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