The liquid air engine

At cryogenic temperatures heat release from a so-called permanent gas by phase change is possible

As a teacher of thermodynamics, I not only read with interest the piece on the liquid air engine (February 2012) issue of this periodical but decided that an important pedagogic point could be made from it. 

I have often emphasised in lectures that in a steam cycle the heat which is converted to work results from phase change and that no such heat release is possible with air hence the need to introduce something capable of chemical heat release into the cycle. This is of course the basis of the Otto and Diesel cycles. At cryogenic temperatures heat release from a so-called permanent gas by phase change is possible so a counter example to my oft-repeated point is provided.

J.C. Jones, University of Aberdeen

Next letter: The McNulty report