A spark plug is an electrical device that fits into the cylinder head and ignites a compressed mixture of air and fuel by means of an electric spark. Spark plugs have an insulated center electrode which is connected by heavily insulated wire to an ignition coil circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder.
Parts of the plug
The top of the spark plug contains a terminal to connect to the ignition coil by means of a spark plug cap. The plug terminal often has a nut on a thin threaded shaft so that they can be used for two different types of connections.
By lengthening the surface between the high voltage terminal and the grounded metal case of the spark plug, the physical shape of the ribs functions to improve the electrical insulation and prevent electrical energy from leaking along the insulator surface from the terminal to the metal case. The disrupted and longer path makes the electricity encounter more resistance along the surface of the spark plug even in the presence of dirt and moisture.
The insulator extends from the metal case into the combustion chamber. The exact composition and length of the insulator partly determines the heat range of the plug.
As the spark plug also seals the combustion chamber of the engine when installed, the seals ensure there is no leakage from the combustion chamber. The seal is a hollow metal washer which is crushed slightly between the flat surface of the head and that of the plug, just above the threads.
The metal case of the spark plug bears the torque of tightening the plug, serves to remove heat from the insulator and pass it on to the cylinder head, and acts as the ground for the sparks passing through the center electrode to the side electrode.
The tip of the insulator surrounding the center electrode is within the combustion chamber and directly affects the spark plug performance, particularly the heat range.
Side electrode, or ground electrode
The side electrode is welded to the side of the metal case.
The center electrode is connected to the terminal through an internal wire and commonly a ceramic series resistance to reduce emission of radio noise from the sparking. The electrons emit from the sharp edges of the end of the electrode; as these edges erode, the spark becomes weaker and less reliable.
The operating temperature of a spark plug is the actual physical temperature at the tip of the spark plug within the running engine. This is determined by a number of factors, but primarily the actual temperature within the combustion chamber. There is no direct relationship between the actual operating temperature of the spark plug and spark voltage
If the tip of the spark plug is too hot it can cause pre-ignition, leading to engine damage. If it is too cold, deposits may form on the insulator, fouling the plug and causing a loss of spark.
A spark plug is said to be "hot" if it is a better heat insulator, keeping more heat in the tip of the spark plug. A spark plug is said to be "cold" if it can conduct more heat out of the spark plug tip and lower the tip's temperature. Whether a spark plug is "hot" or "cold" is known as the heat range of the spark plug. The heat range of a spark plug is typically specified as a number, with some manufacturers using ascending numbers for hotter plugs and others doing the opposite, using descending numbers for hotter plugs.
Reading spark plugs
The spark plug's firing end will be affected by the internal environment of the combustion chamber. As the spark plug can be removed for inspection, the effects of combustion on the plug can be examined. An examination, or "reading" of the characteristic markings on the firing end of the spark plug can indicate conditions within the running engine. The spark plug tip will bear the marks as evidence of what is happening inside the engine. (NGK's spark plug reading chart)
A light brownish discoloration of the tip of the block indicates proper operation; other conditions may indicate malfunction. For example, if the plug is too cold, there will be deposits on the nose of the plug, likely very oily in a 2-stroke moped. Conversely if the plug is too hot, the porcelain will be porous looking, almost like sugar. The material which seals the center electrode to the insulator may boil out. Sometimes the end of the plug will appear glazed, as the deposits have melted.
An idling engine will have a different impact on the spark plugs than one running at full throttle. Spark plug readings are only valid for the most recent engine operating conditions and running the engine under different conditions may erase or obscure characteristic marks previously left on the spark plugs. Thus, the most valuable information is gathered by running the engine at high speed and full load, immediately cutting the ignition off and stopping without idling and removing the plugs for reading.