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:
|W 125||W 9|
|W 225||W 5|
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