London Banter - View Single Post - National Rail and Zones 7-9

Tom Anderson · January 25th 08, 06:04 PM posted to uk.transport.london,uk.railway

On Fri, 25 Jan 2008, wrote:

On Jan 25, 2:50 pm, Tom Anderson wrote:
On Fri, 25 Jan 2008, wrote:
Presumably for trains with few stops the power consumption is
approximately constant regardless of the length of the train because
the main loss will be air drag.

As long as the train never wants to climb a hill, perhaps. If it does, the
old mgh term rears its head.

And you get it back again on the downhills. If we assume a 1000kg car
takes 10kW to maintain 25m/s (about 50mph) on the flat. On a 1 in 50 it
will take 15kW to maintain that speed uphill but only 5kW to maintain
that speed downhill. As the speed is constant the time is the same up
and down so the total energy is the average, i.e. same as on the flat
over the same distance.

Good point. Although speed could only be constant if you had a 15 kW
engine, which would mean you were using less than full power (and i'm not
talking transient overload, since the hills could be long) on the flats.
And that you didn't combine motor and gravity power to go downhill
super-fast. Still, your point stands - you get the energy back.

(You also lose out if you have to use braking on the descent and it's
non-regenerative.

Yes.

tom

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