Electrical System

Contact Breaker or Distributor.—The contact breaker is a switch that is periodically opened and closed in synchronism with valves in the engine cylinder. It is equipped with tungsten contact points, because tungsten has a very high melting point, is me chanically hard, resists corrosion to a high degree and is com paratively inexpensive. Formerly platinum was used almost ex clusively. A small condenser of about 0.2 microfarads capacity is connected across the contact points to reduce arcing and pitting and also to increase the sharpness of the current break and the intensity of the spark.

Coil.—The ignition coil consists of an iron core, a primary winding and a secondary winding. The core is a bunch of iron wires or thin strips. The primary low voltage winding consists of comparatively few turns of comparatively large wire, wound around the core. The purpose of the primary winding is to build up a magnetic flux through and around the core. The secondary or high voltage winding consists of a large number of turns of fine wire, also wound around the core. When the interrupter or breaker is closed, the low voltage electric current flows through the primary winding of the ignition coil and sets up a magnetic flux in and around the core. When next the interrupter is opened, the low voltage electric circuit is opened and the current stops flowing through the primary winding. The magnetic flux which had been set up in and around the iron core now immediately collapses. This collapse of the magnetic flux induces a voltage in the secondary winding, causing a current to jump the gap in the spark-plug. Several thousand volts are required to cause a spark to jump across the gap in the spark-plug, whereas the battery voltage is usually 6 or 12 volts. The voltage in the pri mary winding of the coil is therefore 6 or 12 volts, but the voltage induced in the secondary winding of the coil is many thousands of volts. The coil functions increase the voltage available in the battery to the voltage required by the spark-plug, although at the same time the process results in a reduction of the current.

Distributor.—The distributor rotates and distributes the sec ondary current from the coil to the spark-plugs at the proper time and in the proper sequence. A typical distributor is shown in fig. 12. The case is usually made of some moulded insulation material such as bakelite, resistive to the action of moisture, heat, acids and oils.

Timing.—Shortly before the piston reaches the upper end of its travel on the compression stroke, ignition occurs. Due to the fact that an appreciable time is required for the charge to burn, the timing of the spark to produce maximum turning effort on the crankshaft is required to be somewhat in advance of the top centre position of the piston. This advance varies with speed.

In general, less spark advance is required at low speeds and more at high speeds. 1 wo automatic controls are commonly provided in the distributor. The first adjusts the spark to suit the engine speed, and the second, vacuum control, serves to retard the spark as the load on the engine increases. In addition, manual spark controls may be provided.

Generators and Storage Batteries.

The purpose of the electric generator (q.v.) is to transform mechanical energy from the engine to electrical energy which is used in the various elec tric units of the car, either directly or reserved for later use by means of the electric storage battery, or accumulator. The conventional generator is a direct current two- or four-pole unit equipped with a shunt field. In the two-pole generator a third brush may be mounted between the other two brushes so that, when the speed and load on the generator increase and the field shifts, the third brush will collect a diminishing current supply for the field. Thus the battery charging rate may be kept high at low speeds and be prevented from rising abnormally at high speeds.

In 1940, most American cars used shunt-wound generators of about 35 amp. capacity with both current and voltage regulation. Starting Motor.—The function of a starting motor is to set the moving parts of an engine in motion at sufficient speed until the engine starts operating under its own power. Engine cranking speeds of above 35 r.p.m. are usually required for successful starting in cold weather. Starting motors are generally of the six-volt series-wound type. They are equipped with a special coupling which permits the engagement of a small gear on a shaft extension of the motor with a ring gear either cut on or attached to the fly-wheel. This gear reduction makes possible a starting motor speed i o or 12 times as great as the engine speed during cranking, with a consequent multiplication of torque. Starting motors are designed with a low internal resistance in order to utilize to full advantage the energy contained in the storage battery.