HOT AIR ENGINES, a type of prime mover in which air, which has been heated to a high temperature and pres sure, acts upon a piston and thus con verts thermal energy into mechanical energy. Air, under high temperature and pressure, is admitted into a cylinder where it expands and moves a well fitted piston, supplied with nearly airtight piston rings. Consequently its temper ature and pressure are decreased. The air is then exhausted either into the atmosphere, or carried through a system of pipes and again heated and injected into the engine. When the same charge of air is reheated, and new air used only to replace leakage, the engine is said to be of the closed cycle type, while those of the open cycle type are the ones in which a fresh charge of air is used for each stroke of the piston. Some engines are designed to have the tem. perature change take place at constant volume, others at constant pressure. In some engines a device is used to absorb the heat from the exhaust air and use it to heat the new air. If the engine
is fitted with such apparatus, it is said to be regenerative, if it has no such device it is non-regenerative. The theo retical economy of the hot air engine has often been urged, but there has been no large successful commercial application of the principle, for in the larger sizes steam is much more economical, while the smaller sizes cannot compete with the internal combustion engine. The Rev. Robert Stirling built a comparatively successful engine about 1816, which was used commercially in Dundee, Scotland, after 1827. In 1833 John Ericsson de signed a hot air engine of large size for use in his boat, but after much experi mentation it was pronounced a complete failure. Napier and Rankins did early experimental work in this field, and Stephen Wilcox, Philander Shaw, and S. H. Roper, Americans, separately built and sold a number of the engines.