10 hours. 10 hours to make a piston that seals air in a cylinder.
(And 1 more hour that won’t count because technically at 10 hours I still hadn’t sealed the air yet TT_TT)

I realized that rubber bands would not seal effectively, so I just got some cheap o-rings from amazon instead. But gosh, sealing was way harder than I thought it’d be!

The Process
At first I just created a simple groove in Onshape and experimented with my existing cylinder. But there wasn’t enough space for the o-ring to compress.

What followed was a lot of testing, trying out thicker and thinner o-rings, experimenting with the cylinder size for the power piston to slide smoothly, etc.

I fell down this deep rabbit hole of using a bigger cylinder bore and a thicker o-ring, printing and reprinting with different dimensions to find what worked.

To figure out if a piston seals correctly, the piston should “float” back up once you push it down, as without your compressing force the air can expand once again.

But none of these tests worked. I couldn’t figure it out, haha. I even tried Parker’s handbook of o-rings, but couldn’t figure out how it worked (my own skill issue :’) ).

But when I examined pistons that worked, I realized (after a while) that the problem was that the piston was too small! For the cylinder, anyway, so much so that when I pushed the piston down, it didn’t go in a straight line. It had the ability to tilt, letting air escape. So I made the cylinder smaller, but with enough tolerance to let the piston slide, and used lubricant!! I’m happy that after how many tries, the piston finally seals the air.

However, there’s much to improve on.

• The tolerance between the piston and the cylinder may be too small. There is friction between the two that interferes with performance.
• The groove is a bit too deep. Also the piston can seal air now, there is still quite a bit of air lost.

Hopefully these will be easy fixes. I’m glad I’ve learned from my many trials and errors.

Woah, I got a lot of stuff done!

Here’s a layout of what I finished:

• Designing the real layout of the Stirling engine (linkages that don’t overlap)
• Cylinder and base
• Gear holder
• Holes for screws
• Completed power piston
• Completed displacer piston
• Flywheel

However, after printing the first few parts, I realized a few issues at first:

• The linkages were too thick for the screws
• The displacer piston couldn’t be threaded through the hole of the power piston (that was my mistake)

So, I made the linkages thinner and adapted them to be more screw-friendly, and sliced the displacer piston into multiple pieces so that it would be possible to build and use.

On the second iteration, I printed out the base and an official gear holder to attach. There I found more issues:

• The power piston had too tight of a fit. It couldn’t smoothly slide into the cylinder.
• The flywheel wasn’t heavy enough. The goal is to have the gears spin for an extended period of time once the flywheel is spun, but the gears only rotate about twice.
• The connection between the gear holder and the base had bigger screw holes for smaller screws, so I have to make that connection thinner.

For the future, I think I should add a cavity in the power piston for something like a rubber band. Since I have no o-rings, that’s my cheap alternative for sealing, haha.

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.

I finished the design of the Stirling engine and printed it out! But I’m far from done.

I’ve added:

• A cylinder
• A stand
• Deeper holes to make it easier to screw into the design

Printing this took about 4+ hours, and putting it together who knows how long.

However, there were major flaws that I was able to pinpoint:

• The cranks don’t always move uniformly. Although they are screwed together, as 3d prints, they don’t actually have threads, so the screws are able to rotate independently from the other cranks.
• It’s hard to spin the shafts, and the cranks, at the same time, because of the issue above and friction.
• And…the most obvious…it doesn’t work.

I think it’s because of the aforementioned flaws, and because I was designing the beta engine based on mechanical linkages alone, not size or pressure, etc.

So now, I’ve decided to redesign the beta Stirling engine as a Rhombic Stirling engine, and go deep into the math using formulas from stuff like Schmidt analysis.

Yes…I made another mistake that I had to fix -_-

If you look at the photo in the previous devlog, you’ll notice that the connection between the displacer piston and the crankshaft actually overlap the power piston’s shaft—it moves in front of the connection between the power piston and its crankshaft! This was very hard to model in Onshape.

So I had to improve the linkage so that the power-crankshaft connection moved in the front instead. After a bit of tweaking (and painful double-checking), the linkage seemed alright.

So, I modeled the system in Onshape and it seemed to run well.

Now I have finalized the linkages, making sure that they don’t overlap and run properly, and everything finally seems to check out!

I’ve learned that prototyping, design, and checking my work is very important when it comes to engineering. I should learn to be less careless and spot these mistakes before investing too much time into them!

Although this was frustrating, I’m very happy that I learned that lesson :)

Ok, so I solved an issue while making a mistake. Silly me!
I created another linkage in MotionGen that fulfilled all the requirements from the previous devlog:

• The pistons don’t overlap
• The areas connecting the linkages don’t overlap

Unfortunately, in focusing on this, I forgot a crucial requirement: that the displacer crankshaft should be 90 degrees ahead of the power crankshaft. I had done it the opposite way. Instead, I set it up so that the power piston’s crankshaft was actually 90 degrees ahead.

By that time I had already modeled a prototype in Onshape and started CADing the final. But oh well, I should fix my mistakes anytime.

So! I have redesigned the linkage in MotionGen and it should fulfull all the requirements and have no mistakes (my peanut brain is bound to make one more though :’) ).

I hope to create a working prototype in Onshape to finally build and test it.

I experimented on the linkages for the beta engine and found a way for the displacer and power pistons to not overlap. I did this by:

• simulating different linkage lengths in MotionGen
• using a rotating crank and a slotted link.

Unfortunately, while one problem was solved, another problem appeared: the linkages were overlapping now!

If you look closely, the areas connecting the pistons to the linkages overlap. Obviously this is a big issue that I will have to solve.

So, now I have two goals:

1. To have the power and displacer pistons not overlap
2. To have the linkages not overlap, either. (How many times have I said “overlap” in this devlog?)

I plan to do this by examining the positions of the pistons when they are at their most extremes (when the displacer is at the left-most and when the power piston is at the right-most).

Hopefully this gets solved by today!

Now was the time to delve really deep. Now that I had the layout figured out, I needed to make the mechanism actually function. If you’re interested, the theory is in the photos down below!
In case you can’t read my horrendous handwriting:

1. The hot air moves to the center of the Stirling engine.
2. Because the hot air moves to the colder area, it becomes colder and compresses. The compression moves the power piston.
3. The cold air is then shuttled by the displacer to the hot end. The cold air becomes hotter and expands, moving the pistons and flywheel. The flywheel’s angular momentum continues to move the pistons, returning the cycle to its first stage.

Throughout steps 2-3, the flywheel keeps the momentum and continues to move the pistons. Unfortunately, I haven’t pictured the flywheel in relation to the engine, but you can imagine that the flywheel is at the axle of the crankshaft.

So, from what I knew, the cranks of the pistons needed to be, optimally, 90 degrees offset, with the displacer piston 90 degrees ahead so that it could shuttle the air first.
Then, using the diagrams I drew below, I configured one of the phases into Onshape to hopefully create a working mechanism.
The design is still in the works, but I think I at least have a nice working structure! :)

OK, I’ve decided to stick to the crankshafts for now! I figured that I at least need to know the basics of designing SEs before diving into the more unique variations.
This is less of a design, but more of a lay-out (albeit incorrectly :P). It has:

• The displacer piston (blue), which moves the working fluid through the engine
• The power piston (white), which transmits the output rotational force
• The linkages (aka crankshafts) for each piston (with their corresponding colors)

The difficult part was laying out every part in such a way that they didn’t overlap or cut into each other (at least the linkages). I was focusing more on the structure rather than the function, so the pistons are positioned incorrectly and will overlap each other when put into Onshape’s assembly!

I’m aiming to create a mechanism that actually functions correctly.

Stirling engine redesign

First of all, what is a Stirling engine?
A Stirling engine (SE) is an external combustion engine powered by heat. What did that jumble of words mean? Basically, unlike conventional engines, SEs don’t require fuels inside the cylinder to react. Instead, they use fuel or fluids outside of the cylinder, the aforementioned hot air. Interestingly, SEs can also be powered by cold air, so technically SEs are actually engines powered by temperature differences, not simply heat.

Okay, but what’s a beta Stirling engine?
Stirling engines have so many types! The alpha, beta, and gamma are the main ones. What these random names describe are the different configurations of a SE. The beta engine is a specific structure.

The dilemma
I was working on the SE in Onshape until I reached a pit stop. I was using mechanical cranks, but they cut through the axle! I checked some simulations and found that there had to be literally no axle in between––meaning that the axle would have to be cut. So I’m wondering if I should switch gears to a Rhombic drive mechanism. I’ll delve more into that next devlog. Seeya there! :)

(I know the video assembly looks wonky because it’s not put together properly, but you can see the issue pretty clearly)