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Spark plug advances match combustion developments
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01/04/2007
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It is easy to forget that how advanced combustion processes are affecting downstream suppliers of engine components and systems, writes Roger Bishop.
Valve drives based on cam phasers or valve lift control as well as direct injection systems with wall or air guided combustion are now state of the art. And the latest generation of piezo-controlled injectors is extending the range of unchoked, lean engine operation.
All these developments, and those still to come, are putting new demands on spark plugs. Research engineers at one major supplier, Beru, say there is an ongoing requirement for smaller designs, new types of electrodes, more precise spark positions and a higher dielectric of the spark plug ceramic.
Residual porosity is the main factor determining ceramic properties and to reduce this substantially – so improving the dielectric strength and mechanical solidity of the spark plugs – Beru has modified the fluxes used.
Maintaining the 60,000 to 100,000km service life of smaller (M12) spark plugs requires the sparking distance to be as small as possible, resulting in the need for new electrode geometries. An accuracy of ±0.2mm can now be guaranteed.
In the case of low-cost spark plugs with electrodes based on a nickel alloy, wear and tear is mostly determined by oxidisation, calling for the use of nickel alloys with a stable permanent oxide layer. In the case of high-end spark plugs whose electrodes are sheathed with an oxidisation-stabilised noble metal, permanent contact of the noble metal on the nickel-based spark plug electrodes must be guaranteed. This is not as straightforward as it sounds as the metals have very different coefficients of expansion and to achieve the right results Beru has developed advanced laser welding techniques.
Just as important as the exact spark location is the specified alignment of the body electrode so that mixture formation is not impeded. This requires a defined thread cut in the cylinder head. By aligning, adjusting and positioning the body by means of an optical measuring system, Beru is able to achieves a tolerance of ±15deg.
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Author
Roger Bishop
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