Monday, February 18, 2013

Flying the Prius

I've been driving a Toyota Prius for a little over a year.

Being a hybrid, it is a quirky car.  And, no, it doesn't fly, although the salesman tried to impress me by talking about its coefficient of drag.  I called his bluff; he didn't really know what a coefficient of drag was, but he knew it sounded good.

But even though this is as close as the Prius comes to flying, it still can teach us a lot about my two favorite aerodynamic parameters, essence and finesse.

First the finesse part: finesse is the ratio of lift to drag, what glider pilots call L/D.  Due to a quirk in the mathematics of aerodynamical forces, when you write L/D as a formula everything cancels except the coefficients: the coefficient of lift, and, Nick the Toyota salesman's favorite, the coefficient of drag.  When the latter is small, the ratio is big.

Now the Prius's coefficient of lift is about zero, and dividing zero by a smaller number still leaves zero, but at least morally the Prius's finesse is increased.

Now for essence, a compendium of the various kinds of energy available to the craft.  As a hybrid, the Prius has a gas motor and an electric motor.  It starts on the electric motor, sounding like a MontrĂ©al subway train, but then the gas motor kicks in.  The gas motor is pretty puny, and the thing holds less gas than my 1946 Taylorcraft.  But the Taylorcraft was a 25mpg machine, while the Prius is a 50mpg machine.

What the Prius has instead is a big battery with its own charge meter hidden somewhere in the display menus.  (It's hard to photograph, being a raster scanned projection.)  As you climb the hill, you can watch essence flow through the gas motor and through the traction motor to the front wheels.  As you descend, you can watch the gas motor turn off and the regenerative braking send essence from the front wheels to the battery.

Essence is malleable: chemical essence becomes kinetic essence becomes potential essence which can become kinetic essence again.  In the Prius, potential essence becomes electrical essence and you can watch it happen.

Still, no matter what you do, the gas gauge only goes down.


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