An eponymous, hypothetical heat engine that violates the Second Law of Thermodynamics by continuously turning the heat energy of its surroundings into work. Ex hypothesi it could not in practice be realized.
Description of the Cycle:
A cylinder, open at both ends, is fitted with two sliding pistons - one in each end. A single particle bounces round inside, trapped between the two pistons. It is possible to divide the cylinder into two chambers by sliding down a barrier in its middle.
Step 1: Slide down the partition.
Step 2: Push in whichever piston does not have the particle in its half of the cylinder. Since there is no particle this will require zero work.
Step 3: Slide up the barrier - now the particle may bang against the piston that was pushed in in Step 2.
Step 4: Allow the pressure of the particle's tiny blows to push out said piston. This step gives out work.
This completes the cycle. It is seen that no work is done on the engine while work is given out.
For a detailed and generally fascinating discussion see: "Demons, Engines and the Second Law." Charles H. Bennett. Scientific American. Nov. '87. Page 88.