The ignition of the fuel-air mixture in the petrol engine should, depending on the RPM and the strain, as well as perhaps other factors, occur at exactly the right moment. At high RPM and when cold starting, sufficient ignition energy must be available.
If the ignition switch (on the top left) is switched on, the primary circuit (12V) runs from the positive pole of the battery (on the left), through the primary windings of the ignition coil (second from the left) via the (closed) interrupter contacts in the distributor to the negative pole of the battery. This current builds up a magnetic field in the ignition coil. The smaller the primary current is and the more windings the primary coil has, the longer it takes to build-up the field. In the primary coil the magnetic field generates a voltage, which counteracts the primary voltage (self-induction). The stronger the magnetic field is, the slower is the build up. The closing delay of the contacts should ideally, be calculated in such a way that it is sufficient for the build-up of the magnetic field at any RPM. With a too long closing phase the current is converted into heat. Because the contact-controlled coil ignition can not be changed during operation, it must also be calculated high enough for the maximum RPM. This leads to unnecessary losses at low RPM. Only with the introduction of the transistor-coil-ignition was a variable closing phase intergrated. The interrupter contacts open at a point exactly defined by the ignition adjustment (rotation of the distributor). An oscillation process, whose frequency depends on the primary-, or the secondary coil, and on the condenser then begins. A tension of about 400 V on the primary side is converted into a voltage of around 15,000 to 20,000 V. This is conducted through the rotor to the spark plug (on the right) of the cylinder at the end of the compression stroke. The condenser plays an important role for the quick build-up of the secondary voltage because it absorbs the remaining self-induction tension of the primary windings. The secondary tension increases at the same rate as it decreases in the primary windings.
The ignition tension requirements ...
increase with the compression in the cylinder.
increase with the Lambda-value of the fuel-air mixture.
increase with the distance of the center- and the ground-electrode(s).
increase with the increasing erosion of the center-electrode.
depend on the material used for the center-electrode.