Hot-air engines employing internal combus tion, like compression engines, form a separate class of this type of motor. At the outset of their consideration, however, it is important to note a somewhat arbitrary distinction between them and the internal combustion motors known as gas-engines. The true gas-engine is limited generally to internal-combustion engines, in which the air, mixed with gas or with oil-vapor, is admitted to the cylinder and ignited after its admission. Liquid or gaseous fuels arc essential in this type of engine. lint-air internal-combus tion engines, or, more properly, hot-air products of combustion engines, operate by forcing atmos pheric air through a closed tire, lvbiell may lie and generally is a solid fuel fire, and carrying the air and gases of combustion to the engine cylin der. One form of such air-engines is shown by Fig. 5. The furnace was placed in a chamber strong enough to withstand the pressure. The compressing pump B forced air below the ash-pit up through the tire, where it was expanded by heat and by combustion with carbon. Being ad mitted to the working cylinder against the pis ton, it was exhausted into the chimney. The fur nace had to be charged with fresh fuel through a combination of double doors. D, working on the principle of an air-lock. A hot-air product of combustion engine was described by Sir George Cayley, an Englishman, in 1807 and IS25, and again in 1q37 this inventor patented a modifica tion of the same design and built several engines. none of which gave any marked success. In 18'21 Dr. Neil Arnott (q.v.), the celebrated scientist, took up the same idea and patented a form of engine in which, to avoid the abrading action of the ashes on metal pistons and cylinders, he used oil pistons. Following Cayley and Arnott, number of inventors worked on the idea, among them being the Americans Stephen Wilcox, S. H. Roper, and Philander Shaw, each of whom built and sold a number of engines. Broadly speak ing, the difficulties of operating these engines were so great that they recorded a general fail ure. These difficulties were caused by flue-dust
and the grit in the cylinders, the rapid destruc tion of the working surfaces and valves by the intense heat, and the practical impossibility of lubrication. They were also more bulky than other types of hot-air engines in proportion to the power developed.
The possibilities of the hot-air engine as a competitor of the steam-engine are often urged, but so far it has never reached any practical suc cess which warranted much hope that the compe tition would prove serious. The two sides of the question are fairly and concisely summarized by Prof. F. R. Hutton as follows: "The hot-air engine in small sizes is more eco nomical than the steam-engine of the same capac ity. In larger sizes it has about the same economy as the less economical steam-engine, measured in coal consumed per horse-power. It has the advantage of avoiding the steam-boiler as a magazine or reservoir of energy which may be liberated by accident so suddenly as to be explosive. It can be run by less skilled and expensive labor, and no steam-runner's license is demanded. It is safe and odorless. The objec tions to the hot-air engine are the greater bulk and greater weight for the same power than is required with the steam-engine: the low mean pressure with high initial pressure, which latter compels great strength of structure; the de terioration of heating surfaces exposed to high heats and consequent oxidation: the difficulties of packing and lubricating at high temperatures; the difficulty of regulation closely to varying resistances. If there is any danger to the present supremacy of the steam-engine, it will be in rela tively small plants that a hot-air engine can be a substitute; the gas or internal-combustion en gine is more to be feared titan the hot-air engine proper." A theoretical and practical discussion of hot air engines and heat-engines generally is con tained in Hutton, Heat and Heat-Engines (New York. 1899).