Brake pedal effect on regen in various vehicles?

[NeilBlanchard]

Ask any ecodriver / hypermiler. Ask Wayne Gerdes, who coined the word hypermiler. Every person on EcoModder.com learns this. With my last ICE car, I averaged ALL YEAR, for 7 years 50%+ over EPA Combined. In my EV's, I have averaged about 5.5 miles / kWh over the whole year. (With our Leaf and Bolt EV, I shift into neutral.)

It is physics: the moving car has stored kinetic energy. All the losses are always there. Comparing coasting to regen, you can do a simple thought experiment - coasting uses the kinetic energy to carry the car forward. The only losses are there no matter what.

If you use regen instead, you lose the portion of energy that regen cannot get back - the motor and charging electronics and battery cells all lose energy. So, by definition regen will lose more energy than coasting.

Rolling resistance and aerodynamic drag are the two main losses. Drivetrain losses are primary, and EV's are the best efficiency drivetrains.

Coasting has an even bigger advantage - you accelerate for less time. Again, a thought experiment:

Accelerate to a certain speed ...

... With a car that cannot coast, you have to still add energy (accelerate gently) all the way up until the time that you need to slow the car. Then use regen to slow the car, and regain some of the energy.

... With a car that coasts by default, you can use the kinetic energy to carry the car forward for some of the time, and you then can add energy only when needed. You can anticipate the stop, and so you can coast while you need to slow the car - and then use regen only when you need to stop the car, and regain some of the energy.

Same distance, very similar average speed - coasting uses significantly less energy because of less acceleration, and less regen needed - so less energy lost.

And, if the stop is for a traffic light, you can coast while you watch the light cycle, and you might be able to time things so that you never have to stop the car - and you then do not have to use very much energy to get the car back up to speed.

 

[Pushmi-Pullyu]

Neil, do you know of any published studies that looks at this? I've heard it alleged, but many people believe differently. I'd like to see some data.

Lots of discussion on the Tesla Motors Club forum; for example, this thread.

I don't think there is any reasonable doubt that in general, coasting is more energy efficient than regenerative braking. It's basic physics: No reaction is 100% efficient, and as I recall, real-world testing has shown regen is only about 35% efficient in terms of round-trip energy efficiency. ("Round trip" in this case means first expending energy to accelerate the car, then recapturing a fraction of that energy with regenerative braking.)

But obviously it depends on circumstances. If you are driving down a mountain, or approaching a stop sign or stop light, then obviously you'll want to use regen. Coasting until the last second and slamming on the brakes, when approaching a stop sign or stop light, will reduce the amount of regen you can capture, because under heavy braking some braking force will be friction brakes. With light braking (or, depending on which EV you're driving, regen using "one pedal" driving), all the braking force will come from regen, which maximizes energy efficiency.

Hopefully there is no question that regen is more energy efficient than friction braking? With regen you're recapturing about 35% of the energy used to accelerate the car; with friction braking, you're recapturing 0%.

EDIT: I started writing this before NeilBlanchard posted his comment above. Apologies for repeating much of what he already said, and he said it in greater detail! Thanks, Neil.
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[NeilBlanchard]

To put rough estimates on a hypothetical drive:

Without coasting: accelerate hard for 20% of the time, then accelerate gently for 70%, and then regen for the last 10%. 20 / 70 / 10

With coasting: accelerate hard for 20%, coast 10%, accelerate gently for 25%, coast for 10%, accelerate gently for 20%, coast for 10%, and then regen for the last 5%. 20 / 45 / 30 / 5

When you coast, it really helps to have low rolling resistance tires. Once you make a habit of coasting, you also notice that cooler air temperatures keep you from coasting as well - increased aero drag. So, when you don't coast, you use more energy then, as well.

 

[Martin Williams]

Regenerative braking may help a little, but I doubt if you'd notice if it stopped working.

 

[NeilBlanchard]

No, it would be obvious. It would be very hard for it to stop working - you would stop being able to drive the car; because it uses the drivetrain.

 

[Kendalf]

Accelerate to a certain speed ...

... With a car that cannot coast, you have to still add energy (accelerate gently) all the way up until the time that you need to slow the car. Then use regen to slow the car, and regain some of the energy.

... With a car that coasts by default, you can use the kinetic energy to carry the car forward for some of the time, and you then can add energy only when needed. You can anticipate the stop, and so you can coast while you need to slow the car - and then use regen only when you need to stop the car, and regain some of the energy.

I agree with your thought experiment. Just want to indicate that for cars that have heavy regen by default when you lift off the accelerator (one pedal operation) you can "coast" by feathering the throttle so that the regen doesn't kick in, nor are you accelerating.

 

[Martin Williams]

No, it would be obvious. It would be very hard for it to stop working - you would stop being able to drive the car; because it uses the drivetrain.

Point taken! Mea Culpa. I should have been clearer. You'd certainly notice its effect on the braking of the car! And it would certainly hard to imagine a failure mode that was unnoticeable in that way too. What I tried to say was you'd hardly notice its effect on the mileage if - say - the energy recovered were just thrown away instead of going back into the battery.

I think the London Underground uses regenerative braking, and has done since the year dot. It runs on 600v DC, and I guess the mass of a train makes the kinetic energy worth saving. I don't know the details, but I imagine power from braking come from the motors acting as generators and is simply fed into the power rail where other trains can use it. (The KE of an 8 coach London underground train doing 30 mph is 6kWh. At 60, its 24kWh, and they do a lot of stopping and starting.)

 

[NeilBlanchard]

I agree with your thought experiment. Just want to indicate that for cars that have heavy regen by default when you lift off the accelerator (one pedal operation) you can "coast" by feathering the throttle so that the regen doesn't kick in, nor are you accelerating.

Right - the i3 is really the only one that does this very well. It is a lot harder, and less consistent to coast like this. And, you have to hold your foot on the accelerator pedal all the time you're driving - so no resting. This can lead to sore muscles.

 

[jdbob]

They have to do a lot of coasting in Formula E. When they let off the throttle the car coasts and when they get near the corner there are paddles they use to activate regeneration. If needed then they can also apply the friction brakes.