The Stirling Engine

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 (TH).
  2. Removing heat at constant volume (V2).
  3. Compression at constant temperature (TL).
  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!