The gas engine (Fig. 176), which is coming gradually into use, requires but a small amount of fuel. In a steam boiler, the energy is transmitted to water inside the vessel. In the gas engine, the gas or oil is brought in contact, mixed with the air, and exploded. Gas engines are constructed in somewhat the same way as an ordinary high-pressure steam engine, and are built both as single and coupled engines. The cylinder is specially constructed and is surrounded by a water jacket provided with an ample supply of water to keep it cool (Fig. 177). The piston and rod, guards, connecting rod, crank, and fly-wheel are the same as those of a steam engine. The propulsive force of the gas engine is furnished by an explosion produced by igniting within the cylinder a mixture of air with coal gas, kerosene, gasoline, or alcohol vapor. To have complete combustion, it is necessary to have sufficient air, as the oxygen must combine with the hydrogen and carbon of the fuel. The gas is admitted at every other revolution, since the products of combustion must first be expelled by the piston on its first return stroke. During the second stroke the mixed gases are admitted through a valve, which closes like a pump valve when the piston shoots back. When the piston is at the end of its stroke and has compressed the gases, it closes an electric circuit, which is broken when the piston shoots on its second outward stroke. This produces a spark which ignites the gases, and the operation is then repeated. This method of sparking is classified as a make-and-break system, and should be distinguished from the spark-plug system.

As the force is excited on but one side of the piston, and only once in two revolutions, the gas engine is less steady than the steam engine, which has two impulses for each revolution. This fault is overcome to some extent, however, by the use of heavy flywheels.