Regen and friction brakes - do/how do they work together?

[Cocijo]

Interesting - how does the system know that the brake pedal has been pressed?

 

[Tim Green]

Because the rate of change of speed will have increased.(eg, decelerating faster).
I think you can see the incremental change in regen on the power meter when you press the brake pedal.
I suppose they may use the brake light switch as they already have that as an input to allow disengagement of the Park function.

 

[Deleted member 2439]

I suspect the regen level is increased when the brake pedal is used. However, it will not use the brake light switch, it will use the rate of chang of speed as measured by the speedo system. As this is all in software it is easy to do.

I’m not sure that logic stacks up, think about it. The rate of change of speed, ie deceleration, is very little before braking actually starts. The only way the car “knows” that you are wanting to slow at a grater rate than regen provides will be via the brake light switch being operated.

IMHO, there’s too much assumption in general on this thread (self included) - short of having an EV brake system specialist from MG available then I guess we will never know the actual facts.

 

[Cocijo]

I’ve requested it as a topic for next weeks podcast.

 

[Deleted member 2439]

I'm not incorrect;

How Regenerative Braking Works

Vehicles equipped with regenerative braking systems are able to recapture some of the vehicle's kinetic energy and convert it into electricity. This electricity is then used to charge the car's batteries. How is this possible?

auto.howstuffworks.com

I’m not sure that this reference is entirely credible - in the way in which the article is written. It’s inferring that the electric motor would be run backwards, er, no. In the study of electrical machines, i.e. electric motors, the transition from being under load (known as motoring), to being driven - is known as generating. The connections are exactly the same, the motor is NOT reversed and neither is the polarity of the power to it. However, it acts as a generator and slows down what’s driving it. That’s simply how it works and why the current display goes negative when you slow down - with that negative current charging the battery for the duration of slowing down.

 

[Deleted member 2439]

I’ve requested it as a topic for next weeks podcast.

I’ll listen with interest. It will be interesting to see exactly how the question is asked. ?

 

[Deleted member 3150]

Is there no senior trained MG mechanic on this forum who can definitively answer such questions? If not, is there anyone who could invite one please?

 

[Cocijo]

There is but he’s in hiding ?

 

[Tim Green]

There is a lot of confused info here, and to get an accurate answer would need a response from the actual design engineer. Lincs Robert is correct in general, but the detail is a lot more complicated due to the control electronics required to vary the power required from the motor. When in over-run (ie decelerating), the same control system must be used in reverse to feed power back to the battery. (This must be the case because Kers can be varied by the software). Thus the Kers value could be increased by the software when the vehicle is slowing faster than determined by the nominal Kers value, or it could be triggered by the brake pedal switch.

 

[JodyS21]

I'm not quite sure what we're talking about now lol!

The brake light according to the vehicle rules etc that someone found, said at which point the brake lights must illuminate:
In summary, when the deceleration of the car hits a set threshold the lights must come on (so will be based on the speed and how quick it's reducing and/or a g sensor).
On an old ICE car I imagine it's literally a contact switch that's made when the brake pedal is pressed.

The regen is a separate thing.
The mode setting determines how much regen the system asks for when you lift off the accelerator pedal.

In terms of the brake pedal, somehow the system determines how much regen to request and how much friction brakes to apply. I can't see any reason why the mode switch would alter this:
Whether the brake pedal is simply mechanically connected to the friction brakes as per how it's always been done, we don't know for sure.
Whether the brake pedal just requests regen based on pedal position, we don't know for sure.
Whether the regen system and friction brakes are independent systems both acting from the brake pedal, we don't know for sure.
Whether the brake pedal is all "brake by wire" and the system totally controls electronically when and by how much each of the regen and friction brakes are applied, we don't know for sure.
However it works; the car must be capable of pressuring the friction brakes itself - as it does brake itself, and there must be some kind of mechanical safety measure to apply the friction brakes from the pedal I'd have thought.

 

[Deleted member 2439]

Actually, we do know. There’s a brake servo - which is vacuum operated, with the vacuum coming from an electrically powered pump to replace the missing vacuum that would exist in an ICE car. This allows the brakes to operate, albeit without servo assistance in the event of a total power loss. You can hear the pump working and feel it through the brake pedal when you press that start button.

 

[JodyS21]

Actually, we do know. There’s a brake servo - which is vacuum operated, with the vacuum coming from an electrically powered pump to replace the missing vacuum that would exist in an ICE car. This allows the brakes to operate, albeit without servo assistance in the event of a total power loss. You can hear the pump working and feel it through the brake pedal when you press that start button.

That's how all cars that can operate the friction brake autonomously work isn't it? They use a pump that creates the equivalent of someone applying pressure from the brake pedal?

 

[Tim Green]

The normal procedure would be for the car to have a standard braking system: that is a hydraulic system with added servo operation to reduce pedal pressure. Any automatic system driven by software would be just added to the above system. Thus an electronically controlled valve would be added to the servo so that the radar driven automated system can control the brakes. The regen system is totally separate but also interactive as described earlier. I have the same system on my Jaguar XK8 (less the regen of course) and both systems work pretty well.

 

[Deleted member 2439]

The normal procedure would be for the car to have a standard braking system: that is a hydraulic system with added servo operation to reduce pedal pressure. Any automatic system driven by software would be just added to the above system. Thus an electronically controlled valve would be added to the servo so that the radar driven automated system can control the brakes. The regen system is totally separate but also interactive as described earlier. I have the same system on my Jaguar XK8 (less the regen of course) and both systems work pretty well.

It’s more likely that the ABS modulator block will apply the brakes as necessary as determined by whichever brain makes the decision. Think of the way the rear cross traffic alert works in practice. My Volvo XC60 had a collision avoidance system that worked that way.

 

[RossN]

And here for all these past years I thought it was done by very small dwarfs inside the wheels. You learn a lot on here

 

[Deleted member 2439]

And here for all these past years I thought it was done by very small dwarfs inside the wheels. You learn a lot on here

Indeed. It‘s interesting the way that safety features really seem to have become standard in cars whilst more fashionable items like a sunroof have become options.

 

[Kithmo]

And here for all these past years I thought it was done by very small dwarfs inside the wheels. You learn a lot on here

Hamsters actually.

 

[Gomev]

I'm not quite sure what we're talking about now lol!

The brake light according to the vehicle rules etc that someone found, said at which point the brake lights must illuminate:
In summary, when the deceleration of the car hits a set threshold the lights must come on (so will be based on the speed and how quick it's reducing and/or a g sensor).
On an old ICE car I imagine it's literally a contact switch that's made when the brake pedal is pressed.

The regen is a separate thing.
The mode setting determines how much regen the system asks for when you lift off the accelerator pedal.

In terms of the brake pedal, somehow the system determines how much regen to request and how much friction brakes to apply. I can't see any reason why the mode switch would alter this:
Whether the brake pedal is simply mechanically connected to the friction brakes as per how it's always been done, we don't know for sure.
Whether the brake pedal just requests regen based on pedal position, we don't know for sure.
Whether the regen system and friction brakes are independent systems both acting from the brake pedal, we don't know for sure.
Whether the brake pedal is all "brake by wire" and the system totally controls electronically when and by how much each of the regen and friction brakes are applied, we don't know for sure.
However it works; the car must be capable of pressuring the friction brakes itself - as it does brake itself, and there must be some kind of mechanical safety measure to apply the friction brakes from the pedal I'd have thought.

The part about the deceleration and brake lights coming on is only for regen when not applying brakes, otherwise the lights come on when brake is applied, just like an ICE car.

 

[Deleted member 2439]

The part about the deceleration and brake lights coming on is only for regen when not applying brakes, otherwise the lights come on when brake is applied, just like an ICE car.

Er, but when “applying brakes”, the brake light will come on - if done by the drivers foot anyway. Also when emergency brake application is done by the car. I don’t think it will be done on any cruise control intervention though.

 

[Gomev]

Er, but when “applying brakes”, the brake light will come on - if done by the drivers foot anyway.

That's what I said