? "daestrom" ?????? ??? ??????
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"Tzortzakakis Dimitrios" wrote in message
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? "daestrom" ?????? ??? ??????
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Nice thing about the newer solid-state control systems (AC-Generator/
DC-Traction) is the ability to control wheel-slip. In the old days it
took a skilled engineer (the train-driving kind) to get maximum power
without slipping a lot (and wasting a lot of sand). Now modern units
have speed sensors on each individual wheel set and control the power
flow to individual traction motors. As soon as a wheel set starts to
slip it can redirect power flow to other traction motors to prevent the
slipping set from 'polishing the rail'. This prolongs life of the
wheels and rail and actually improves the maximum tractive effort a
locomotive can deliver. And when hauling 100+ cars of coal in a unit
train up grade, tractive effort is what keeps you moving.
I have no idea about train driving, but in Germany I got a local train
from a small city to Mannheim, and the Lokfuehrer (train driver) was
driving it like a race car... He accelerated fully to 130 km/h, and when
he was close to the next stop, he braked fully, too. It had one E-Lok,
and two cars. Also, the ICE starts like a race car. It's longer than 500
m, 12 cars, and I think it accelerates to 100 km/h in 10 seconds.
There is little doubt that electric trains are faster than other types as
far as acceleration and overall speed. :-)
Yes, because as the germans say-"Sie nehmen Strom direct aus der
Leitung"-They draw power directly from the wire. So it's a higher impulse
current than any on board diesel can provide;_)
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Some diesel-electric unitl have six axles and six traction motors. The
trade-off is between how much power you can get to the traction motors
and how much weight you can keep on the wheels to keep them from
slipping. Sand is okay for starting and some special situations, but you
can't carry enough to use it for an entire run. But of course too much
weight and you need more axles to protect the rail from damage
(depending on the size of the rail being used).
But isn't a locomotive by itself heavy enough? Like 120 tons and above,
with fuel and all?
(Check at www.wartsila.com some large diesels). In our new power station,
they have installed two 50 MW, 70,000 HP two-stroke diesels. To see how
2-stroke diesels work, look in www.howstuffworks.com..
I'm quite aware of how a 2-stroke works, as the large EMD's (654 series,
up to V-20 cylinder) that have been around for years are exactly that.
Also how the turbo-charger works, the four different lube-oil pumps
(scavenging, piston-cooling, main, and soak-back). Not to mention the
fuel injectors, overspeed trip, high-crankcase pressure shutdown, and
air-start systems to name a few of the various components. And
Westinghouse air brakes with several variations, and the MU (multi-unit)
interface used to connect several locomotives together and allow them all
to be 'driven' from one cab.
'
Of course you are, but I thought there might be other members of the group,
that don't. I didn't know until I read the article. The large, 15,000 HP, 11
MW diesels we have here at our local power station, have a final steam
stage, for better efficiency. The URL of our local college, where I got my
degree, is
www.teiher.gr , but I'm not sure if they got an english version.
But the trouble with overall weight is the combination of weight, power
and rail capacity. When you get to larger units, the rail used on a lot
of roads can't handle more than about 50,000 lbm per wheel set. That
means you're limited to about 100 tons for a unit with just 2 axles per
truck (4 total). Go up to a 120 ton and you need 3 axles per truck. But
a 100 ton, 4-axle unit has 12,500 lbm per axle, while a 120 ton, 6-axle
unit has only 10,000 lbm per axle. If the wheel friction coefficients are
the same, the 4-axle unit can develop 25% more tractive effort when
starting before slipping wheels.
Of course if the 120 ton, 6-axle unit has more overall horsepower, then
even though it develops less tractive effort at low speeds, it can achieve
a higher speed when loaded to it's rated tractive effort. Below a certain
speed, the maximum you can pull is dictated by wheel slip. Then you're
limited by tractive motor cooling up to a second point. Beyond that, the
overall horsepower becomes the limit. Once you're 'horsepower limited',
you can go faster, but only if you can reduce the amount of tractive
effort needed (i.e. you want to go faster, you have to pull fewer cars or
not climb as steep a grade). This 'hp limited speed' is in the range of
just 15 to 20 mph for a lot of 4-axle units, somewhat faster for 6-axle
units.
With typical freight trains in the US, they look at the steepest grade on
the road and figure out enough locomotive units and maximum cars to just
be horsepower limited on that grade. So while the train may go faster on
less steep sections or level grade, it'll be at notch 8 (full throttle)
and struggling to make about 15 mph up the steepest part of the route.
And stalled if one of the locomotive units dies.
So more hp means you may be able to pull it faster, but you can't always
pull as much.
Kind of 'weird' until you work out a few problems, but that's how it
works.
In Germany, they have special locomotives for freight trains, and special
for passenger ones. The former desingned for larger traction power, the
latter for higher speed. I have more experience with ships, since there are
no railroads in Crete, but there's a lot of sea, and islands in Greece:-)
I'll never forget my trip to Rhodes, where my batallion was situated, by
rail from Korinthos (the infamous boot camp) and with ship to Rhodes. She
was full of soldiers and commuters:-)
NB.:There are railroads in continental Greece.
--
Tzortzakakis Dimitrios
major in electrical engineering
mechanized infantry reservist
hordad AT otenet DOT gr
NB:I killfile googlegroups.