Powering engines by the changing volume of air as it changed temperature was first envisioned by Henry Wood in his patent of 1759. His idea was to pump heated air into a large cylinder, cool the air, and let the atmosphere do the work on the inward stroke of the piston.

The first to build a working model of Wood's proposal was Sir George Cayley in 1807. An improved design by Cayley was produced by the Caloric Engine Company in England and the Roper Caloric Engine Company in the United States. Further technological advancements by the Rev. Robert Stirling in 1816 earned him a spot in history as the "inventor" of the hot air or "Stirling" engine.

John Ericsson, builder of the ironclad U.S.S. Monitor, developed many different hot air engine designs, beginning with his 1826 British Patent. Our engine is based on his patent of 1888, and was built by the Rider Ericsson Company.

Ericsson engines were used strictly to pump water; the smaller engines such as ours were used in homes and small businesses. The water was pumped from a well or cistern into an overhead tank where it was stored for later usage.

The operation of the stirling engine is not complicated. There are no carburetors, ignition systems, valves, or other complicated mechanisms. Stirling engines run off of the expansion of air as it is heated, and the contraction of the same air as it is cooled. The source of heat can be wood, fuel oil, sunlight, or geothermal sources. Cooling can be achieved from water, air, or even ice cubes!

In the colored diagram, "A" is the power piston; it moves up as the air in the cylinder is heated, and moves down when the air is cooled. The diameter of the piston is 6 inches and the stroke is 2 7/8 inches. "B" is the displacer; it is about 1/4 inch smaller than the diameter of the cylinder. The displacer moves the air from the hot side of the cylinder to the cold side and vice versa, but more on that later. "C" is the firebox where wood, natural gas, or other fuels are burned. "D" is the water jacket. Water that is being pumped by the engine is first sent into the water jacket to provide cooling.

The cycle of the Ericsson engine are as follows: Air at the bottom of the cylinder (E) is heated, thus expanding and forcing the piston (A) upward. At this time the displacer (B) is driven downward to the bottom of the cylinder. Since the displacer is of a smaller diameter than the cylinder, the hot air rushes around the displacer to the cool end of the engine (F). Once in the top end of the cylinder, the hot air begins to contract, sucking the piston downward. Now the displacer moves upward, forcing all the cool air from the top end of the cylinder into the bottom end. Here the air is heated and the cycle begins again.