A spark plug, or a plug is an electric device which is used for delivering current from an ignition system to the combustion chamber of a spark-ignition engine to instigate the compressed fuel by an electric spark. A plug consists of a metal threaded shell, electrically separated from a central electrode by a porcelain insulator. The central insulator is connected by a heavily insulated wire to the external terminal of an ignition coil which is also called magneto. The metal shell of the plug is screwed into the cylinder head of the engine and hence electrically grounded. Read on further for the parts of a plug.
The top of the plug has a terminal to join to the ignition system. As the time went on, the variations in the terminal configurations have also changed. The precise terminal construction depends upon the use of the plug. Most of the passenger car plug wires snap onto the terminal, but some have eyelet connectors which are fastened onto the plug under a nut. Some plugs manufacturer design a plug that have a bare thread which is common for motorbikes and ATV.
The primary part of the insulator is usually made from sintered alumina or (Al2O3). It is a very sturdy ceramic material with high dielectric strength. It is printed with the name of the manufacturer and identifying marks, and then it is glazed to improve the resistance to surface spark tracking. The functions of insulator are to provide mechanical support and electrical insulation for the central electrode and also providing an extended spark path for flashover protection. Consequently, it helps in extending the terminal above the cylinder head to make it more accessible.
By stretching the surface between the high voltage terminal and the grounded metal case of the plug, the physical structure of the ribs helps in improving the electrical insulation and preventing the electrical energy from leaking along the insulator surface from the terminal to the metal case. This disrupted path makes the electricity confront more resistance even in the presence of dirt and moisture.
As the plugs also seal the combustion chamber or the engine during its installation, seals are needed to make sure that there is no leakage from the combustion chamber. The internal seals of the plugs are made of compressed glass or metal powder, but typically it was made from a multi-layer blaze. The external seal is a typical crush washer, but sometimes it is manufactured by using a cheaper method of a taper interface and simple compression to attempt sealing.