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 Powertrain Control Module (PCM) with the ratio of oxygen in the exhaust stream to atmospheric oxygen. The 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.
SEQUENTIAL FUEL INJECTION
In sequential fuel injection, each injector is energized independently and in engine firing order. Injector pulses are controlled by the PCM and are based upon an ignition reference pulse and a camshaft position signal from the distributor.
MODES OF OPERATION
There are several operating parameters that the 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 PCM.
STARTING MODE
When the key is first turned "ON", the PCM will turn "ON" the fuel pump relay for two seconds. The fuel pump energizes, building up fuel pressure to the fuel rail. The 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 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 PCM will completely turn "OFF" the fuel. The PCM holds this injector rate as long as the throttle remains wide open and the engine rpm is below 600. If the throttle position becomes less than 60%, the 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 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 58°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 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 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 PCM can cut "OFF" fuel completely for short periods of time.
BATTERY VOLTAGE CORRECTION MODE
When the battery voltage is low, the 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.