On Wed, Sep 03, 2003 at 10:07:39AM +0100, Karsten M. Self wrote: > on Sat, Aug 30, 2003 at 02:19:39AM -0700, Paul Johnson (baloo@ursine.ca) wrote: > > On Sat, Aug 30, 2003 at 01:44:43AM +0200, Arnt Karlsen wrote: > > > ..2 reason diesel-electric locomotives are popular; they are > > > about as clean as your average power utility, and they dont > > > put heavy loads on the power grids. > > > > Nope, and nope. Diesel electrics are popular because they give the > > most bang for the buck. Vastly more efficient than gasoline engines > > and mechanical transmissions (it's 2003, why can't I get a diesel > > electric car?, with fewer moving parts than the steam engines it > > replaced. This makes them dirt cheap and bloody reliable. The > > railroads really couldn't give a damn about how much electric they're > > using since they're not having to string thousands apon thousands of > > miles of overhead lines (another costly expense railroads don't bother > > with unless they can get economic benefit from the typically heavier > > and faster trains that electrified lines run). > > Electric traction offers a few benefits: > > - Quieter. > - Less (near zero) right-of-way (RoW) pollution. > - Better high-speed performance. > - Fewer ventilation issues for tunnels or enclosed operations (e.g.: > RR terminals). > - Ability to power all axels. Well, that last is a benefit of multiple-unit stock rather than electric traction per se. The current trend for passenger stock in the UK is for diesel-hydraulic multiple units with all axles powered. > This last provides several benefits. One is to reduce the amount of > slam between cars as the train accelerates or decellerates. > Ordinarially, cars pack into the locomotive as the train slows, and > start moving incrementally as the train starts (incidentally > distributing the startup load). More significantly though, for long > trains, is the elimination of the stringlining problem. This is the > result of having all your motive force at one end of a long line of > cars, while drag is distributed through the train. On a long curve (or > series of curves), the tendency is for the string to straighten out. > This has resulted in several derailments in mountainous regions, > notabily Dunsmuir in northern California (near Mt. Shasta, just south of > the Oregon border). ...and model railways everywhere! Another benefit is that the locomotives are simpler, being essentially less than the electric part of a diesel-electric, and the electrical components also benefit from not having to share accommodation with a diesel engine, so reliability is greater, and maintenance less complicated (although the maintenance of the routes gets harder). > Downsides of electrification are notably the cost of electrifying the > RoW, and aquisition of stock. > > Also, back in my intern days at SMUD, I recall that the city's light > rail system operated in power blocks seperated (though not fully > independent of) the city grid, largely because of the acceleration and > deceleration power draws and surges. The London Underground used to maintain their own completely independent power stations until not all that long ago. Nowadays they take power from the national grid, using a large number of small substations supplying fairly short sections of line. The substations appear to have a fairly high impedance, and the voltage on the rails can vary over a range of nearly 2:1 as a result, while the effect on the main grid is reduced. Be funny if all the PCs in the City blew up when a tube train went past... -- Pigeon Be kind to pigeons Get my GPG key here: http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x21C61F7F
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