A diesel engine does not have an ignition system, because due to the strong compression the air is heated so much that after the fuel is injected into the cylinder it ignites spontaneously.
Schematic diagram of the Motronic ignition system (models since 1986)
1 - ignition switch;
2 - ignition coil;
3 - high voltage distributor;
4 - ignition cables;
5 - spark plug tips;
6 - spark plugs;
7 — control device;
8 — battery.
Modern BMW petrol engines are equipped with an electronically controlled ignition system, the control unit of which is integrated into the engine control unit. It does not have a conventional ignition distributor. Instead, the distribution of high voltage to the individual cylinders is carried out by fixed electronic elements. In modern engines, high voltage is generated in six separate ignition coils, which are located above the spark plugs. The operation of the ignition system is synchronized by signals that the pulse generator sends to the control unit. The pulse generator is located at the front of the cylinder block and displays the number of revolutions of the crankshaft.
To determine the right moment (lead angle) the ignition control unit uses an electronic database. Thanks to this, the ignition system easily adapts to various engine operating conditions.
Ignition System Database Features
1 — ignition timing angle;
2 — load;
3 — number of revolutions.
The ignition timing is also adjusted by knock sensors. Since fuel consumption is minimal at high compression ratios, modern engines compress the air-fuel mixture as much as possible. However, due to high compression, uncontrolled combustion, called detonation, can occur. Over time, detonation can cause engine damage. To prevent this phenomenon, a knock sensor is needed, which detects uncontrolled combustion and informs the control unit, which ensures that the ignition timing is shifted in the retarded direction. BMW engines have two knock sensors screwed into the cylinder block, one sensor for cylinders No.1, 2, 3, and the other for cylinders 4, 5 and 6. Thanks to knock control, the engine can run on fuel that is less resistant to detonation (91 octane gasoline), although this will produce less power. When using gasoline with an octane rating of 98, the engine power increases.
All ignition system components are matched to each other and always provide the optimum ignition timing. Adjusting the ignition timing is no longer possible.
When working on the electronic ignition system, it is necessary to pay attention to safety precautions to avoid injury and damage to system parts.
(The original article is posted on the resource «bmwman»)