Vauclain compound

The Vauclain compound was a type of compound steam locomotive that was briefly popular around 1900. Developed at the Baldwin Locomotive Works, it featured two pistons moving in parallel, driving a common crosshead and controlled by a common valve gear using a single, complex piston valve.

The claimed advantage for this arrangement, as with other compounding arrangements, was greater economy due to lower fuel and water consumption. In practice uneven forces at the crosshead produced excess wear, with increased maintenance costs thus offsetting any fuel economies. The integration of the compounding system into the smokebox saddle made conversion to conventional engines straightforward, so most Vauclain compounds were so converted and led normal lives thereafter. The only known operable example is Manitou and Pikes Peak Railway #4, which is preserved on-site along with the static display of its sister #5.

The key to the Vauclain compound is its valve system. In essence, there is an extra system of valves, concentric with the usual middle steam passage in conventional single expansion piston valves. This passage connects the high-pressure cylinder exhaust to the low-pressure cylinder intake. The driving pistons are rigidly connected to either side of the crosshead, so that they move in concert. As high-pressure steam is admitted to one side of the high-pressure cylinder, the low-pressure steam exhausted from the other side is passed through the valves to the opposite side of the engine and into the low-pressure cylinder; finally the exhaust steam from the opposite side of the low-pressure cylinder escapes through the center part of the valve to the blastpipe.

The high- and low-pressure cylinders were mounted in vertical line with each other, with the piston rods in parallel. Usually the low-pressure cylinder was on the bottom, but clearance issues sometimes caused it to be put on the top. In the former case, the valve cylinder was mounted directly inboard from the high-pressure cylinder; in the latter case, the valves were also placed inboard, but at a level between the two power cylinders. The placement of the valves necessitated an inside-connected valve gear, and the Stephenson pattern was used (being the dominant type of the era anyway). One extra appliance required was a starting valve, manually controlled, which allowed admission of high-pressure boiler steam directly to the low-pressure intake. Without this, the low-pressure cylinders would have to actually work against atmospheric pressure.

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