Electric Ignition by Battery and Induction-Coil.—The first system is the cheaper; but it exacts the most painstaking care in maintaining the parts in proper working condition. It comprises three essential elements—a battery, a coil, and a spark-plug (Fig. 12). The battery may be a storage-battery, which must, consequently, be recharged from time to time; or it may be
a primary battery which must be frequently renewed and carefully cleaned. The induction-coil is fitted with a trembler or interrupter, which easily gets out of order and which must be regulated with considerable accuracy. The spark-plug is a particularly delicate part, subject to many possible accidents. The porcelain of which it is made is liable to crack. It is hard to obtain absolutely perfect insulation; for the terminals deteriorate as they become overheated, break, or become foul (Fig. 13). In oil-engines, especially, soot is rapidly deposited on the terminals, so that no spark can be produced. In benzine or naphtha motors, such an accident is less likely to happen. In automobile-motors, however, the spark-plug only too often fails to perform its function. The one remedy for these evils is to be found
in the most painstaking care of the spark-plug and of the other elements of the ignition system.
Fig. 14.—Magneto ignition apparatus.
Fig. 15.—General view and details of a magneto ignition apparatus.
Ignition by Magnetos.—Magneto apparatus, on the other hand, are noteworthy for the regularity of their operation. They may be used for several years without being remagnetized, and require no exceptional care. Magneto ignition devices are mechanically actuated, the necessary displacement of the coil being effected by means of a cam carried on a shaft turning with half the motor speed (Figs. 14 and 15). At the moment when it is released by the cam, the coil is suddenly
returned to its initial position by means of a spring. This rapid movement generates a current that passes through terminals, which are arranged within the cylinder and which are immediately separated by mechanical means. Thus a much hotter circuit-breaking spark is produced, which is very much more energetic than that of a battery and induction-coil, and which surely ignites the gaseous mixture in the cylinder. The terminals are generally of steel, sometimes pointed with nickel or platinum (Fig. 16). The only precaution to be observed is the exclusion of moisture
and occasional cleaning. For engines driven by producer-gas magneto-igniters are preferable to cells and batteries. In general, electrical ignition is to be recommended for high-pressure engines.