The connection is designed as an SAE connection or a 4 mm thread. The ignition cable or a rod coil is plugged into the connection. In both cases a high voltage coupled here must be transported to the other end of the spark plug.
The ceramic insulator has two tasks. It serves primarily for insulation, whereby it prevents a flashover of the high voltage to the vehicle mass (= minus), and conducts combustion heat to the cylinder head.
Creepage current barriers
The wave-shaped creepage current barriers on the outside of the insulator prevent the leakage of voltage to the vehicle mass. In doing so, they extend the path to be travelled and increase the electrical resistance. Therefore it is guaranteed that the energy takes the path of least resistance - the path through the middle electrode.
In order to ensure the electromagnetic compatibility (EMC) and thus the fault-free operation of the onboard electronics, a glass melt is used inside the spark plug as interference suppression.
Middle electrode with copper core
The middle electrode of a standard spark plug is comprised mostly of a nickel alloy. From the end of this electrode the spark must jump over to the earth electrode. Middle electrodes from NGK have a copper core, which improves the heat dissipation.
Metal housing with thread
The metal housing also plays an important role in the heat dissipation of the spark plug. Its thread is always rolled for NGK spark plugs. As opposed to cut threads, this has the advantage of no sharp edges which can damage the threaded bore in the cylinder head.
The seal ring prevents combustion gas from emerging past the spark plug through high combustion pressures. In the process, it prevents pressure losses. Moreover, it conducts heat to the cylinder head and evens out the different expansion properties of the cylinder head and spark plug housing.
The inner seals create a gas-tight connection between insulator and metal housing. For this purpose, a talcum ring is enclosed between two additional seal rings. During the production of the spark plug it breaks down, ensuring an optimal seal.
The earth electrode of a standard spark plug is made of a nickel alloy. It represents the opposite pole of the middle electrode in normal function.