Steam & Excursion > Fact or fiction?

Decades ago, I read a story about moving a steam engine dead in train, that needed to be set out enroute, but it had to go into a spur track from the other direction. A few miles from the stop, the messenger or rider, moved the reverser to the opposite direction the engine was moving, opened the throttle, and the pistons pumped the boiler full of air. When they made the stop, it had enough pressure to move itself into the spur. Can this be done, or is it BS ?

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BS.

'Nuff said...

​W.



Edited 1 time(s). Last edit at 09/23/16 07:23 by wcamp1472.

Care to explain why this would not work?

I too have seen that story, as I recall it was one of the semi-well-know rail preservationists in the 50's or 60's when railroading was a bit looser and some engine abuse was a bit more accepted.

The valve alternates the ends of the piston from pressure to exhaust, so each stroke essentially
empties whatever residual pressure is left. In operation this allows full differential pressure to
exist over the piston less any exhaust back pressure. So, yup it'd be BS but hardly an old wives
tale cause they don't talk about stuff like this.

Out

The all-wise and all seeing pundits have pronouned the process BS. However, I've actually done it (with a 4-6-0 which has 67" drivers). Extended running of course isn't possbile, but if all you needed was to initiate movement for coasting into that spur, it would do just fine.

Besides, why go through all of that trouble? Just do a "drop!"

Now that'd be something I'd like to see:)

I have a small model stationary steam engine I made.  I spun the crankshaft with a drill and it inflated a rubber glove attached to what would normally be the input.  When I spun the crank the other way it evacuated the glove.  My coworkers and I are easily amused.

Hmm.  Normally you are admitting steam for a portion of the piston stroke and then it is cut off and its expansive force continues to push.  Meanwhile on the other side steam is being pushed out the exhaust.  So if we reverse the valves from the direction of motion it should draw air in from the exhaust nozzle on the expanding side of the piston.  Meanwhile, on the other side, air is being compressed until the valve opens in which case it's forced out, goes through the superheaters, throttle (presumably open) and into the boiiler.  Whether you could build up sufficient pressure to be of any use or how long it would take is another matter but it does seem like you could in fact build up some pressure this way.  What am I missing?

What you're missing is LUBRICATION !!

If hydrostatic lubricator, you have no continuous  steam flow to get oil into the cylinders....Also, the steam delivery pipes from the throttle are now blowing air and oil, if any, the wrong way, -----back into the boiler.

To be a 'compressor', you'd need the reverser set for full stroke in order to suck-in enough air to get any beneficial compressing going on.
With a power reverse, on a dead engine, you're S.O.L......

If 'Mechanical' lubrication ( pump operated),  you are dribbling oil into the passageway ( off of the 'spoon' above the piston valves) and the insufficient amount of oil is not lubricating the valves or the flashing Pistons.....& the wind is blowing the oil back towards the throttle.

Running dry, the rings on both the piston and the valves will galled themselves badly, before you build up sufficient air pressure in the storage tank to move the engine.  The ruined rings will now not compress any air, all,you have accomplished is ruining the rings and bushings, etc..... "Go directly to Cheyenne, DO NOT pass GO, do not collect $200 ...."

If you're towing the engine a sufficient distance, with rods and valve gear fully functional, the reciprocating components' rings are going to be shot, anyway, and you will not be able to compress air, or move, once under steam power.  
Possibly ruining the cylinder and valve bushings,,the packing rings and the Pistons/ valves themselves.
Centering the valve gear for towing, does nothing for the full strokes of the dry Pistons...

Yes, the Frenchman, LeChatelier, improvised the so-called 'water brake' where he injected water into the passages of the system, compressing air and cooling/lubricating the Pistons, but the compressed air is vented up a separate exhaust pipe.
i saw these water brakes used very successfully on the Mt. Washington Cog Railway, all the way down the mountain,   for over a century... 

Similarly, heavy duty highway trucks can be fitted with the Jacobs Brake, 'Jake-Brake', that also forces compressed air up the exhaust stack....noisy, ----- and some communities forbid its use going down the hills in their towns.  
But, all this is not what goes on in the simpler, reciprocating steamers.

In this thread it makes nice for "Nice theoretical possibility" discussions; but, such a practice in 'actuality', is ruinous to precious steam engines being towed 'dead', even for very modest distances.
So, as they say on "Myth Busters": it's totally false....

As Hotwater said in the beginning, it's totally false....& it's still totally false...
Don't even think about trying it...

​W.

[ I ought to become a Myth Buster for steam locos, on T.O.....]



Edited 2 time(s). Last edit at 09/24/16 06:34 by wcamp1472.

> What you're missing is LUBRICATION !

So all agreed that towing the engine forward
with the gear in full reverse will gradually
raise the air pressure inside the boiler,
like the story said, except that rings etc
will dry out?

On the big end stroke the cylinder is connected to BP and the rod end is connected
to exhaust. OK so we turn it around, the rod end is now connected to BP and big end
to exhaust. So you pull the locomotive forward and the rod end tries to suck air out
of the BP line which is at atmospheric pressure, the valve train changes state and
the rod end slightly sucks from the exhaust atmosphere and big end now connects
to the BP line. It's drawn away and again creates a slight vacuum on the big end
side. Where in all this are you getting any air to pressurize the boiler? A two piston
compressor would require checks to pull in the air when the vacuum was created
and another to hold the expelled air in what is the BP line, t'aint happening.

Out

Huh?  What is BP?  Boiler pressure?  With reversed valves, the air is getting pushed out to the boiler by the same valve events that normally admit steam.  And air is getting pulled in at the opposite end the same way steam would normally be exhausting.

You might be on to something but I am having a difficult time understanding what you are trying to say.  You don't need check valves if you have a valve train opening and closing valves at the right times.

> Where in all this are you getting any air to
> pressurize the boiler?

Let's oversimplify things for now. Say the
engine's valve gear has no lap, no lead,
no exhaust lap or clearance. So when
we put it in reverse, it's at 100% cutoff.
As the piston makes a stroke in one direction,
the cylinder behind it is open to exhaust
for the full stroke; as the piston moves
in the other direction, that part of the cylinder
is connected to the inside of the boiler
for the full length of the stroke (assuming
the throttle is open).

So we're looking at the right side of the engine;
gear is in reverse and we're ready to tow
the engine forward. Say the crankpin starts
at 9 o'clock. As the piston moves forward,
air from the smokebox flows into the back
end of the cylinder; when the crankpin reaches
3 o'clock the cylinder is full of air at 14.7 psi.

As the crankpin continues past 3 o'clock and
the piston starts moving back, the back end
of the cylinder is connected to the boiler. The
cylinderful of 14.7-psi air all goes into the boiler.

That's air that wasn't inside the boiler before,
and it's never going to be lost. As the engine
continues forward, the boiler is gaining air
at each stroke-- its pressure is increasing
from the 14.7 psi it had before.

So eventually the air inside the boiler is at
20 psi absolute. Each time the piston reaches
the end of a stroke, frontward or backward,
the boiler is connected to a cylinder
full of 14.7-psi air. So some air flows
out of the boiler into the cylinder.
But it all comes back on the next stroke,
along with the new air.

Think changing to a real engine, with
lap and lead, will change things?



Edited 1 time(s). Last edit at 09/24/16 14:41 by timz.

Yes but understand that whatever air is moved back into the line between cylinder and the throttle valve(s)
is exhausted on the opposite valve action, you're just moving air back and forth. Doesn't matter which position
the reverser occupies (other than centered) you're just moving air back and forth. The throttle is open to the
dry pipe and into the superheater and on and on so there's nothing to keep your pumped air from going right
back out the exhaust on the opposite stroke.

Out

> there's nothing to keep your pumped air from going right
> back out the exhaust on the opposite stroke.

Except the valve. As long as each end of the cylinder
is connected to the boiler or to the smokebox--
never to both at the same time-- the boiler
will gain pressure.

The only source of air is the boiler, you have what you've got, there's no more to add
and building pressure would take adding volume. V1P1T1=V2P2T2 and since V1 and V2
are the same and T isn't going to rise much nither will P, or pressure.

Out

> V1P1T1=V2P2T2

That's for a given quantity of air--
a given mass/weight of air.
As we pull the engine forward,
more air is being pushed into
the boiler.

Let's look at it stroke by stroke. Same
oversimplifying assumption-- zero lap,
zero lead, zero exhaust clearance.
We're looking at the right side of the engine
as we start to tow it forward, with the
valve gear in reverse and the crankpin
starting from 9 o'clock.

We've opened and closed the
cylinder cocks, so both ends of the
cylinder are at 14.7 psi absolute,
as is the inside of the boiler.

For the length of that first stroke--
as the crankpin goes from 9 o'clock
to 3 o'clock-- the front of the cylinder
is connected to the inside of the boiler
and the back of the cylinder is connected
to the inside of the smokebox. So
a cylinderful of air is shoved from the front
of the cylinder into the boiler, raising the
pressure inside the boiler to 14.8 psi
or some such. When the crankpin reaches
3 o'clock a new cylinderful of 14.7-psi air
behind the piston has entered from outside.

As the piston starts back, the valve connects
the boiler to the back of the cylinder-- so
a bit of air flows from the boiler into the
back of the cylinder. The pressure in both
becomes 14.799 psi, or some such.

During the rearward piston stroke, that air
is compressed into the boiler, raising its
pressure to 14.899 psi or some such. The
front of the cylinder contains a new load
of 14.7-psi air that entered from outside.

So we've made one driver rotation and
everything's the same as it was, except
the boiler pressure is a bit higher. If we
continue pulling the engine forward, the
pressure inside the boiler will continue
to increase.



Edited 1 time(s). Last edit at 09/24/16 16:12 by timz.

Dude, for the last time. The P or pressure port of the cylinders is connected to the boiler, all you're doing is moving
air in an out of the boiler, you're not building pressure. You don't have an external suction port, you talk cylinder cocks,
first you have no way of opening them and second how the hell would you time the opening and closing if you did?

Out

Yes,....

1.     With NO 'discharge check valve' , you have NO WAY of trapping the air that is moved into the 'receiver tank'.
You have only a pump that gets no accumulated pressure in the boiler ----- thus, the air cycles in and out, up and down the steam delivery pipes, following the piston's movements

All reciprocating air compressors require a discharge check valve, of some type, downstream of the piston....
Upstream valving cannot help you retain trapped air in the receiver tank.

2.      What have the cylinder cocks have to do with the discussion?
         What does callum mean: 'first you have no way of  opening them' ?

Several schemes for operating them are common:
lever operated manual linkage, air-opened, and steam closed types....the manual linkage can be operated at any time, the others require substantial pressure.

The esoteric discussion about what should be possible has no application in the real-world experience.
The hypothetical musings about a myth are farcical. Give it up.

The only analogus comparison is with crosshead/driver crank-driven water pumps ....all are dependent on the pump discharge checks in order to maintain unidirectional water flow.  In fact, all boiler water pumping devices utilize these boiler mounted check valves.

​W.