Principle of Operation
Principle Of Operation
Fuel Management delivers fuel from the tank to the intake ports of the engine. To accomplish this, fuel supply must be available to the fuel injectors. Then the fuel must be injected in the precise amount and at the correct time. The ECM does not directly monitor fuel supply, although it does control it. The ECM controls and monitors fuel injection.
The Fuel Pump supplies fuel when it receives operating voltage from the Fuel Pump Module. The ECM Relay supplies voltage to the Fuel Pump Module (M3-located in the trunk, above right wheel well, M3 C-behind right rear interior side trim panel).
The ECM controls the activation of the fuel pump module. After the ignition is switched ON, the ECM provides voltage for the fuel pump module and the voltage is maintained (pulse width modulated) with the presence of the engine speed signal.
A high output fuel pump is utilized to match the fuel supply demands of the S54 engine (5 Bar). The ECM will cycle the voltage signal (0 - 120 Hz) to the fuel pump module. The fuel pump module will cycle the voltage to the pump (to reduce the speed). The fuel pump will operate at low speed during idle and part load. The pump will run at full speed during start-up (approx. 20 seconds) and full load.
The power to the fuel pump module will be switched off in the event of an airbag activation. The MRS III control module will signal the ECM over K-bus and CAN bus for this purpose.
The Fuel Injectors will be opened by the ECM to inject pressurized fuel into the intake ports. The ECM Relay supplies voltage to the Fuel Injector Relay, the Fuel Injector Relay supplies operating voltage to the injectors. The ECM controls the opening by activating the ground circuit for the Solenoid Windings. The ECM will vary the duration (in milli-seconds) of "opening" time to regulate the air/fuel ratio.
The ECM has six Final Stage output transistors that switch ground to the six injector solenoids. The Injector "triggering" is first established from the Crankshaft Position/RPM Sensor.
The ECM is programmed to activate the Final Stage output transistors once (per cylinder) for every working cycle of the engine (Full Sequential Injection).The ECM calculates the total milli-second time to open the injectors and triggers them independently.
During start up, the ECM recognizes the Camshaft Position (Cylinder ID) inputs. The camshaft positions are referenced to the crankshaft position. This process "times" the injection closer to the intake valve opening for increased efficiency.
When activated, each injector delivers the full fuel charge at separate times for each cylinder working cycle.
The Camshaft Position input is monitored by the ECM during start up. There will be an effect on injector timing if this input is missing when the engine is started.
When KL15 is switched "off", the ECM discontinues voltage to the Fuel Injector Relay and deactivates the six Final Stage transistors to discontinue fuel injection.
The Injector "open" Time to maintain engine operation after it has been started is determined by the ECM (programming).The ECM will calculate the injector timing based on a combination of the following inputs:
The injection ms value will be regulated based on battery voltage. When cranking, the voltage is low and the ECM will increase the ms value to compensate for injector "lag time". When the engine is running and the battery voltage is higher, the ECM will decrease the injection ms value due to faster injector reaction time.
Cold starting requires additional fuel to compensate for poor mixture and the loss of fuel as it condenses onto cold intake ports, valves and cylinder walls. The cold start fuel quantity is determined by the ECM based on the Engine Coolant Temperature Sensor input during start up.
During cranking, additional fuel is injected for the first few crankshaft revolutions. The ECM recognizes the Camshaft Positions and precisely times the Full Sequential Injection. After the first few crankshaft revolutions, the injected quantity is metered down as the engine comes up to speed.
When the engine is cold, optimum fuel metering is not possible due to poor air/fuel mixing and an enriched mixture is required. The Coolant Temperature input allows the ECM to adjust the injection ms value to compensate during warm up and minimize the the injected fuel at engine operating temperature.
When the engine is at idle, minimum injection is required. Additional fuel will be added if the ECM observes low engine rpm and increasing throttle/air volume inputs (acceleration enrichment). As the accelerator pedal is actuated, the ECM monitors acceleration and rate of movement. The ECM will increase the volume of fuel injected into the engine by increasing the injection ms value. The "full throttle" position indicates maximum acceleration and the ECM will add more fuel (full load enrichment).
As the accelerator pedal is released, the ECM decreases the injection ms value (fuel shut off) if the rpm is above idle speed (coasting). This feature decreases fuel consumption and lowers emissions. When the engine rpm approaches idle speed, the injection ms value is increased (cut-in)to prevent the engine from stalling. The cut-in rpm is dependent upon the engine temperature and the rate of deceleration.
The HFM signal provides the measured amount of intake air volume. This input is used by the ECM to determine the amount of fuel to be injected to "balance" the air/fuel ratio.
The Air Temperature Signal allows the ECM to make a calculation of air density. The varying voltage input from the NTC sensor indicates the larger proportion of oxygen found in cold air, as compared to less oxygen found in warmer air. The ECM will adjust the amount of injected fuel because the quality of combustion depends on oxygen sensing ratio (details in Emissions).
The Crankshaft Position/RPM signals the ECM to start injection as well as providing information about the engine operation. This input is used in combination with other inputs to determine engine load which increases/decreases the injection ms value. Without this in-put, the ECM will not activate the injectors.
When KL15 is switched "off", the ECM discontinues voltage to the Fuel Injector Relay and deactivates the six Final Stage transistors to cease fuel injection.
Injection "Reduction" Time is required to control fuel economy, emissions, engine and vehicle speed limitation. The ECM will "trim" back or deactivate the fuel injection as necessary while maintaining optimum engine operation.
As the throttle is closed during deceleration, the ECM decreases the injection ms value (fuel shut off) if the rpm is above idle speed (coasting).This feature decreases fuel consumption and lowers emissions.
When the engine rpm approaches idle speed, the injection ms value is increased (cut-in)to prevent the engine from stalling. The cut-in rpm is dependent upon the engine temperature and the rate of deceleration.
The ECM will deactivate the injectors to control maximum engine rpm (regardless of vehicle speed). When the engine speed reaches 8000 rpm, the injectors will be deactivated to protect the engine from over-rev. As the engine speed drops below 8000 rpm, injector activation will be resumed. This feature does not protect the engine from a forced over-rev such as improperly downshifting a manual transmission equipped vehicle (driver error).
Maximum vehicle speed is limited by the ECM reducing the injection ms value (regardless of engine rpm). This limitation is based on the vehicle dimensions, specifications and installed tires (speed rating).
The ECM will also protect the Catalytic Converters by deactivating the injectors.
If the ECM detects a "Misfire" (ignition, injection or combustion), it will selectively deactivate the Final Stage output transistor for that cylinder(s). On the MS S54 system, there are six individual injector circuits resulting in deactivation of one or multiples. This will limit engine power, but protect the Catalytic Converters.
Fuel Injection Control Monitoring is performed by the ECM for OBD II requirements. Faults with the fuel injectors and/or control circuits will be stored in memory. This monitoring includes:
- Closed Loop Operation
- Oxygen Sensor Feedback
These additional corrections are factored into the calculated injection time. If the correction factor exceeds set limits a fault will be stored in memory.
When the criteria for OBD II monitoring is achieved, the "Malfunction Indicator Light" will be illuminated.