Some Generalizations about Design

The three basic types of stirling described above may employ a wide assortment of possible crank drive mechanisms, or none at all, as in free piston stirling engines. The main requirements are high mechanical efficiency and simplicity, with other important considerations including good dynamic balance, the ability to operate with minimal lu­brication, and compactness. But how does one choose among the numerous options? It is easy to become too absorbed in these matters. There is no one-and-only way to build a good small stirling engine. As I look back on my early work with the rhombic drive, I now realize that even my first working engine could have been developed into a very useful machine, had I continued to develop it, rather than move on to other ideas.

Superimposed upon the mechanical design is the thermodynamic design, which in a stirling engine is concerned primarily with getting the maximum amount of heat into and out of the engine through the heater and cooler, and recycling the maximum amount of heat in the regenerator, with as little temperature drop, pressure drop and dead volume as possible. These desired qualities often conflict with each other, and so judgment and prior experience are required.

Thermodynamic design also includes questions about phase angle and the mo­tions of the pistons as imparted by the crank mechanism, or mechanical design. So we have come full circle back to questions of mechanical design.

Indeed, upon examination the stirling presents the designer with so many inter­related variables that a sort of mental paralysis can set in. How can one rationally design an engine when there is so much one doesn’t know that seems essential?

In my early stirling work, I was deeply troubled by these matters. For example, I la­boriously charted out hot and cold space volume variations for every 15° of crank rota­tion for four versions of rhombic drive geometry, for the Philips 102C bell crank engine, and for several versions of alpha engine. There were seemingly significant differences between them, and I worried that without computer analysis or expert assistance there could be no hope of designing a good stirling. Through experience I learned the follow­ing useful things:

1. Read the relevant literature, but do not become overwhelmed by it. Much of it deals with fine points of theory that are of marginal use to the engine designer, whose pri­mary concerns are such practical problems as making a more efficient burner, improv­ing the life of a bearing, or sealing the pistons with Iess friction and leakage. These sorts of challenges are what must be solved to get a stirling up and running, into the field for testing, and perhaps into some suitable niche for commerce. Only then does further refinement become useful or possible.

2. Beware of the common idea that existing gasoline or diesel engines, or oilless com­pressors, can be readily converted into stirlings. Plenty of people have tried this idea (in­cluding yours truly), and it usually fails. It simply entails too many compromises, as most of these machines have inadequate seals, excessive friction and unsuitable lubrication systems for use as a stirling.

3. Talk to or correspond with the authors of interesting books or articles or others who are involved in building stirling engines. Once you have read the useful literature and done some independent thinking on the subject, people in the field will be happy to talk with you and share ideas. You will learn a great deal more in this way than you will from merely reading the literature.

4. Study and restudy the design and performance data of every real stirling for which you can find such information.

5. Keep it simple, Most everyone’s early designs are too complex and impractical, and most do not run. The essential starting point is a prototype that runs.

6. Keep your program focused. Select an engine size that is appropriate for your uses and stick with it, developing it as far as possible. Building prototypes of differing sizes is extremely wasteful of time, energy, and enthusiasm. It is the practical, not the theoreti­cal, problems of scaling that will prove the more frustrating.

7. Pay great attention to mechanical details. Make sure the piston(s) seal well. The engine should have a "bouncy" feel as it is turned over compression (like that of a good model airplane engine), and the seals should be able to hold most of the compressed gas at top dead center for four or five seconds. If the compression feels "mushy", the engine will run poorly, if at all. Take care to keep friction as low as possible. Never be satisfied with binds or kinks in the mechanism. With the mechanical details done well, then one knows to look into the heat exchangers and burner for the answers to poor performance.

8. Take great pains to get the heat into and out of the working gas; you can never have too much active surface area, especially in engines charged with air.

9. Become your own machinist. It will get you quality parts on time, and encourage design simplicity. Or perhaps l should say, redesign simplicity. Countless times I have stood idle at my lathe, lazy as always, and mentally redesigned a complex part into a simpler one, before I could muster the enthusiasm to begin making it. Another advan­tage of doing one’s own machine work is that formal drawings are unnecessary; mini­malist sketches will serve perfectly well.

10. Be on the lookout for subtle problems that can absorb incredible amounts of power, such as heater conduction losses or crankcase pumping losses. Great patience is often required to solve these problems, and a little luck helps, too.

11. Get your prototype out in the field for tests as soon as possible. It is the best pos­sible reality check.

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