Backpressure

Exactly what does the backpressure gauge tell you and what is backpressure?Robert Learmont
Future and Current BNSF Engineer (I'm only 12 years old but drove some real BNSF locomotives and plan to work for a railroad when I grow up.)

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ummm, Backpressure gauge tells you the backpressure, and back pressur eis what the guage is saying???
I dunno, i am as curious and clueless as you are.....

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Dispatcher on April 18th, 2003 on the NEC. First multiplayer operation of the NEC. History is made.

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Backpressure generally refers to the final exit exhaust pressure of the steam from a steam engine.As it is not directly exhausted,from a loco's cylinders to atmosphere,but through pipework to the chimney,and usually throttled through draught producing nozzles,it is a measureable pressure.Ideally this pressure is a vacuum,as in marine plant,but the constraints of a loco means it is usually above atmospheric pressure.Does it matter?I hear you ask,yes,because it can affect the overall efficiency of the engine,and the work the fireman has to do shovelling the fuel.Imagine a loco running flat out with the regulator wide open.The boiler pressure is say 250PSI,and the back pressure of the exhaust steam is 30PSI.In effect the work being done by the loco is being achieved by a pressure difference of 220PSI.A skillfull driver will throttle the regulator in,so that the steam pressure to the cylinders is say 230PSI,with the back pressure falling to 10PSI.The engine still produces the same power with the same relative pressure drop of 220PSI,so the steam is being utilised more efficiently,and you're using less steam too.This means that less fuel is required than in the original set of pressures,as steam was wasted,by being exhausted from the cylinders at a higher backpressure pressure than necessary for full power operation.
Indeed,with a high backpressure,the engine is theoretically having to fight against that same backpressure,in order to produce its power. This explanation has been simplified,as the expansion of steam is not a simple subject,but I think it suffices to clarify back pressure.IE run a steam engine at the lowest obtainable back pressure,for whatever power output(speed) you're running at.

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A discussion of back pressure cannot be divorced from that of draft. The diameter and lenght of the blast pipes affect the backpressure. The greater the velocity of the exhaust steam, the greater the impact on draft. But constricting the diameter of the blast pipes in order to increase velocity increases the backpressure on the cylinders. The ideal situation was to run with the exhaust openings as wide as consistent with allowing adequate draft. Professor Goss and others spent a number of years around the TOC (20th, that is) testing the ideal front end proportions and came up with a set of recommended relations between the stack diameter, height, and boiler penetration and the boiler diameter, blast pipe diameter, etc.
In England, more than the US, a interesting solution to the backpressure problem was created by using a variable blast pipe, with a component that functioned like a popoff valve: when the pressure against the blast pipe became great enough, the valve would open, allowing the steam to escape through an additional opening.

Bill Hobbs

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