Fuel System
FUEL INJECTION SYSTEM DESCRIPTIONFuel metering is obtained by controlling the injector pulse duration during each engine revolution. The pulse duration is varied by the engine control module (ECM) according to several sensor inputs. The sensed control inputs form two groups - primary and secondary Primary control inputs are intake mass air flow (engine load) and engine speed; secondary control inputs consist of engine coolant temperature, cranking signal, throttle movement and position and exhaust oxygen content. The injector pulse is then trimmed for battery voltage. Except during "cranking" and rapid throttle application, all six injectors are pulsed once per engine revolution (twice per engine cycle). Half of the fuel requirement is delivered at each pulse and the pulse duration is recalculated before each succeeding injection.
Fuel metering strategies are held in memory (EPROM) in the ECM and form an engine load versus engine speed matrix. The load and speed range of the engine is divided into 16 loads and 16 speeds (256 memory sites). Digital numbers representing injector pulse duration in milliseconds fill each site and cover the entire engine load and speed range.
NOTE: The sites are numbered 0 to 15 (16 total). Site 0 is less than overrun load, and low engine speed. Site 15 is higher than full load and high engine speed. The load and speed sites are purposely chosen to extend beyond the operational envelope of the engine.
Injectors Wave Form:
All fuel injectors are pulsed simultaneously by the ECM "output stage". To reduce power consumption, the current drawn by the injectors is controlled by the ECM. The injectors are opened by a relatively large "turn on" pulse and are held open for the required duration by a series of smaller "hold on" pulses.
Additional fuel injection controls are used for overrun fuel cut-off, engine overspeed prevention and wide-open-throttle during cranking fuel cut-off.
The ECM monitors its output signals to the fuel injectors and its input signals from sensors for on-board diagnostics.
FI Primary And Secondary Control:
Load Cart:
FUEL INJECTION PRIMARY CONTROL
Fuel metering is controlled primarily as a function of engine load and speed. Engine load is sensed by a mass air flow sensor located in the engine air intake before the throttle housing. Engine speed is sensed by a crankshaft sensor located behind the engine damper. In addition to engine speed the sensor supplies crankshaft position inputs to the ECM for ignition timing. The ECM processes the input from the mass air flow meter and the crankshaft sensor to access pulse duration from the fuel metering strategy. Usually the load and speed at which the engine is running will be between sites. A function known as two dimensional interpolation is used to calculate the correct pulse duration for the betweensites engine condition.
FUEL INJECTION SECONDARY CONTROL
Secondary fuel metering control adjusts for engine coolant temperature, cranking signal, throttle movement and position and exhaust oxygen content.
Cranking And After-start Enrichment
The ECM provides fuel metering enrichment for cranking and after-start conditions.
The ECM recognizes engine cranking from the engine speed input (less than 200 rpm) and increases the injector pulse frequency to three injection pulses per engine revolution. The pulse duration is determined by engine coolant temperature. Cranking fuel metering is canceled at 250 rpm above a coolant temperature of 42°C (107°F) and at 500 rpm below the same temperature. At that point, injective reverts to one pulse per revolution and after-start enrichment is then applied. The pulse duration, and the rate at which the enrichment is decreased back to the warm-up phase is dependent upon engine coolant temperature.
Warm-up
The programmed warm-up enrichment provides extra fuel during engine warm-up based on the engine temperature measured by the coolant temperature sensor. The injector pulse duration is increased above the fully warm requirement when the coolant temperature is less than 85°C (186°F).
Acceleration Enrichment
When the ECM senses that the throttle is opening (throttle position sensor input), the injector pulse duration is lengthened by an amount dependent upon the rate at which the throttle is opened and on engine coolant temperature. This acceleration enrichment prevents a momentary lean condition that can cause driveability or exhaust emission problems. If the throttle is opened rapidly, a single "extra" injector pulse of about 5 milliseconds is generated to improve engine response.
Full Load Enrichment
If the ECM senses a full throttle input from the throttle position sensor, full load enrichment is applied and closed loop operation is temporarily canceled.
Deceleration Leaning
When the ECM senses that the throttle is closing (throttle position sensor input), the injector pulse duration is shortened dependent on the rate at which the throttle closed. This action prevents a momentary rich condition that can cause exhaust emission problems.
ECM Functions Based On TPS Input:
Oxygen Sensor Characteristic:
Closed Loop Operation
In order to significantly reduce exhaust emission, the exhaust system incorporates 3-way catalytic converters that oxidize CO and HC, and reduce NOx. These converters operate efficiently only if engine combustion is as complete as possible. It is generally accepted that optimum combustion occurs with an air/fuel ratio of 14.7:1 (Lambda = 1). A closed loop system between fuel injection, ECM control and exhaust oxygen content feedback is used to maintain the air/fuel ratio as close to 14.7:1 as possible.
In response to the oxygen sensor voltage, the ECM continuously drives the air/fuel ratio rich-lean-rich by adding to, or subtracting from, the injector pulse duration determined from the main fuel metering strategy.
NOTE: The oxygen sensor voltage switches abruptly at an air/fuel ratio of 14.7:1 (Lambda = 11).
Closed loop operation is canceled by the ECM during the following tunctions:
- full load enrichment
- after-start enrichment
- warm-up, below 35°C (95°F)
- deceleration fuel cut-off
Oxygen Sensor Feedback
Oxygen sensor feedback is an ECM output voltage that is used for technician monitoring of closed loop operation. Oxygen sensor feedback can only be accessed through serial communications using JDS or PDU. The dampened or "average" feedback voltage is used to measure the amount of "dynamic" correction applied by the ECM to the fuel metering strategy in response to exhaust gas oxygen content,
NOTE: Oxygen sensor feedback may also be called Lambda feedback or integrator voltage.
The range for oxygen sensor feedback is 0-5 volts, representing a fuel metering strategy correction of -22 % to +22 %. The normal "average" feedback voltage is 2-3 volts.
Oxygen Sensor Feedback Voltage Correction:
Oxygen Sensor Feedback:
Battery Voltage Correction
Because the time to achieve full lift of the injector pintle decreases as voltage increases, the amount of fuel delivered by the injector for a given pulse duration is dependent upon the injector operating voltage.
The ECM is programmed with a voltage correction strategy. The supply voltage is monitored by a software routine and the correction applied to the pulse duration.
ADDITIONAL FUEL INJECTION CONTROLS
Overrun Fuel Cut-off
In order to improve fuel economy end aid in controlling exhaust emission, the ECM cancels fuel injection during engine overrun conditions. The ECM determines overrun conditions from inputs received from the throttle position sensor, crankshaft sensor and coolant temperature sensor.
With the throttle closed and the engine speed above 1100 rpm, the ECM cancels fuel injection, provided that the coolant temperature is above 30°C (86°F). The engine speed must always reach 1500 rpm first for overrun fuel cut-off to occur. In order to smooth the transition back to power, the ECM retards the ignition timing and shortens the injector pulse duration as the throttle opens and fuel injection is reinstated.
Engine Overspeed Control
An engine overspeed control function limits the maximum engine speed to 6300 rpm by canceling fuel injection. Fuel injection is reinstated at a slightly lower engine speed.
Wide-open-throttle During Cranking
If the ECM senses that the throttle is wide open (throttle position sensor input) during cranking (less than 200 rpm), fuel injection is canceled to help clear a flooded engine.
Adaptive idle Fuel Metering
In order to ensure optimum performance, the ECM contains an adaptive idle fuel metering software function that automatically makes a baseline correction to the idle fuel metering strategy, throughout the life of the vehicle. The total available trim to the nominal injector pulse duration is ±10 %. Adaptive idle fuel metering is performed by the ECM at idle, only when diagnostic trouble codes (DTC) are cleared, and certain preconditions are met. If the DTCs are cleared and the preconditions are met, the ECM cancels purge flow and adapts the fuel metering strategy. Between adaptations, there is a delay of approximately eight minutes. The correction is applied across the entire engine speed and load range.
NOTE: With the incorporation of adaptive idle fuel metering, the idle trim potentiometer was removed from the mass air flow sensor.
Adaptive idle fuel metering preconditions:
- throttle is closed
- engine speed is below 1000 rpm
- road speed is below 6 kmh (3 mph)
- engine coolant temperature is above 76°C (170°F)
- idle speed adaptive delay is complete (vehicle speed reached 3 mph for approximately 100 yards traveled)
- closed loop air/fuel ratio control is operating and in control
- oxygen sensor feedback (HO2SFB) voltage outside of the normal range (2-3 volts)
If the ECM loses its battery voltage supply for any reason, the stored correction will be lost and fueling will revert to the programmed values.