Fuel Delivery and Air Induction: Description and Operation

Port Fuel Injection Metering System:

Alternating Synchronous Double Fire (ASDF) Fuel Injection Diagram:

PURPOSE
The purpose of Port Fuel Injection is to deliver the correct amount of fuel to the engine under all operating conditions.

CONSTRUCTION
The fuel supply system is made up of the following components.
1. Fuel Filter
2. Fuel Injectors
3. Fuel Lines And Tank
4. Fuel Pressure Regulator
5. Fuel Pump
6. Fuel Pump Relay
7. Fuel Rail
8. Idle Air Control Valve
9. Throttle Body
10. Throttle Position Sensor.

OPERATION
The fuel supply system begins with gasoline in the fuel tank. An electric fuel pump, located in the fuel tank with the fuel level gauge sending unit, pumps fuel to the fuel rail through an in-line filter. The pump is designed to supply fuel at a pressure above the pressure needed by the injectors. A pressure regulator located on the fuel rail keeps the fuel available to the injectors at a constant pressure. The unused fuel is returned to the fuel tank by a separate line.

ALTERNATING SYNCHRONOUS DOUBLE FIRE (ASDF) FUEL INJECTION OPERATION
For better fuel vaporization and improved emission control, fuel is injected at the intake valves while they are in the closed position. To do this, the fuel injectors are alternately pulsed in pairs so that injection for each cylinder begins 75 degrees Before Top Dead Center (BTDC). Injectors for cylinders 1 and 4 are electrically paired, as are the injectors for cylinders 2 and 3. Each pair of injectors is pulsed twice (double fire), once on the compression stroke and once on the exhaust stroke for each cylinder. Injector pulses are synchronized with the ignition reference (2X) pulse to the ECM. A second signal, the 1X reference pulse, is used to sequence the injector pairs.

MODES OF OPERATION
The ECM uses voltage inputs from several sensors to determine how much fuel to give the engine. The fuel is delivered under several conditions, called "Modes." All modes are controlled by the ECM, and are described below.

STARTING MODE
When the ignition is first turned "ON", the Electronic Control Module (ECM) turns "ON" the fuel pump relay for two seconds, and fuel pump builds up pressure. The ECM also checks the coolant temperature sensor and the throttle position sensor, and determines the proper air/fuel ratio for starting. This ranges from approximately 1.5:1 at -36°C (-33°F) to 14.7:1 at 94°C (201°F) engine coolant temperature. The ECM controls the amount of fuel delivered by changing how long the injectors are turned "ON" and "OFF." This is done by pulsing the injectors for very short times.

CLEAR FLOOD MODE
If the engine floods, it may be cleared by pushing the accelerator pedal down all the way. The ECM then will completely turn "OFF" the fuel. The ECM holds this injector rate as long as the throttle stays wide open, and the engine speed is below approximately 637 rpm. If the throttle position becomes less than approximately 75%, the ECM returns to the starting mode.

RUN MODE
The run mode has two conditions, called "OPEN LOOP" and "CLOSED LOOP."

OPEN LOOP
When the engine is first started, and engine speed is above 637 rpm, the system goes into "OPEN LOOP" operation. In "OPEN LOOP," the ECM ignores the signal from the Oxygen sensor, and calculates the air/fuel ratio based on inputs from the coolant temperature and manifold pressure (MAP) sensors.
The system will stay in "OPEN LOOP" until the following conditions are met:
1. The Oxygen sensor has a varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature)
2. The Coolant temperature is above a specified temperature.
3. A specified amount of time has elapsed after starting the engine.

CLOSED LOOP
The specified operating conditions vary with different engines, and are stored in the memory calibration module (Mem-Cal). When these conditions are met, the system goes into "CLOSED LOOP" operation. In "CLOSED LOOP," the ECM calculates the air/fuel ratio (injector on-time) based on the signal from the Oxygen sensor. This allows the air/fuel ratio to stay very close to 14.7:1.

ACCELERATION MODE
The ECM responds to rapid changes in throttle position and air flow, and provides extra fuel.

DECELERATION MODE
The ECM responds to changes in throttle position and air flow, and reduces the amount of fuel. When deceleration is very fast, the ECM can cut off fuel completely for short periods.

BATTERY CORRECTION MODE
When battery voltage is low, the ECM can compensate for the weak spark delivered by the integrated direct ignition (IDI) module by:
1. Increasing the injector pulse width.
2. Increasing the idle speed.
3. Increasing the ignition dwell time.

FUEL CUTOFF MODE
No fuel is delivered by the injector when the ignition is "OFF." This prevents dieseling. Also, fuel is not delivered if no reference pulses are received from the ignition module, which means the engine is not running. This prevents flooding. Fuel is cut-off at engine speeds above 6900 rpm regardless of throttle position, to prevent engine damage.