The ideal moment to ignite 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 the combustion of the mixture would provide the piston with a return force, which the piston would transfer to the engine. However, it would be a mistake to ignite the mixture when the cylinder piston is at TDC. It takes a certain time to ignite the air-fuel mixture and create the appropriate pressure in the cylinder (approximately 1/3000 sec). For this reason, ignition must occur in advance, 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 timing must become more and more early so that the ignition of the air-fuel mixture occurs at the same time (see illustration 1.0).
In addition, the rate of ignition and combustion of the air-fuel mixture depends on its composition. For example, with a slightly depressed accelerator pedal, the mixture in the combustion chamber is more difficult to ignite and burns more slowly. For this reason, the ignition point must be earlier.
In some situations, a late ignition is necessary, when the ignition point 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).
The ignition control for vehicles with four and six-cylinder engines is carried out by the fuel injection system control unit. For this reason, fuel injection and ignition systems have the same designation. Cars with a five-cylinder engine have a fully electronic ignition system (VEZ) and a separate ignition control unit, functioning in close connection with the fuel injection control unit (see illustration 1.0b).
Models with a 66 kW four-cylinder 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.
Models with a six-cylinder engine are equipped with an MPI or MPFI ignition system that operates without an ignition distributor. Every two cylinders of a six-cylinder engine, located opposite each other, have a common ignition coil, which produces an ignition spark only when it is needed by the corresponding cylinder. Another, not used, but also harmless to the engine, spark is produced on the exhaust stroke.
The ignition system consists of:
- A) spark plugs;
- b) ignition coils with output stage;
- V) control unit for the ignition and fuel injection system (excluding cars with a five-cylinder engine);
- G) sensors that supply various information to the control unit.
The supply and disconnection of power to the ignition coil is provided by a Hall sensor, which for cars with four- and five-cylinder engines is located in the ignition distributor, and with six-cylinder engines - on the back of the left cylinder head (see illustrations 1.0c and 1.0g).
In addition to the Hall sensor, vehicles with a six-cylinder engine have an engine speed sensor and an ignition timing sensor. They are both inductive type sensors.
With the exception of 66 kW engines, all others have knock sensors. Using these sensors, the ignition timing is adjusted in such a way as to prevent detonation of the fuel (see illustrations 1.0e 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, the following safety precautions must be observed. This is especially true for people with a pacemaker.
- do not touch the wires, including those on measuring instruments, until the ignition is turned off;
- disconnect the terminals from the battery only after turning off the ignition;
- turn off the ignition before washing the engine;
- do not leave the booster charger connected when forcibly starting the engine from it with a dead battery for more than one minute. The voltage in this case should not exceed 16.5 V;
- disconnect the power plug of the ignition system in case of its failure and, if necessary, towing the car to the workshop;
- do not connect capacitor ignition coils to terminal 1/-;
- replace the faulty ignition distributor runner only with a new one of the appropriate type. The slider of 4- and 5-cylinder engines has a resistance of 1 kOhm and is marked R1;
- connect to thick high voltage wires of 4- and 5-cylinder motors with only 1 kΩ resistance;
- use spark plug tips with a resistance of 5 kOhm;
- Disconnect the battery when performing welding work on the vehicle.
Attention! The control unit or ignition system output stage is damaged when trying to crank the engine with the starter with the high voltage wire disconnected. To avoid this, we recommend disconnecting the output stage connector on 4- and 5-cylinder engines (next to the ignition coil), and for 6-cylinders, both output stage plugs (on the rear bulkhead of the engine compartment) with the ignition off (see illustration 1.0z and 1.0k).
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