Wednesday, August 30, 2006

Recovering Energy, Part Deux

It has come to my attention, through my friend Simon, that perhaps I was too glib and a tad misinformed regarding the power characteristics of jet engines. There are a lot of variables to consider, and simply saying stating the thrust of an engine doesn't tell you all you need to know (unless you're on the Senate Arms Committee).

Here is a cool reference on jet engine ratings, and Wikipedia has a nice page on jet engines. It seems that maximum thrust is not constant with airspeed .. but it doesn't go down linearly either, so you're still better off in the power game than with piston engines.

Simon also thought my main point about energy and driving was a bit muddied, so I'll restate it here.

Constant acceleration requires linearly increasing power .. until you are working against drag, in which case it takes, uhh, quartic power? Exercise for the reader: drag goes as speed squared, so power to move against drag goes as speed cubed.. so what about accelerating against drag?

Similarly, constant deceleration requires linearly decreasing power. Since a hybrid can only recover a maximum amount of power, the deceleration you can invoke that does not dissipate power goes as 1/speed. Lift off the gas at 60 MPH and you're maxing out the Civic Hybrid regenerator, ditto for touching the brakes at 30 MPH. And standing on them at 5 MPH only barely gets you there (but there's not enough energy at 5 MPH to care about anyway).

This is not the way I want to train myself to drive, nor would I like it as a passenger. I just hope we don't have a generation [sic] of hybrid drivers with their eyes glued to the regen meter while they're braking.


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