Fuel Delivery and Air Induction: Description and Operation
GENERAL DESCRIPTION - MULTI PORT FUEL INJECTION (MFI)Fuel Injection Control.:
Injector Control.:
PURPOSE
The purpose of the fuel control system is to deliver the correct amount of fuel to the engine under all operating conditions. Fuel is delivered to the engine by individual fuel injectors mounted in the ports of the intake manifold.
FUEL SYSTEM DESCRIPTION
The SATURN fuel system is comprised of a polyethylene fuel storage tank. This tank is equipped with a submersible electric fuel pump, reservoir, and fuel level sending unit module attached at the top of the fuel tank. This module is held in place with a cam lock ring and sealed with a rubber O-ring seal. Fuel is drained from the tank by removing the filler tube at tank and pushing the check ball into the tank.
Fuel enters the fuel storage tank through a lead tin plated steel filler neck attached to the tank with a short section of rubber hose. This system is sealed from the environment with a fuel filler cap.
Fuel is delivered from the pump to the engine and from the engine back to the tank by way of nylon fuel lines with quick connect line connections. Before entering the engine, fuel passes through an in-line fuel filter also equipped with quick connects. Fuel vapor generated within the fuel storage tank is stored in a charcoal vapor canister where it is later purged and consumed by the engine.
BASIC OPERATION
The PCM controls the amount of fuel delivered by changing how long the injectors are turned "ON" and "OFF" every 360 degrees of crankshaft revolution. This is called Simultaneous Double Fire (SDF) injection.
The PCM uses one injector driver circuit to control the injectors. The current in this circuit is allowed to climb to a peak of eight amps, then is very quickly reduced to two amps to hold the injector open.
During closed loop operation, the main control sensor is the oxygen (02) sensor, which is located in the exhaust manifold. This sensor indicates to the PCM how much oxygen is in the exhaust gas, and the PCM changes the fuel to the engine based on this information by controlling how long the fuel injectors are pulsed. The best mixture to minimize exhaust emissions is 14.6:1 air/fuel ratio, which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a Closed Loop system.
Injector pulses are synchronized with the ignition reference (2X) pulse to the PCM. All injectors are pulsed once each time number 1 cylinder is 60 degrees before top dead center (BTDC).
MODES OF OPERATION
The PCM uses voltage inputs from several sensors to determine how much fuel to deliver to the engine. The fuel is delivered under one of several conditions, called modes. All the modes are controlled by the PCM, and are described below.
STARTING MODE
When the ignition is first turned to the "ON" position, the PCM turns "ON" the fuel pump relay for two seconds, allowing the fuel pump to build pressure. The PCM also checks the coolant temperature sensor and throttle position sensor, to determine the proper air/fuel ratio for starting. This ranges from approximately 0.8:1 at -40°C (-40°F) to 14.6:1 at 104°C (220°F) engine coolant temperature.
CLEAR FLOOD MODE
If the engine floods, it may be cleared by pressing the accelerator pedal all the way to the floor. The PCM then completely turns "OFF" the fuel. The PCM holds this injector rate as long as the throttle stays wide open, and the engine speed is below approximately 400 rpm If the throttle position becomes less than approximately 75 percent, the PCM returns to the starting mode.
RUN MODE - OPEN LOOP
When the engine is first started, and engine speed is above 400 RPM, the system goes into Open Loop operation. The PCM ignores the signal from the oxygen sensor, and calculates the fuel required based on inputs from the coolant temperature and manifold absolute pressure (MAP) sensors. The system stays in open loop until the following conditions are met:
^ The oxygen sensor has varying voltage output, showing that it is hot enough to operate properly depending on temperature.
^ The coolant temperature sensor is above a specified temperature.
^ A specific amount of time has elapsed after starting the engine.
RUN MODE - CLOSED LOOP
Specific values for the above conditions exist and are stored in the memory calibration module. When these values are met, the system goes into Closed Loop operation. In closed loop, the PCM calculates the fuel required in the same manner as open loop but corrects for small errors based on the signal from the oxygen sensor. This allows the air/fuel ratio to stay very close to 14.6:1. The best mixture to minimize exhaust emissions is 14.7:1 air/fuel ratio, which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the injection system is called a closed loop system.
ACCELERATION MODE
The PCM responds to rapid increases in throttle position and manifold pressure (MAP), providing extra fuel needed for smooth acceleration.
DECELERATION MODE
The PCM responds to decreases in throttle position and manifold pressure (MAP) reduces the amount of fuel. When deceleration is extended, the PCM cuts "OFF" fuel completely when power is not needed.
BATTERY VOLTAGE CORRECTION MODE
When battery voltage is low, the PCM compensates by:
^ Increasing the injector pulse width to provide proper fuel.
^ Increasing the idle rpm to increase generator output.
FUEL CUT-OFF MODE
To prevent possible engine damage from overspeed, the PCM cuts "OFF" fuel from the injectors when engine speed is above 6750 rpm in drive or while vehicle is moving. In neutral or while stopped, the engine speed is limited to 4000 rpm.
FUEL SYSTEM COMPONENTS
The fuel metering system is made up of the following parts:
1. Fuel supply components (fuel tank, pump and lines)
2. Fuel pump electrical circuit
3. Fuel rail assembly, including:
a. Fuel injectors
b. Pressure regulator
4. Throttle body assembly, including:
a. Idle Air Control (IAC) valve
b. Throttle position sensor (TPS)
BASIC SYSTEM OPERATION
The fuel is delivered to the fuel rail and circulated through the system by an in-tank fuel pump. The fuel gage sending unit is mounted on the side of the pump module. Fuel is pumped through a fuel supply line to an in-line fuel filter. A pressure regulator on the fuel rail keeps the fuel pressure at the injector between 214-306 kPa (31-44 psi) depending on intake manifold vacuum. Excess fuel, not needed by the injectors is returned to the fuel tank by a separate line. The fuel pump is controlled by the PCM through the fuel pump relay.
FUEL SUPPLY COMPONENTS
The fuel supply is stored in the fuel tank. An electric high pressure fuel pump, located in the fuel tank with the gage sending unit, pumps fuel through an in-line fuel filter to the fuel rail assembly. The pressure regulator, part of the fuel rail assembly, keeps fuel available to the injectors at a controlled pressure. Unused fuel is returned to the fuel tank by a separate line. Nylon fuel supply and return lines are routed along the left hand side of the vehicle from the fuel tank. Quick connects have been installed at the tank, fuel filter and engine for ease of assembly and serviceability.
FUEL PUMP ELECTRICAL CIRCUIT
When the ignition is first turned to the "ON" position, without the engine running, the PCM turns the fuel pump relay "ON" for two seconds. This builds up the fuel pressure quickly. If the engine is not started within two seconds, the PCM shuts the fuel pump "OFF" and waits until ignition reference pulses are present. As soon as the engine is cranked, the PCM turns the relay "ON" which powers the fuel pump.