Free Piston engine for more cheap energy than the Rhombic Stirling to support solar and go off grid?

In this video I show you the beginning of the investigation into which Stirling engine concept could become the successor to the Rhombic Stirling.

The Stirling engines I have built so far are too complex for energy self-sufficiency and therefore too expensive and maintenance-intensive.

I would now like to extensively investigate which engine concept would be best for my next project to support the solar plant in times of low sunshine!

Free Piston Stirling engines do not have a kinematic drive mechanism and are therefore much simpler with less friction and wear.

They require no rolling or plain bearings and the only moving parts are the piston and the displacer.

Free Piston engines are available in many different versions, some of which are maintenance-free and extremely durable.

The very first Free Piston Stirling engine was described in a British patent of 1876 by Davy Postle.

Modern Free Piston Stirling engines are associated with the names William Beale and Sunpower.

They commercially marketed the Free Piston Stirling engines and pushed its development very far.

Nowadays, Free Piston engines are mainly used as refrigeration machines in cryogenic systems.

Free Piston Stirling engines have no kinematic mechanism coupling the reciprocating elements to each other or to a common rotating shaft.

This eliminates the need for bearings and simplifies the engine to a great extent.

The piston and displacer move entirely in response to the working gas or other spring forces acting upon them.

The Free Piston engine is a dynamic, resonant system operating at a more or less constant frequency and is self starting.

It requires only a slight, random pertubation to set it in motion.

There are no major side thrusts on the piston and displacer which relieves the lubrication and wear problem sufficiently to permit the use of gas lubricated pistons.

The cylinder can be hermetically sealed which is also an substantial advantage oft he Free Piston concept.

Another important characteristic of Free Piston engines is that they are more or less self regulating.

In addition to these advantages, the Free Piston Stirling also has some drawbacks:

The lack of a rotating shaft makes it necessary to use the motion directly or to employ a linear generator.

Free Piston engines do not have heavy piston side forces or other demanding bearing requirements.

The resulting possible use of gas lubricated bearings are characteristically high precision components with very small clearance.

A piston running exactly centrical is also not easy to achieve and must be ensured by precisely coordinated measures.

In engines of substantial capacity the spring forces involved are so great that mechanical springs would need to be so massive as to dominate the system.

Gas springs on the other hand have hysteresis and leakage losses and they do not provide positive suspension when the engine is not running.

The analysis of Free Piston engines is complicated by the fact that the dynamics of the piston and displacer are caused by the fluid processes their motion causes.

This leaves fewer known starting parameters and therefore the analysis more difficult.

Free Piston Stirling engines utilize the prinziple of vibrating systems.

The necessary requirement for the operation is the dynamic equilibrium of forces.

The resolution of the complicated interdisciplinary actions inside a Free Piston engine is complex and best approached numerically on a computer.

The vibration theory and the analysis of linear and non-linear systems is a difficult subject to handle.

The involved several masses with multiple springs and damping devices don't make things any easier.

Free Piston Stirling engines involve few moving parts and appear to be quite simple and straightforward.

Their proper design and operation requires substantial engineering expertise and a very high level of craftsmanship.

The advantage that they do not require a kinematic drive mechanism is partially negated by the problems of piston centering and balancing the forces.

The major problems I had with the Rhombic Stirling with friction, wear and leakage of the piston running surfaces would also continue to exist with the Free Piston Stirling.

The Free Piston Stirling also involves difficulties in design and production that I can hardly estimate and that could prove to be unsolvable.

The uniquely elegant concept is very promising but the more I have looked into it, the more my doubts have grown as there are many complexities.

What do you think of the Free Piston Stirling, please let me know and write your opinion in the comments.

Thank you very much for your attention!


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