On Sun, Aug 31, 2003 at 04:28:20AM -0700, Paul Johnson wrote: > On Sun, Aug 31, 2003 at 03:32:13PM +1200, cr wrote: > > On Sunday 31 August 2003 07:48, Pigeon wrote: > > > ...and because (in Britain) the introduction of diesel-hydraulics was > > > cocked up badly, for silly reasons like the idea coming from those > > > nasty Germans we'd just fought a war against; also because of > > > commonality of transmission technology with straight electrics; > > > worldwide, a lot of country B seeing country A using diesel-electrics > > > and copying them, often buying similar designs from the same > > > manufacturer, making the whole process easier and cheaper. > > > > I don't normally disagree with you Pigeon ;), but here I do. If hydraulics > > had proved reliable, and cheaper, or significantly better in any way, d'you > > think they wouldn't have been more widely adopted by now? They were > > certainly tried in enough areas. So was the 68000, and look what a POS chip PCs ended up using... it is all too common for non-technical considerations to result in a technically suboptimal solution becoming the most widely used method. So we get VHS for VCRs, telescopic forks for motorcycle front suspensions, Windoze, incandescent light bulbs... > > > Diesel-hydraulics are in many ways a much better idea than the > > > diesel-electric - simpler, more reliable, lighter, no need for complex > > > high-power electrical control gear, inherently good resistance to > > > wheel slip and transmission overheating which makes them good choices > > > for heavy freight trains and/or heavily graded routes. The reasons > > > they're not more widely used are historical/political rather than > > > technical. > > > > You're not a Great Western enthusiast by any chance? <vbeg> > > Me - Southern. ;) Certainly as far as technical developments are concerned. As for the more general view, I'm both Western and Midland - coming from "border territory"! > > > Hydraulic transmission does seem to be making a comeback, though - > > > diesel multiple units in Britain since the 80s have all used hydraulic > > > transmission, with great success. It's far more reliable than the > > > mechanical transmissions on older DMUs, small and light enough for > > > underfloor mounting, and much cheaper than the experimental > > > diesel-electric version that was also built. > > > > Well, automatic cars all have hydraulic torque-convertor transmission. It > > works fine for low powers (by which I mean, up to a few hundred HP). It > > doesn't seem to work so well for high powers. Hence OK for railcars, not > > (apparently) for locomotives at higher powers. I admit that my main knowledge is of British traction, but it has to be said that the hydraulic transmissions were about the only bits of the British hydraulics that didn't fail - even the crappily-built British licensed clones of German-designed transmissions. (I'm excluding overheating due to (often silly) failures of parts in the cooling systems.) German and American hydraulics seem to have been similar. > And I'm not terrably fond of hydraulic transmissions. They're not > terrably reliable compared to manual mechanical transmissions. I > won't own another car with a hydraulic transmission, if the slightest > thing is wrong, the car becomes difficult to drive. Whereas if I'm > controlling the clutch and gearshift myself, the vehicle becomes > considerably more robust. Well, this is a good indication of the differences between rail and road applications... The car autobox replaces the clutch (and effectively some of the gearing) with a torque converter (hydrodynamic transmission), coupled to a gearbox with automatic gear selection which is also, usually, hydraulically controlled and actuated. The clutch-bit, the torque converter, is generally extremely reliable unless abused; the unreliability comes from the added complexity of the gearbox component. In the rail situation, the extra complexity is there in one form or another whatever form of transmission you use; the components rapidly become too heavy for manual operation, and multiple working demands some degree of automation / power operation, so that source of unreliability cannot be eliminated. Hydraulic transmission, however, allows complexity to be minimised. The most successful transmissions on the Western Region hydraulics, and the type used on modern DMUs, provide a variable-ratio drive beyond the range of a single torque converter by having several of different ratios, which are drained or filled as required. This eliminates any need to mechanically shift gears, greatly reduces the number of moving and wearing parts compared to a gearbox, and produces a transmission unit of high reliability and simple maintenance requirements. > > And *also*, in places like Switzerland (or New Zealand), where there are long > > continuous grades, regenerative braking can put a lot of power back into the > > grid and save on train brake blocks. I believe some US diesel-electrics > > are equipped with regen braking but they just have to 'waste' the power > > produced. > > Yup, they call it rheostatic braking. Basically, the juice just gets > burned off by what amounts to giant oven coils under massive fans (the > big fans you see on most American diesel-electrics). Virgin have just introduced some flashy new diesel units with this system... not long ago the waves were breaking over the sea wall at Dawlish and shorting out the roof-mounted oven coils. The onboard computer detected this, and brought the entire train to a halt. > Portland's MAX > uses a similar system of braking to send power back on to the line. > My school used to share a (small) substation with the trains, you > could tell when a train was slowing down on the line up the street > because the lights would get slightly brighter, and if a particularly > full train (and thus heavy, requiring full throttle to accellerate > reasonably well) would make the lights slightly dimmer. This effect limits the ability of the London Underground, at least, to use regenerative braking - too many trains doing it at once, the supply voltage rises too high, and there's a risk of trains under power going too fast... > > Incidentally, third-rail works fine for multiple-units, but has distinct > > limitations for single-loco trains in any place where there are, say, a lot > > of railway crossings. (Or, as in NZ, largely unfenced tracks in some > > areas). Good point, though of less weight in Britain - everything's fenced, all new passenger stock is multiple-unit and freight operators prefer diesels for greater flexibility. > > The Southern Railway's early electric locos had a > > motor-flywheel-generator set ingeniously wired in series with the traction > > motors to overcome the break in traction. But that's DC, I'm not sure it > > would work so well on AC. Given that the technology of the time made a rotary convertor a good solution anyway, it was a brilliant idea. It'd work just as well on AC or DC. It would still be quite a good solution, though I suspect a dirty great bank of batteries that you could also go on/off shed with would be preferred. > > Also of course, 3kV third rail is a bit more dangerous than 600v DC. Well... 600V DC is considered to be in a pretty dangerous range: much below that and you're more able to take it; much over that and it tends to throw you off before it does much harm. Third-rail voltages tend to make you lock on and fry you. With bottom contact, it'd probably be safer than the existing system. -- 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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