During the first half of the nineteenth century the Stirling brothers and John Ericsson made significant attempts to design hot air engines which could provide an economical alternative to high-pressure steam. They believed that it would be possible to gain fuel efficiencies superior to those obtained with steam by employing a heat exchanger within the air engine cycle, erroneously supposing that such a heat exchanger could eliminate the large loss of heat incurred in the condenser of the steam engine. The (...) Stirling brothers called their heat exchanger an economizer because it prevented the waste of heat, but Ericsson coined the term regenerator because he imagined that the power of the heat (caloric) could actually be regenerated, i.e. re-used in the engine to generate mechanical work. Ericsson's term became commonly accepted, so that Stirling's economizer is now also referred to as a generator. These attempts to attain high horsepower outputs with hot air engines were abandoned after the dramatic failure of Ericsson's notorious paddle steamer in the early 1850s. Further ventures were aimed at the more limited goal of low horsepowers and generally ignored the regenerator principle. (shrink)