It is possible to convert mechanical work into heat with virtually 100% efficiency, as, for example, when work is done against a frictional force. There is, however, an upper limit to the efficiency of an engine that turns heat energy into directed mechanical work. This upper limit is achieved by an ideal heat engine operating in a Carnot cycle between a hot source and a cold sink. Bearing in mind that the performance of a real heat engine will fall short of that of an ideal engine we shall use the theory of the Carnot engine to investigate the problem of cooling the Channel rail tunnel that links Dover to Calais.