a. (Mach.) Designating, or pertaining to, any engine (called an ) in which the heat or pressure energy necessary to produce motion is developed in the engine cylinder, as by the explosion of a gas, and not in a separate chamber, as in a steam-engine boiler. The gas used may be a fixed gas, or one derived from alcohol, ether, gasoline (petrol), naphtha, oil (petroleum), etc. There are three main classes: (1) gas engines proper, using fixed gases, as coal, blast-furnace, or producer gas; (2) engines using the vapor of a volatile fluid, as the typical gasoline (petrol) engine; (3) oil engines, using either an atomized spray or the vapor (produced by heat) of a comparatively heavy oil, as petroleum or kerosene. In all of these the gas is mixed with a definite amount of air, the charge is composed in the cylinder and is then exploded either by a flame of gas (flame ignition -- now little used), by a hot tube (tube ignition) or the like, by an electric spark (electric ignition, the usual method is gasoline engines, or by the heat of compression, as in the Diesel engine. Gas and oil engines are chiefly of the stationary type. Gasoline engines are largely used for automobile vehicles, boats, etc. Most internal-combustion engines use the Otto (four-stroke) cycle, though many use the two-stroke cycle. They are almost universally trunk engines and single-acting. Because of the intense heat produced by the frequent explosions, the cylinders must be cooled by a water jacket (water-cooled) or by air currents (air cooled) to give the maximum thermodynamic efficiency and to avoid excessive friction or seizing. [ Webster 1913 Suppl. ]