IGNITION, in gas engines, the firing of the explosive mixture of gases — in the cylinder of the reciprocating engine; in the pressure chamber of the rotary engine. In the earliest types of gas engine ignition was accomplished by an outside flame kept burning before a slide door, which was opened when the cylinder had been charged. After compression of the gases was introduced, the ignition was secured by a platinum tube projecting into the cylinder, and heated red-hot by an outside flame blown into it. In the modern gas engines ignition is ac complished by a "fat)) electric spark, produced either by a battery, a magneto, or a small dynamo. There are two systems of electrical ignition: the ajump-spark," or high tension system; and the °make-and-break" or low ten sion system. In the first, the electric current is taken into the cylinder through a "plug," at the inner extremity of which is a break, or "gap" in the continuity of the metal conductor, across which gap the current is forced to leap. This gap ranges in width from one-third inch where a coil is used in the circuit, •to one-sixty-fourth inch where there is no coil. The fact that the explosion of
the gaseous mixture is not actually instan taneous, but requires an appreciable time, has led to the use of two sparking plugs in cylinders of large capacity. In this case one of the plugs is a double pole member, and the firing current is sent first through this, going afterward through the ordinary plug. A gain of power ranging as high as 30 per cent has been noted as a result of this double firing.
The make-and-break system is in use chiefly in small marine or motorboat engines. Within the cylinder are two polar extremities of the battery conductor, and one _of these is moved by a mechanism outside of the cylinder, so as to make contact with the other, and ihis con tact is immediately and suddenly broken, when a spark is produced. This method does not re quire as strong a current as that necessary ,to make the leap across the gap in the spark plug, but with a coil in the circuit the spark is big enough and hot enough to be entirely depend able. See AIRPLANE ENGINE ;' AUTOMOBILE • EN INTERNAL COMBUSTION ENGINE.