The ideal moment for ignition of the air-fuel mixture is when the piston of the cylinder has reached TDC, providing maximum compression of the mixture, and is ready to move back. The gases formed during combustion of the mixture would provide the piston with a return force, which the piston would transmit to the engine. However, it would be a mistake to ignite the mixture when the piston of the cylinder is at TDC. A certain time is required for ignition of the air-fuel mixture and creation of the corresponding pressure in the cylinder (approximately 1/3000 sec). For this reason, ignition must occur early, at the moment the piston approaches TDC, so that the resulting pressure acts on the piston immediately after it leaves TDC. As the engine speed increases, the ignition advance angle must become increasingly early so that the ignition of the air-fuel mixture occurs at the same time (see illustration 1.0).
In addition, the ignition and combustion speed of the air-fuel mixture depends on its composition. For example, when the accelerator pedal is pressed lightly, the mixture in the combustion chamber is more difficult to ignite and burns more slowly. For this reason, the ignition moment must be earlier.
In some situations, late ignition is necessary when the ignition moment of the mixture must be shifted to a later time, when the cylinder piston has already passed TDC, for example, when descending in forced idle mode, when the engine is running without load (see illustration 1.0a).
Ignition control in vehicles with four and six-cylinder engines is performed by the fuel injection control unit. For this reason, the fuel injection and ignition systems have the same designation. Vehicles with a five-cylinder engine have a fully electronic ignition system (VEZ) and a separate ignition control unit that functions in close connection with the fuel injection control unit (see illustration 1.0b).
Models with a four-cylinder 66 kW engine have a Mono-Motronic ignition system with automatic ignition timing.
The 85 kW four-cylinder engines are equipped with a Digifant ignition system with automatic ignition timing control.
Models with a six-cylinder engine are equipped with an MPI or MPFI ignition system that operates without a distributor. Each two cylinders of a six-cylinder engine, located opposite each other, have a common ignition coil that produces an ignition spark only when it is needed by the corresponding cylinder. Another, unused, but also harmless to the engine, spark is produced during the exhaust stroke.
The ignition system consists of:
- a) spark plugs;
- b) ignition coils with output stage;
- c) ignition and fuel injection system control unit (excluding vehicles with five-cylinder engines);
- d) sensors that supply various information to the control unit.
The supply and disconnection of power to the ignition coil is provided by the Hall sensor, which in cars with four- and five-cylinder engines is located in the ignition distributor, and in cars with six-cylinder engines - on the rear of the left cylinder head (see illustrations 1.0b and 1.0g).
In addition to the Hall sensor, cars with a six-cylinder engine have an engine speed sensor and an ignition timing sensor. Both are inductive type sensors.
With the exception of 66 kW engines, all others have detonation combustion sensors. With the help of these sensors, the ignition advance angle is adjusted in such a way as to prevent detonation of fuel combustion (see illustrations 1.0d and 1.0g).
Attention! The knock sensor mounting bolt must be tightened with great precision, observing a tightening torque of 20 Nm. Otherwise, the sensor will not work.
To avoid electric shock, as well as failure of the ignition system as a whole or its components, it is necessary to observe the following safety measures. This especially applies to people with a pacemaker.
- do not touch any wires, including those on measuring devices, until the ignition is turned off;
- disconnect the battery terminals only after the ignition is turned off;
- turn off the ignition before washing the engine;
- do not leave the fast charger connected when forcibly starting the engine from it with a discharged battery for more than one minute. The voltage should not exceed 16.5 V;
- disconnect the ignition system power plug if it fails and if it is necessary to tow the car to a workshop;
- do not connect to terminal 1/- of the ignition coil capacitors;
- replace a faulty distributor rotor only with a new one of the appropriate type. The rotor of 4- and 5-cylinder engines has a resistance of 1 kOhm and is marked R1;
- connect only 1 kOhm resistors to thick high voltage wires of 4 and 5 cylinder engines;
- use spark plug caps with 5k ohm resistance;
- disconnect the battery when performing welding work on the vehicle.
Warning! The control unit or the ignition system output stage is damaged if you try to turn the engine over with the starter while the high-voltage wire is disconnected. To avoid this, it is recommended to disconnect the output stage plug for 4- and 5-cylinder engines (next to the ignition coil), and for 6-cylinder engines - both output stage plugs (on the rear bulkhead of the engine compartment) with ignition off (see illustration 1.0z and 1.0k).
(This publication is borrowed from the resource AudiManual)