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Spark Plug 2

In the petrol engine there is only one way of initiating combustion of the air-fuel mixture to get the whole load moving, namely through the spark plug. The first spark plug is to be found on Lenoir's engine in 1860, the first internal combustion engine also produced in numbers. Here, town gas was ignited.

The Lenoir engine (model below) had no compression stroke yet. Instead a mixture with air was sucked into the cylinder for a short part of the stroke and this mixture was ignited without compression, so that useful work could be done during the remaining stroke. So it did not depend on an ignition point, because the piston was already on the same path as after the ignition of the mixture.


A Ruhmkorff coil is always being loaded and unloaded. The latter in particular produced sparks, one of which eventually ignited the mixture. Later, Nikola Tesla, Frederick Simms and Robert Bosch successfully modified the concept of the spark plug, the latter soon produced it on a large scale, combinable with a high-voltage magneto ignition.

So the spark plugs are connected to the cables, here on a Ford V8 from 1930 . .

Then there were the Lodge brothers, who among other things produced spark plugs, which were indispensable as Golden Lodge for a long time perhaps when driving an Alfa. Here in Europe KLG- less, but Champion spark plugs are much more known. And of course, General Motors also had its own supplier in its subsidiary ACDelco.

It is not only the extremely high number of ignitions that a spark plug must withstand without problems during its service life, but also high pressures and temperatures. Of the electrons, the ones with mass usually are connected directly to the thread. The centre electrode connected to the high voltage is usually inserted from above, together with the insulator. In the previous chapter we explained the ionization before the spark jumpes over.

Incredible temperatures of several 10,000°C during spark formation are difficult to measure because of the closed combustion chamber and the speed of the processes. Just like the result is several 10,000 V, which a normal oscilloscope often only displays as a 5,000 V long voltage needle. Of course, the possibility of measuring longer and hotter voltages depends on the voltage level to create ignition sparks.

This was not yet the focus of development for the first spark plugs. What mattered here was the reliability of the formation of ignition sparks. Among other things, the self-cleaning temperature plays a major roll. It is said that a spark plug can 'burn free', provided a certain amount of energy is supplied according to the conditions in the combustion chamber. There is no uniform minimum temperature for this, not more than an average value of 500°C.

In the three pictures you can see how spark plugs can help to dissipate heat. On the far right is a so-called cold plug, more suitable for highly stressed engines. It has a very small breathing space and the metal thread is connected to the insulator in a very short way to improve heat dissipation. In the middle, the connection point is already shifted upwards, on the far right it is practically no longer there.

In the past, e.g. at Bosch, the heat value was included in the designation. According to this, a cold candle had a high heat value, corresponding to its ability to give off a lot of heat to the insulator. The today typical heat value index for such a spark plug would be rather small. If it is too small, the self-cleaning temperature may not be reached.

Never install spark plugs that are not approved by the manufacturer. Even garages should only do so in case of certain problems in the context of tuning. As already mentioned, if the heat rating is too low, you run the risk of breaking down, if it is too high, you risk a veritable engine damage. Spark plugs should be changed as seldom as possible, especially not first if there are ignition problems. Because they often hide the real causes.

Removed spark plugs reveal more about the engine in which they have worked than about themselves. The times when they had to be fawn brown are long gone. Actually, only dry black betrays a rich mixture and dampness suggests oil. To determine the wear of a spark plugs is extremely difficult. In any case, it is not possible to determine it from natural soiling, i.e. that which has not been removed by self-cleaning.

The prescribed replacement intervals become longer and longer. At the very top of the picture you can see that both electrodes end up as pins. These can then be made of iridium, for example, a material that was added to the original meter in Paris as an alloy material. So very durable. This also means that their tips are preserved. Some people are already talking about 160,000 instead of 60,000 km maintenance interval. We ourselves have easily managed 100,000 with new spark plugs in the glove compartment.

Earlier spark plugs used porcelain as an insulator, surrounding an electrode made of a nickel-chrome alloy. They had to be changed every 1,500 km at the latest. The electrode materials were still available as standard, but small amounts of platinum and silver have long been added for more demanding applications. The insulator consists of sintered aluminium, which has the same thermal expansion as the rest of the spark plug. In the meantime, the grooves have long since been added to make it more difficult to conduct external current through dirt.

Carefully insert spark plugs when screwing them in. After all, there is an internal thread in aluminium which you can mess up. Especially do not overtighten the short threads, which are even more common on classic cars. Because it can happen that such a spark plug, including plug and cable, is thrown out and the motor just pops at the spot. By the way, if repairing grease the drill or tap with grease to absorb chips.

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