On a diesel locomotive, moving the throttle increases the amount of electricity being fed to the traction motors, speeding them up and speeding up the train. On a steamer, which is more direct drive, how does opening the throttle relate to an increase in speed? Does it work just like pushing the gas peddle in your car?
Thanks to Chris / CopCarSS for my avatar.
The throttle opens a valve that allows more steam to go to the cylinders. See number 14 in the diagram:
http://trumpetb.net/loco/locoworks.html
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
When you open the throttle, steam pressure from the boiler enters the pipes to the cylinders and goes into the cylinder itself... this pressure starts to move the piston, which increases the volume the steam is occupying on that side of the throttle valve.
But only a certain amount of steam can pass through the throttle at any time, depending on how far open the throttle is. If the moving piston creates more volume than the steam can occupy by passing through the throttle, the pressure will be less than what is in the boiler, thus the piston will not be pushed as fast as it would if the full boiler pressure were against the head all the time. The piston will only move as far as the available pressure of the expanding steam will move it.
The locomotive moves slower or faster depending on the restriction of the throttle allowing steam to maintain the pressure on the piston.
Semper Vaporo
Pkgs.
If I'm understanding that correctly, to increase speed the fastest and the smoothest, the engineer would would be watching the steam pressure guage closely to get the most bang for his buck?
Murphy Siding If I'm understanding that correctly, to increase speed the fastest and the smoothest, the engineer would would be watching the steam pressure guage closely to get the most bang for his buck?
It is the fireman's responsibility to maintain maximum steam pressure that is needed for the highest steam usage that is required.
The steam pressure gauge is on the wrong end of the system to measure the pressure at the cylinders and the loss of pressure in the whole system due to the volume of steam used in the cylinder is too small to be of an indication of operating effeciency. Though a poor (or spiteful) engineer can certainly wear out a fireman by not being respectfull of the fireman's ability to keep up with the steam usage.
Just like today's Dismal engineer knows not to just throw the throttle to notch 8 and leave divots in the rails, the steam engineer knows how to handle the throttle via experience and "feel".
To start a steamer. the engineer sets the "Reverser lever" (sometimes known as the "Johnson bar") to full forward (or backward) and then "cracks" the throttle open to let steam into the cylinders. This lets the pressure build gradually and when there is enough pressure to move the piston (and thus to move the wheels) the pressure will drop slightly which keeps the piston from moving too fast/far and the wheels losing their grip on the rails. As the train gains momentum the engineer can open the throttle a bit more to keep the pressure in the cylinders up and thus move the train faster.
As the train approaches the speed desired, the engineer then begins to shorten the cut-off by moving the Reverser lever back toward the middle. This cuts the supply of steam to each cylinder earlier in the cycle so that the piston is being moved by the residual steam in the cylinder at the moment the valve closes off the inlet. (As noted by the reduction in the "stack talk" sound volume).
The engineer might actually then reduce the throttle setting a little bit and control the speed of the engine with the Reverser lever allowing a little bit more or less steam to accumulate in the cylinder before cut-off occurs.
Euclid Murphy Siding If I'm understanding that correctly, to increase speed the fastest and the smoothest, the engineer would would be watching the steam pressure guage closely to get the most bang for his buck? It is the fireman's responsibility to maintain maximum steam pressure that is needed for the highest steam usage that is required.
To be a bit more clear, I would say that both the fireman and engineer often check the steam pressure. Generally, they want full pressure avaiable without wasting it by lifting the pop valve due to over-pressure.
But to get the "most bang for the buck" when working the engine, the engineer is most concerned with throttle which controls delivery of steam to the cylinders, and the variable cut-off setting which controls how long the cylinder valves are open to admit steam into the cylinders during the piston stroke.
For starting, the variable cut-off has the cylinder valves open for much of the pistion stroke. Then as speed increases, the variable cut-off ends the cylinder valve admission time during the piston stroke earlier than at the lower speeds. That ending of the steam admission is the "cut-off."
There are tradeoffs between the effect of throttle and variable cut-off, and the engineer must excersize skill in balancing these tradeoffs for optimum performance.
But even after the cut-off, the steam that was admitted into the cylinder continues to expand and push the piston away from it.
She who has no signature! cinscocom-tmw
Several years back, I was talking with a former fireman who mentioned one engineer who liked to run with the reverse in full forward--and wore the fireman out bailing coal, despite the fireman's requesthat he run properlyto run properly.
Johnny
To follow-up on Johnny's post just above and this portion of Euclid's:
Euclid . . . There are tradeoffs between the effect of throttle and variable cut-off, and the engineer must excersize skill in balancing these tradeoffs for optimum performance. But even after the cut-off, the steam that was admitted into the cylinder continues to expand and push the piston away from it.
Somewhere in the 3-part Trains series* in April - June 1967 on the D&H and its love affair with Consolidation types (2-8-0's) -
http://trc.trains.com/Train%20Magazine%20Index.aspx?articleId=66427&view=ViewIssue&issueId=5915
was this statement (or similar - probably in the June issue):
"Steam can be used either expansively or extensively."
It's the cut-off which governs that.
This was mentioned in a story about using the steam extensively during a test to beat another train to a junction (the rest of the story's longer than that, but not pertinent here).
- PDN.
Our community is FREE to join. To participate you must either login or register for an account.