A question of stages

Is the engine Andrew Porter describes in his comments on thermodynamics (

, 12 February) two-stage?

The efficiency of a single-stage thermodynamic engine cannot exceed the Carnot value, which is only a function of the temperature difference — at most 50 per cent for current technologies/materials.

Anyway, we should be seeking efficient engines to burn hydrogen or otherwise use it in a fuel cell, in anticipation of wider use of this energy source. We also need safe methods of storing and retrieving the gas.