Tuesday, August 29, 2006

Recovering Energy

Reversing the acceleration profile is Not Smooth!

Let me explain. Part of the fun of how calculus, physics, and getting your driver's license all happen at roughly the same time in high school is how nicely these disciplines all mesh together.

F=ma explains so much, like how constant power becomes decreasing acceleration with speed. (And, coincidentally, how stating jet engine performance in terms of thrust instead of power explains their speed advantage over pistons and propellors.)

You can think of it many different ways, but the fact that kinetic energy goes as the square of speed means that the same acceleration takes more power at higher speed. This is why you never feel like you have enough power*.

Similarly, the amount of energy dissipated by constant deceleration goes linearly with speed. Gears mean nothing in braking - you get the same deceleration from your brakes regardless of speed, but they heat up faster if you are braking from speed.

Woa, woa, woa.. gears mean nothing unless you want to recover energy instead of dissipating it! Hybrids use regenerative braking to recharge their batteries. And just as there is a limit to how much power they can add for acceleration, there is a limit to how much power they can absorb.

Which means that just lifting off the gas on the freeway pegs the regen meter .. and just touching the brakes at 30 MPH pegs the regen meter .. and you basically have to stand on the brakes at 5 MPH to register at all.

Oops. This is not the way I or any passenger wants to be driven around. Have you ever been in a car with someone who saves their hardest braking for the end? Not Fun (and very disconcerting on the track, I might add).

So now I'm wondering how many hybrid drivers are going to let their driving habits be governed by the energy gauges in their cars? Kind of a scary thought..

* "When you can lay two black stripes between the exit of one corner and the entrance of the next, then you have enough power" - Mark Donohue


Blogger Simon Quellen Field said...

This is why people are looking into putting big capacitors in parallel with the battery.

Not only do you get more immediate current for acceleration, but you can quickly dump energy into the capacitor during deceleration.

2:40 PM  

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