Other Rhombics

The relative success of the 65cc engine inspired a series of other rhombic designs, some actually built, others not. Among the engines actually built was a small 11cc demonstrator engine designed for an article in Model Engineer magazine. This engine employes exactly the same rhombic geometry as the 65cc engine, but scaled-down to half size. The piston is cast iron, machined to a close clearance (about 0.0006 inch, or 0.015 mm) fit in the honed steel cylinder. The displacer does not seal against the cyl­inder at all, but rather has 0.015 inch (0.38 mm) per side annular gap, which forms the cooler, regenerative annulus, and heater. The engine ran well from the start, producing 8.8 watts at 1980 rpm, atmospheric, and no modifications have been made to it. The entire project, including the article resulting from it, consumed only a month and a half to complete.

A second rhombic was a 100cc engine intended to be the successor to the origi­nal 65cc machine. It incorporated quiet delrin synchronizing gears, internal aluminum bronze heater tins, a separate pressurizable butter case, and clearance seals. However, the internal cross sectional area of the heat exchangers was too restricted, and the press fit between the heater wall and the internal fins produced an inadequate thermal bond, consequently initial performance was mediocre. At that time I was so very impatient to move on to such new ideas as the yoke drive that I simply abandoned this engine without any development whatsoever. Looking back on this episode, I am struck by my seemingly unlimited energy and enthusiasm, and how readily I squandered them.

I would also note that the 100cc was too different from its 65cc predecessor; it was indeed a new engine in almost every respect, with all the headaches that entails. A far better approach would have been to modify the 65cc engine one step at a time, so as to learn what was an improvement and what was not.

A third rhombic was a 300cc test engine built under a DOE appropriate technology grant to make a simple, low pressure, high speed hot air engine of 100 watts output. Although much larger than the 65cc rhombic upon which it was based, this engine was very much closer to that original engine in design than was the ill-fated 100cc machine described above. The DOE engine used off-the-shelf piston rings (re-machined for lower outspring and friction), a plain annular heater with inner sleeve, and a greatly larger bore (4.54 inches, 11.53 cm) than stroke (1.125 inches, 2.86 cm). It produced 112.4 watts peak power at 1150 rpm, atmospheric, and free speed was 2000 rpm. Testing under

Pressure was attempted, but was unsuccessful since the crankcase cover distorted un— der pressure, causing excessive shaft seal friction. By this time the grant funds had run out, and the balance of my effort on this engine was devoted to demonstration on a wood stove. This large rhombic presented few problems, other than finding the proper end gap for the rings, and the proper mixing tube and orifice diameters for the propane burner under which the power testing was conducted. There were signs that the friction could be further reduced, and the sealing improved, and I believe with more develop­ment time this engine would prove to have considerable potential. On the other hand, it was at the upper size limits of what could comfortably be made on my machine tools, so I did no more with it and eventually sold it to the University of Calgary.

During this DOE work, it occurred to me that the rhombic drive lent itself to use in a low pressure pancake-shape engine. lf one greatly increases the bore relative to the stroke (6:1 to 8:1 ratios), and uses the cylinder head, rather than the cylinder side, for the heater, then a very compact engine with ample heater surface area is possible. Several new versions of both the 65cc and the 11cc engines were designed along these lines, and overall dimensions were significantly reduced. A 1 10cc version of the 11cc engine was begun, featuring a 4 inch (10.2 cm) bore with the same 0.56 inch (1.4 cm) stroke. Overall height would be a mere 5 inches (12.7 cm), which is actually less than that of the 1 1cc engine upon which it is based. Even with an unfinned heater, there is sufficient surface area to produce 100 watts output, charged with air at atmospheric pressure. Al­though never completed, I still believe this design represents an excellent way to make a simple, compact, high speed, low pressure stove-top air engine.

Other Rhombics

The 11cc rhombic (above), and the 100cc rhombic (left).

Other Rhombics

On the left is the 65cc rhombic, with its bore/stroke ratio of 2.125 to 1. On the right is an engine of the same swept volume, but with a bore/stroke ratio of 6 to 1. The dotted line shows the further height reduc­tion possible if the top, rather than the side, of the cylinder cap is made the "heater".

Other Rhombics Other RhombicsBelow is a sectional view of the 300cc rhombic.

Other Rhombics

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *