TIMED FUEL INJECTION SYSTEM
injection systems for gasoline engines inject a measured amount of fuel in timed bursts
that are synchronized to the intake strokes of the engine.
Timed 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.
operation of a mechanical-timed injection system (fig. 4-43) is as
high-pressure electric pump draws fuel from the fuel tank and delivers it to 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 tank.
metering unit is a pump that is driven by the engine camshaft. It is always in the same
rotational relationship with the camshaft, so it can be timed to feed the fuel to the
injectors just at the right moment.
injector contains a spring-loaded valve that is opened by fuel pressure, injecting fuel
into the intake at a point just before the intake valve.
throttle valve regulates engine speed and power output by regulating manifold vacuum,
which, in turn, regulates the amount of fuel supplied to the injectors by the metering
common type of timed fuel injection is the electronic-timed fuel injection, also known as
electronic fuel injection (EFI) (fig. 4-44). Anelectronic
fuel injection system can be divided into four subsystems:
delivery system of an EFI system includes an electric fuel pump, a fuel filter, a pressure
regulator, the injector valves, and the connecting lines and hoses.
ELECTRIC FUEL PUMP draws fuel out of the tank and forces it into the pressure regulator.
FUEL PRESSURE REGULATOR controls the amount of pressure entering the injector valves.
sufficient pressure is attained, the regulator returns excess fuel to the tank. This
maintains a preset amount of fuel pressure for injector valve operation.
FUEL INJECTOR for an EFI system is a coil or solenoid-operated fuel valve. When not
energized, spring pressure keeps the injector closed, keeping fuel from entering the
engine. When current flows through the injector coil or solenoid, the magnetic field
attracts the injector armature. The injector opens, squirting fuel into the intake
manifold under pressure.
induction system for the EFI typically consists of a throttle valve, sensors, an air
filter, and connecting ducts.
valve regulates how much air flows into the engine. In turn, it controls engine power
output. Like the carburetor throttle valve, it is connected to the gas pedal. When the
pedal is depressed, the throttle valve swings open to allow more air to rush into the
The EFI sensor
system monitors engine operating conditions and reports this information to the computer.
A sensor is an electrical device that changes circuit resistance or voltage with a change
in a condition (temperature, pressure, position of parts, etc.). For example, the
resistance of a temperature sensor may decrease as temperature increases. The computer can
use the icreased current flow through the sensor to calculate any needed change in the
injector valve opening. Typical sensors for an EFI system include the following:
gas or oxygen sensor
air temperature sensor
Since some of
these sensors were discussed in the section on computerized carburetor systems, we will
only concentrate on the sensors that are particular to the EFI system. These sensors are
THROTTLE POSITION SENSOR is a variable resistor connected to the throttle plate shaft.
When the throttle swings open for more power or closes for less power, the sensor changes
resistance and signals the computer. The computer can then enrich or lean the mixture as
AIR FLOW SENSOR is used in many EFI systems to measure the amount of outside air entering
the engine. It is usually an air flap or door that operates a variable resistor. Increased
air flow opens the air flap more to change the position of the resistor. Information is
sent to the computer indicating air inlet volume.
INLET AIR TEMPERATURE SENSOR measures the temperature of the air entering the engine. Cold
air is more dense, requiring a little more fuel.
air is not as dense as cold, requiring a little less fuel. The sensor helps the
computer compensate for changes in outside air temperature and maintain an almost perfect
air-fuel mixture ratio.
CRANKSHAFT POSITION SENSOR is used to detect engine speed It allows the computer to change
injector openings with changes in engine rpm.
from the engine sensors can be either a digital or an analog type output. Digital signals
are on-off signals. An example is the crankshaft position sensor that shows engine rpm.
Voltage output or resistance goes from maximum to minimum, like a switch. An analog signal
changes in strength to let the computer know about a change in condition. Sensor internal
resistance may smoothly increase or decrease with temperature, pressure, or part position.
The sensor acts as a variable resistor.
operation of an electronic-timed injection system is as follows:
is fed by a high-pressure electric fuel pump to the injectors that are connected in
parallel to a common fuel line.
fuel pressure regulator is installed in-line with the injectors to keep fuel pressure
constant by diverting excess fuel back to the tank.
injector contains a solenoid valve and is normally in a closed position. With a
pressurized supply of fuel behind it, each injector will operate individually whenever
electric current is applied to the solenoid valve.
electronic computer sends the electric impulses and provides the proper amount of fuel.
The computer receives a signal for the ignition distributor to establish the timing
sending electric current impulses to the injectors in a sequence timed to coincide with
the needs of the engine, the system will supply fuel to the engine as it should.