compressed air 3 cylinder radial engine (Working)

thingiverse

I was inspired by Tom Stanton's work. Unfortunately, I didn't see a working multicylinder engine, so I decided to improve my drawing skills and design one myself. After opting for a flat twin, it wasn't much extra effort to make a radial, and since it's effectively a two-stroke, it could be self-starting, so that's what I did. The engine uses bearings, O-rings, airsoft ammo, springs, 2mm rods, and M2 bolts. The crank bearing has the following dimensions: outside 15mm, inside 6mm, thick 5mm. The rod bearings are 5mm outside, 2mm inside, and 2.5mm thick. All bolts are M2, which is actually too small to tap nicely into the plastic. There are 24 X M2 x 10mm, 1 X M2 x 12 (cranks), and 6 X M2 x 15. The pushrods are 32mm long, and the springs are 17mm long with a 5.4mm outer diameter and a 0.5mm wire thickness. The O-rings are 4.5mm inside and 8.5mm outside. I tried to make it have a 3x 2.5cc displacement (11mm stroke, 17mm bore). I haven't used gaskets yet between the cylinders and heads, which could improve efficiency. The cam was my first attempt, and it seems to work well enough. It's purring nicely at 1 bar: https://www.youtube.com/watch?v=g_-GaT5-Jkg And sounds better at 4 bar: https://www.youtube.com/watch?v=pnACFU9a8LI Some performance data (not very accurate): prop 9x4 APC C2. At 1 bar, it's 930 rpm; at 2 bars, it's 2250 rpm; at 3 bars, it's 3000 rpm; at 4 bars, it's 3960 rpm; at 5 bars, it's 4500 rpm; and at 6 bars, it's 5250 rpm, at which point the prop became loose, drastically decreasing performance. I've added a new crank. It uses an M3 bolt in the center to keep the two halves together, allowing for a 12mm thick propeller or so, and it held up the 8 bar I threw at it. It will spin a 13x8 prop at 3000 rpm at 6 bars and will go up some at 8 bars. I've seen 3250 rpm with no issues except frozen fingers. It does move air. It sounded more alive with the smaller prop, but more stable at these speeds. It's still plastic.