Engine management system - siemens (all except NAS) (Operation): Description and operation

Operation

General

Engine Starting

When the ignition switch is in position II a power feed is connected from the ignition switch to the ECM. The ECM then initiates 'wake up' routines and energises the main and fuel pump relays. If the ignition switch remains in position II without the engine running, the ECM de-energises the fuel pump relay after approximately 2 seconds. When the ignition switch is in position II with the engine running, or position III, the fuel pump relay is permanently energised.

When the engine cranks, the ECM initiates fuelling and ignition to start the engine. Provided a valid mobilisation signal is received from the immobilisation ECU, the ECM maintains fuelling and ignition control of the engine as necessary to meet driver demand. If no mobilisation code is received from the immobilisation ECU, or the code is invalid, the ECM stops the engine after 2 seconds.

The electrical circuit from the fuel pump relay to the fuel pump is routed through the fuel cut-off inertia switch, located below the E-box in the engine compartment. In the event of a collision the switch breaks the circuit to prevent further fuel being delivered to the engine. The switch is reset by pressing down the centre of the rubber cover on the top of the switch.

During the start sequence, the ECM also illuminates the MIL, as a bulb check, for 4 seconds after the ignition switch turns to position II or until the ignition switch turns to position III.

Engine Stopping

When the ignition switch is turned to position I, the ECM switches off the ignition coils, injectors and fuel pump to stop the engine. The ECM continues to energise the main relay until the power down functions are completed. Power down functions include engine cooling, referencing the IAC valve stepper motor and memorising data for the next start up. When the power down process is completed, the ECM de-energises the main relay and enters a low power mode. In the low power mode, maximum quiescent drain is 0.5 mA.

Fuelling Control

The ECM controls the amount of fuel injected into the engine by adjusting the duty cycle of the fuel injectors. The amount of fuel required is a rolling process determined from maps of engine speed against mass air flow. The value from the map is then corrected for engine coolant temperature, throttle position, vehicle speed and any adaptive value stored in memory.

Mass air flow is calculated using engine speed, inlet air temperature and inlet air pressure. The engine speed indicates the volume of air flowing into the cylinders; the inlet air temperature and inlet air pressure indicate the density of the air. The pressure of the inlet air varies according to the following: l The position of the throttle valve (driver input). l The atmospheric pressure (altitude and weather conditions). l The mechanical condition of the engine (volumetric efficiency).

To ensure the accuracy of the amount of fuel injected, the ECM monitors the fuel injector power supplies and, if necessary, adjusts the fuel injector duty cycle to compensate for low battery voltage. At lower voltages, fuel injector response is slower and, unless compensated for, results in a leaner AFR than intended.

The ECM operates the fuel injectors during their related cylinder's induction stroke, in cylinder firing order. Fuel injector timing is determined from the CMP and CKP sensor inputs.

Fuelling control operates in either closed loop or open loop.

Closed Loop Fuelling Closed loop fuelling is used for the following conditions: l Idle. l Lower mass air flows. l Cruise.