Steam

[NW Mailing List]

Sounds about right.  However, below that speed point, the drawbar HP  
of the 1.6 SDs will begin to exceed that of the A because the A's  
engines can't absorb all the HP that the boiler is producing, whereas  
the traction motors of the SDs will be able to absorb all the  
electric energy that the Diesel engines can produce down to a pretty  
low speed.  (At starting, 1.6 SDs will exert well over 160,000 lbs  
tractive effort v. the 115,000 of the A.)  The modern 6-axle Diesel,  
particularly, with AC drive, is the ultimate "drag" engine ever.   
(I've heard that AC loco powered coal drags in Wyoming sometimes  
climb grades at three mph!)  The modern steam loco is a good  
speedster - whatever it can start, it can move at a pretty good  
clip.  I read somewhere that it took 3 GP 7s(?) to replace one NKP  
Berkshire in fast freight service on a relatively flat RR, which  
makes sense because the HP is about the same for both set-ups.

Since a steam engine is a constant torque engine (within limits), the  
drawbar HP will drop as the loco speed drops below the maximum HP  
speed (where the engine is converting to tractive effort all the  
steam that the boiler can produce).  Due to "breathing" constraints,  
the steam engine will also be unable to absorb all the HP that the  
boiler can produce above a certain speed.  I've read somewhere that  
TE begins to drop when steam engine piston speed exceeds 400 feet per  
minute, but I'd have to think that's empirical.  Improved valving and  
steam pipe design would obviously make a difference here.  (cf,  
Chapelon and Porta.)

Note that a steam loco produces its maximum horsepower at one speed.   
An electric loco or a loco with an electric (or hydraulic)  
transmission can deliver its maximum HP to the rail over a fairly  
broad speed range.  In automotive terms, a steam loco is like a car  
with a single gear; a Diesel loco is like a car with an automatic  
tranny.

Nevertheless, I still fantasize about a double 2-10-2 Beyer-Garratt  
with a boiler based on that of a VGN AG, with all modern  
conveniences, such as computer-controlled poppet valves, etc., maybe  
1 HP and 2 LP compound cylinders on each engine.

Finally, I'm more impressed by the performance of the late Ys.  The A  
is a good textbook design, but who could have imagined the  
performance improvements that N&W was able to make over the years in  
the USRA 2-8-8-2??

pete groom

On May 28, 2008, at 5:46 PM, NW Mailing List wrote:

All,

The N&W Class A above 30 mph has roughly the same Drawbar Horse Power  
as 1.6 SD70M-2 over the rest of the speed range.  If anyone is  
interested.

John Rhodes


On Wed, May 28, 2008 at 6:33 PM, NW Mailing List <nw-mailing- 
list at nwhs.org> wrote:

 > Figure three SD45s can haul a 7500 ton train at 51 mph, then. How  
fast could three 4300 hp SD70s haul it? This is not quite 20% more  
power than the SD45s. It takes 50% more to haul it 62 mph. So only  
20% more would fall far short of 60 mph.



You're forgetting that SD70s have a higher TE curve... thanks to  
radial trucks (bolsterless HTCR II vs Flexicoil) and more  
sophisticated traction motors (D90TR vs D77) and alternator (AR20/CA7  
vs AR10/A7)

Robb Fisher
RFDI


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