Fuel Delivery and Air Induction: Description and Operation
Fuel Metering System:
COMPONENTS
The fuel metering system consists of the following components:
^ Fuel injectors
^ Throttle body
^ Fuel rail
^ Fuel pressure regulator
^ PCM
^ Crank sensor
^ Cam sensor
^ Idle Air Control (IAC) valve
^ Fuel pump
^ Fuel pump relay
BASIC SYSTEM OPERATION:
The fuel metering system starts with fuel in the tank. An electric fuel pump, located in the tank with the gauge sending unit, pumps fuel to the fuel rail through an in-line fuel filter. The pump is designed to provide fuel at a pressure above the pressure needed by the injectors. A pressure regulator in the fuel rail keeps fuel available at the injectors at a constant pressure. Unused fuel is returned to the fuel tank by a separate line.
SEQUENTIAL FUEL INJECTION OPERATION:
The purpose of port fuel injection systems, is to improve emission control, performance, and fuel economy. With sequential fuel injection systems this is accomplished by not only delivering the fuel at the intake valves, but by timing each injector individually, using the ignition signal as a reference. Each injector is pulsed each revolution of the crank shaft, and timed to fire on the compression and exhaust cycles of the engine for the best fuel atomization possible.
MODES OF OPERATION:
The PCM 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 PCM, and are described below.
Starting Mode: When the ignition is first turned "ON", the Powertrain Control Module (PCM) turns "ON" the fuel pump relay for two seconds, allowing the fuel pump to build up system pressure. If the engine is immediately started, the fuel pump will continue to operate. If the key is left in the "ON" position, but the engine is not started, the PCM will turn the pump "OFF." The fuel pump will remain OFF until the PCM detects the crank signal, or in the case of a faulty fuel pump relay, the oil pressure switch detects approximately 4 psi of oil pressure which will then bypass the fuel pump relay.
Fuel delivery from the port injectors (which deliver fuel in all operating modes) is controlled by changing the amount of time the injectors are turned on or "pulsed" by the PCM. The PCM checks the coolant temperature sensor, throttle position sensor, and the crank signal, and determines the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33° F) to 14.7:1 at 94° C (201° F).
Clear Flood Mode: If the engine floods, it may be cleared by pushing the accelerator pedal down all the way. The PCM then will completely turn off the fuel. The PCM holds this injector rate as long as the throttle stays wide open, and the engine speed is below approximately 600 rpm. If the throttle position becomes less than approximately 65%, the PCM 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 400 rpm, the system goes into "Open Loop" operation. In "Open Loop", the PCM ignores the signal from the Oxygen sensor, and calculates the air/fuel ratio based on inputs from the coolant temperature, throttle position, and the mass air flow (MAF) 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 PCM 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 PCM responds to rapid changes in throttle position and manifold pressure or air flow, and provides extra fuel.
Deceleration Mode: The PCM responds to changes in throttle position and air flow, and reduces the amount of fuel. When deceleration is very fast, the PCM can cut off fuel completely for short periods.
Battery Correction Mode: When battery voltage is low, the PCM can compensate for the weak spark by:
1. Increasing the amount of fuel delivered.
2. Increasing the idle speed.
3. Increasing the ignition dwell time.
Fuel Cut-Off 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.