Multipoint - EFI System (Part3. GASOLINE INJECTION)

An injector for a gasoline injection system is simply an electrically operated fuel valve. When energized by the computer, it must open and produce a uniform fuel spray pattern in the intake manifold. When re-energized, it must close quickly, without leakage.

Injector Construction

Most modern injectors are made of metal and plastic. Rubber O-rings seal joints where parts fit together. Usual­ly, the injector fits into a hole machined into the intake manifold. However, as will be discussed in the next chapter, some systems have the injector in the throttle body assembly.

Cutaways show important components of an injector. (Chrysler)

Refer to this illustration.

1. ELECTRIC TERMINALS (electrical connection for circuit between injector coil and computer).
2. INJECTOR SOLENOID (armature and coil that opens and closes valve).
3. INJECTOR SCREEN (screen filter for trapping debris before it can enter injector nozzle).
4. NEEDLE VALVE (end of armature shaped to seal against needle seat).
5. NEEDLE SEAT (machined surface around the hole in end of injector against which the needle valve tip presses to form a seal).
6. INJECTOR SPRING (small spring that returns nee­dle valve to closed position).
7. O-RING SEAL (rubber seal that fits around outside of injector body and seals in intake manifold).
8. INJECTOR NOZZLE (injector outlet that produces fuel-spray pattern).

EFI injector operation. A-Current through injector coil builds magnetic field. Magnetism attracts and pulls up on armature to draw injector needle off its seat. Gasoline sprays out. B-Current flow stops when computer breaks circuit. Injector valve closes stopping fuel spray.

Gasoline Injector Operation

In a simplified way, Above picture illustrates the operation of a gasoline injector. When the computer sends current to the injector coil, the coil develops a magnetic field. Like an electromagnet, the field attracts and pulls on the injector armature. The armature moves up into the coil's field. The needle is then lifted off its seat and let’s fuel spray out the nozzle into the intake manifold.
When the computer shuts off current to the injector coil, the magnetic field collapses. This lets the injector spring push down on the armature forcing the needle against its seat. This blocks fuel flow.

Injector Pulse Width

Injector pulse width refers to the length of time or dura­tion that the injector is open. The computer controls injector pulse width. A long pulse width richens the fuel mixture because more fuel would spray into the intake manifold on each cycle. A short pulse width would lean the mixture because the injector would be kept closed longer between pulses.

Injector pulse width means the amount of time that computer sends current to injector to keep valve open. A-Short pulse causes less fuel spray because injector valve is not open long percentage of time. Mixture is leaner. B-Long pulse keeps valve open more of the time. Mixture is richer

Above picture illustrates short and long injector pulse widths. Note that the square sine wave (sine represent­ing voltage change) denotes the pulse width. When the wave moves up from zero, indicating voltage supply to the injector, the injector is open. When the wave moves back down to zero (base line), the injector is closed because there is no voltage and current flow.
When the square wave is shorter, Fig.A, the injector pulse width is shorter and the fuel mixture is leaner. When the square wave is longer, Fig.B, the pulse width is longer and the mixture is richer.
With many systems, the computer cycles the injec­tors open and closed several times a second. By chang­ing the percentage of ON and OFF times, it can control the air-fuel mixture ratio.