Vauclain Compound - Design

Design

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.

Each side of the locomotive had its own separate engine, as with conventional locomotives. This eliminated the connections between sides characteristic of cross-compound engines, where the low and high pressure cylinders were on opposite sides. The direct flow of steam between the high and low pressure cylinders on each side of the locomotive eliminated the need for a receiving chamber to store the steam. It also avoided subjecting the locomotive frame to unequal forces from separate high and low pressure sides. The whole package was compact and took up little more space than conventional equipment. Indeed, at first glance from the side the Vauclain compound can be mistaken for a conventional simple locomotive; the only giveaways are the unusual arrangement of the crosshead and (viewed from the front) the extra cylinder.

The scheme did not produce a more powerful locomotive; the maximum practical tractive force was governed by the weight on the drivers, and this did not change. The advantage was efficiency: the compounding reduced the steam required for the same performance. According to Baldwin's standard sizing tables, the high pressure cylinder on the compound was about 70% the diameter of the single cylinder of the conventional engine; therefore, steam consumption for the same stroke and degree of cutoff was about half that of the conventional engine. In practice lesser results were generally achieved, with Baldwin in 1900 reporting trials by various railroads showing fuel savings of 17% to 45%. In order to produce symmetrical forces, the low pressure cylinders had to be about 66% larger in diameter than the high, or about 20% larger than conventional cylinders; for small drivers this could lead to the clearance issues mentioned earlier, thus requiring the low pressure cylinders to be on top.