Fuel Injection Control
FUEL INJECTION CONTROL- The ECM 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 engine starts.
- 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 ECM 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.
1) 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 factors + 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 mass air flow sensor and the engine speed (rpm) monitored by the crankshaft position sensor.
- Duration of fuel injection during engine starts Determined according to the engine coolant temperature detected by a signal emitted from the engine coolant temperature sensor to improve starting ability.
- Voltage correction time Compensates for the fuel injector's time lag affected by the battery voltage.
2) Correction factors
Correction factors 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 factors are classified as follows:
(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.
(2) Start increment coefficient:
Increases the amount of fuel injected only when cranking the engine, which improves starting ability.
(3) Engine coolant temperature increment coefficient:
Used to increase the amount of fuel injected in relation to a signal emitted from the engine coolant temperature sensor for easier starting of a cold engine. The lower the engine coolant temperature, the greater the increment rate.
(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.
- The amount of fuel to be compensated for depends on the water temperature during engine starting.
(5) Full increment coefficient:
Increases the amount of fuel injected by a signal emitted from the throttle position sensor in relation to a signal emitted from the mass air flow sensor.
(6) Acceleration increment coefficient:
Compensates for time lags of air flow measurement and/or fuel injection during acceleration to provide quick response.
3) Air-fuel ratio feedback coefficient "alpha"
This feedback coefficient utilizes the front oxygen sensor's electromotive force (voltage) as a signal to be entered into the ECM. When low voltage is entered, the ECM 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 stoichiometric mixture 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 stoichiometric mixture ratio as possible on which the three-way catalyst acts most effectively. (CO, HC and NOx are also reduced when the air-fuel ratio is close to stoichiometric mixture ratio.)
4) 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 ECM receives a signal emitted from the oxygen sensor, the basic amount of fuel injected is corrected so that it is close to the stoichiometric mixture 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.