Abstract
Modern engines and driving conditions have imposed serious problems on the spark plug designer. Colder or ‘harder’ spark plugs are now required yet they must resist cold fouling in city driving. These conditions are largely being met by spark plugs in which the insulator core nose protrudes beyond the steel shell.
Design features are discussed and techniques for engine evaluation of spark plugs are described.
If the spark plug temperature exceeds 950°C it is liable to become a source of pre-ignition though the actual pre-ignition temperature will depend on the fuel used. Transient pre-ignition from hot areas and deposits in the combustion chamber will increase the temperature of even very cold spark plugs and may force them into run-away pre-ignition.
Detonation and the relationship of the spark plug to run-on are discussed.
Improper installation and operation of ignition systems will precipitate apparent spark plug problems. Variations between identical engines may require careful investigation.
Two-stroke engines have their own problems of pre-ignition and fouling and some are now operating on surface discharge spark plugs. Some require small size spark plugs.
Racing engines and Wankel engines have particular requirements which do not necessarily relate to other engines.
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