4. MAIN METERING SYSTEM. With the engine
warmed up and running at 2,000 rpm, slowly cover part
of the air horn with a piece of stiff cardboard The
engine should speed up slightly, since this action causes
a normal operating main metering system to discharge
Do NOT use your hand to cover the air horn
when performing this test.
Name the seven basic carburetor systems?
What system maintains a steady working supply
of fuel to a constant level in the carburetor?
What device acts as a damper to keep the throttle
from closing too quickly when the accelerator
pedal is suddenly released?
What sensor in a computerized carburetor
system measures intake vacuum and engine
GASOLINE FUEL INJECTION
LEARNING OBJECTIVE: Identify and
describe the different gasoline fuel injection
A modern gasoline injection system uses pressure
from an electric fuel pump to spray fuel into the engine
intake manifold. Like a carburetor, it must provide the
engine with the correct air-fuel mixture for specific
operating conditions. Unlike a carburetor, however,
PRESSURE, not engine vacuum, is used to feed fuel
into the engine.
This makes the gasoline injection
system very efficient.
A gasoline injection system has several possible
advantages over a carburetor type of fuel system. Some
advantages are as follows:
Improved atomization. Fuel is forced into the
intake manifold under pressure that helps break fuel
droplets into a fine mist.
Better fuel distribution. Equal flow of fuel
vapors into each cylinder.
Smoother idle. Lean fuel mixture can be used
without rough idle because of better fuel distribution
and low-speed atomization.
Lower emissions. Lean efficient air-fuel mixture
reduces exhaust pollution.
Better cold weather drivability. Injection
provides better control of mixture enrichment than a
Increased engine power. Precise metering of
fuel to each cylinder and increased air flow can result in
more horsepower output.
Fewer parts. Simpler, late model, electronic fuel
injection system have fewer parts than modern
There are many types of gasoline injection systems.
Before studying the most common ones, you should
have a basic knowledge of the different classifications.
Systems are classified either single- or multi-point
injection and indirect or direct injection.
The point or location of fuel injection is one way to
classify a gasoline injection system. A single-point
injection system, also call throttle body injection (TBI),
has the injector nozzles in a throttle body assembly on
top of the engine. Fuel is sprayed into the top center of
the intake manifold
A multi-point injection system, also called port
injection, has an injector in the port (air-fuel passage)
going to each cylinder. Gasoline is sprayed into each
intake port and toward each intake valve. Thereby, the
term multi-point (more than one location) fuel injection
An indirect injection system sprays fuel into the
engine intake manifold. Most gasoline injection
systems are of this type. Direct injection forces fuel into
the engine combustion chambers. Diesel injection
systems are direct type.
There are three basic configurations of gasoline fuel
injectiontimed, continuous, and throttle body.
TIMED FUEL INJECTION
Timed fuel injection systems for gasoline engines
inject a measured amount of fuel in timed bursts that are
synchronized to the intake strokes of the engine. limed
injection is the most precise form of fuel injection but is
also the most complex. There are two basic forms of
timed fuel injection-mechanical and electronic.
The basic operation of a mechanical-timed
injection system (fig. 4-43) is as follows: