Brake pedal input and regenerative braking

[AMason86]

Hi all,

A question about braking. Specifically, what does pressing the brake pedal actually do.

Firstly, my understanding is based on the Tesla Model 3. In that car there are "Low" and "Normal" regen modes. As far as I know, pressing the brake pedal has no influence on how much regenerative braking is done in either mode. In "Normal" you seem to get a full hit of regen and this enables one-pedal driving. In "Low" you get less regen, but if you want to stop in reasonable time, you need to press the brakes and this will engage the friction brakes.

Granted, I saw this on a video of someone demonstrating just that behavior, but I know Tesla like to frequently change thing with OTA updates. But, given that understanding. How does it work in the Mach E?

In my Toyota Hybrid the system works always by blending the regen and friction brakes together (presumably for driver expectation and probably because the motors are not powerful enough to stop the car in good time).

I understand there are 3 drive modes and each provides different levels of regenerative braking when you come off the throttle.

Does pressing the brake pedal in any of the modes bring on more regen?

Is there any brake blending going on? i.e. mixing friction and regenerative brakes together by varying amounts to achieve deceleration?

Many thanks

Alex

Sponsored

 

[Mach-Lee]

Yes, it's a blended brake pedal system (with an electronic brake booster). Unless it's a panic stop, normally when you press the brake pedal it's just an input and not hydraulically linked. The hydraulic pump is controlled to vary the actual hydraulic pressure going out to the wheels. The car decides if it will use regen only, a mixture of regen and hydraulic, or hydraulic only. The amount of regen you get depends on the drive mode, I think this is true when using the brake pedal as well. Basically it applies the brakes for you in the best way (blended) based on your requested input with the pedal. You can vary the amount of regen with it.

 

[noway]

The drive mode will change how much regen and how quickly is being applied when releasing the accelerator pedal. Silent drive mode gives the least amount, all the way to 1 pedal driving which will give the full regen available regardless of which drive mode is selected. When in 1 pedal mode the maximum amount of regen is always applied when releasing accelerator pedal.

Since different drive modes, using "L" (B) mode and 1 pedal drive mode changes the maximum brake force applied by releasing accelerator pedal it also changes how much brake pedal pressure is required before friction brakes is used. In very slow speeds (around walking speed) the friction brakes is always used, so even in 1 pedal mode the friction brakes will still be applied when going to a full stop even when not touching the brake pedal.

This is all computer controlled, in normal operation the brake pedal is just a sensor. When the brake system is running it will disconnect the hydraulic connection between the brake pedal and the actual brake pistons and use both accelerator pedal and brake pedal as inputs to determine how much regen brakes to use and how much friction brakes to apply. The friction brakes as controlled by an electric pump with some solenoids that will apply hydraulic pressure individually to each wheel. This also means that, yes, the brake system is fully software controlled and can be updated OTA.

 

[AMason86]

Many thanks guys, it confirms what I have been thinking. Very helpful!

 

[Frankie]

When in 1 pedal mode the maximum amount of regen is always applied when releasing accelerator pedal.

This is true for Teslas (that max out at 60-75 kw of regen) but it's not true for Porsches (which max out near 200 kw IIRC), have you seen what you're saying documented for the Mach-E? Personally from what I have seen I would think it is not true, and releasing the brake pedal in 1 pedal mode results in ~75 kw of regen because any more and you're getting into braking thats so abrupt it causes traffic problems, but the Mach-E is actually capable of more regen, 100 kw+.

 

[Frankie]

I think this is true when using the brake pedal as well.

I wouldn't think this was true. If you want 75 (or 50, or 100, or whatever) kw of braking it seems like a waste to do it all via regen in one mode but partially friction in a different mode.

 

[noway]

I have not been able to get more than 70kW of braking, more than that and friction brakes are used. This is for sure a virtual limit based on what it can take in different conditions, and might even be lower at certain conditions. It might also be conditions where it can do more braking. I have not seen any difference in the maxium brake force from regen generated from releasing brake pedal in 1 pedal mode depending on drive mode, but there might be difference in the ramp up and pedal characteristics. I would think, speculating, that if it can do more than 70kW of braking force at specific conditions (for example at high speed, cold battery, low charge percentage, ..) it will still give consistent brake power from releasing accelerator and adding more regen braking before friction brakes kick in when pressing brake pedal instead. Having different characteristics on the accelerator would just result in unsafe and unpredictable situations, not knowing how much brake force it will use.

But, 1 pedal mode is not necessary designed to be the most energy efficient mode. Usually when pressing brake pedal in 1 pedal mode it means brake force is required fast, otherwise one would not do it, so it could still be optimized for safety by being able to immediately provide brake force from friction brakes anyway. Still speculations from my side, need to test that to see what it does.

 

[JamieGeek]

I'm guessing the brakes on the Mach-E work similarly to Ford's other cars with plugs. Probably very close to what the Focus Electric did which is described here:

 

[SnBGC]

I'm guessing the brakes on the Mach-E work similarly to Ford's other cars with plugs. Probably very close to what the Focus Electric did which is described here:

I think it's very similar still but perhaps a next generation design.
I don't see the elastomer spring so maybe they are using a torsion rod instead? Possibly up towards the pivot point of the pedal assembly?

There still is a push rod though. So, whoever said the system was 100% brake-by-wire isn't totally correct. Was that Sandy Munro?

Looks like an electrically boosted master cylinder (vs vacuum boost in gasoline vehicles or hydro boost in diesels...)

 

[bFWEAD]

The drive mode will change how much regen and how quickly is being applied when releasing the accelerator pedal. Silent drive mode gives the least amount, all the way to 1 pedal driving which will give the full regen available regardless of which drive mode is selected. When in 1 pedal mode the maximum amount of regen is always applied when releasing accelerator pedal.

Since different drive modes, using "L" (B) mode and 1 pedal drive mode changes the maximum brake force applied by releasing accelerator pedal it also changes how much brake pedal pressure is required before friction brakes is used. In very slow speeds (around walking speed) the friction brakes is always used, so even in 1 pedal mode the friction brakes will still be applied when going to a full stop even when not touching the brake pedal.

This is all computer controlled, in normal operation the brake pedal is just a sensor. When the brake system is running it will disconnect the hydraulic connection between the brake pedal and the actual brake pistons and use both accelerator pedal and brake pedal as inputs to determine how much regen brakes to use and how much friction brakes to apply. The friction brakes as controlled by an electric pump with some solenoids that will apply hydraulic pressure individually to each wheel. This also means that, yes, the brake system is fully software controlled and can be updated OTA.

Dumb question,, but when are the brake lights actuated? Do they illuminate when regen occurs?

 

[Murse-In-Airy]

Dumb question,, but when are the brake lights actuated? Do they illuminate when regen occurs?

There’s a limit. Not every time you let off the accelerator. But when you let off it quickly. The car senses how much slowing down its doing/ how many negative g’s are being applied. In my simplified mind, there’s a mercury switch like an old thermostat. When enough mercury sloshes forward it makes the contact and the lights come on. A slow deceleration doesn’t slosh the mercury enough to complete the circuit.

I know. I know. This isn’t the way it actually works. But if you go for a drive at night and watch your mirrors, it’s easy to see when the brake lights come on. It’s not EVERY time your slowing down. But it is every time your slowing down quickly.

 

[breeves002]

When in 1 pedal mode the maximum amount of regen is always applied when releasing accelerator pedal.

I have found this to be false. I have only tested this one way, but in whisper mode on my extended range AWD it does about 70kW of regen when I let off the pedal, however if I add braking I see spikes up to about 90kW briefly. I tested it several times like this and it was the same each time.

 

[BMT1071]

I think it's very similar still but perhaps a next generation design.
I don't see the elastomer spring so maybe they are using a torsion rod instead? Possibly up towards the pivot point of the pedal assembly?

There still is a push rod though. So, whoever said the system was 100% brake-by-wire isn't totally correct. Was that Sandy Munro?

Looks like an electrically boosted master cylinder (vs vacuum boost in gasoline vehicles or hydro boost in diesels...)

I don't think total brake by wire would meet FMVSS. Many vehicles with ADAS use braking systems similar to this. Minus the regen braking of course.

 

[Mach-Lee]

I think it's very similar still but perhaps a next generation design.
I don't see the elastomer spring so maybe they are using a torsion rod instead? Possibly up towards the pivot point of the pedal assembly?

I don't think there's a spring external to the HCU. There might be one on the input pushrod or internally. No brake-by-wire, more like indirect hydraulic with mechanical backup.

 

[SnBGC]

I don't think there's a spring external to the HCU. There might be one on the input pushrod or internally. No brake-by-wire, more like indirect hydraulic with mechanical backup.

That makes sense and would be a good place to put the spring......inside the HCU.

Sponsored