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The spark plug

The spark plug is necessary to reliably ignite the air-fuel -mixture in the Otto engine. Thereby, the self-cleaning temperature of 400°C should be quickly reached and the full throttle temperature of 850°C should not be exceeded.
Working against the above mentioned conditions, is the stress placed on the spark plug of temperatures up to 3000°C, very high pressures and ignition tension. In spite of the significant chemical influences, e.g., through the still existing sulphur in the fuel or oil-carbons, the plug changing intervals are continuously being extended.

Spark plugs have a fine pitch thread

The ignition spark jumps across from a center electrode to one or more earth electrodes. To see this, please click on 'sectional drawing - earth electrode'! The fuel-air mixture between the center- and the earth electrode is, first of all, ionized, i.e. prepared for the sparking. The height of the ionization tension increases when the spark plug gap becomes larger, this can be seen in the oscilloscope display as a higher firing line. A higher pressure in the combustion chamber, e.g., through higher compression, can also increase the voltage. An earlier ignition point has more the opposite effect.

Bigger spark plug gap -> requires higher ignition voltage

The center electrode is surrounded by an insulator sleeve whose length influences the heat value. If the insulator is short and has only a small breathing area, the heat can be quickly transferred to the thread, and thus to the cooled cylinder head. One speaks of a "cold" plug for engines on which high demands are made and have a high heat-value (low heat-range code). Should the insulator sleeve be long, then it's the other way around. The spark plug heat-value must be suited to the respective engine. E.g., a spark plug with the description 'W8DC' is a Bosch product for relatively low demand Otto engines.


Aluminium oxide insulator, nickle-, manganese-, chrome- or silver alloy electrodes, with platinum on the electrode tips.

Wear and tear

There is hardly any component in the motor vehicle, where it is so difficult to judge how worn out it is through visual examination. Visual deposits are often not of primary importance. They are taken care of by the self- cleaning temperature, or can be removed, e.g, by sand-blasting. So, when must the, sometimes pretty expensive, spark plugs be replaced? According to the manufacturers data of course. Indeed, there have been successful attempts to leave them in the engine for twice as long as prescribed. Apart from that, the manufacturers are now recommending longer and longer intervals. One must be aware, when testing the plug, that with a pot-shaped center electrode, the the sparking will take place on the edge. The fact is, the rounding of this edge is a measure of how far worn down it is. Should the center electrode look like the one in picture 4, one should perhaps, check the plug-gap ...

Don't try to conceal ignition disorders by installing new spark plugs


If you're looking for spark plugs for older vehicles and can't find their description any more, you could possibly also use one of the newer types. Take into consideration however, that some manufacturers have changed their descriptions. Bosch, e.g., has even turned the heat range identification number around to fit in with the competition:

W95W 10
W 125W 9
W175W 7
W 225W 5
W240W 4

Changing the spark plugs

Great care is to be taken, also in engines whose spark plugs can be reached quite easily. Even loosening them can pose problems, the breaking off of parts of the spark plug can mean a possible re-drilling and/or the tapping of a new thread. Because nowadays actually only aluminium cylinder heads exist, also when screwing them in a lot depends on working precisely, quick fault detection and avoiding the use of any sort of force. Neither oil nor grease should be used as an aid, because the plug could become so tightly lodged that the above problems could occur. 08/11

Information about heat values

More information about spark plugs

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