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Gasoline Fuel Injection Systems

Two Basic Systems

       There are two basic injection systems that are used:
 

CIS - Continuous Injection System
 

The continuous injection system will spray fuel at all times when the fuel
pump is running.  The injectors are closed by spring pressure, and opened
by the pressure of the fuel.  The amount the injector opens (and the amount
of fuel that will be injected into the intake) depends on the pressure in the
system.  These systems are controlled by an airflowsensor located in the air
intake.  As airflow increases into the engine (as the throttle is opened) the
air flow plate will rise.  Notice in the diagram that the airflow sensor plate is
attached (through the use of a lever) to the bottom of the fuel distributor
control plunger.  As the plunger is forced upwards, more pressure is allowed
to flow from the fuel distributor, through the lines, to the injectors.  This
system does not have to rely on computer control to maintain air/fuel ratios.
Newer systems had some added electronic components to increase system
efficiency through computer monitoring.
 

Electronic Fuel Injection

    Electronic Fuel Injection systems use fuel injectors that are controlled by an electric current.
These injectors are actually small solenoids - with a small coil winding that will produce a
magnetic field when energized - and a moveable iron core (pintle) that will move to allow fuel to
spray out the injector nozzle.  Because a solenoid only has two positions - open or closed - the
amount of fuel required is metered by the length of time that the coil winding in the injector will
receive an electric current.  This is known as injector pulse width.
 


Here is a typical fuel injector.  Notice the coil winding surrounding
the central iron core (plunger and pintle).  Note that the injector
also has a very fine filtering screen located at the top end to
reduce the possiblity of the injector sticking in the open position.
There is a spring that pushes the plunger down and blocks the
flow of fuel when the solenoid is no longer energized.
 

How Much Fuel?

    There are two basic methods used to determine how much fuel is required:
 

            Air Flow

This is a Mass Air Flow (MAF) sensor which is located in the
air intake between the air filter and the throttle plate.  It measures
the amount of air entering the engine - in terms of volume and
density.  It does this in a very simple way.  A heated wire (or grid)
is located in the intake.  The computer control circuit will attempt
to keep this wire heated to a specific value.  If the incoming air is
heavy and cold, much more electric current is required to keep the
wire heated to the pre-determined temperature, and the computer
will use the amount of current required to keep the wire hot as
part of the injector pulse width calculation.



            Engine Vacuum

This is a Manifold Absolute Pressure (MAP) sensor.  It has a
vacuum line attached from the intake to the fitting on the sensor.
There is an electrical connection from the MAP sensor to the
computer.  Most MAP sensors produce a frequency signal - that
is a certain number of pulses per second.  The frequency will
change with the amount of vacuum applied to the sensor.
Rather than measure vacuum, however, the sensor will read
manifold pressure.  This is important because if we use a turbo
or super charger, we no longer have vacuum in the intake -
we have a positive pressure - and the computer must be able
to receive that signal.


Next - TBI vs PFI

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