- The triggering of the ignition spark by the ignition interrupter (ignition contact, with contact ignition) or Hall sensor (contactless ignition)
- The distribution of the high voltage generated in the ignition coil to the spark plugs
- Occasionally, the engine speed limitation.
The name of the assembly is based on the only externally recognizable function of high-voltage distribution, the other functions take place inside the assembly, invisible from the outside.
Modern engines do not have an ignition distributor, ignition interrupter or Hall sensor, but have fully electronic ignition systems that do without moving components. The ignition timing is calculated by a control unit; the spark is generated by a single or double spark coil (s) (one ignition coil for each two cylinders, so-called " wasted spark ").
The assembly is usually housed in a housing: The head part consists of the distributor cap , onto which the ignition cables leading to the spark plugs and the high-voltage cable coming from the ignition coil are attached in the ignition sequence of the engine . There are contact surfaces on the inside of the cap. The distributor rotor (also called “distributor finger”), which is attached to the distributor shaft and has a contact surface attached to its tip, rotates under the distributor cap . At the time of ignition, the contact surface on the distributor rotor is located directly next to the contact surface of the corresponding cylinder in the distributor cap. However, there is no direct contact; the narrow gap between the contacts is jumped over by the high voltage. Although there is no mechanical abrasion on the contact surfaces, the erosion of the high-voltage spark causes wear on the contact surfaces of the rotor and cap.
An interference suppression resistor is often integrated in the distributor rotor to reduce high-frequency interference. Various motors (up to around the mid-1980s) with contact or transistor ignition have distributor rotors with a centrifugal speed limiter that works by spring force: Above a defined speed, the high voltage generated is dissipated to ground (body) and therefore no ignition voltage is passed on to the spark plugs.
Under the distributor rotor, mostly covered by a plastic cap, there is the ignition interrupter , the ignition capacitor (often attached to the outside) and the devices for the speed-dependent adjustment of the ignition timing (by negative pressure or by centrifugal force). In today's standard contactless ignition systems, the ignition interrupter is replaced by an electromagnetic transmitter (Hall transmitter), which does not necessarily have to be located in the distributor, but can also be located on the crankshaft or the flywheel.
The distributor shaft is mounted in the lower part of the housing . At the upper end it has a receptacle for the distributor rotor, onto which it is attached so that it cannot twist. In the case of a contact-controlled ignition system, the distributor shaft has a number of ignition cams corresponding to the number of cylinders in the engine. In a four-cylinder engine, the cross section of the distributor shaft in the area of the ignition cams is a square with strongly rounded corners. The ignition cams activate the breaker contact . With the contactless ignition, a rotating panel is installed that controls the Hall sender. At the lower end of the distributor shaft is a coupling piece that drives it.
Since the distributor shaft rotates at camshaft speed (= half the crankshaft speed) in four-stroke engines, it is often driven directly by the camshaft, with the distributor being arranged at one end of the camshaft. For Volkswagen boxer engines z. B. the shaft of the distributor and the oil pump is driven by helical gears from the camshaft. This was widespread on camshaft engines.
In various modern ignition systems such as B. the Motronic from Bosch , the distributor only has the function of high voltage distribution. The ignition timing and speed limitation are determined directly by the engine control unit .
When the distributor shaft rotates, the ignition contact is opened and closed by each ignition cam (the number of which is equal to the number of cylinders in the engine). When the contacts open, the circuit of the ignition coil primary winding is interrupted. The magnetic field then collapses in it and induces a high voltage in the secondary winding of the ignition coil , which is conducted from the ignition coil (standard designation: terminal 4) to the central terminal 4 in the cap of the ignition distributor. The distributor rotor located on the upper part of the distributor shaft conducts the high voltage to the connections of the ignition cables leading to the spark plugs . The distributor shaft rotates at camshaft speed (for four-stroke engines) or crankshaft speed (for two-stroke engines).
The ignition point is the moment at which the ignition contact interrupts the primary circuit of the ignition coil and the high voltage is induced in the secondary winding by the collapsing magnetic field of the primary winding. This jumps over to the spark plug in the form of a spark and ignites the fuel-air mixture.
The point in time at which the contacts are lifted and thus the ignition point is changed by turning the distributor housing.
Since it takes a short time for the maximum combustion pressure to be reached after ignition by the ignition spark, the ignition point is always shortly before top dead center of the piston in the work cycle so that the maximum pressure acts on the piston at the right moment. A change in the ignition point therefore has an effect on the performance and service life of the engine. Therefore, the manufacturer's information must be observed.
In order for the engine to achieve optimum performance and exhaust emissions in all speed ranges, the ignition point must be adjusted depending on the operating status.
- Vacuum adjustment
The adjustment is particularly effective when accelerating in the lower speed range. It takes place via a membrane box (vacuum box) on the ignition distributor, which is controlled by the vacuum near the throttle valve in the intake tract . The contact plate of the interrupter is rotated in the direction of pre-ignition via a pull rod connected to the membrane. Some distributors have a second “retardation box” for the idle / overrun operating state, which shifts the ignition point back in the “retarded” direction to improve exhaust gas, but is subordinate to the advance ignition adjustment with vacuum or centrifugal force.
- Centrifugal adjustment
It only comes into effect in the upper speed range. On the distributor shaft there is a rotating axle plate with flyweights attached to return springs , which are pushed outwards as the speed increases . Here, the ignition cam on the distributor shaft is turned in the direction of "advance ignition" via additional mechanics. The shape and mass of the flyweights and the spring characteristic of the return springs create an ignition advance characteristic that is adapted to the engine.
- Speed limitation
In order to prevent the engine from rotating too high, a centrifugal speed limiter is also installed in some designs. With a special distributor finger, this ensures that the high voltage is discharged directly to ground above the cut-off speed. In the absence of an ignition spark, the engine then begins to “stutter” and the speed is reduced again. In modern vehicles, the speed limitation is implemented via the electronic engine control, which no longer allocates fuel to the engine above a maximum speed. To avoid damage to the vehicle catalytic converter, only the latter variant is possible in catalytic converter vehicles, since otherwise unburned fuel can get into the catalytic converter and damage it.
The correct function of the high-voltage part of an ignition system can be recognized by removing a spark plug connector and a separate spark plug held against ground ( caution high voltage! ). A stable connection is very important. If this is interrupted, even for a short time, the resulting voltage peaks can destroy electronic components. With older contact-controlled ignition systems with a purely mechanical breaker, this risk does not exist.
The ignition point can be checked with a simple test lamp while the vehicle is stationary. When the engine is running, it is determined with a stroboscopic lamp or a test computer. The diagnosis with a motor testers determining an oscilloscope image of the ignition system is, however, preferable to get an accurate picture of the entire ignition system.
The contact distance of the interrupter is usually 0.4 mm. Due to wear on the bearings of moving components and age-related changes in the force of the return springs (flyweights), there are deviations in the ignition curve. Already 0.1 mm of bearing play has a major influence on the motor performance and the concentricity of the motor. For older or historic vehicles, ignition distributors have therefore been developed in which the ignition contact has been replaced by a sensor. The flyweights were replaced by electronics. The adjustment characteristics are preprogrammed for different motors and for different purposes and can be selected via a switch. In some cases, these ignition distributors recognize the smallest angle errors in the rotary movement via a sensor and correct them by shifting the ignition timing.