The winding of the ignition coil is current flowing through it. Since the voltage comes directly from the on-board battery and is therefore quite low, the primary winding is also called the low-voltage winding. If the current flow is suddenly interrupted, the magnetic field changes ; This induces a voltage pulse in the secondary winding in accordance with Lenz's rule (see induction ). The ratio of the two windings is designed so that a voltage of 10,000 to 25,000 volts is induced on the secondary winding ; therefore the secondary winding is also called high-voltage winding. This voltage is transmitted to a spark plug in the cylinder . A spark occurs on the spark plug. To attenuate the sparking caused by the induced voltage of several hundred volts at the UK, the ignition capacitor is connected in parallel to the ignition contact .
The UK is usually located in the "basement" of the ignition distributor , which distributes the ignition voltage generated by the ignition coil to the spark plugs via the rotating distributor rotor ("distributor finger").
The ignition interrupter consists of the following components:
- Base plate with axis
- Fixed contact (anvil) on the base plate
- The interrupter lever (hammer) with a sliding piece made of polyamide , hard paper or another insulating material sits on the axis . The cams of the distributor shaft operate the interrupter lever via the slide.
- Loose contact with connection cable on the breaker lever
Both the breaker contact (loose and fixed contact) and the sliding piece running on the cam of the distributor shaft wear out. The consequences of wear are opposite: the wear on the sliding piece can compensate for the erosion on the contact. However, the contact opens increasingly too late (the slider is now on the higher, rounder part of the cam at the point of ignition) and, due to the steeper voltage rise, experiences a higher contact erosion even with an intact ignition capacitor .
The contact is exposed to a high load: it has to switch currents of up to 5 amps; voltages of up to 500 V are applied to it. At a speed of 6000 min −1 of a four-cylinder engine, the contact switches 12,000 times per minute (corresponds to a frequency of 200 Hz ).
Hiring and testing
The contact distance is set by moving (rotating) the base plate in the ignition distributor to a specified value (see manual of the respective engine); for many four-cylinder vehicles, a value of around 0.35 to 0.4 mm is correct. When new, the contact distance can be measured with a feeler gauge ; A dwell angle tester should be used for contacts that have already been run . The ignition point must then be set by turning the ignition distributor : this is at a certain angular position of the crankshaft that is specified by the engine manufacturer (approx. 5 degrees before top dead center (TDC) of the first cylinder). At this angle, the contact opens and triggers the ignition.
If the engine load and engine speed increase (acceleration), the ignition must be "earlier": The advance angle must increase: from around 4 to 7 degrees before TDC to 40 and more before TDC. This adjustment is achieved by rotating the spring-loaded base plate of the ignition distributor via a vacuum adjustment or with a centrifugal adjuster on the drive shaft of the distributor. The vacuum adjustment by means of a vacuum unit, which is connected to the intake tract , adjusts the ignition point depending on the engine output or load, while the centrifugal adjuster works depending on the speed. Usually both types of adjustment can be found together.
The function of these centrifugal force and vacuum adjustments can be checked on a running engine with a stroboscope , which is triggered capacitively or inductively via the ignition cable of the first cylinder. A test lamp connected to the breaker contact can also be used to check the ignition angle with the engine stopped (static).
Increased contact wear should be checked immediately and is usually caused by a faulty ignition capacitor. A faulty ignition coil is also less common. Due to the material migration during contact erosion, an increase on the ground side caused by this material migration indicates the ignition capacitor as a source of error, while an increase on the contact side indicates the ignition coil as a source of error.
Contact-controlled ignition systems are no longer used in newer engines because of the increased wear and tear and the associated maintenance costs. At the end of the 1960s , the first contact-controlled transistor ignition systems were used, which already greatly reduced contact wear. Later non-contact sensors ( Hall sensors ) were used for control. Today, the ignition coil (s) are controlled by combined engine electronics for injection and ignition, which receive signals from sensors about the angle of rotation of the crankshaft , the applied load and the speed , and trigger the ignition spark via power semiconductors in accordance with a programmed ignition map .
- Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden, 2003, ISBN 3-528-23876-3
- Rudolf Hüppen, Dieter Korp: Car electrics all types. Motorbuchverlag, Stuttgart, 1968, ISBN 3-87943-059-4