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 TM 9-8000
Section V.
FUEL INJECTION SYSTEMS
FOR GASOLINE ENGINES
4-25.
the engine camshaft. It is always in the same rotational
General.
Fuel injection systems are an
relationship with the camshaft so that it can be timed to
increasingly popular alternative to the carburetor for
feed the fuel at just the right moment to the injectors.
providing an air-fuel mixture.
They inject, under
There is one injector for each cylinder. Each injector
pressure, a measured amount of fuel into the Intake air,
contains a spring loaded valve that is opened by fuel
usually at a point near the intake valve. Fuel injection
pressure injecting fuel into the intake at a point just
systems provide the following advantages.
before the intake valve. The throttle valve regulates
engine speed and power output by regulating manifold
(1) Fuel delivery can be measured with
vacuum, which in turn regulates the amount of fuel
extreme accuracy, giving the potential for improved fuel
supplied to the injectors by the metering unit.
economy and performance.
c. Electronic-Timed Fuel Injection (Fig. 4-64). The
(2) Because the fuel is injected at the intake
operation of electronic-timed fuel injection is somewhat
port of each cylinder, fuel distribution will be much better
different than the mechanical-timed system described
and fuel condensing in the manifold will not be a
in the previous subparagraph. In an electronic system,
problem.
all of the fuel injectors are connected in parallel to a
common fuel line that is fed by a high-pressure pump
(3) There is no venturi to restrict the air Intake,
from the gas tank. A fuel pressure regulator also is
making it easier to keep volumetric efficiency high (para
installed in line with the injectors to keep fuel pressure
constant by diverting excess fuel back to the gas tank.
Each injector contains a solenoid valve and is normally In
(4) The fuel injector, working under pressure,
a closed position. With a pressurized supply of fuel
can atomize the fuel much finer than the carburetor,
behind it, each injector will operate individually whenever
resulting In improved fuel vaporization. There are
an electric current is applied to its solenoid valve. By
three basic configurations of gasoline fuel Injection:
sending electric current impulses to the injectors in a
timed, continuous, and throttle body.
sequence timed to coincide with the needs of the engine,
4-26.
the system will supply gasoline to the engine as it should.
Timed Fuel Injection Systems.
The system is fitted with an electronic computer to serve
this function and the function of providing the proper
a. General.
Timed fuel Injection systems for
amount of fuel. The computer receives a signal from the
gasoline engines inject a measured amount of fuel in
ignition distributor to establish the timing sequence. The
timed bursts that are synchronized to the Intake strokes
engine is fitted with a variety of sensors and switches
of the engine. Timed injection is the most precise form
that gather information such as:
of fuel injection but it is also the most complex. There
are two basic forms of timed fuel injection: mechanical
and electronic. The operation of the two are very
(1) Intake air temperature
different and will be covered separately In the two
following subparagraphs.
(2) Engine speed
b. Mechanical-Timed Injection (Fig. 4-63). The
(3) Manifold vacuum
mechanical-timed injection system uses a high-pressure
pump that draws fuel from the gas tank and delivers it to
(4) Engine coolant temperature
the metering unit. A pressure relief valve is installed
between the fuel pump and the metering unit to regulate
fuel line pressure by bleeding off excess fuel back to the
gas tank. The metering unit is a pump that is driven by
4-40
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