Engine Braking

[ThrillerAuthor]

The owner's manual says that you can put the shift into Low at any speed, and the electric motor regenerates when the car slows. With an internal combustion engine, most sources say that it's better to wear out the brakes on hills than use the engine to brake. What's the consensus on using Low gear to brake with the electric motor? Is it bad for the motor?

 

[jmueller065]

Whenever you press the brake pedal in the FFE you're using the motor for a brake. It does the exact opposite of powering the wheels: Using the motor to generate electricity to put back into the battery.

Putting the car in L just regen's more than normal. Even in D if you just take your foot off the accelerator you're using the motor to brake (you see the circle spinny thing on the battery--unless you've turned that off).

Thus putting the trans in L doesn't wear anything out any more (actually probably a little less as more regen means that it will use the conventional brakes less).

Note that you'll probably never have to replace the brake pads as the FFE only ever uses them below about 5 mph or so (unless you frequently panic stop).

 

[michael]

Whether you get your regeneration by putting the car in L or by applying the brake pedal is basically a matter of personal preference.

I prefer leaving it in D, giving the car the feel of an ICE with automatic transmission, but some prefer the "one pedal" approach.

 

[WattsUp]

What's the consensus on using Low gear to brake with the electric motor? Is it bad for the motor?

No, this is like asking if it is bad for a generator to generate.

None of what you know about ICE vehicles applies to EVs. There are no "gears" in the FFE (meaning, like a traditional ICE drivetrain). The "L" position on the "gear shift" is purely a software setting that controls how much electric resistance the motor has to being turned (like a generator). It is not bad for the car in the least.

 

[kalel14]

But would it make any difference? If your braking score is 98% without messing with the gearshift, why bother? There is only a fixed amount of kinetic energy to recapture as the car slows. At 98%, you've got about all you can. The drive train has re-absorbed almost all the energy pushing the car forward, and you barely used conventional brakes. Dropping to low can't summon more energy, it just might absorb it at a different rate, although it would make sense that braking programming would override "gearing" programming since you stopped adding forward momentum.

 

[jmueller065]

At 98%, you've got about all you can. The drive train has re-absorbed almost all the energy pushing the car forward, and you barely used conventional brakes.

Note quite. The % value regen does not indicate a % of all the energy in the system. e.g. a 100% brake score doesn't mean that all of your energy used accelerating was put back into the battery (minus losses).

The software is telling you a % of the regen it is programmed to use. This is nowhere near what it could. In theory the car should be able to brake as hard as it can accelerate (and thus put as much power back into the battery as possible) but it doesn't. The actual value put back into the battery is some fraction of that.

Thus getting a 100% brake score does not mean exactly what you think it means; it simply means you drove the car as close as you possibly could to what the software algorithms want you to drive; nothing more.

 

[kalel14]

I'm saying the car has X amount of kinetic energy as it is moving forward. To stop, all that energy has to be dissipated. You have wind resistance, gravity, tire adhesion, etc., conventional brakes, and drive train braking. Assuming for the sake of argument that no conventional braking takes place, and all the smaller parasitic sources are constant, all the remaining momentum of the car has to be absorbed by the drive train. Changing the way the drive train absorbs the energy might make the car slow faster or slower, but it can't change the amount of energy the system absorbs. It doesn't have anywhere else to go.

 

[jmueller065]

I'm saying the car has X amount of kinetic energy as it is moving forward. To stop, all that energy has to be dissipated. You have wind resistance, gravity, tire adhesion, etc., conventional brakes, and drive train braking. Assuming for the sake of argument that no conventional braking takes place, and all the smaller parasitic sources are constant, all the remaining momentum of the car has to be absorbed by the drive train. Changing the way the drive train absorbs the energy might make the car slow faster or slower, but it can't change the amount of energy the system absorbs. It doesn't have anywhere else to go.

True but the friction brakes are used more than you think.

 

[Foulwind]

So the FFE (My car) only uses the front wheels for brake regen?.
I have a few hills in my commute, and living in a Redwood forest, (Santa Cruz Mountains, CA.) I have to climb out of a small valley then go up over HWY17 8 miles up, 8 miles down from the summit, (Commute to work) then same commute home only the hills are at the end of my commute. If, when coasting to a stop in traffic can I get a little more regen by shifting into "L" and on the down slope using the "L"?

 

[kalel14]

I'm going to guess that if you already have a high braking score, that indicates that you are not using the conventional brakes, and you are already recovering virtually all the stored energy from the car's momentum.

 

[WattsUp]

If, when coasting to a stop in traffic can I get a little more regen by shifting into "L" and on the down slope using the "L"?

You get no more regen than if you manually used the brake pedal to accomplish the exact same level of motor resistance. It's all the same thing.

The brake pedal in the FFE simply modulates the level of motor resistance until you "request" more braking than can be accomplished that way, in which case the mechanical brakes are also engaged. This is a nice user-interface, as it helps blur the difference between the two types of braking.

So, again, shifting to L is just like pressing the brake pedal a little bit so there is a little bit more motor resistance. It does't make the regen "better".

 

[oilerlord]

I'm going to guess that if you already have a high braking score, that indicates that you are not using the conventional brakes, and you are already recovering virtually all the stored energy from the car's momentum.

Not even close.

Even though the brake coach will display a gratifying "100%" regen score, only a small amount of the energy that went into getting the vehicle moving actually returns back into the battery from regenerative braking. If you did recover "virtually all of the stored energy from the car's momentum", you would be driving a near-perpetual motion machine. Friction losses and inefficiencies in the conversion of kinetic to stored energy still waste most of the energy you used to get the vehicle in motion.

Driving for best efficiency is about preserving momentum and using brakes (regenerative or otherwise) as little as possible.