In January of this year a significant buzz was created involving a New Hampshire inventor, Dean Kamen, and a project known as "Ginger" or "IT". Rumor has it that "Ginger" is some kind of scooter powered by a Stirling engine. 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_L).
4. Adding heat at constant volume (V₁).

The Stirling engine is very efficient, but it is still less efficient than the ideal Carnot engine. It's important to realize that to create a perpetual motion machine you'd need either a lower-temperature of 0 K or a higher-temperature of infinity. In practice you can't come close to that, so even with an ideal engine the efficiency with reasonable temperatures is conserably 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 less efficient than an ideal engine, so... It is a good design, but take such claims with a rather large grain of salt!