Principle of Operation
Principle of Operation
Ignition Management provides ignition to the combustion chambers with the required voltage at the correct time. Based on the combination of inputs, the ECM calculates and controls the ignition timing and secondary output voltage by regulating the activation and dwell of the primary ignition circuit. The ECM controls and monitors the primary ignition circuit as well as the secondary ignition output (Misfire Detection).
The ECM has a very "broad" range of ignition timing. This is possible using a Direct Ignition System, or sometimes referred to as "Static Ignition System". Reliability is also increased by having separate individual ignition circuits.
The Ignition Control is determined by the ECM (load dependent). The ECM will calculate the engine "load" based on a combination of the following inputs:
The dwell time will be regulated based on battery voltage. When cranking, the voltage is low and the ECM will increase the dwell to compensate for saturation "lag time". When the engine is running and the battery voltage is higher, the ECM will decrease the dwell due to a faster saturation time.
The Crankshaft Position/RPM signals the ECM to start ignition in firing order (1-5-3-6-2-4) as well as providing information about the engine operation. This input is used in combination with other inputs to determine engine load which advances/retards the ignition timing. Without this input, the ECM will not activate the ignition.
Cold start is determined by the ECM based on the engine coolant temperature and rpm during start up. A cold engine will crank over slower than a warm engine, the ignition timing will range between top dead center to slightly retarded providing optimum starting.
When starting a warm engine, the rpm is higher which results in slightly advanced timing. If the engine coolant and intake air temperature is hot, the ignition timing will not be advanced reducing starter motor "load".
During cranking, the ECM recognizes the Camshaft Position (compression stroke) and activates a single ignition per cylinder. The ignition timing will be progressively advanced assisting the engine in coming up to speed.
As the engine speed approaches idle rpm, the timing remains slightly advanced to boost torque.
When the engine is at idle speed, minimum timing advance is required. This will allow faster engine and catalyst warm up.
The timing will be advanced when the ECM observes low engine rpm and increasing accelerator/air volume inputs (acceleration torque). As the throttle is opened, the ECM advances the timing based on engine acceleration and at what rate. The ECM will fully advance timing for the "full throttle" position indicating maximum acceleration (torque).
The Air Flow Volume signal provides the measured amount of intake air volume. This input is used by the ECM to determine the amount of timing advance to properly combust the air/fuel mixture.
The Air Temperature Signal assists the ECM in reducing the risk of detonation (ping). If the intake air is hot the ECM retards the ignition timing. If the intake air is cooler, the ignition timing will be advanced.
As the throttle is closed, the ECM decreases the ignition timing if the rpm is above idle speed (coasting).This feature lowers the engine torque for deceleration. When the engine rpm approaches idle speed, the timing is slightly advanced to prevent the engine from stalling. The amount of advance is dependent upon the engine temperature and the rate of deceleration.
EMISSION OPTIMIZED - IGNITION KEY OFF
"Emission Optimized Ignition Key Off is a programmed feature of the MS S4 ECM. After the ECM detects KL 15 is switched "off, the ignition stays active (ECM Relay/voltage supply) for two more individual coil firings. This means that just two cylinders are fired - not two revolutions.
This feature allows residual fuel injected into the cylinders, as the ignition key is switched off, to be combusted as the engine runs down.
When KL15 is switched "off the ECM operating voltage is removed. The ECM will maintain a ground to the Engine Control Module Relay for a few seconds to maintain ignition coil activation.
Knock Control
The use of Knock Control allows the ECM to further advance the ignition timing under load for increased torque. This system uses three Knock Sensors located between cylinders 1 & 2, cylinders 3 & 4 and cylinders 5 & 6.
Knock Control is only in affect when the engine temperature is greater than 35 °C and there is a load on the engine. This will disregard false signals while idling or from a cold engine.
Based on the firing order, the ECM monitors the Knock Sensors after each ignition for a normal (low) signal.
If the signal value exceeds the threshold, the ECM identifies the "knock" and retards the ignition timing (3°)for that cylinder the next time it is fired.
This process is repeated in 3° increments until the knock ceases. The ignition timing will be advanced again in increments right up to the knock limit and maintain the timing at that point.
If a fault is detected with the Knock Sensor(s) or circuits, the ECM deactivates Knock Control. The "Malfunction Indicator Light" will be illuminated, the ignition timing will be set to a conservative basic setting (to reduce the risk of detonation) and a fault will be stored.