To elaborate, full rated boiler pressure doesn't make it all the way to the cylinders. Some of the rated pressure is lost due to cooling along the way, inefficiency of the valves, and other causes. 85% is the constant used in the U.S. to represent cylinder pressure. It's based on 19th century experimentation, but that's still something of an arbitrary value. The British system opted to use 80% instead, giving slightly lower T.E. figures in result. In practice actual cylinder pressure could be different; I recall reading somewhere that the Central Pacific 4-8-0 locomotives with their complex Stevens double slide valves could manage more like 93% pressure in the cylinders. Piston valves can be more efficient than slide valves and should also give better results.
Superheaters of course throw this even farther out of whack.
For something like the 2-8-0's at Knotts, they only run at extremely low speed and not at full pressure since the valves aren't lifting in any of the videos I see. It's safe to say these engines aren't working at nearly their full power.
An alternate method of working out horsepower is adding up the rolling resistance of the entire train (including locomotive and tender), and add in curve and grade resistance. That gives you the force being exerted; multiply it out times the speed and divide by the constant given and you get the same thing. That's handy in cases like Knotts where the equipment isn't working at full power.
Edit-------------
Also Baldwin, at least in the 19th century, liked to keep things conservative. Baldwin guaranteed that its locomotives could handle the stated loads, and did so by estimating on the side of caution. For example, loads were calculated at an adhesion ratio of .225 instead of the more common .25, and sand use could push that even higher. I've read a great many examples, including some within Baldwin's own catalogues, of locomotives routinely doing more work (pulling heavier loads or at higher speeds) than strict adherence to Baldwin's figures would allow for.
Edited 1 time(s). Last edit at 12/27/2011 01:09PM by James.
