Crankshaft sensor

The crankshaft sensor is integrated in the radial sealing ring. The crankshaft sensor picks up the position of the crankshaft by means of a multi-pole sensor wheel bolted onto the flywheel. The Digital Engine Electronics (DME) uses this to calculate the engine speed. The camshaft sensor and the crankshaft sensor are needed for fully sequential fuel injection (fuel injection takes place individually optimized for each cylinder at the specific ignition point).

The signal from the crankshaft sensor means that the Digital Engine Electronics (DME) evaluate the crankshaft acceleration. The crankshaft acceleration provides an indication of the combustion quality of individual cylinders. The multi-pole sensor gear has 58 magnetic pole pairs as well as a reference point. The reference point on the multi-pole sensor wheel is shown by a magnetic pole pair that is twice as long.

The reference point enables detection of the top dead center of the 1st cylinder. By monitoring the individual pairs of magnetic poles, the Hall effect sensor delivers a certain number of signals to the Digital Engine Electronics (DME).

The active crankshaft sensor detects the direction of rotation of the crankshaft as well as the air gap in relation to the multi-pole sensor gear.

The following graphic shows the engine N55 as example.

Fig 1: Identifying Crankshaft Sensor Components

Courtesy of BMW OF NORTH AMERICA, INC.

Fig 2: Identifying Crankshaft Sensor And Multi-Pole Sensor Gear

Courtesy of BMW OF NORTH AMERICA, INC.

The Digital Engine Electronics (DME) use the scanned signals to calculate the engine speed.

For the engine start, the Digital Engine Electronics (DME) checks the following conditions:

• error-free signal from the crankshaft sensor and camshaft sensor
• both signals must be detected in a specific chronological sequence.

This process is referred to as synchronization and is only performed when the engine is started. Only the synchronization enables Digital Engine Electronics (DME) to activate the fuel injection correctly. The engine will not start without synchronization.

If the crankshaft sensor signals fails (with the 1st crankshaft revolution) or an invalid synchronization is detected for the engine start, the diagnosis starts immediately. The camshaft sensor signals are read here. If 12 flanks on the camshaft are read and the fault is still there, a fault is stored.

The transition from a high to a low phase is signalled by a change in the magnetic field. These changes are counted in the Digital Engine Electronics (DME). The difference between 2 changes of the magnetic field is 6° crank angle.

Fig 3: Crankshaft Sensor And Camshaft Sensor Signal Waveform

Courtesy of BMW OF NORTH AMERICA, INC.