The spark plug provides suitable electrodes inside the cylinder, which can be jumped by a high voltage from an induction coil or magneto to produce a spark for igniting the fuel mixture. The two electrodes are insulated from
each other, the center one being surrounded by insulating material (usually porcelain) while the other is attached to the steel shell, which is grounded when the plug is screwed into the cylinder head.
The conditions under which a spark plug must operate are severe. It must be able to withstand an electrical pressure of 12,000 to 14,000 volts under high compression pressures and temperatures 40 to 45 times per second when the engine is running at high speed. The pressure in the combustion chamber from the explosion of the fuel charge tends to force the plug from the cylinder head. At the same time the, heat of combustion, which in high compression
engines may reach a maximum of 2500° F. or more, tends to burn and distort the electrodes, which would change the spark gap setting. In addition, the surfaces of the insulator may become overheated and burned, which materially
decreases the resistance of the insulator and usually causes electrical leakage around the plugs. Internal strains due to sudden changes in temperature tend to crack the insulator. It is therefore of highest importance that correct design and material enter into the construction of the spark plug.
High compression pressures and increased speeds, which result in higher temperatures, necessitate the use of a spark plug which will dissipate heat very rapidly. The extent to which a plug will dissipate heat depends on the length of insulation exposed to the combustion gases. Hence, for engines which develop a lot of heat, a plug with short insulation — or a cold plug – should be used. For low compression or low speed engines a plug with a long insulation — or a hot plug – should be used. The following
illustration shows three plugs of various heat ranges and also the path of heat conductions.
The heat characteristics of an engine are not the only thing that have to be considered in the selection of a spark plug. The conditions under which motorcycles are operated have an important bearing on the selection. Exceptionally
severe service, such as continuous runs in mountainous country or running at high speeds for long distances, will require a colder plug than that used for ordinary service.