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Electrical System

ignition, voltage, gap, battery, current and magneto

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ELECTRICAL SYSTEM The carburation and manifolding system delivers the proper mixture of fuel and air to the engine. There the chemical energy is converted into mechanical energy through the process of com bustion. The ignition system functions in starting this very process by igniting the combustible mixture in the cylinder.


The spark-plug provides ignition of the charge. It consists of an electrically insulated rod surrounded by a shell which screws into a cylinder. Projecting into the cylinder is a pair of short wires, one an extension of the central rod and the other attached to the shell, and between the pair there is a short gap (.015 to -025 in.) across which the spark passes. These two wires are the electrodes. The electrodes are made of a heat resisting metal alloy. The outer shell is usually made of steel, threaded on the outside to permit screwing it in place in the cylin der. The insulator, which surrounds the central electrode, is either some ceramic material or mica, and is selected to with stand high temperatures, resist mechanical strains, and to possess high insulating properties. This last requirement is very impor tant for electric current must jump nowhere except across the spark gap. The insulating material must also be a relatively good heat conductor, and proportioned in size and shape to permit the dissipation of heat into the shell. By this means the temperature of the insulator and electrode is kept down to a point where the material itself will not be injured, and is kept below the point where it might cause ignition of the charge at the wrong time. Nevertheless, it is just as necessary to maintain the temperature of the insulator sufficiently high to burn any liquid oil or fuel from its surface in order to prevent electrical leakage across the surface. The spark-plug has practically supplanted earlier types of ignition devices, such as hot tubes and make-and-break spark mechanisms, as far as common practice in motor vehicle engines is concerned.

Ignition System.

In order for the spark to jump the spark gap, a high voltage is necessary. This is obtained either from a magneto or a battery ignition system.

Magneto.—Magneto ignition is used on many European cars and on many aeroplanes where the additional weight of a storage battery is sometimes undesirable. The magneto is fundamentally an electric generator using permanent magnets for the field, and equipped with an integral transformer for the purpose of in creasing the voltage to a value high enough to jump the gap in the spark-plug. Either the coils in which the currents are gen erated may rotate between the stationary field magnets, or the field magnets may rotate within the stationary coils. At the proper time the current which is induced in the primary coils by the relative rotation of the field magnets and the coils is interrupted by means of a circuit-breaker mechanism operated by a cam. A sudden flux change then occurs which induces a current flow in the secondary windings of the magneto, of sufficiently high voltage to jump the gap of the spark-plug in series with this secondary winding. A distributor built integrally with the magneto distributes the secondary current to the various cylinders in the proper order. (See also HIGH TENSION MAGNETO.) Battery Ignition System.—The mechanism of a battery igni tion system appears different in form from the magneto, but functions in a very similar way. The engine drives an electric generator. The electric energy generated is either used immedi ately in the electrical system of the car or is stored in a battery, or accumulator. The ignition system draws on the low voltage battery for an electric current, which passes through an inter rupter and through the low voltage winding of a transformer, or coil. Current is induced in the high voltage winding of the same coil and passes through a distributor, and thence through the spark-plug electrodes, jumping the spark gap.

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