Fuel Injection Control
Fuel Injector Duration:
FUEL INJECTION CONTROL
ECU receives signals emitted from various sensors to control the amount of fuel injected and the fuel injection timing. Sequential fuel injection control is utilized over the entire engine operating range except during standing starts.
As for injection timing, the ECU controls the starting of injection with leading to the signal emitted from crank angle sensor 1.
The starting of injection is changed between BTDC 500 (deg) and BTDC 100 (deg) under various conditions.
The amount of fuel injected by the injector valve is dependent upon the length of time it remains open. The optimum fuel injection timing is determined by transmitting a signal to the injector from the ECU according to varying engine operations. Feedback control is also accomplished by means of a learning control. As a result, the fuel injection control system is highly responsive and accurate in design and structure.
The sequential fuel injection system is designed so that fuel is injected at a specific time to provide maximum air intake efficiency for each cylinder. In other words, fuel injection is completed just before the intake valve begins to open.
FUEL INJECTION CHARACTERISTICS
Fuel injection timing is basically expressed as indicated below:
1. During engine starts:
Duration of fuel injection
= Duration of fuel injection during engine starts
2. During normal operation:
Basic duration of fuel injection x correction factor + voltage correction time
^ Basic duration of fuel injection The basic length of time fuel is injected. This is determined by two factors--the amount of intake air detected by the air flow sensor and the engine speed (rpm) monitored by the crank angle sensor.
^ Duration of fuel injection during engine starts Determined according to the engine coolant temperature detected by a signal emitted from the water temperature sensor to improve starting ability.
^ Voltage correction time compensates for the fuel injector's time lag affected by the battery voltage.
CORRECTION COEFFICIENTS
Correction coefficients are used to correct the basic duration of fuel injection so that the air-fuel ratio meets the requirements of varying engine operations.
These correction coefficients are classified as follows:
Air Fuel Ratio Characteristic:
1. Air-fuel ratio coefficient:
Allotted to provide the optimum air-fuel ratio in relation to engine speed and the basic amount of fuel injected.
Start Increment Characteristic:
2. Start increment coefficient:
Increases the mount of fuel injected only when cranking the engine, which improves starting ability.
Engine Temperature Increment Characteristic:
3. Water temperature increment coefficient:
Used to increase the amount of fuel injected in relation to a signal emitted from the water temperature sensor for easier starting of a cold engine. The lower the water temperature, the greater the increment rate.
After Start Increment Characteristic:
4. After-start increment coefficient:
Increases the amount of fuel injected for a certain period of time immediately after the engine starts to stabilize engine operation.
Full Increment Characteristic:
5. Full increment coefficient:
Increases the amount of fuel injected by a signal emitted from the throttle sensor in relation to a signal emitted from the air flow sensor.
Acceleration Increment Characteristic:
6. Acceleration increment coefficient: Compensates for time lags of air flow measurement and/or fuel injection during acceleration to provide quick response.
Air Fuel Ratio Feedback Operation:
AIR-FUEL RATIO FEEDBACK COEFFICIENT "ALPHA"
This feedback coefficient utilizes the O2 sensor's electromotive force (voltage) as a signal to be entered into the ECU. When low voltage is entered, the ECU judges it as a lean mixture, and when high voltage is entered, it is judged as a rich mixture. In other words, when the air-fuel ratio is richer than the theoretical air-fuel ratio, the amount of fuel injected is decreased. When it is leaner, the amount of fuel injected is increased. In this way, the air-fuel ratio is compensated so that it comes as close to the theoretical air-fuel ratio as possible on which the three-way catalyst acts most effectively. (CO, HC and NOx are also reduced when the air4uel ratio is close to theoretical air-fuel ratio.)
Learning Control System:
LEARNING CONTROL SYSTEM
In a conventional air-fuel feedback control system, the basic amount of fuel injected (according to engine speed and various loads) is stored in the memory. After the ECU receives a signal emitted from the O2 sensor, the basic amount of fuel injected is corrected so that it is close to the theoretical air-fuel ratio. This means that the greater the air-fuel ratio is corrected, the lesser the control accuracy.
In Subaru engines, however, an air-fuel ratio learning control system constantly memorizes the amount of correction required in relation to the basic amount of fuel to be injected (the basic amount of fuel injected is determined after several cycles of fuel injection), so that the correction affected by feedback control is minimized. Thus, quick response and accurate control of variations in air-fuel ratio, sensors' and actuators' characteristics during operation, as well as in the air-fuel ratio with the time of engine operation, are achieved. In addition, accurate control contributes much to stability of exhaust gases and driving performance.