Spark Plug Construction
The top of the spark plug contains a terminal to connect to the ignition system. The exact terminal construction varies depending on the use of the spark plug. Most passenger car spark plug wires snap onto the terminal of the plug, but some wires have spade connectors which are fastened onto the plug under a nut. Plugs which are used for these applications often have the end of the terminal serve a double purpose as the nut on a thin threaded shaft so that they can be used for either type of connection. These are a necessary part of the spark plug.
The main part of the insulator is made from porcelain. Its major function is to provide mechanical support for the centre electrode, whilst insulating the high voltage. It has a secondary role, particularly in modern engines with deeply inaccessible plugs, in extending the terminal above the cylinder head so as to make it more readily accessible.
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 tip of the insulator, the part from the metal body of the plug to the center electrode protruding into the combustion chamber, must resist high temperatures whilst retaining electrical insulation. To avoid over-heating the electrode, it must also offer good thermal conductivity. The porcelain of the main insulator is inadequate and so a sintered aluminium oxide ceramic is used, designed to withstand 650°C and 60,000 V. The exact composition and length of the insulator determines the heat range of the plug. Short insulators are "cooler" plugs. "Hotter" plugs are made with a lengthened path to the metal body, by isolating the insulator over much of its length with an annular groove. Older spark plugs, particularly in aircraft, used an insulator made of stacked layers of mica, compressed by tension in the centre electrode. With the development of leaded petrol in the 1930s, lead deposits on the mica became a problem and reduced the interval between needing to clean the spark plug. Sintered aluminium oxide was developed by Siemens in Germany to counteract this.
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 typically made by the use of a multi-layer braze as there are no braze compositions that will wet both the ceramic and metal case and therefore intermediary alloys are required.
The metal case (or the "jacket" as many people call it) 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. As it acts as the ground, it can be harmful if touched while igniting.
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 tip can be made of a combination of copper, nickel-iron, chromium, or precious metals. In the late seventies, the development of engines reached a stage where the "heat range" of conventional spark plugs with solid nickel alloy centre electrodes was unable to cope with their demands. A plug that was "cold" enough to cope with the demands of high speed driving would not be able to burn off the carbon deposits caused by stop-start urban conditions, and would foul in these conditions, making the engine misfire. Similarly, a plug that was "hot" enough to run smoothly in town, could actually melt when called upon to cope with extended high speed running on motorways, causing serious damage to the engine. The answer to this problem, devised by the spark plug manufacturers, was a centre electrode that carried the heat of combustion away from the tip more effectively than was possible with a solid nickel alloy. Copper was the material chosen for the task and a method for manufacturing the Copper cored center electrode was created by Floform. The center electrode is usually the one designed to eject the electrons (the cathode) because it is the hottest (normally) part of the plug; it is easier to emit electrons from a hot surface, because of the same physical laws that increase emissions of vapor from hot surfaces (see thermionic emission). In addition, electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, i.e. from a sharp point or edge rather than a flat surface (see corona discharge). It would be easiest to pull electrons from a pointed electrode but a pointed electrode would erode after only a few seconds. Instead, the electrons emit from the sharp edges of the end of the electrode; as these edges erode, the spark becomes weaker and less reliable. At one time it was common to remove the spark plugs, clean deposits off the ends either manually or with specialized sandblasting equipment and file the end of the electrode to restore the sharp edges, but this practice has become less frequent as spark plugs are now merely replaced, at much longer intervals. The development of precious metal high temperature electrodes (using metals such as yttrium, iridium, platinum, tungsten, or palladium, as well as the relatively prosaic silver or gold) allows the use of a smaller center wire, which has sharper edges but will not melt or corrode away. The smaller electrode also absorbs less heat from the spark and initial flame energy. At one point, Firestone marketed plugs with polonium in the tip, under the questionable theory that the radioactivity would ionize the air in the gap, easing spark formation.
Side electrode, or ground electrode:
The side electrode is made from high nickel steel and is welded to the side of the metal case. The side electrode also runs very hot, especially on projected nose plugs. Some designs have provided a copper core to this electrode, so as to increase heat conduction. Multiple side electrodes may also be used, so that they don't overlap the center electrode.