speed and cutoff

[nw-mailing-list at nwhs.org]

John,

I can take a couple of guesses at the figures, but I'm
not at all sure of the details, so take all this with
a fairly large grain of salt, say about 10 lbs or so. 

In general terms, the boiler will evaporate water from
the tender plus condensate from the feedwater heater. 
This is total evaporation and is the quantity usually
tabulated in most test reports.  However, auxiliaries
will use between 7% and 10% of the total generated
steam (stoker, generator, air pumps....) before it can
get to the cylinders.  The figures I'm using are total
evaporation and should probably be reduced by about 7%
if you want to estimate the amount of steam used by
the machinery and exausted from the system.

An A producing 5,300 dbhp at 40 mph (not necessarily
its maximum capability) would be generating about
100,000 to 105,000 lbs of steam/hr (total evaporation
including feedwater heater).  This would be the
nominal amount of steam exhausted, less an amount
tapped off for the feedwater heater and auxiliaries. 
I don't believe the cutoff point would matter after
about 20 mph or so as far as steam quantity/hr is
concerned.  At maximum output above that point, the
machinery would be using all the steam the boiler
could produce at any speed and at any DBHP.  If you
want to haul a heavy train at 35 mph, a lighter train
at 45 mph, or time freight at 60 mph, you would still
use the same amount of steam/hr at maximum capacity
for each respective speed.  The cutoff would be
progressively shorter for each of these three
examples.  The amount of DBHP is a balance between the
declining DB pull, increasing train resistance, and
speed.

Under similar conditions a J producing about 5,000
dbhp at 40 mph (also not necessarily its maximum
capacity) would be generating between 95,000 and
100,000 lbs of steam/hr, total evaporation.  Maximum
output at 70 mph would be something on the order of
4,300 DBHP, but the steam consumption would still be
about the same with a shorter cutoff.

The Y6 is more of a problem.  I've been trying to
figure this one for several years.  Best I can do is
that at 5,500 dbhp, the Y6 would generate about 97,000
to 100,000 lbs of steam/hr.  Bud Jeffries has more
information on this, but I believe he found that the
Y6 was run more by throttle than by cutoff. 

THe S1 producing about 1,200 DBHP at maybe 25 mph
would use about 40,000 lbs of steam/hr.  Switchers
don't usually get used continuously for long periods,
so I believe they would considered over-cylindered
compared to road power.  Their critical speed (the
speed at which the cylinders use all the steam the
boiler can produce in full gear) is pretty low.  I
doubt any of the normal ratios would hold regarding
their performance much above 15 mph.

Your question regarding cutoff is a good one.  There
probably is a numerical relationship between cutoff
percent and mean effective pressure in the cylinders,
which would give the figures you're looking for.  I've
seen several tables indicating a relationship, but it
seems to vary according to each source, and there are
a lot of conditions applied.  So I'm not sure what it
is.

Help from others here??!!

Hope this helps,
Dave Stephenson



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