Steam & Excursion > Valve pilot question.

When older large engines were rebuilt they got valve pilots. What does a valve pilot do?

In a nutshell, the traditional valve pilot, as I understand it, tells the engineer how long of a cutoff the engine can be worked at a given speed, for maximum power. Any cutoff longer than that is a waste, at best, or even damaging to the engine, at worst. If maximum power is not needed a cutoff shorter than that can be used for additional economy. Many roads simply had back pressure gauges and told their hoggers to run with it at a minimum. One version I particularly like and have actually used is the "control gauge" which is a two-needle device showing back pressure and steam chest pressure on the same face, with two respective scales. Theoretically these gauges were designed so that the maximum economy was achieved by hooking up the engine to where the two needles lined up with each other. If less than maximum power was needed the engine was hooked up further to where the back pressure needle was lower than the steam chest.

In most of these cases each road would dynomometer each class of locos to determine the ideal numbers/settings for these gauges. Each class was different. 15 lbs of back pressure might be a good max for one type and be way to high on another class. And of course there were always some of the older hoggers who "never had the gauges before, and we don't need'em now!" Older and smaller steamers typically never had any of this equipment, and the guys knew how to listen to and feel the engine. There is something to be said for that, too.

Scott Gordon, MRSR

tomstp Wrote:
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> When older large engines were rebuilt they got
> valve pilots. What does a valve pilot do?


If I remember correctly, the loco valve pilot was a sort of mechanical computer that gave the locomotive engineer an indication of how to best set the locomotive reverse gear for optimum performance and economy. I believe that when the Espee's GS-4s' came new from Lima that they were equipped with them. I believe the indication was shown as an added dial or needle on the speed recorder unit. Exactly how it worked, I do not know. I do not believe that Espee kept the devices as I don't think the 4449 still has one.

see:

http://www.trainorders.com/discussion/read.php?10,1637993,1638059#msg-1638059

Bob

The SP had removed all valve pilots by the end of 1952. Maintenance on the appliance was not justified as it was a significant amount of effort to keep all the pieces lubricated and in tune. Each change of driver diameter over 1.5 inches required a change in valve pilot cams in the cam box. I have never seen a photo of an SP engine after 1952 with a valve pilot. Even by the summer of 1952, valve pilots intact on locomotives were rare.