Pam Stephens, age 13, of Utica, N.Y., for her question:
HOW DOES A CAR'S IGNITION WORK?
Most gasoline engines use ignition systems which set fire to the fuel with an electric spark. Such an ignition system must have a source of electricity and a lot of additional equipment that goes to work when a key is turned in a car's ignition switch.
Most automobile gasoline engines use battery ignition. Its main parts are the battery, the coil, the distributor and the spark plugs. The battery supplies a low voltage current.
The coil, or spark coil, increases the voltage of the current. Inside the coil are two coils of wire. The primary coil is made up of several hundred turns of wire which is about as big around as a straight pin. It is connected to the battery.
The secondary coil is connected to the distributor. When the flow of low voltage in the primary coil is broken for an instant, a high voltage surges through the secondary coil.
To break the low voltage current, a pair of breaker points is included in the primary circuit. These breaker points are opened and closed quickly by a rapidly revolving cam or uneven wheel inside the distributor housing.
The breaker points remain closed for less than five thousandths of a second for each spark. These breaks in the primary circuit are timed to take place near the top of each piston's compression stroke as the fuel mixture is squeezed in the cylinder for firing.
A capacitor attached to the two breaker points draws off electricity that might otherwise cause a spark between the points when they open. The capacitor also helps give a clean electrical break that is important in obtaining a hot, or high voltage, spark in the cylinder.
The high voltage from the secondary coil runs to the distributor. There a whirling rotary switch sends the current to the spark plugs, one after another.
The electricity must be increased to a high voltage. The spark must be hot enough to set fire to the tightly compressed gasoline air mixture in the cylinder.
Magneto ignition is used in most airplane piston engines. The magneto generates the electricity, transforms it into high voltage and then distributes the electricity to the spark plugs.
The advantage of the magneto system is that it does not have a battery to run down or wear out. Its biggest disadwantage is that, when the engine is being started, the magneto turns so slowly that it does not produce a hot spark, and an extra source of high voltage must be provided.
In diesel engines the fuel mixture is tightly compressed. The compression itself produces heat to ignite the fuel.
In jet engines the fuel is ignited by the heat of the combustion chamber walls, or by spark plugs..