One magazine hyped the Stirling engine as an "almost-perpetual motion machine".

The cycle in the Stirling engine involves:

1. Expansion at constant temperature (T_h).
2. Removing heat at constant volume (V₂).
3. Compression at constant temperature (T_c).
4. Adding heat at constant volume (V₁).

The Stirling engine is very efficient, but no engine, not even an ideal engine, would be a perpetual-motion machine. To create a perpetual motion machine you'd need either a lower-temperature reservoir of absolute zero or a higher-temperature reservoir of infinite temperature. In practice you can't come close to that, so even with an ideal engine the efficiency with reasonable temperatures is considerably less than the 100% necessary for a perpetual motion machine.

Moral: Even an ideal engine would not live up to hype like "an almost-perpetual motion machine". A Stirling engine is a good design, but take such claims with a rather large grain of salt!