New Simplified Heat Engine

This site describes a new engine design having these key benefits:

  • It demonstrates the principle of heat engines
  • It is incredibly simple (even for heat engines)
  • You can build it from scratch

The new design is substantially simpler than a conventional Stirling engine. Until now, ultra simple heat engine designs were generally devices that rocked or vibrated but did not rotate. This new design is extremely simple and rotates. (This site assumes familiarity with conventional Stirling engines. To learn about Stirling engines search for them at google.)

The new design unifies the displacer and flywheel. Unification is achieved by placing the flywheel inside the displacer chamber, placing the hot and cold plates adjacent to each other instead of parallel to each other, and using a D-shaped displacer attached to the flywheel. Although a low temperature differential (LTD)
model is described, the concept extends to HTD configurations.

With the chamber held horizontally (like a pie) the hot and cold sides are to the front and rear respectively instead of top and bottom. A rotating flywheel/displacer rotates the air in the chamber between the hot and cold regions. An external piston is the only other significant moving part and is oriented parallel to the chamber instead of perpendicular.

This configuration has several advantages over the conventional configuration, including:

  • Less points of friction – 5 bearing joints instead of 8 or more
  • Fewer parts – simplifies time and cost of construction and improves reliability
  • More compact – allows use in tighter spaces

The five bearing points in comparison to eight in a conventional Senft LTD are:

  • Top crankshaft bearing
  • Bottom crankshaft bearing
  • Connecting rod bearing
  • Yoke bearing
  • Piston

The compactness comes from eliminating the 90 degree relationship of the flywheel to the displacer chamber.

Although this design has less points of friction than a conventional Stirling engine, it is slightly less efficient as it lacks dwell time and a regenerator, and chamber air turbulence is lower. It is not the first pseudo-rotary heat engine but it is the simplest. This site shows the simple parts needed to build the engine. The model shown has a 6 inch chamber diameter.

Note 1: We do NOT offer a kit of any type – however you can use to design and make many of the parts.

Note 2: I did this project many years ago and am not active in this area – working on other projects now.

Rotary Stirling Engine Background History

I was introduced to Stirling engines at the 5th annual Cabin Fever Expo on 1/27/01 in Lebanon, PA. I have an interest in design optimization and was challenged by the task of further simplifying a conventional Stirling. At first I concluded that substantial work had already been done to create the simplest possible design and that a large simplification was not likely. A few weeks later the idea of placing the flywheel inside the chamber occurred although at first I did not see how it could work. Then the full picture fell into place, benefits were analyzed, and finally a CAD design and construction began. The prototype first ran on 4/8/2001. By 5/6/01 the engine ran with a differential of approximately 17C. On 5/12/01 the differential was reduced to 8C. (I estimate that further reduction should be relatively easy to achieve with better precision machining and some small alterations.) This is my first engine.

I would like to thank Larry Simms of Thailand for his technique of using brass tubing for the cylinder and James Senft for writing his excellent books on Stirling engines.

Following are some possible variations and ideas for future development:

  • Build an HTD version
  • Optimize the design mathematically (the current prototype roughly follows Senft’s LTD measurements). For example, a thinner chamber may be more effective and a smaller diameter piston should work better.
  • Reduce gap between displacer and plates by making displacer more precisely.
  • Improve seal mechanism to reduce its friction.
  • Add a method to increase chamber air turbulence.
  • Add a method to achieve an advantage similar to regeneration, if possible.