Re: starting a steam loco

In article <1191861838.747667.261820@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Stephen Furley <furles@xxxxxxxxxxxxxxxxxx> wrote:
On 8 Oct, 13:49, "R.C. Payne" <rc...@xxxxxxxxxxxx> wrote:

I am familiar with the mode of operation of this (Newcomen) type of
engine, but I would query the statement, "Steam was allowed to rise from
the boiler into the cylinder". I would have thought some sort of
positive induction would be needed to get the steam to displace the air
that would inevitably leak into the cylinder when the engine sits idle
for some time. I can see one option being to lift the weight[1] (or
halt the engine with the weight in the raised position and have a catch
to hold it there in order to start the engine next time)

You've raised a good point here, and I'm not sure of the answer. If

That's certainly how it would end up if the engine were left standing with
the pump rod down and the cylinder up..

you start with a cylinder full of air. I would have thought that you
could get some steam into it simply by the fact that it was suspended
over the boiler, so it would contain a mixture of air and steam. If
you were to condense the steam, you would create some drop in
pressure, though far from a vacuum, and the piston would move down,
until the pressure of the air underneath it in the cylinder was equal
to the pressure of the atmosphere above. If you then opened the valve
this would allow the condensate to run back into the boiler; the pump
rods would fall, raising the piston, and this would draw in fresh seam
from the boiler; rather more than found it's way into the cylinder on
the first stroke, and so the piston would be moved down further when
the steam was condensed, which would allow it to draw in still more
steam on the third stroke etc., until after a few strokes the engine
would be working the full length of its stroke. Does this sound
reasonable?

It could work. OTOH, given that pumping engines were rarely stopped - and
then only for maintainance work - would't it be easier to stop the engine
with the pump rods raised and the piston near the bottom of the cylinder.
That's where it would naturally end up at the end of the "vacuum" stroke
(the "inside" stroke, in Cornish-engine speak: not sure if the same terms
were used for the Newcomen engines, but it would make sense it they
were..). Don't put on steam to destory the vacuum immediately, but instead
lash the beam to the framework of the engine-house and pin the pump-rods
across the top of the shaft (you're going to do this anyway if you're
working on the engine - the last thing you want is the pump-rod breaking
away half-way through when you're standing in amongst the gubbins[1]).
Once you've done that, /then/ you destroy the vacuum.
Once you've finished, unpin the rodding, open the valve to admit steam to
the cylinder and - very carefully - unlash the beam. The engine should now
work.

[1] The Cornish pumping engine at East Pool has a //most// impressive dent
in the valve casing, dating from the day the pump rod snapped and the
inside stroke took place a bit faster than intended. Ouch.

--
Andy Breen ~ Not speaking on behalf of the University of Wales, Aberystwyth
Feng Shui: an ancient oriental art for extracting
money from the gullible (Martin Sinclair)
.

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