Right. But we're talking about large trucks, not a train. Unless
drafting, each truck has to deal with wind resistance. But, with
drafting, your point makes the argument that rails would make a
significant difference.
Maybe the US should be financing a new kind of rail system or modify the
existing one. One where each freight car is self powered and can exit
the tracks and use rubber tires for "the last mile."
Peri
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------ Original Message ------
From: "Lee Hart" <[email protected]>
To: "Peri Hartman" <[email protected]>; "Electric Vehicle Discussion
List" <[email protected]>
Sent: 05-Dec-20 10:34:05 AM
Subject: Re: [EVDL] 'All's not well with UK public charging' / Autonomy
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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