AC> Then there is the beaut feature of
AC> positive creep whereby even more traction is obtained by
AC> deliberate, micro slippage possible thanks to microprocessors.
GP> This works the opposite way around from the way you have written
GP> it! The amount of friction between wheel and rail reduces
GP> drastically when slipage occurs, the electronics takes the motors
GP> to the point of slippage and then drops back the current and then
GP> advances it again. The cycle repeats continuously, but the point
GP> of it is to keep the wheels immediately below the slipping point
GP>  for the highest percentage of the time. (same thing I know:-)
Umm..., no. I was told on a plant trip to EMD that they achieve
maximim tractive effort at 125% speed; that is, the wheels turn 1.25
times as fast as the rails go by. They call it "controlled creep".
AC> However few locos are allowed to run steadily at optimum speed.
AC> In the real world it is stop, start, retard, accelerate. Steamers
AC> do not take so kindly to this and that is why for locos of
AC> comparable HP, the steamer achieves lower average speeds over
AC> a duty cycle, than do the diesels/ elecs.
You are misapplying the term "comparable horsepower". Remember,
steam locos do not have "a" rated horsepower as a Diesel does;
they cannot. As any motor turns faster, its power increases (up
to a point). To rate a Diesel, the Diesel motor is reved up to
its maximum, and rated there. For a steam loco, the "motor" is
the drive axles themselves. Any horsepower rating is thus good
for one speed only; and the "rated horsepower" is actually the
maximum obtainable at the best speed. At all other speeds, it
must be less; so the "comparable" steam loco will always be
underpowered relative to the Diesel whose maximum is (almost)
always available.
On the other side of the coin, if you consider comparable
tractive effort, the steam engine will be overstrong relative to
the Diesel. Available steam TE is mantained higher as you
accelerate relative to the Diesel, again due to the math.
As an expansion engine (as opposed to a combustion engine), the
reciprocating steam "motor" increases its efficiency at lower
power output. (This is done not by throttling, but by adjusting
the valve timing to use less steam, and expand it more.)
Therefore, at the usual utilization rate of 70% of full power, a
steam engine /of proper size/ will be effective and efficient.
GP> We're comparing apples and pears here. A 1997 steam
GP> locomotive could be designed with individual axle drive
GP> on two axle bogies, with positive creep, MUing,
GP> single driver, modern ashpan and grate operation, etc.;
GP> all the advantages that the Diesel and electric have.
Why? A Beyer-Garatt (sp?) type engine, perhaps burning
pulverized coal or a coal/water slurry, would work quite nicely.
Note that on a steam loco, the axles are usually MUCH closer
together, so a 4-coupled steamer may have a similar rigid
wheelbase to a C-type (3 axle) traction truck. In World War 1,
there were several types of articulated steam loco used on the
"trench" railroads. Some 0-6-0 locos had a ZERO rigid wheelbase!
These mechanisms could be adapted to modern use. A more powerful
steam locomotive could replace several MUed Diesel units.
Reggie Arford
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 X SLMR 2.1a X Confused? Just wait 'till I try to explain it!
--- Maximus 3.01
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