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Ignition Methods.—The other essential auxiliary group of the automobile power plant, and one absolutely necessary to insure engine action, is the ignition system, or the method employed of kindling the compressed gas in the cylinder to produce an explosion and useful power. The ignition system has been fully as well developed as other parts of the automobile, and at the present time practically all ignition systems follow principles which have become standard through wide acceptance. The essential elements of any ignition system are: First, a simple and practicable method of current production; second, suitable timing apparatus to cause the spark to occur at the right point in the cycle of engine action; third, suitable wiring and other apparatus to convey the current produced by the generator to the sparking member in the cylinder. Dry batteries are the simplest method of current generation, but storage batteries are mostly used at the present time.
Two distinct types of mechanical generators are in common use and, while their principles of action are practically the same, they differ somewhat in construction and application. The forms first used to succeed the battery were modifications of the larger dynamo electric machines used for delivering current for power and lighting. Later developments resulted in the simplification of the dynamo by which it was made lighter and more efficient, and the magneto igniter is the form furnished on some modern power plants. A dynamo uses electro-magnets to produce a magnetic field for the armature to revolve in and is necessarily somewhat heavier and larger than a magneto of equal capacity because the field in the latter instrument is produced by perma. nent magnets. An important advantage in using the magneto form of construction is that the weight of the windings is saved because the permanent magnets retain their magnetism and do not require the continual energizing that an electro-magnet demands.
The dynamo construction is superior where a continual drain is made upon the apparatus, as in modern lighting and electric starting systems, because if a magneto is used continuously the magnets are liable to lose some of their strength, and as the
magnetic field existing between the pole pieces decreases in value the amount of current delivered by the apparatus diminishes in direct proportion. When electro-magnets are used the constant flow of electrical energy through the windings keeps them energized to the proper point, and, as current is continuously supplied, the strength of the magnetic field remains constant. The dynamo form of generator is utilized where currents of considerable value are needed, such as in electric lighting systems now so widely used on automobiles.
When the device is depended upon only to furnish ignition current the magneto is preferred by some engineers because it is simpler and lighter than the dynamo, and also because it may be made in such form that it will comprise a complete ignition system in itself. When a dynamo is utilized the conditions are just the same, as far as necessary auxiliary apparatus is concerned, as though batteries were used and one merely substitutes a mechanical generator in place of the chemical cells. The same auxiliary apparatus necessary in one case is employed in the other as well. Anyone familiar with the basic principles of internal combustion engine action will recognize the need of incorporating some device in the ignition system which will insure that the igniting spark will occur only in the cylinder that is ready to be fired and at the right time in the cycle of operations. There is a certain definite point at which the spark must take place, this having been determined to be at the end of the compression up stroke, at which time the gas has been properly compacted and the piston is about to start returning to the bottom of the cylinder again. Timers or distributors are a form of switch designed so that hundreds of positive contacts which are necessary to close and open the ignition circuit may be made per minute without failure.
When the device is employed to open and close a low-tension circuit it is known as a commutator or timer, and when used in connection with current of high voltage they are called secondary distributors. Certain constructional details make one form different from the other and while they perform the same functions they vary in design. Such distributing devices are
Fig. 13.—The Battery Ignition System of the Dodge Brothers Motor Car
is Typical of Present Day
always driven by positive gearing from the engine and are timed so that the sparks will occur in the cylinders at just the proper ignition time. The usual construction is to use a fixed insulated case which carries one or more contact members disposed around its periphery and a central interior revolving member or cam which contacts with the points on the body of the device to close any desired circuit. The location of a device of this character is clearly shown in Fig. 13, which also shows the other parts of the ignition system.
The current obtained from the dry or storage battery or low-tension dynamo or magneto is not sufficiently powerful to leap the gap which exists between the points of the spark plug in the cylinder unless it is transformed to a current having a higher voltage. The air gap between the points of the spark plug has a resistance which requires several thousand volts pressure to overcome, and as a battery will only deliver six to eight volts, it will be evident that, unless the current value is increased, it could not produce a spark between the plug electrodes. The low voltage current is transformed to one of higher potential by means of a device known as the induction coil, plainly shown in Fig. 13. The current from the battery is passed through a primary coil in this device and is composed of a number of layers of coarse wire. This induces a higher voltage current in the secondary coil wound around it and composed of many turns of very fine wire. With the high tension system of ignition the spark is produced by a current of high voltage jumping between two spark plug points, which break the complete circuit that would exist otherwise in the secondary coil and its external connections. The spark plug is in a simple device which consists of two terminal electrodes carried in a suitable shell member, which is screwed into the cylinder. The secondary wires from the coil are attached to terminals at the top of a central electrode member which is supported in a bushing of some form of insulating material. Some employ a molded porcelain as an insulator, while others use a bushing of mica. The insulating bushing and electrode are housed in a steel body, which is provided with a screw thread at the bottom, by which it is screwed
into the combustion chamber. The current entering at the top of the plug cannot reach the ground, which is represented by the metal portion of the engine, until it has traversed the full length of the central electrode and overcome the resistance of the gap between it and the terminal point on the shell. All wiring of a modern battery ignition system is shown at Fig. 13.
The magneto is a simple form of dynamo and a mechanical
Fig. 14.—The Dual Magneto Ignition System Used on Maxwell
generator of electricity in which permanent magnets are used to produce the magnetic field and between which the armature revolves. The permanent magnets are called “field magnets" and at their ends are provided with cast iron shoes which form the walls of the armature tunnel and which are known as pole pieces. A magneto consists of two compound horse shoe magnets attached to the pole pieces which collect and concentrate the magnetism upon the armature. The armature is shuttle-shaped