The process of burning fuel and air powers most of the vehicles in the U.S. and around the world. At its most basic level, this combustion is a chemical process. The burning of fuel and air requires first that the two gasses be mixed, compressed and then ignited by the addition of heat.
The required heat can come from a number of sources. In a diesel engine, the burning reaction is started by the heat caused as the mixture is heated by very high compression. In a gasoline engine, ignition is usually accomplished by an electric spark that is made to arc or jump across the spark plug’s terminals. It is the current of the spark, flowing through the resistance of the gap, which causes the heat needed to start combustion.
The required heat can be delivered in other ways; Ford, for example, has been experimenting with an engine that uses laser beam energy to start the combustion process. Very early engines, built before the invention of the ignition coil, used hot tubes or even flames to start the combustion. The point is that this heat has to be added at exactly the right time and in the right location for greatest engine efficiency. Every technician needs to know how this works.
The business end of a spark plug
Understanding ignition systems, and how they really work, begins with a look at the business end of a spark plug. For typical spark plugs, there is a single center electrode and a “J” shaped ground electrode. Depending upon the polarity of the voltage applied, it is possible for the arc to travel from the center electrode to ground or the other way around. Usually, it is the center electrode that is made negative. This way the spark travels to the center electrode from the ground electrode.
Normally there is not a conductive path between center and the ground electrodes. The goal of the ignition system is to create that conductive path through the air or whatever else is between the electrodes. The molecules of air cannot normally support conduction because the outer layers of their atoms/molecules have no free electrons. To make the air conduct requires a voltage that is high enough to literally rip the electrons loose from atomic forces that are holding them. This process is called ionization.
It takes not only high voltage, but a concentrated high voltage to accomplish ionization. This is where the design of the spark plug and its electrodes becomes important. In order to concentrate the high voltage, sharp edges must be present. When the plug is new, the center electrode is a perfect cylinder with a flat top. The top edge of that center electrode is sharp and helps to concentrate the voltage.
In current technology there are two things that can be done to improve the performance of the spark plugs. The first is to go to smaller, more pointed electrodes through the use of very high temperature metals like iridium. Sometimes called “fine wire” plugs, these spark plugs have center electrodes that are half the size and diameter of ordinary plugs