Branden Davis, age 16, of Mesa, Ariz., for his 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 a 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 gasoline engines in automobiles use a 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. The distributor sends the current on to the spark plugs, one after another. The plugs carry the current into the cylinders where the fuel mixture is ignited as each piston nears the top of its compression stroke.
Here's how it works:
The battery stores electricity in specially designed compartments or cells. The electric current travels by means of a cable to the coil.
Inside the coil are actually two coils of wire. The primary coil, which is the one connected to the battery, is made up of several hundred turns of wire about as big around as a straight pin. The similarly constructed secondary coil is connected to the distributor. When the flow of low voltage in the primary coil is interrupted for an instant, a high voltage surges through the secondary coil.
High voltage is essential because the spark must be hot enough to set fire to the compressed gasoline air mixture in the cylinder. To interrupt the low voltage current, a pair of breaker points are opened and closed by a rapidly revolving cam, or uneven wheel, inside the distributor housing. The points stay closed for less than five thousandths of a second for each spark. The breaks are timed to take place just as the fuel mixture is most tightly squeezed in the cylinder.
A capacitor draws off electricity that might otherwise cause a stray spark between the breaker points when they open. The capacitor also helps maintain a clean electrical break that is important in obtaining a hot, or high voltage, spark in the cylinder.
Magneto ignition is used in most piston engines in airplanes. The magneto generates the electricity, transforms it into high voltage and then distributes the current to the spark plugs.
The advantage of the magneto system is that it doesn't have a battery to run down or wear out. Its biggest disadvantage is that, when the engine is being started, the magneto turns so slowly that it does not produce a hot spar, and an extra source of high voltage must be provided.
In diesel engines the fuel mixture is very tightly compressed. The compression itself produces heat to ignite the fuel.
In jet engines fuel is ignited by the heat of the combustion chamber walls, or by spark plugs.