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Electrical System

Spark Plug Components

The terminal stud, insulator, shell and electrodes (center and ground) represent the most important parts of a spark plug, and are vital to a spark plug's performance and life span. If any of these critical parts are varied, the operating characteristics of the spark plug are also varied.

Terminal Stud: The steel terminal stud is melted, gas-tight, into the insulator with a special conductive glass seal that also serves as the electrical connection to the center electrode. To the end projecting out of the insulator, a terminal nut is attached which connects to a spark plug wire.

Insulator: The insulator houses both the center electrode and the terminal stud. It is made of a special ceramic material and its function is to insulate the center electrode and terminal stud from the shell. The special ceramic also ensures high resistance to electrical breakdown.

The material used by Bosch for the spark plug insulator contains aluminum oxide. After it has been stoved and glazed, this special ceramic ensures high insulation resistance, good thermal conductivity and both mechanical and chemical strength.

Shell: The shell is made of steel and its function is to secure the spark plug in the engine's cylinder head. The upper part of the shell has a hexagonal section to which the spark plug wrench is applied, and the lower part is threaded. The surface of the spark plug shell has an electroplated nickel coating to prevent corrosion, to keep the thread free and to prevent seizing. Depending on the design of the shell, the spark plug might also include a seal ring. After the insulator has been inserted into the spark plug shell, it is crimped and heat-shrunk into position.

Electrodes: Electrodes (ground and center) must have good heat-dissipation properties and must also be resistant to wear, in order to maintain the required ignition voltage over the service life of the spark plug.

Ground Electrode: Different part plugs feature:

• Different numbers of ground electrodes
• Ground electrodes of varying dimensions
• Different ground electrode materials

The ground electrode(s) are welded to the shell and usually have a rectangular cross-section. The ground electrode's service life is highly dependent upon its ability to dissipate heat. In the ground and center electrodes, suitable composite materials are employed to improve thermal dissipation and extend service life. And, thicker ground electrodes and multiple electrodes can both be used to extend spark plug service life.

CENTER ELECTRODE:
The center electrode projects from the insulator nose. It is melted, gas-tight, into the insulator with a special conductive seal. The electrode has a slightly smaller diameter than the bore in the insulator nose. This is necessary in order to compensate for the different internal expansion that takes place between the electrode material and the insulator ceramic. The air gap produced is subject to very close tolerances and is of great importance with regard to the heat range of the spark plug.

The center electrode can be made of many different materials. Center electrodes made of precious metals such as platinum (Bosch platinum) are smaller in diameter than the compound electrodes that have a copper core and nickel-alloy jacket (Bosch Super).

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