Archive for April, 2010

by Engineman Preserved Wook

I reckon that steam railroading in the Northern Rockies is not considered wildly exciting by some languid artistes of late-modern steam; the Rosebud coal burned by NP and Goat Line engines simply didn’t allow the big articulated Z-5s to curdle the mountain airs the way the fellows did down in Wyoming on their UP Big Boy runs more than fifty years since, now.  But, the story of railroading throughout the Rocky Mountains is a tale all its own, and the scenery inarguably was a lot more gorgeous up there in Montana.  So this link to the story of the Logan-Bozeman, MT, Low Line is a valuable historical resource:


Engineman Wook

[all text-rights reserved & all others revert to holders

[21 April 2010]


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reposted by Engineman Preserved Wook
[I am reposting this Wikipedia article here as the link does not quite seem to be operational — EPW]
From Wikipedia, the free encyclopedia:
[In a steam engine, cutoff is the point in the piston stroke at which the steam inlet valve is closed. On a steam locomotive the adjustable cutoff of steam at the inlet valve is controlled by the reverser.  More steam is admitted for a longer part of the piston travel-distance to start heavy trains.  Less steam is needed underway and for only a shorter part of piston travel-distance in order to maintain efficient velocity at road speed. — ed]

The point at which the inlet valve closes and stops the entry of steam into the cylinder from the boiler plays a crucial role in the control of a steam engine. Once the valve has closed, steam trapped in the cylinder expands adiabatically. The steam pressure drops as it expands. A late cutoff delivers full steam pressure to move the piston through its entire stroke, for maximum start-up forces. But, since there will still be unexploited pressure in the cylinder at the end of the stroke, this is achieved at the expense of engine efficiency. In this situation the steam will still have considerable pressure remaining when it is exhausted resulting in the characteristic “chuff chuff” sound of a steam engine. An early cutoff has greater thermodynamic efficiency but provides less average force on the piston and is used for running the engine at higher speeds. The steam engine is the only thermodynamic engine design that can provide its maximum torque at zero revolutions.

1 Explanation
2 Control mechanism on locomotives
2.1 Reversing lever
2.2 Screw reverser
2.3 Steam reverser
2.4 Enginemen’s terminology
3 References
4 Sources


Schematic Indicator diagram of pressure in a steam locomotive cylinder. Note that pressure in the cylinder declines after cutoff as the steam pushes the piston down its bore. Cutoff is one of the four valve events. Early cutoff is used to increase (more…)

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