Holley COMMANDER 950 - Manual (Page 7)

7
1.6 Fuel Injectors
There are basically two approaches in delivering the fuel to the engine:
• Above the throttle plate as in throttle body injection
• In the intake port toward the intake valves as in multi-point injection
The fuel injector is continuously supplied with pressurized fuel from the electric fuel pump. The pressure to the injector is
maintained constant by the fuel pressure regulator. The fuel injector is an electric solenoid valve that when driven by the ECU
delivers a metered quantity of fuel into the intake manifold. The ECU controls the fuel flow by opening and closing the injector.
The time the injector is open is defined as pulse width. The time the injector is driven into an open condition is determined by
the following sensor inputs:
• Engine RPM • Oxygen sensor feedback voltage
• Throttle position (TPS) • Intake air charge temperature
• Manifold absolute pressure or mass air flow • Battery voltage
• Engine coolant temperature
1.6.1 Bottom-Fed Fuel Injector
NOTE: This is typically used in TBI systems.
This electromagnetic valve meters fuel into the intake manifold proportional to the air being induced into the engine. When the
valve is energized, the electromagnetic force generated by the solenoid lifts the pintle/ball from the seat. Fuel under pressure
is then injected into the throttle body bore. For throttle body injection, a hollow conical spray is required to aim the metered
flow around the throttle valve.
Bottom-Fed Fuel Injector Top-Fed Fuel Injector
1.6.2 Top-Fed Fuel Injector
NOTE: This is typically used in MPFI systems.
When the ECU activates this electromagnetic valve, the injector meters and atomizes fuel in front of the intake valve. The fuel
enters the top and is discharged via the metering orifice at the bottom at high pressure. The spray geometry and cross
sectional area is specific to the engine application. For MPFI system, a solid spray geometry is required to avoid fuel wall
wetting.
In general there are three injector metering design configuration:
§ 1
Pintle injector. This is one of the first fuel injector designs applied to automotive fuel engine management
technology. The fuel flow is metered via an annular orifice between the pintle and the seat. The tip of the pintle
has the function of generating the required spray geometry. Pintle type injectors are very susceptible to carbon
deposit and have slowly been replaced by director plate metering technology. A conical seat between the
plunger and the seat achieves the seal of these injectors.
§ 2 Disk injector. The disc injector design is different from the types of injectors discussed above because a disc
replaces the plunger. Sealing is achieved by seating the disc against the protrusion of the metering orifice. The
main advantage of the disc is the lower mass and it is perceived that it can reciprocate at higher frequencies than
a plunger. Recent advances in solenoid and plunger manufacturing technology have significantly reduced the
weight disparity between the disc and plunger designs. These injectors make use of the director disc design to
achieve the required flow and spray geometry