Spark plugs exist in every combustion engine. The function of the spark plug is to do just as the name implies--provide a spark to the combustion chamber for rotational movement of the engine. This spark will ignite, combust or explode a flammable liquid, and create a pressure that drives the piston downward.
An electrical current is conducted through the main electrode of the spark plug from a generation source, whether it is a battery, generator or alternator. On automobiles, this current is transformed into a higher voltage from the coil. On small engines, there is a magneto coil that provides the current. The current is created by a magnet embedded in the flywheel.
As the engine cycles through a full revolution, the spark plug fires into the combustion chamber or chambers, depending on the number of cylinders that the engines has. The spark firing is controlled by the distributor cap. A small set of points--a switch--is opened and closed to provide the current flow from the high voltage coil to each spark plug. This in turn fires the spark plug to ignite the gas in each cylinder.
The construction of the spark plug has not changed since its inception of the first combustion engine. All spark plugs have the same basic construction. An electrode--the center part of the spark plug that carries the current--runs the entire length of the spark plug. The top of the electrode is where the plug wires connects on to. The electrode is insulated from the rest of the body with a glass insulator. The glass is the white part of the spark plug, and the brand name is generally printed on the glass insulator.
There is a threaded metal portion of the plug where the spark plug is screwed into the combustion chamber. This is also known as the ground point of the spark plug. At the bottom of all spark plugs you will see a small metal piece that looks like the letter "J" laying on its side. There is a gap between the bottom of the electrode and this ground point. This is the gap of the spark plug that is referenced for every engine specification. The size of the air gap regulates how large a spark will be fired into the chamber.
Many times when a spark plug will not fire there may be debris that has built up around the lower portion of the electrode and the grounding point. This will cause the spark plug to short out and not create a spark. The air gap is vital for the spark to occur.
