Stirling engine at 40°C / 5°C

What's happening, step by step

1. Heating (0°→90°): The displacer sits near the cold (right) end, so most of the air is in the hot (left) chamber bathed by 40°C water. The air warms, expands, and pressure rises — pushing the power piston outward to the right. This is the power stroke; mechanical work leaves through the connecting rod.
2. Transfer to cold (90°→180°): The crankshaft carries the displacer leftward, shuttling air past it (around its loose-fitting edges) into the cold chamber on the right.
3. Cooling and compression (180°→270°): Air now sitting against the 5°C wall loses heat and contracts; pressure falls. The flywheel's stored momentum pushes the power piston back inward at this lower pressure — so it costs less work to compress than was extracted during expansion. The net work over the cycle is the difference.
4. Transfer to hot (270°→360°): Displacer moves rightward again, shuttling the now-cool air back into the hot chamber. The cycle repeats.

The 90° phase offset is the whole trick

The displacer and power piston are both linked to the same crankshaft, but their crank pins are 90° apart. That offset means whenever the gas is hot (high pressure), the power piston is in the middle of moving outward; whenever the gas is cold (low pressure), the piston is moving inward. Pressure pushes outward more than it resists being pushed back in — net positive work per cycle.

From flywheel to iron

The flywheel transmits its rotation through a drive belt to a small friction wheel pressed against a ~60 g iron target (2 cm cube). Friction converts mechanical work directly into heat, concentrated in a much smaller mass than the warm reservoir that supplied the original energy. That's how you get the temperature "amplification": you trade total energy (most stays in the warm envelope as waste heat) for energy density in the target.