All this talk of trains and hills reminds me of this simple explanation. Not sure how accurate it is but it has James may so it 'feels' more accurate. https://www.youtube.com/watch?v=KbUsKWbOqUU On 12/09/2016 2:51 PM, Robin Humble via luv-main wrote:
On Sun, Sep 11, 2016 at 02:22:39PM +1000, Russell Coker via luv-main wrote:
example if train stations were placed above shops (like Glenferrie) then kinetic energy would be converted to potential energy as the train goes uphill to the station and it would take less electrical energy to get up to speed when it leaves the station. the Montreal metro does that. https://en.wikipedia.org/wiki/Montreal_Metro the tracks drop steeply after each station and then rise again before the next (flat) platform. not sure how much it dips, but I remember it seemed like a lot.
dunno why the idea isn't more widespread. maybe 'cos you need a fair sized dip. back of the envelope says you need a 14m drop to get you up to 60km/h, or 20m up to 72km/h (ideal, excluding friction etc.) mgh = 0.5mv^2 60km/h = 16.7m/s => h=14.2m, or 72km/h = 20m/s => h=20.4m
with a big dip, if the train stops or slows down at the bottom then maybe it can't climb back up again...? the Montreal metro is also notable because they have rubber wheels on concrete tracks. very bouncy and noisy, but perhaps it means they can get traction on steeper slopes than steel wheels on rails.
obviously the steeper you can make the gradient and the closer you can put it to the station the longer the train will be kept at maximum speed. a shallow gradient spread out between stations doesn't really help that much - you need the steepest possibly climb/drop to the station to minimise travel time.
I suspect the "big dip" idea is impractical for steel wheels on (potentially) wet rails, and a small dip doesn't help all that much 'cos of the v^2 term.
here's another idea. if Melb trains carried onboard storage (super-capacitors, flywheels, batteries, ...) then they could store the majority of the braking energy and deploy it from on-board storage when leaving the station rather than pulling it from the overhead power cables. presumably trains already do "re-gen" on deceleration, but currently dump it back into the (unhappy) overhead grid?
electric cars are probably better than 64% efficient at re-gen these days, especially on 4wd models. https://www.tesla.com/blog/magic-tesla-roadster-regenerative-braking trains with steel wheels do better.
but frankly it sounds like improving the Melb trains overhead power system would be a better idea.
cheers, robin _______________________________________________ luv-main mailing list luv-main@luv.asn.au https://lists.luv.asn.au/cgi-bin/mailman/listinfo/luv-main
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