Designing a free-piston Stirling engine

Why a gas spring

Most hobby Stirlings load the power piston with a weight or mechanical spring. Weight degrades performance. Pressurizing the space above the piston gives a spring that compresses as the piston rises and returns it, with nothing to wear out; the engine becomes a gas spring on both sides of the piston in a sealed vessel.

Why helium

Helium's low molecular mass and high thermal conductivity let the working gas heat and cool faster, which is why high-performance Stirlings use it.

Why vertical

The planar spring works horizontally but carries side-loading; vertical mounting lets gravity assist the displacer and removes that load.

Why a burner, not a campfire

Fire restrictions often rule out campfires; a propane torch or butane stove is available almost anywhere, so the engine is designed around it.

Cost targets

Copper-coil cooling instead of a water jacket (jackets were expensive and prone to leaks). Displacer quoted at ~$130 machined; ceramic would be ~$500, heavier and brittle.

Design log · Jul 26, 2025

Snapshot posted to the channel community tab mid-design; expected to change.

• Spring sizing: for a ~0.4 kg displacer at 5 Hz, k = (2π·5)² × 0.4 ≈ 394 N/m. The spring sets the displacer's natural frequency, which has to match the engine's thermodynamic cycle.
• Power piston: AIRPEL M24D25.0D (double-acting, 24 mm bore, 25 mm stroke). One port sealed so the trapped air acts as an internal gas spring; the other port at the base runs vertically straight into the engine dome.
• Dome cap: hemispherical, chosen for airflow and mechanical fit.
• Plumbing: minimize T-junctions (flow losses); don't feed two pistons symmetrically unless they're isolated. Two piston layouts considered, the single high-efficiency piston favored.
• Alternator: must be matched to piston mass and stroke to convert motion efficiently.
• Feasibility for ~30 W: tight sealing, a high temperature gradient, low dead volume, minimal friction, and accurate spring-to-piston alignment.