Fuel Delivery and Air Induction: Description and Operation

Closed Loop System Diagram:

Fuel Supply System Diagram:

BASIC SYSTEM OPERATION

The basic function of the fuel control system is to control fuel delivery to the engine. Fuel is delivered to the engine by individual port fuel injectors mounted in the intake manifold near each cylinder's intake valve.
The main control sensors are the Oxygen (O2) sensors, located in both the right and left exhaust manifold. The O2 sensor provides the ECM/PCM with the ratio of oxygen in the exhaust stream to atmospheric oxygen. The ECM/PCM modifies the air/fuel ratio by changing the injector signal sent to each individual injector. By constantly measuring the exhaust stream oxygen level and adjusting the injector pulse width, the air/fuel ratio is kept very close to 14.7 to 1. This is the most efficient mixture to minimize exhaust emissions. A 14.7:1 ratio allows the Catalytic Converter to operate the most efficiently. The constant tailoring of the air/fuel ratio based upon oxygen sensor feedback is referred to as "Closed Loop" operation.

MODES OF OPERATION

There are several operating parameters that the ECM/PCM looks at to determine how much fuel to give the engine. Based on voltage signals from several sensors, fuel is delivered to the engine under one of several operating "modes". All of the modes are controlled by the ECM/PCM.

STARTING MODE

When the key is first turned ON, the ECM/PCM will turn on the fuel pump relay for two seconds. The fuel pump energizes, building up fuel pressure to the fuel rail. The ECM/PCM then checks the coolant temperature sensor, throttle position sensor, and crank signal, and determines the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36°C to 12.0:1 at 94°C.
The ECM/PCM controls the amount of fuel delivered in the STARTING mode by changing how long the injectors are turned on. This is done by pulsing the injectors for very short times.

CLEAR FLOOD MODE

If the engine floods, clear it by pushing down the accelerator pedal all the way. With this signal, the ECM/PCM will completely turn OFF the fuel. The ECM/PCM holds this injector rate as long as the throttle remains wide open and the engine rpm is below 1000. If the throttle position becomes less than 80%, the ECM/PCM returns to the Starting Mode.

RUN MODE

The RUN mode has two conditions: OPEN LOOP and CLOSED LOOP.
When the engine is first started and rpm is above 400 rpm, the system goes into OPEN LOOP operation. In OPEN LOOP, the ECM/PCM will ignore the signal from the Oxygen sensor and calculate the air/fuel ratio based primarily on inputs from the coolant and MAP sensors.
The system will stay 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.
2. The coolant temperature is above 80°C.
3. A calibrated (about 60 seconds) time has elapsed after starting the engine. This delay allows the sensors to stabilize.
When these conditions have been satisfied, the system goes into CLOSED LOOP operation. In CLOSED LOOP, the ECM/PCM will calculate 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

The ECM/PCM looks at changes in throttle position and manifold pressure to provide extra fuel.

DECELERATION MODE

The PCM looks at changes in throttle position and air flow, and reduces the amount of fuel. When deceleration is very fast, the ECM/PCM can cut off fuel completely for short periods of time.

BATTERY VOLTAGE CORRECTION MODE

When the battery voltage is low, the ECM/PCM can compensate to maintain acceptable system performance by:
^ Increasing the injector ON time to maintain the proper amount of fuel delivery.
^ Increasing the idle rpm.
^ Increasing 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 seen from the distributor (indicating the engine is not running). This prevents flooding.