Formerly known as the "Oscillating Ringbom", the latest engine to come out of DeKalb Tech is now referred to as "Thumper" because of the distinctive "thumping" sounds created as the displacer piston reaches the end of its travel in both directions.

Ringbom Stirling cycle engines have no mechanical linkage to actuate the displacer piston. Pressure changes inside the engine caused by the movement of the power piston act on an oversized displacer pushrod to create movement of the displacer piston.

All Stirling cycle engines, including the Ringbom , are "closed cycle" engines. There is a fixed quantity of air inside and there is no intake or exhaust. The air inside is heated and cooled every revolution. When air is heated its pressure increases and when air is cooled its pressure decreases. This variation in pressure, acting on the power piston is what converts thermal energy (heat) into mechanical energy (rotation of the crankshaft). Because of leakage, average pressure inside the engine stabilizes equal to the pressure outside the engine (atmospheric).

The displacer piston is the clever mechanism that facilitates the heating and cooling of the air inside. The displacer piston serves to shuffle the air back and forth between the hot end, where it is heated, and the cold end, where it is cooled. In most Stirling engines the displacer piston is actuated by mechanical linkage from the crankshaft. Typically the linkage provides for the displacer piston to lead the power piston by 90 degrees.

In the case of the Ringbom Stirling, there is no mechanical linkage to the displacer piston. Instead, the displacer pushrod is large enough so that pressure inside the engine, acting on the area of the pushrod, results in a force sufficient to move the displacer piston. As you recall from your knowledge of hydraulics, pressure times the area being pushed against by that pressure, equals the force available to do work. The force in this case has to exceed the weight of the piston in order to lift it. (The displacer piston in THUMPER hangs vertically.)

As the power piston moves up on its compression stroke the resulting increase in pressure causes the displacer piston to move up as well. (An o-ring cushions the sudden stop and accounts for the thumping sound.) This displaces the air into the hot end of the displacer cylinder where it is heated. Rotational inertia from the flywheel provides the energy for this compression stroke.

As the power piston passes over Top Dead Center, pressure from the now hotter air pushes the power piston down on its expansion stroke. This is the conversion of thermal energy into mechanical energy.

Approximately midway through the expansion stroke, the pressure inside the engine falls below atmospheric pressure outside the engine. (Remember that the average pressure in the engine equals atmospheric.) As the power piston continues its down stroke, internal pressure continues to drop and pressure outside the engine pushes the displacer piston down. ( Again, that thumping sound!) Air is displaced into the cold end of the engine where it cools and its pressure drops further.

The power piston rounds Bottom Dead Center, starts the compression stroke again, and the cycle repeats. . .

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