One of the important machinery features identified during
the 2000 field season was the port engine's valve chest, the device that regulates the
flow of steam into the main engine cylinder. Denbigh Project volunteer Gene
Shimko, an engineer with considerable professional experience working with reciprocating
engines, provided invaluable assistance in the identification and analysis of this
particular feature of the wreck.
This diagram shows a valve chest similar to Denbigh's,
taken from a contemporary engineering textbook. The valve chest is a rectangular box atop
the engine cylinder, the upper part of which is also shown. The view here is looking from
the right side of the ship; the bow would be toward the right of the picture.
"Live" steam, piped into the valve chest directly from the boiler, is shown here
in bright red.
There are two valves in the chest, a Cutoff (or Expansion) Valve and the Main Slide Valve.
The main slide valve assembly sits directly on top of the cylinder and slides on a
machined face. The cutoff valve consists of two blocks connected by a double reverse
screw. The cutoff valve slides within the main slide valve assembly, sitting directly on
the top of the main slide valve ports. In effect, the Cutoff Valve regulates live steam to
the Main Valve ports, which in turn passes it into alternate ends of the engine cylinder,
pushing the piston back and forth. At the same time, the spent steam from the previous
stroke (black) is exhausted out of the cylinder to the condenser, where it will be
converted back to liquid water and returned to the boiler.
here for a 113kb GIF image showing this animation frame-by-frame.
The Cutoff and Main Valves are themselves powered by the
rotating paddlewheel shaft via eccentrics, so that once set in motion the whole assembly
should continue to operate as long as sufficient steam is provided. The precise timing of
the Cutoff Valve could be adjusted by the threaded rod shown running through its center;
this could be used to make the engine operate more efficiently. Denbigh's engine
was considered "double acting" because steam pushed the piston both forward and
Valve chest illustration from William S. Auchincloss, Practical Application of
the Slide Valve and Link Motion (New York: 1869).