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All Tests

Spark Plug 1

Now please take a look at this splendid spark plug above, the long thread and the small tips on the middle and ground electrode. This is where all efforts in connection with the ignition come to a culmination point. This is where it is decided whether the spark is strong and in the meantime frequent enough and whether the timing was well chosen.

The long thread stands for the long distance that had to be covered from the outer plug contact to the inside of the combustion chamber. It is not unusual for this spark plug to be connected to a much longer single ignition coil (picture below) in a channel specially created in the cylinder head for this purpose. However, the long thread in the aluminium cylinder head also provides more grip than a short one.

It's really just a matter of one or more sparks that are supposed to jump over. To do this, a kind of power line must be run through the air in between. Whenever this is attempted, e.g. on fluorescent tubes or xenon light, a large voltage surge is needed to separate protons and electrons from each other in the atoms in between. Charged particles, i.e. ions, are created.

The tension requirement is further aggravated by the fact that this process does not take place under normal pressure, but in the combustion chamber in the compressed air-fuel mixture. In addition, low outside temperatures worsen this process, e.g. when the engine is to start. This is because the starter must now drive a much heavier engine. Its power consumption reduces the power available for ignition.

In any case, a cold battery makes it harder to provide sufficient power. It should be proven, so to speak, that a Fiat 500 Nuova (picture below), because of its basic electrical equipment, was not able to run the engine at -25°C, no matter how well the battery was charged. However, the electronics have considerably improved the voltage supply. More on this later.

You may have heard that a Xenon lighting system, for example, only needs about 35 watts after the first strong current surge, which is considerably less than a more conventional halogen lighting system. This is also the case with ignition, but a first surge of about 10,000 - 20,000 volts is required.

In order to produce this, a so-called ignition coil exists, whereby it remains unclear whether this is responsible for one, a part or all spark plugs. If a coil is put under voltage at both ends, its height is not so important at first. You may be able to apply 5 volts and measure it, but if you switch off the power supply, there will be a short voltage surge from the coil of possibly several hundred volts.

Of course an oscilloscope is once again required for this. This opens up the possibility of generating an even higher voltage with our on-board power supply of at least 12 volts. But remember, this only becomes noticeable when we switch off the power supply. And so it was for a long time with the contact-controlled ignition system.

However, one coil was still not sufficient. What would be? The required ignition voltage depends on the distance between the two electrodes and, as already mentioned, on the pressure in the combustion chamber. In the past, dodgy salesmen often sold people so-called ignition amplifiers, and then drew sparks before their eyes over a distance of up to three centimetres and more.

By the way, the alleged amplifier sometimes consisted of only one knob in a slightly larger housing, which was looped into the high voltage line. A certain amplification cannot even be denied, because the electrons usually collect before they overcome this obstacle as well. In the past, a certain distance between the high-voltage plug and the ignition coil sometimes helped a weak candidate to start up after all.

Do not touch the ignition system during operation.

Of course this so-called 'spark gap' is out of fashion, especially when there is no high voltage cable at all for single spark coils. But we had stopped at one coil. From now on we call this coil primary coil because it is operated with 12V and we add another one, the secondary coil. Ideally, it would be enclosed by the primary coil with its windings.

In addition to the primary coil, we now use three other possibilities for storage or amplification, the secondary coil with the soft iron core and the capacitor. It is important here that the second coil has many more windings than the existing one. As a rule, the tension increases in the ratio of the windings. One speaks of a voltage requirement of about 20,000 volts. But why is it that this is not always perceived when measuring with an oscilloscope?

This picture is drawn by the oscilloscope. We left out the axes. The upward deflections are voltages of an otherwise time-related recording. From 1 the primary current flows through the switch at the bottom left of the spark plug. Precondition: The ignition switch on top next to the battery is also switched on. At 2 again 0 volts are reached. The magnetic field no longer absorbs any further energy, the additional current heats the ignition coil (unnecessary).

Now the switch in the primary circuit should open. This causes the magnetic field around the primary coil to collapse, resulting in that decisive high voltage pulse in the secondary coil. The magnitude of this voltage pulse cannot always be read correctly in the oscilloscope image. It is very pointed, which is why it is also called an 'ignition voltage needle'. In any case, this high voltage is no longer necessary if the area between the centre and ground electrode is sufficiently ionised. So up to point 3 a spark can be seen. This is called 'burning' of the spark, this part is the 'burning voltage line'.

The voltage does not come to rest so suddenly when the spark is extinguished, similar to the phenomenon described above when one coil is switched off. Here, after all, two coils are still involved and a capacitor is still at work. So it takes time with the so-called 'decay process' before the next ignition spark is prepared at some point.

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