47. A direct current must always be used for charging the storage battery. In case only an alternating current is available, an alternating-current rectifier of some kind must be used for changing it into direct current.

1

The voltage of a storage cell varies from about 2.5 when fully charged to about 1.7 when completely discharged. The pressure falls rapidly about 10 volt when the battery first begins to discharge after being fully charged. When the voltage has dropped to about 1.7 per cell, the battery should be recharged, because it retains only a comparatively small amount of electric energy at this pressure, and the voltage drops rapidly during discharge after getting this low. The life of the battery is shortened by discharging below 1.7 volts per cell.

Storage batteries for ignition purposes are generally made up of either three cells in series, giving an average pressure during discharge of about 6 volts, or two cells in series, with an average discharge pressure of about 4 volts. These two pressures are adapted to meet the requirements in operating induction coils.

SPARK COILS

48. There are two distinct methods of forming a spark for ignition purposes by means of the electric current. One method employs a current of low voltage, such as that produced by an ordinary dry-cell battery, and the spark is obtained by making and breaking the circuit. Systems using this method are known as low-tension, or make-and-break, ignition systems. The other method makes use of a current of very high voltage and the spark is formed by causing this current to jump an air gap in a spark plug. Systems using this method are known as high-tension, or jump-spark, ignition systems.

49.

While a spark can be obtained from an ordinary battery current in a low-tension system, it is possible to increase its intensity by making use of an inductance, or kick, coil. A kick coil consists of a single coil of wire wound about a bundle of soft-iron wires, known as the core.

When a low

tension, or low-voltage, current flows through such a coil, an action known as self-induction takes place, and any rapid change in the strength of the current is opposed. Consequently, when the circuit is broken, the current continues for a longer time across the space between the separated contact points; and when the points are separated quickly, the current also continues across a longer space or a wider gap. By using such a coil with a battery, a spark may be produced that is hot enough to ignite a combustible gaseous mixture. A kick coil, suitably mounted, is shown in Fig. 18.

50. In a high-tension system of ignition, a very high voltage is required to cause a spark to jump the air gap, hence when only current of low voltage is available, such as a battery current, it is necessary to employ some means for converting the current into high-tension current. In marine-engine ignition, an induction coil is used for this purpose. An induction, or jump-spark, coil consists really of two coils of wire, one wound about the other, and the whole surrounding

FIG. 18

a core composed of a bundle of soft-iron wires. The inner, or primary, coil is made up of a few turns of coarse, insulated wire, while the outer, or secondary, coil is made up of a large number of turns of fine, insulated wire.

If a current of low voltage is caused to flow through the primary coil and then stopped suddenly, a high-tension current is set up in the secondary winding. A high-tension current is also induced when the primary current is suddenly increased. In practice, the battery current is run through the primary winding and the circuit is repeatedly broken by means of a magnetic vibrator or an interrupter. The secondary winding is connected to the spark plug or spark plugs, and a high-tension current carried to the spark gap or gaps at regular intervals.

51. In Fig. 19 is shown a typical jump-spark coil enclosed in a wooden box a. The coil proper consists of the core b, the primary coil c, and the secondary coil d. The coil is provided with the magnetic vibrator e, which automatically makes and breaks the primary circuit. The battery or lowtension magneto is connected to the low-tension terminals f and g, hence the current must pass through the vibrator e and the bridge h in order to flow through the battery. With the vibrator in the position shown, the circuit is closed and a current will flow through the primary coil c. As soon as a current flows through the coil c, the core b becomes magnetized and the soft-iron piece i draws the vibrator to it, separating the vibrator points at j and breaking the primary circuit. This

operation induces a secondary current of high voltage in the secondary coil d, which is connected to the external circuit by means of the terminals k and 1. One of these secondary terminals is usually connected to the spark plug and the other to the engine frame, thus completing the secondary circuit.

52. When the contact at j is broken, a spark is formed at that point if no provision is made to prevent it. For the prevention of such a spark, with the accompanying injury to the contact points, a condenser m is connected around the point j. The condenser consists of alternate layers of tin-foil and waxed paper and has the property of storing up electric energy; hence, when the contact points are separated, the current flows back into the condenser instead of forming an arc over the space.

Thus, a spark is prevented and the primary current stopped more suddenly, which causes a stronger secondary current to be produced. The energy stored in the condenser is given back to the primary circuit.

ELECTRIC GENERATORS

53. Classification.-The electric generators commonly employed for ignition purposes are divided into two principal classes, namely, those that generate a continuous, or direct, current, and those that generate an alternating current. The former are known as direct-current generators, dynamo-electric generators, or simply dynamos. The latter are known as magneto-electric generators, or simply magnetos. There are two classes of magnetos: (1) those that generate a low-tension current for the make-and-break type of ignition system and for delivery to both vibrator and non-vibrator induction coils, by which the low-tension primary current is transformed into a high-tension current, which is led to the spark plugs by the heavily insulated secondary wiring, and (2), those that generate a high-tension current, embodying within themselves all the elements necessary to the production and distribution of such current, thereby making the use of induction coils unnecessary.

54. Essential Parts of an Electric Generator.-As the simplest mechanical motion is rotation, electric generators use this principal for sweeping the conductors through the magnetic field. There are essentially two parts to such a machine: the field magnet, wherein is produced the necessary magnetism; and the armature, on or near whose surface the working conductors (those that cut the lines of force) are arranged. These two parts are rotated relatively to each other, it being immaterial, except for convenience, which is stationary and which is rotated.

A single conductor can seldom be made to generate a desired voltage, so that on an armature a number of conductors are usually connected up in series and in parallel, in the same way as electric batteries, until the required voltage and currentcarrying capacity are obtained.

55. Dynamos.-The field magnets of a dynamo may be permanent magnets or electromagnets. As a usual thing, they are electromagnets and consist of soft iron wound with wire, through which current flows, thus magnetizing the iron. Ignition dynamos are generally of the self-exciting, shunt-wound type, the magnetizing current being obtained from the dynamo itself. The armature core is usually cylindrical in shape and built up of disks of the proper size punched from sheet iron. This core is then wound lengthwise with a number of coils of wire in which the current is produced when the armature is rotated. The current is collected from the armature winding by means of copper brushes.

A diagrammatic view of a typical shunt-wound dynamo is shown in Fig. 20. The armature a rotates between the magnet poles b, and the current is collected by means of the brushes c. The copper segments d rotate with the armature and are called commutator bars. As the ends of the coil wound on the field magnet are connected to the brush terminals, the field coil

FIG. 20

R

forms a shunt, or by-pass, for part of the current delivered by the armature. The remainder of the current flows through the external circuit R, which in this case consists of the wiring of the ignition system.

56. A dynamo must usually run at very high speeds in order to generate enough current for ignition purposes and, therefore, is not efficient at low speeds. On account of this fact, the dynamo is ordinarily used in connection with a storage battery. The generator and battery are so connected that they operate in conjunction with each other; that is, the system is so wired that when running at average or high speeds the dynamo supplies the current and keeps the storage battery charged, but when running at low speed the storage battery