Peri Hartman via EV wrote:
If nearly all the savings is from reducing wind resistance, by drafting,
then the rails aren't really needed. We could build more expensive
infrastructure, but a cheaper solution is often better. In other words,
the focus should be on making drafting possible, not reducing rolling
resistance.
The equation for the tractive power required is
HP = rolling resistance + power train loss + aerodynamic drag
where rolling resistance = R W V / 375
power train loss = C W V^2 / 375
aerodynamic drag = Cd A V^3 / 146,625
and R=tire roll resistance (~0.01 rubber tires, 0.001 steel wheels)
W=vehicle weight, lbs
V = velocity, mph
C = loss coefficient (~0.0002 for I^2R, gears, bearings,
stirring oil in gearboxes, etc.)
Cd = drag coefficient (0.2-0.4)
A = frontal area, sq.ft.
If you plug these into a spreadsheet and play with the numbers, you'll
see that in a lightweight rubber-tired vehicle, drag predominate at high
speeds. But in a train, weight predominates at just about any speed.
My understanding (via Bob Rice) is that trains are so massively heavy
that wind resistance hardly matters, unless you're talking about very
high speed passenger streamliners like the Acela. Trains can roll on for
many miles without power on flat ground.
And, rolling resistance is so low that a healthy person can push a
railroad car on a flat track. He routinely pushed trolley cars around
when he worked in the trolley museum.
So, the tractive force required by a locomotive is dominated by weight x
rolling resistance, plus a factor for the grade.
Lee
--
Ring the bells that still can ring
Forget your perfect offering
There is a crack in everything
That's how the light gets in.
-- Leonard Cohen, from "Anthem"
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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