Corliss Steam Engine

This is by far my most complex steam engine model, with twice as many parts as any other others. It shares that characteristic with real steam engines of the same design.

Common, older steam engine designs use a single valve that alternates between letting steam in and letting it out on both sides of the piston. These engines are simple, reliable, and cheap, but relatively inefficient in their use of energy and water.

The Corliss design uses a completely separate set of valves for letting steam in and letting it back out again, one on each side, for a total of four separate valves. The inlet and exhaust valves work in different ways, each optimized for its own function.

The exhaust valves are similar to those in older designs, in that they move in a smooth harmonic motion (sine wave pattern) derived from the rotation of the flywheel. They remain open for nearly the entire stroke of the piston, so steam can get out the whole time the inactive side of the piston is closing up.

But the inlet valves are completely different. Instead of remaining open to push the piston during its entire stroke, they are opened only for a short time right at the beginning of the cycle. During this short time a pulse of high-pressure steam fills the piston cavity, after which the valve is closed and the steam expands, pushing the cylinder as the steam pressure and temperature drop. By the time the piston reaches the other end and the exhaust valve opens, the energy content of the steam has been largely used up, and little goes to waste out the exhaust.

In a traditional engine, the speed is regulated by adjusting the steam pressure, but in a Corliss engine the steam is always full-pressure. Instead the speed is controlled by varying the length of time that the inlet valves are open. Under light load the valves stay open only for a short time, while under higher load or faster speed, the valves are kept open longer. This reduces efficiency, but gains power. (In this model I do not include the fairly complex governor and variable-timing mechanism. Instead, the model shows how the valves would be working when the engine is running at maximum efficiency under light load.)