Multiport Fuel Injection (MFI) System

Fuel Injector:

FUEL INJECTOR
The fuel injectors are small solenoids that allow fuel to be injected into the intake manifold. The injectors receive the ground circuit from the ECU in terms of "Injection Duration". The voltage applied to the electrical windings pulls the needle valve up allowing fuel to flow through the nozzle.
WARNING: The positive circuit is always "Hot" at the injector terminals when the battery is connected.

Fuel Injection Control System:

BASIC FUEL INJECTION CONTROL
The amount of fuel injected or the amount of injector duration is determined by a programmable value mapped in the ECU's ROM memory. In other words the ECU writes a program based for injection duration based on input signals sent by the crank angle sensor and the air flow meter.

Fig. 10 Fuel Injection Compensation:

FUEL INJECTION COMPENSATION
Fuel injector duration is compensated to improve engine performance under various conditions, for instance;
1. Increased Duration (more fuel)
a. During engine warm-up.
b. While starting engine.
c. During acceleration.
d. Hot-engine operation.
2. Reduced Duration (less fuel)
a. During deceleration.

Fig. 11 Closed Loop Control:

MIXTURE RATIO FEEDBACK
The mixture ratio feedback operation is provided to control the air fuel mixture at the stoichiometric point. This allows the three-way catalyst to operate most efficiently. The ECU adjusts the injector duration based on a voltage signal sent by the oxygen sensor mounted in the exhaust stream. When the ECU is controlling the air-fuel mixture this is called "Closed-Loop Operation". The system is considered in "Open-Loop Operation" (not controlling A/F mixture) when the ECU senses the following conditions:
1. Deceleration.
2. Engine starting.
3. Engine idling.
4. O2 sensor or circuit malfunction.
5. Low O2 sensor temperature.
6. High speed or high load operation.

MIXTURE RATIO SELF-LEARNING
The ECU monitors O2 sensor voltage and will regulate injector duration to provide a basic mixture ratio as close to the theoretical mixture as possible. Manufacturing differences and changing operating conditions (engine wear, injector flow, etc.) will affect the the mixture ratio.
The difference between the basic and theoretical mixture ratios is quantitatively monitored and computed in terms of "Fuel Injection Duration" to automatically compensate for differences in the two ratios. This allows the system to compensate at a faster rate than if it had to rely only on a pre-programmed ratio.

Fig. 12 Fuel Injection Timing:

FUEL INJECTION TIMING
When the engine is cranking and for a short period after start up, fuel is injected into all cylinders twice per cycle simultaneously.
Under all other engine operating conditions fuel is injected sequentially (once a cycle for each cylinder in the firing order).
Fuel is shut off at high-speed driving (to prevent engine damage from over-revving) and during deceleration.