3D Printed Engine Days 1-3

Three days ago, I began work on my project. I am designing a Stirling Engine, which is an external combustion engine. That means it takes heat from outside the engine and converts it into useful work (for example, rotating a wheel to generate electricity or drive a mill). The other main type of engine is an internal combustion engine, such as the one in your car. They generate work (spin your car’s wheel) from heat created inside the engine.

Stirling engines come in two configurations: displacer and two piston. In the two piston type, there are two pistons that power the engine. In the displacer type, there is one displacer piston and one power piston. The displacer piston moves air from the hot side of the engine to the cold side, while the power piston moves up and down due to the expansion and contraction of the air. Initially, I am constructing a displacer type stirling engine. Depending on how long it takes, I may also design a two piston engine as well.

On the first day of my project, I spent much of my time learning how to use the CAD (computer-aided design) software, Onshape. Onshape is free and cloud based, so my designs can be accessed from any computer with a web browser. It’s the Google Docs of CAD software. Luckily, it also has some built in tutorials, so I started there. After spending a few hours completing those, I moved on to researching my engine design.

I knew that I wanted to begin with a Stirling Engine, since they are relatively simple, useful, and widespread in physics education. After some quick research I learned about the two possible configurations. I opted to begin with the displacer variety, as it only requires one chamber and would be easier to print. Using some basic schematics that I found online (pictured below), I began to model my engine.

Credit: American Stirling Company

After a few hours of working, I had created this:

My original displacer engine design

The green piston is the power piston, the yellow is the displacer, and the gray cylinder is the piston container.

Satisfied with my work for the day, I headed over to the Small Hall Makerspace to do a test print of a random object and ensure that everything was in order. Unfortunately, there was an issue with the printer’s extruder. It had somehow gotten jammed the night before, so I was unable to print anything that day. I have been told that it was a known problem and should be fixed in a day or so, but I actually haven’t been back to check yet.

For the next two days, I have only been working on my engine design. The second day was mostly spent redoing everything that I had done on the first day. In order to maximize efficiency, the engine must have as little friction as possible. However, everything must also be airtight, as there is supposed to be no transfer of gas between the engine and the outside world. So, the pistons must be flush with the walls but have minimal friction. Since 3D printers are good at printing straight lines and bad at curves, I redid my cylindrical design and made rectangular prisms instead.

The updated design

After switching over to the cube based model, I then spent the third day creating the tower (dark gray), the crankshaft (light gray), and fan (red). I opted to have my engine do something beyond spinning, so now it should spin a fan when powered. There were also some updates to parts that needed to be made. I had to bore through the pistons and towers so that they could be connected to the crankshaft. It took a surprisingly long time to figure out how to design everything properly both with the program and with functionality.

In the next coming days, I need to create cylinders that connect the pistons to the crankshaft and continue refining the design. I realized yesterday that there’s no real world way to get the displacer in the container currently (funnily enough you can only move objects through walls on the computer), so I need to create a removable bottom for the container. Likely, I will also need to decrease the distance that both pistons move through, as I don’t think there will be enough heat to create so much expansion in the gas. After these refinements, I will be printing and making additional adjustments as necessary, and possibly working on a two piston engine.