Fuel Delivery and Air Induction: Description and Operation

Fuel Metering System:

The fuel metering system consists of the following components:
1. Fuel pump electrical circuit.
2. Fuel supply components, including:
a. Fuel lines and pipes.
b. Fuel pump.
c. Fuel tank.
3. Fuel rail assembly, including:
a. Fuel injectors.
b. Fuel pressure regulator.
4. Throttle body assembly, including:
a. Idle air control valve.
b. Throttle position sensor.


BASIC SYSTEM OPERATION:


The function of the fuel metering 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 intake manifold near each cylinder.
The main control sensor is the Oxygen (O2) sensor, which is located in the exhaust manifold. This sensor tells the Electronic Control Module (ECM) how much oxygen is in the exhaust gas, and the ECM changes the air/fuel ratio to the engine by controlling the fuel injectors. The best mixture to minimize exhaust emissions is 14.7:1, which allows the catalytic convertor 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.

MODES OF OPERATION

The ECM monitors information from several sensors to determine how much fuel to give the engine. The fuel is delivered under one of several conditions called "modes." All the modes are controlled by the ECM and are as follows.

^ 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 600 rpm. If the throttle position becomes less than approximately 80%, the ECM returns to the starting mode.

^ Run Mode:

The run mode is either "Open Loop" or "Closed Loop."
When the engine is first started and the rpm is above 400, the system goes into "Open Loop," the ECM ignores the signal from the O2 sensor, and calculates the air/fuel ratio based on inputs from the coolant temperature and Manifold Absolute Temperature (MAT) sensors.
The system stays in "Open Loop" until the following conditions are met:
1. The O2 sensor has a varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature).
2. The coolant temperature sensor is above a specified temperature.
3. A specified amount of time has elapsed after starting the engine.
The specific values for the above conditions vary with different engines, and are stored in the memory calibrator (Mem-Cal) portion of the ECM. When these values 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 O2 sensor. This allows the air/fuel ratio to stay very close to 14.7:1.

^ Acceleration Mode:

When the driver pushes the accelerator pedal, air flow into the cylinders increases rapidly, while fuel tends to lag behind. To prevent possible hesitation, The ECM increases the pulse width to the injectors to provide extra fuel during acceleration. The amount of fuel required is based on throttle position, manifold air pressure and engine speed.

^ Fuel Cut-Off Mode:

To prevent possible engine damage from overspeed, the ECM cuts off fuel from the injectors when the engine speed is above 6200 rpm with the vehicle in any forward gear position, or in reverse. The ECM also cuts off fuel to the injectors at vehicle speeds above 115 mph, to prevent tire damage.

^ Deceleration Mode:

When the driver releases the accelerator pedal, air flow into the engine is reduced. The corresponding changes in throttle position and manifold air pressure are relayed to the ECM, which reduces the injector pulse width, to reduce fuel flow. If the deceleration is very rapid, or for long periods (such as long closed throttle coast-down), the ECM shuts fuel completely OFF, to protect the catalytic convertor.

^ Converter Protection Mode:

The ECM constantly monitors engine operation, and estimates condition that could result in high converter temperatures. If the ECM determines the converter may overheat, it causes the system to return to "Open Loop" operation and enriches the fuel mixture.

^ Battery Correction Mode:

When battery voltage is low, the ECM can compensate for weak spark by:
1. Increasing the amount of fuel delivered.
2. Increasing idle rpm.
3. Increasing ignition dwell time.