Dual Action Internal Stirling Engine Concept





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Published on Aug 12, 2012

A simplified computer model of a Stirling engine with dual action on the power piston.

The two biggest factors that limit performance/efficiency of a Stirling engine are friction and air leaks. Some of the friction and almost all of the air loss occur at the sliding seals required around the two pistons and/or their working shafts. While there are still two seals in this design, neither offer a path to the environment for air leaks. One seal is nothing more than a grommet around the displacer swing arm, the other seal is the power piston itself, and both are sealing the two air spaces from each other (there is also a seal around the wiring, but that is a non-moving seal, and as such is not an issue). Any incidental leakage between the two air spaces equals out between cycles, giving zero net leakage.

The dual action concept allows the engine to produce up to twice as much power, while using more of the available heat source, creating a much more efficient engine. In a standard Striling engine cycle, the heat being applied to the hot end is not being used during the compression cycle due to the displacer being at the hot end. In a dual acting design, heat not being used during one side of the engine's compression cycle is being used by the other side of the engine which is simultaneously in it's expansion cycle. Power is being produced twice per full cycle, with one side pushing against the power piston at the same time the other side is pulling it.

Friction is kept to a minimum in this design by reducing the number of friction points. Rather than the 4 sliding seals required in a normal dual action design, by placing all parts internally, we are left with only a single sliding seal. The use of a swing arm with roller bushings to move the second displacer allows a simplified linkage with very low side loading on rulon coated shafts.

Incorporating a generator into the flywheel, and using the available dead space for it's components (not shown in this model other than a simplified coil), eliminates even more friction due to the lack of connecting mechanisms between the engine and generator. All remaining dead space will be taken up with cowling (also not shown in this simplified model).

This computer model shows a diaphragm, which gives zero air loss, but a well machined power piston such as an Airpot actuator would probably give better performance, even with the miniscule leakage that might occur between internal air spaces, which would balance out anyway.

I will be attempting to build a small working model, will post results win or fail. I hope to be able to construct it in such a way that the top displacer assembly can be removed, allowing testing to be done on both a single fully internal design, and the dual action internal design to see what (if any) performance improvements might be acheived.


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