Wednesday, November 24, 2010

On Regenerative Braking and Coasting

Having to hold the accelerator pedal exactly in one position to be able to coast is difficult and tricky, and should not have to be learned.  Driving long distance like this is not good for efficiency, or for the leg muscles.  I do a lot of ecodriving, and coasting is by far the most efficient way to roll -- in an EV, you would use zero energy, and reclaim the potential energy directly.

If you have to use the brakes, then you have accelerated too much.  Regenerative brakes should only be used to slow the car in unanticipated situations, and at the last moments to come to a stop.

Not only is ecodriving much more efficient, but it also helps to improve traffic flow.  The worst thing for traffic flow is also the least efficient way to drive: accelerate hard and then brake hard.  This sets up lots of oscillations in the traffic flow, which causes many drivers to apply their brakes for no apparent reason.

Smooth predictable driving results in smooth and predictable traffic flow, and it is the most efficient way to drive.

Heated brakes are to be avoided, and having hot brakes for normal driving is the clearest indicator that the driver can improve their efficiency.

Coasting uses the weight and momentum of the car in the best way possible.  So, it makes sense that making it as easy as possible to coast -- by just lifting your right foot completely off the accelerator to coast will predictably; and by the way, provide a couple of moments to relax the muscles in the driver's leg, too.

All the braking should be engaged by the brake pedal; pure and simple.  On an EV, all the regenerative braking should be used to regain as much of the energy as possible -- but this is less efficient than coasting, by definition; so it should not be the way you drive to maximize range on an EV.  So, as much braking as possible should come from regeneration, and the engineers need to integrate the hydraulic brakes to provide emergency braking and stop the car at slow speeds; when regen cannot.

Yes, the data is out there -- how do you think a Honda CRX HF gets 118MPG?

Coasting is better for several reasons:

When you coast you are getting most of the energy that it took to accelerate back; as you only lose from aero and rolling drag.

With regenerative braking, you lose the aero and rolling drag AND from the losses of the generator/charger/batteries, too.

More importantly, in many situations if you cannot coast easily -- it is too easy to accelerate and then immediately brake.  So, you over accelerate and then have to over brake.

Think about it: there are three possible modes of driving, right?

1) Accelerating
2) Coasting
3) Decelerating

Accelerating uses energy, depending on the weight of the car, the steepness of the grade, and the rate of acceleration.

Coasting uses no added energy, and it uses the accumulated momentum / kinetic energy gained by the acceleration.  It only loses energy to aerodynamic and rolling drag.

Decelerating loses energy to energy to aerodynamic and rolling drag and either to losses in the regen, and/or converting kinetic energy to heat in the brakes.

To be the most efficient, we need to minimize the energy it takes to accelerate and the energy lost through braking, and we need the car to lose a minimum amount of kinetic energy by being as low aerodynamic and rolling drag as possible.

To cover the most distance with the least energy, we need to accelerate up to a speed that will then allow the car to coast as close to the end as possible, and then use regen to regain some of the remaining kinetic energy.  The brakes needs to stay as cool as possible.

Of course, cruising longer distances and/or up hills requires some additional acceleration; either to maintain a constant speed, or to climb a hill / slope.  You can do pulse and glide instead of constant acceleration (using the terrain as possible) and climbing hills well requires what I call "swooping".  This involves accelerating ahead of the uphill slope (when gaining speed takes less energy) and then use this to help carry speed up the hill.  Think how a bicyclist would climb a hill, and you'll understand.

Coasting downhill is a no-brainer, and it certainly is easier to do this when you don't have to constantly fine tune your foot on the accelerator pedal.  If you go too fast, then use the regenerative brakes, on the brake pedal!  And prepare to "swoop" if there is an uphill.

If coasting is the most efficient way to cover distance, then it should be the easiest mode to achieve; not the hardest.  If all the regenerative braking is integrated into the brake pedal, and lifting your right foot off the accelerator lets you free-wheel coast -- then you will quickly learn how to maximize the time spent coasting.  You will learn the dynamics of your car, on the routes you routinely drive, and you will maximize your range / efficiency; ICE or EV.

Sincerely, Neil


  1. Certainly coasting is more efficient than braking. However if you are constantly having to play with the accelerator pedal in order to coast the answer isn't putting regen on the brake pedal, it's increasing the neutral zone in the throttle map. There should be a reasonable amount of pedal throw between accel and regen which should allow you to coast when possible. I much prefer doing most of my driving with a single pedal.

  2. Sure, but a small advantage of having all the regen on the brake pedal is that you can rest your leg during long coasts. You use muscles in your leg even to hold the pedal just so...

    The other thing that is at odds with putting regen on the accelerator, is if they put "automatic creep" in the programming, to simulate an automatic transmission on an ICE car, then as you push the accelerator down, you get creep first, then regen, then neutral, then acceleration... Talk about complicated and confusing!

    I say keep it simple and predictable.

    Sincerely, Neil

  3. Creep is another matter, I'm not in favor of creep at all. However I think Tesla has creep in the Roadster as well as regen on the A-pedal and it doesn't seem to be a problem. Foot fatigue can be handled with cruise control much better than regen on the brake pedal. Long periods of driving often don't require any use of the brakes.

  4. I hope that all EV's allow for an easy / default way to do freewheel coasting. Many of the EV's we have seen so far do not have this; and they seem to think that lots of regenerative braking on the accelerator pedal is the best way to go.

    I've heard accounts of people driving the Mini E for the first time, and practically knock their heads on the windshield, it is so abrupt. And anything that makes it too hard to coast is going to get less range than it would if it is easy to coast.

    Lots of regen on the accelerator "encourages" an inefficient style of driving; and the range will suffer.

    Sincerely, Neil

  5. I've also heard that after they get used to the strong regen they love it. I know the Tesla crowd loves single pedal driving, as do I. Efficient driving has to be learned no matter where you put the regen. I love my A-pedal regen though I would like a larger neutral pedal area. I'm hoping to be able to program my controller to give me that at some point.

  6. Well, maybe for performance driving; sure. That's what race car drivers do: accelerate and then brake. But for ecodriving, coasting is best, so how ever this is done most easily and consistently.


  7. Neil,

    I agree accelerator pedal is for accelerating, no pedal is for coasting and brake pedal is for braking. I believe that strong but proportional regen should be applied with the brake pedal and friction braking should be reduced by getting the pedal to apply full regen before the pads engage. I also like the idea of a third control for EV's. Some sort of hand control that is a small lever that springs back to zero, but can be moved to allow for the application of regen only. This control can be used for the controlled application of the brakes in a controlled braking situation. For instance if you are going down a steep hill, you could use this to scrub off some speed and insure that you are only using regen to reduce speed. It can also be used to slow down when you see a traffic light ahead and you know you need to loose some speed in order to reach the light when it turns green, but again this lever is easier to used than with your leg and it insures that friction braking is not applied.

    Nice article. I agree with everything except that there is a difference between ecodriving and hypermiling an ICE. I like your description of ecodriving and I agree that it does smooth out the traffic, but hypermiling an ICE is terrible for the traffic flow. Ecodriving is all that is needed for an EV.

  8. Thanks for your comments, Mike. In my mind, ecodriving is the same as hypermiling. Extreme P&G with engine off coasting) is a problem in traffic, but coasting in neutral is much easier to fit into the natural traffic flow, in my opinion.

    Using the kinetic energy of any vehicle, and especially an EV which tend to be heavier than an otherwise similar ICE vehicle, is the most efficient way to "regain" the energy is to free wheel coast. Regenerative braking is the next best way.

    This points to all EV's to be front wheel drive (or all wheel drive). The FVT eVaro has the best implementation of coasting and regenerative braking that I know of. They only use the friction brakes (which are inboard, by the way!) in emergencies, and to hold it in place when stopped.

    Sincerely, Neil

  9. I'm happy with what works easily and predictably -- my ecodriving habits are based on driving a manual shift car, so I may be expecting something different than someone who has always driven an automatic.

    Another good point about coasting when you take your foot off of the pedals, is that on long drives on the highway, you can rest your muscles and/or stretch or move your leg around a bit while coasting. If you have to hold your foot in this coasting "detent" then you cannot do this; though maybe putting it into neutral would be possible.