Spark plug failure due to a combination of strong magnetic fields and undesirable fuel additives

Abstract

Spark plugs were a key invention for the production of an effective spark-ignition engine. All essential components constituting the modern spark plug have been present from the beginning of the twentieth century, including the two electrodes: the anode is connected to the earth trough the metal casing and separated by a ceramic insulator from the central cathode, which connects to the pulse generator. Most modifications introduced later relate to the materials and to the configuration of the electrodes. Presently, the insulator is produced from sintered alumina. The cathode has a copper core to increase thermal conductivity; the surface may be alloyed with silver, gold and platinum to increase the resistance to high-temperature corrosion. The anode is subject to high-temperature corrosion by combustion gases and electro-erosion by the spark which transports metal ions in a plasma state. Nickel alloys are mainly used, although platinum alloys are found in high-performance spark plugs. The efficiency of the engine is strongly dependent on the firing efficiency of the plugs, requiring an intense, localised spark, which is typically formed between sharp edges or protuberances on the electrode surfaces; ‘‘diffuse’’ arcs form if such protuberances are absent, producing a distribution of the spark in a series of small, individual arcs along the edge of the electrodes. Sharp edges are exposed to the corrosive environment and allow limited heat conduction, provoking rapid deterioration of the electrode tips. Erosion occurs by the formation of shallow craters by local melting and ion transport in the electric arc; under more severe conditions, microcracking of the electrode has been observed. The use of multiple anodes presents a partial solution to this problem, as it multiplies the number of edges from which sparks can originate. In general, a spark plug design with high firing efficiency results in a limited lifetime. It is therefore desirable to reduce initial efficiency in favour of improved lifetime efficiency.