Ok, I’ve been working on the Rhombic Stirling engine for a while now!

I tried to use math to figure out the lengths of the linkages for the optimal dimensions, and a phase offset of 90 degrees. I even tried to use MotionGen. Unfortunately, the math was faulty––the website I found it on even calculated the wrong answers for the practice questions. And MotionGen wasn’t working. The minutes of Google searching wasn’t getting me anywhere in troubleshooting, and I just gave up.

So I resorted to good ol’ guessing and checking XD. Like I stated earlier, I will be using Schmidt analysis to determine if the engine will really run though, so no worries! I’ll still use math…

After a few iterations I was able to reach one I was happy with. It not only ran smoothly in Onshape’s assembly, but also allowed the displacer to be about 90 degrees ahead of the power piston, as is optimal.

However, figuring this out was not as easy as for the crankshafts. For the Rhombic Stirling, to determine the phase angle, one must:

1. Mark the gear in some way.
2. Turn the gear until the power piston is at the topmost of its stroke. Take note of the position of the mark on the gear.
3. Turn the gear until the displacer piston is at the topmost of its stroke. Again, take note of the position of the mark on the gear.
4. Measure the angle between the position of the first mark and second mark.

Fortunately for me, my 4th-5th iteration produced a 90 degree phase angle, which is lucky for a guess and checker :).

The first two photos are missing a linkage for clarity. The third features the full linkage.