Fuel Delivery and Air Induction: Description and Operation

FUEL INJECTION CONTROL SYSTEM
In this system, ECM controls the time (amount) and timing of the fuel injection from the fuel injector into the throttle body according to the signals from the various sensors so that suitable air/fuel mixture is supplied to the engine in each driving condition.
Injection Timing
There are two types of injection timing. One is "synchronous injection" in which injection is synchronous with the camshaft position sensor (CMP sensor) signal and the other is "asynchronous injection" in which injection takes place independently of the CMP sensor signal.






- Synchronous injection: Normally, the injector injects fuel at every CMP sensor signal. But when the engine coolant temperature is low immediately after its start, the injection time for one ignition cycle is divided into some and injection takes place accordingly.





- Asynchronous injection: When the throttle valve is opened from its idle position, the injector injects fuel in addition to synchronous injection independently of the CMP sensor signal.
Injection Time (amount of injection)





The factors to determine the injection time are the basis injection time which is calculated on the basis of the engine speed and the intake manifold pressure (amount of the intake air) and various compensations which are determined according to the signals from various sensors that detect the state of the engine and driving conditions.

Fuel cut
Fuel injection stops (with operation of the injector prevented) when decelerating (i.e. when the throttle valve is at idle position and the engine speed is high), so that unburned gas will not be exhausted and it starts again when above conditions are not met. The fuel injection also stops when the engine speed exceeds about 7,000 r/min to prevent over-run which affects the engine adversely and it starts again when the engine speed reduces to less than about 6,800 r/min.





Air/fuel ratio feed back compensation (Closed loop system)
It is necessary to keep the air/fuel mixture close to the theoretical air/fuel ratio (14.7) to obtain efficient performance of the 3-way catalytic converter and high clarification rate of CO, HC and NOx in the exhaust gas. For that purpose, ECM operates as follows. It first compares the signal from the heated oxygen sensor with a specified reference voltage and if the signal is higher, it detects that the air/fuel ratio is richer than the theoretical air/fuel ratio and reduces fuel. On the other hand, if the signal is lower, it detects that the air/fuel ratio is leaner and increases fuel. By repeating these operations, it adjusts the air/fuel ratio closer to the theoretical air/fuel ratio.
1. When oxygen concentration in the exhaust gas is low, that is, when the air/fuel ratio is smaller than the theoretical air/fuel ratio (fuel is richer), electromotive force of the heated oxygen sensor increases and a rich signal is sent to ECM.
2. Upon receipt of the rich signal, ECM decreases the amount of fuel injection, which causes oxygen concentration in the exhaust gas to increase and electromotive force of the heated oxygen sensor to decrease. Then a lean signal is sent to ECM.
3. As ECM increases the amount of fuel injection according to the lean signal, oxygen concentration in the exhaust gas decreases and the situation is back to above 1).
This control process, however, will not take place under any of the following conditions.
- At engine start and when fuel injection is increased after engine start
- When engine coolant temperature is low
- When highly loaded and fuel injection is increased
- At fuel cut
- When heated oxygen sensor is cold
- When engine is running at high speed (higher than about 4000 r/min).