regenerative mode and all you will be left with is conventional brakes. That will put things into perspective.
The brakes we have are a hybrid type. Both conventional and regenerative are in use at the same time. Regenerative are dependent on the readings in the brake line pressure.

Now, Tesla on the other hand, decided not to go this way. The regenerative braking occurs only when you let go of the gas pedal. In other words, you can drive a Tesla around by mostly using one pedal. The brakes themselves are independent and don't affect regenerative cycle.

As far as brake heat generating electricity, this is something from the BMW book. I attribute it to German over-engineering.

wlhg

Does simply taking your foot off the accelerator cause regenerative breaking. Is it proportional to how much you ease off on the accelerator or is it an all or nothing situation. I understand that applying the brakes further increases regenerative braking and that hard breaking will finally engage the disks.

salimshah

Just have you passenger note the charge meter while you look at the road ahead and brake as needed.

Observervation is when the foot is taken off the gas pedal in 'D' the regeneration process happens. You get additional slowdown. When you apply brakes additional level of kinetic energy (vehicle forward motion) is transferred [the meter maxes out]

More severe situation is when the driver applies more foot pressure on brakes to halt the vehicle then actual brake-pads - heat etc come in the play.

Since I am a recent convert to hybrid, I an still adapting to the different system. To the annoyance of my family, I had developed the good practice of applying brakes and letting the pressure off [modulating the brakes to allow cooling] and that does not work well with hybrids. Here a long extended gentle pressure on brakes is better to dump kinetic energy to regenerate dc voltage.

How much of the Gas engine energy is diverted to the battery is based on the complex ecu function and battery status.

In reverse:
I have not looked at the charge gauge.

Salim

MikeInOr

Yes, taking your foot off the accelerator causes regeneration and a bit of drag that slows the car slightly. With a light touch on the accelerator you can control how much regeneration and drag the the electronics will impose.

There is a long gradual hill on my route home from work. If I take my foot completely off the accelerator the car will slow down. I have learned how much pressure (it is just a little pressure) on the accelerator I need to maintain my speed without accelerating or decelerating. Even though my foot is applying a bit of pressure on the accelerator to keep the car at a steady speed the car is in regeneration mode, the engine is off and the battery is charging while going down the hill.

F250

I, too, am still getting used to the regen brakes on our 450h we bought a year ago. It's taking me much longer to adapt to them because I don;t even drive the vehicle every week (it's my wife's vehicle). One thing I have learned thus far, though, is that when we take off from the driveway with the brake system cold, the first two stops create quite a last minute "grab" on the rotors when coming down to a complete stop... UNLESS I back off the pedal pressure quite a bit right as the vehicle gets to a crawl speed. Once the first two stops are completed, the grip of the pads on the rotors is much less severe and I can continue with my typical smooth stops without jerking the passengers around at the end of the stop. Personally, too, I would prefer that the regen brakes only engage when the brake pedal is pressed so that I could coast down hills with no "engine braking", but that's just my own personal preference and driving style, and I certainly am not implying that my approach would be more efficient by any means.

salimshah

Few additional details:

For regenerative braking we are converting the kinetic energy into electrical (generator function). When the kinetic energy is high we can rob (really recover) more of it and as the speed slows down there is very little to be robbed. [The spin rate of the generator goes down to a point we can not generate and thus can not brake. Taking a guess, that number will be below 10mph. Lets say it is 7mph and so you need hydraulic brakes to slow down below that speed and the last bit of halt. Nothing says you can not use hydraulic brake all the time ,, but that is where the controller decides based on the programming.

Regeneration during coasting:
My guess again is that if the recovery is only done during braking, you will most likely not generate enough [unless the vehicle is driven by my mother in law ,,, it is gas or brakes for her ]. Normally when you coast, the vehicle slows down due to drag or you get engine braking on a down wards slope. Small energy is recovered but the time duration is much longer.

Salim

F250

Though I understand the concept behind the thought, there is simply no way this is a surface corrosion issue. The problem with surface corrosion on the first couple of stops typically occurs when a vehicle is left unused for long periods of time (generally greater than a week or two), and is typically highlighted by the brakes actually having slightly LESS grab (and a scraping noise) until the corrosion particles are physically scraped off the rotor or drum surface so that the pads/shoes can get a more consistent bite. I do not have any of the scraping noise associated with surface corrosion related cold braking.

The last second "grab" I'm experiencing is quite sudden, and although I can sense when that same grab action is initiated after the first two stops, it's not nearly as aggressive in its "grip" even though the overall stopping power remains intact. It has to be a situation where the specific OEM pad chemistry simply grips more effectively until the pads heat up. It's a known fact that most street pads lose some of their grip strength as they get hot, anyway.

Back to the outside-vs-inside thing. Our Lexus is kept in a completely closed garage which generally runs around 76°F, and since my main floor's HVAC unit is in the garage, the space is essentially under complete climate control and is drier (humidity-wise) than outside conditions. That said, I've never experienced this kind of sudden and last second "grab" on any of my other vehicles, all of which either are or have been stored outside. Over the years, that has been equally true for my Ford Superduty vehicles, a VW Jetta TDI, multiple BMW's, multiple Infiniti's, multiple Nissan's, multiple Toyota's, a Honda, a Volvo, and a Subaru -- not to mention that I also have never experienced it with any of the rental vehicles I use in conjunction with my business travel.. Additionally, on those rare occasions where I had either one of my Superduty vehicles, a 4Runner, or the Volvo in the garage for extended periods during repair efforts, none have ever exhibited the "cold grab" condition. I should add that I also experience the "cold grab" in the Lexus even when on road trips where the Lexus is left in an outside parking lot overnight. Inside-vs-Outside just doesn't explain it.

That's why I credit the Lexus regenerative braking system as being the root cause of my "cold grab" condition -- it's the only "new and unique" variable in my equation.

salimshah

Just a thought and this may have little to do with regen/hybrid. There is a known phenomena, where a stamp of the brake pad is left on the disc. This causes grab action on those spots. Why this happens, is attributed to discs getting hot and pads remaining in contact as they cool. [pads do not move back in and after breaking ... only due to wheel spin the run out pushes them back].

You can confirm if that is your problem by taking the wheel off and either feeling the disc or changing your viewing angle to see a change in reflection [back side is hard].

If that is the problem how do you fix it. Turning the disc will work and a quick remedy is to burn it off and that requires sharp braking in a short space of time [heat up the disc]. That can be problematic in hybrid but a few emergency stops can possible work for you.

Salim

PS: After reading Andy's post (post #16), if the grab happens ONLY once on the start of the day then we can say it is surface rust/oxidation. If it happens repeatedly during the driving (same day) then imaging of the pad on the disc needs to investigated.

Salim

riredale

Salimshah, I think your family members have been correct in the past--you don't need to worry about cooling conventional disk brakes.

As you know their sole purpose is to convert kinetic energy into heat, and only when the automotive magazines do repeated panic stops from, say, 100mph do they start to see brake fade from excessive heat.

There are some good shots on the old "Top Gear" TV show showing exotic cars braking heavily on a turn. You can see the front disk rotors glowing a dull red.