Dead Ends and New Beginnings

Why didn’t I simply stay with the 65cc rhombic engine and develop it further? I cer­tainly understood that it had a great deal of unrealized potential, even as I moved away from it. An extended surface area heater, additional regenerator volume, increased working pressures, anti-friction bearings on the connecting rods, and other more or less obvious modifications would all have substantially improved its performance. But there was too much I still did not know about other types of stirling to let me comfortably settle on one design for development. At this point, I had become more interested in exploring new ideas than in developing a practical engine.

The first attempt to see if a simpler approach might work as well as the rhombic drive resulted, in the Spring of 1975, in a 38cc V-2 gamma type engine, which incorpo­rated the existing displacer dome, burner, heater, and regenerator of the 65cc rhombic. This new engine was quite easy to make, and it had excellent dynamic balance. Once afew initial bugs were worked out, the most serious of which was a leaky crankcase casting that wasted considerable power pumping air, the V-2’s output and speed were (with elevated pressure) encouraging enough to spur the design of a largerV-2 gamma engine. Fortunately, before too much effort was wasted in this enterprise, comparative thermal efficiency tests were conducted which showed the V-2 to be less than one third as efficient as the 65cc rhombic. Fuel in to shaft power out, the rhombic showed an ef­ficiency ofjust under 5%, whereas the V-2 showed just over 1%. Excluding stack losses and heater head radiation losses, the rhombic showed 13.5% thermal efficiency on air, and 16.7% on helium; whereas the V-2 showed only 4.5% on air, and 6.1% on helium.

I suspect the low compression ratio inherent in the gamma type engine effectively mag­nified the deficiencies of the regenerator, and was the main cause of these poor thermal efficiencies. In any event, my enthusiasm for the gamma type engine faded rapidly with this knowledge.

A second and more successful attempt at simplicity was a 15cc alpha engine com­pleted in November 1976. It featured simple annular heat exchangers like those used inthe 11cc rhombic, and very close-fitting cast iron clearance pistons running unlubri­cated in honed steel cylinders. Other than the care required in machining the piston-to — cylinder fits, the engine was extremely easy to make. With the conventional alpha phase angle of 90° between the pistons’ motion, compression seemed so high that I seriously doubted whether the crank discs which served as flywheels would have sufficient inertia to sustain operation. What a delightful surprise to start this engine and have it run faster and faster until it reached 3600 rpm, far faster than any stirling I had built up to that time. Like the 65cc rhombic, this little alpha engine became a testbed for various modifications, such as a tubular heat exchanger system, a variable dead space speed controller, and a new yoke drive mechanism.

One interesting set of tests involved changing the phase angle in increments from 60° to nearly 180°. The engine’s performance changed dramatically with phase angle.

At narrower angles the engine’s compression ratio became extremely high, it became

Dead Ends and New Beginnings

Harder to start, it required a higher hot end temperature to run at all, but when it was hot enough to run it did so with great power and speed. As the phase angle increased, the engine became more docile, easier to start, more of a low temperature engine, with lower compression and power. Although all of these differences could have been antici­pated by theory alone, to experience them first hand in a real engine was most satisfy­ing.

Another interesting test was a power comparison between the small rhombic and the alpha that Jim Senft and I conducted in Athens, Ohio, in May 1980. These engines had similar expansion space volumes, and I expected similar outputs using the same burner. But the alpha produced only 6.5 watts, or 75% of the power of the rhombic. Ini­tially, I attributed this difference to some inherent superiority of the rhombic drive. Only later did I realize that the alpha heater was in fact somewhat shorter, and it provided only 75% of the internal surface area of the rhombic.

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