Engine management system - MEMS (Ignition system): Description and operation

IGNITION SYSTEM

The ECM determines the optimum ignition timing based on the signals it receives from the following sensors:

1. Crankshaft position sensor - Engine speed and crankshaft position
2. Manifold absolute pressure sensor - Engine load
3. Engine coolant temperature sensor - Engine temperature
4. Throttle position sensor - Throttle pedal released
5. Intake Air Temperature sensor - intake air temperature

The engine management system uses no centrifugal or vacuum advance, the timing being controlled by the ECM.

Spark distribution is achieved by means of a rotor arm and distributor mounted at the No.4 cylinder end of the inlet camshaft.

Basic Ignition Timing

Crankshaft Position (CKP) Sensor

The speed and position of the engine is detected by the CKP sensor which is bolted to, and projects through, the engine adapter plate adjacent to the flywheel.

The CKP sensor is an inductive sensor containing a coil and a permanent magnet which provides a magnetic field. The sensor is situated such that an air gap exists between it and the flywheel. Air gap distance is critical for correct operation.

The flywheel incorporates a reluctor ring which consists of 32 poles spaced at 10° intervals, with 4 missing poles at 0°, 50°, 180° and 240°. The missing poles tell the ECM when to operate the injectors. When the flywheel rotates and a pole passes the CKP sensor, it disturbs the magnetic field inducing a voltage pulse in the coil. This pulse is transmitted to the ECM.

By calculating the number of pulses that occur within a given time, the ECM can determine the engine speed. The output from the CKP sensor, when used in conjunction with that from the Manifold Absolute Pressure (MAP) sensor, provides idle stabilisation and reference for injection timing.

If the CKP sensor fails, the engine will stop and cannot be restarted.

Manifold Absolute Pressure (MAP) Sensor

The MAP sensor is located within the ECM and detects manifold pressure via a hose connected to the inlet manifold. A fuel trap is fitted in the hose from the inlet manifold to prevent fuel and oil residues from entering the MAP sensor.

The sensor converts pressure variations into graduated electrical signals which can be read by the ECM. Increases and decreases in the manifold pressure provide the ECM with an accurate representation of the load being placed on the engine, allowing the ECM to adjust the quantity of fuel being injected and the ignition timing to achieve optimum fuelling of the engine.

If the MAP sensor fails, the ECM enters a default mode (called ’Pressure Back-up’), using a substitute value determined from engine speed and throttle position.