All About Engines

The Petrol Motor 211 In the early engines the explosive mixture was fired by a hot tube, but with the improvement of electrical apparatus this has been completely re- placed by the electric spark. This is produced between the ends of a rod and one or two wires of nickel, nickel-steel, or platinum, fixed in a plug which is screwed into the end of the cylinder. The rod down the centre is insulated from the metal body by mica washers. Sometimes, for high speeds, two sparking plugs are used to ignite the explosive mixture at two points and secure a more rapid explosion. If, for example, an engine is running at a thousand revolutions a minute, then, since there are two strokes to a revolution, each stroke takes only three-hundredths, or ’03 of a second. The explosion, therefore, must be extremely rapid if it is to have any effect upon the piston, which, imme- diately it has passed the end of the stroke, will move away at an average rate of a thousand feet a minute, or nearly 17 feet a second. The spark is produced either by an induction coil and accumulator or by a magneto, which is really a small dynamo having permanent magnets instead of electro-magnets for the field magnets. The armature is wound to give a high-tension current, and the magneto is driven from the main shaft. The spark is produced by a metal stud on a rotating ebonite disc making contact with a fixed stud, and in view of the high speed of the piston it must be accu- rately timed. For if the explosion takes place before the piston has finished its stroke a “ back fire ” occurs