Introduction: Four Cylinder Air Engine, Experimental

About: Our grandkids keep me busy!

A few months ago (February, 2017) I posted a normal and slow motion video of a four cylinder air engine on my YouTube channel ( After experimenting with the design, given its weight and air flow requirements, I put in on the shelf and decided not to publish it. Since then, I've had a number of requests for the STL files, so here they are. Remember, this engine is experimental. so be prepared for precision filing and fine adjustments if you decide to print and experiment with it.

This design utilizes a spinning rotor to direct air into each of the four cylinders. The rotor contains four holes, spaced equally along the length of the rotor, and rotated 90 degrees from each other. "Gear Crankshaft.stl" and "Gear Rotor.stl" are designed to maintain synchronization between the rotor and the crankshaft to make sure air is delivered to the cylinders at the proper time. The design depends on cylinder blow by for exhaust, and as such it is not too efficient, but it does make an interesting conversation piece.

As usual, I probably forgot a file or two or who knows what else, so if you have any questions, please do not hesitate to ask as I do make mistakes in plenty.

Designed using Autodesk Fusion 360, sliced using Cura 2.7.0, and printed in PLA on an Ultimaker 2+ Extended and a dual extrusion Ultimaker 3 Extended.

Step 1: Print and Prepare the Parts.

I have attached a PDF file containing the list of printed parts which includes the part names, part counts, print settings and colors I used.

You will need to have a 1/4 thread compressor adapter and compressor to operate this model.

Prior to assembly, test fit and trim, file, sand, etc. all parts as necessary for smooth movement of moving surfaces, and tight fit for non moving surfaces. Depending on the colors you chose and your printer settings, more or less trimming, filing and/or sanding may be required. Carefully file all edges that contacted the build plate to make absolutely sure that all build plate "ooze" is removed and that all edges are smooth. I used small jewelers files and plenty of patience to perform this step.

Step 2: Assemble the Block.

Assemble a piston by placing the smaller end of one "Connecting Rod.stl" into one "Piston.stl", and secure in place with one "Piston Pin.stl". Repeat this process for the remaining 3 pistons. Test fit the piston assemblies in "Block.stl" to make sure they slide smoothly with no binding or grabbing.

Position on "Journal Main.stl" as shown in "Block.stl". Slide one piston assembly into the cylinder from the top of the block as shown. Center the large hole in the connecting rod over the hole in the journal main, then press one "Journal Rod.stl" into the hole in journal main as shown. Test to make sure the assembly rotates with ease. Repeat this process for the remaining pistons and journals carefully noting the orientation of each journal assembly as shown.

Press "Cap Crankshaft.stl" onto the first main journal as shown.

Press "Gear Crankshaft.stl" onto the last main journal as shown.

Press "Head.stl" onto the block assembly as shown.

Slide "Rotor.stl" into the head as shown.

With the rotor aligned as shown and the first journal rotated such that the piston is at the top of its stroke, press "Gear Rotor.stl" onto the rotor as shown. The position is important for successful operation of the engine.

Thread a compressor fitting into "Adapter Compressor.stl" as shown.

Press the adapter assembly into the head as shown.

Step 3: Test the Engine.

I've included a slow motion video of the engine in operation using an air compressor with about 8psi of air pressure.

That's how I made my Four Cylinder Air Engine.

Hope you like it!