THE gas-engine differs from both the steam and hot-air engine in the character of the expansion of the elastic fluid employed and in the mode of applying the heat. In the one the fire is used to convert water placed in a vessel exterior to the engine into steam, which, let into the cylinder, moves the piston by its expansive force; and in the other it is used to expand a volume of air contained in the cylinder or adjacent chamber.
In both, the heat is applied outside of the working cylinder, but the peculiarity of the gas-engine is that the heat is developed within the cylinder itself. A mixture of gas and air is by the operation of the engine drawn into the cylinder, and then exploded, the heat generated expanding the products of combustion, which, exerting a pressure against the piston, give it motion. Simple as is this mode of converting heat into work, the practical realization of it has been found to be exceedingly difficult, and it is only within a very few years that thoroughly serviceable machines have been constructed. The most economical result is obtained from expanding gases when the pressure they exert is a continuous and gradually diminishing one, such as that of steam in the steam-engine. With an explosive mixture, like that in the gas-engine, the expansion takes place with great rapidity, producing a sudden and unsustained pressure, from which it is difficult to get either an economical result or a steady operation of the mechanism. This rapidity of expansion can be decreased, and the pressure obtained approximated to that of the steam-engine, by altering the proportions of air and gas so as to produce a quick combustion instead of an explosion, and by introducing the gaseous mixture into the cylinder gradually, instead of all at once; and it is in this direction that the improvements have taken place which make the latest forms of gas-engine so superior to their predecessors.
Among the first engines to obtain a moderate degree of success were those of Hugon (1858) and Lenoir (1860). Neither of these was, however, very economical in the use of gas, and, previous to the engine of Otto and Langen in 1867, none were produced that were at all satisfactory in this respect. This was, however, objectionable, owing to the intolerable din it made when in operation.
While this engine was at best but a very qualified success, it has led the way to a machine which is very far from being so. In the Otto silent gas-engine, introduced a few years since, and now made