electronic fuel injection systems only inject fuel during part of the engine's
combustion cycle. The engine fuel needs are measured by intake airflow past a
sensor or by intake manifold pressure (vacuum). The airflow or manifold vacuum
sensor converts its reading to an electrical signal and sends it to the engine
control computer. The computer processes this signal (and others) and calculates
the fuel needs of the engine. The computer then sends an electrical signal to
the fuel injector or injectors. This signal determines the amount of time the
injector opens and sprays fuel. This interval is known as the injector pulse
Throttle body injection systems have a throttle body assembly mounted on the
intake manifold in the position usually occupied by a carburetor. The throttle
body assembly usually contains one or two injectors.
port fuel injection systems, fuel injectors are mounted at the back of each
intake valve. Aside from the differences in injector location and number of
injectors, operation of throttle body and port systems is quite similar with
regard to fuel and air metering, sensors, and computer operation.
Port-type continuous injection systems (CIS) have been used on many import
vehicles. These systems deliver a steady stream of pressurized fuel into the
intake manifold. The injectors do not pulse on and off as in port and throttle
body systems. In CIS, the amount of fuel delivered is controlled by the rate of
airflow entering the engine. An airflow sensor controls movement of a plunger
that alters fuel flow to the injectors. When introduced, CIS was a mechanically
controlled system. However, oxygen sensor feedback circuits and other electronic
controls have been added to the system. CIS systems that have electronic
controls are commonly referred to as CIS-E systems.
ELECTRONIC FUEL INJECTION SYSTEM Electronic fuel injection (EFI) has proven
to be the most precise, reliable, and cost effective method of delivering fuel
to the combustion chambers of today's vehicles. EFI systems must provide the
correct air/fuel ratio for all engine loads, speeds, and temperature conditions.
To accomplish this, an EFI system uses a fuel delivery system, air induction
system, input sensors, control computer, fuel injectors, and some sort of idle
typical EFI fuel delivery system, fuel is drawn from the fuel tank by an in-tank
or chassis-mounted electric fuel pump. Before it reaches the injectors, the fuel
passes through a filter that removes dirt and impurities. A fuel line pressure
regulator maintains a constant fuel line pressure that may be as high as 50 psi
in some systems. This fuel pressure generates the spraying force needed to
inject the fuel. Excess fuel not required by the engine returns to the fuel tank
through a fuel return line.
THROTTLE BODY VERSUS PORT INJECTION
Throttle Body Fuel Injection
some auto manufacturers, TBI served as a stepping stone from carburetors to more
advanced port fuel injection systems. TBI units were used on many engines during
the 1980s and are still used on some engines. The throttle body unit is similar
in size and shape to a carburetor and, like a carburetor, mounts on the intake
manifold. The injector(s) spray fuel down into a throttle body chamber leading
to the intake manifold. The intake manifold feeds the air/fuel mixture to all
TBI Operation The basic TBI assembly consists of two major castings: a
throttle body with a valve to control airflow and a fuel body to supply the
required fuel. A fuel pressure regulator and fuel injector are integral parts of
the fuel body. Also included as part of the assembly is a device to control idle
speed and one to provide throttle valve positioning data.
throttle body casting has ports that can be located above, below, or at the
throttle valve depending on the manufacturer's design. These ports generate
vacuum signals for the manifold absolute pressure sensor and for devices in the
emission control system, such as the EGR valve, the canister purge system, and
fuel pressure regulator used on the throttle body assembly is similar to
a diaphragm-operated relief valve. Fuel pressure is on one side of the diaphragm
and atmospheric pressure is on the other side. The regulator is designed to
provide a constant pressure on the fuel injector throughout the range of engine
loads and speeds. If regulator pressure is too high, a strong fuel odor is
emitted and the engine runs too rich. On the other hand, regulator pressure that
is too low results in poor engine performance or detonation can take place, due
to the lean mixture.
fuel injector is solenoid operated and pulsed on and off by the vehicle's engine
control computer. Surrounding the injector inlet is a fine screen filter where
the incoming fuel is directed. When the injector's solenoid is energized, a
normally closed ball valve is lifted. Fuel under pressure is then injected at
the walls of the throttle body bore just above the throttle plate.
TBI Advantages Throttle body systems provide improved fuel metering when
compared to carburetors. They are also less expensive and simpler to service.
TBI units also have some advantages over port injection. They are less expensive
to manufacture, simpler to diagnose and service, and don't have injector balance
problems to the extent that port injection systems do when the injectors begin
However, throttle body units are not as efficient as port systems. The
disadvantages are primarily manifold related. Like a carburetor system, fuel is
still not distributed equally to all cylinders, and a cold manifold may cause
fuel to condense and puddle in the manifold. Like a carburetor, throttle body
injection systems must be mounted above the combustion chamber level, which
eliminates the possibility of turning the manifold design for more efficient
Port Fuel Injection
systems use one injector at each cylinder. They are mounted in the intake
manifold near the cylinder head where they can inject a fine, atomized fuel mist
as close as possible to the intake valve. Fuel lines run to each cylinder from a
fuel manifold, usually referred to as a fuel rail. The fuel rail assembly on a
PFI system of V-6 and V-8 engines usually consists of a left- and right-hand
rail assembly. The two rails can be connected either by crossover and return
fuel tubes or by a mechanical bracket arrangement. Fuel tubes crisscross between
the two rails. Since each cylinder has its own injector, fuel distribution is
exactly equal. With little or no fuel to wet the manifold walls, there is no
need for manifold heat or any early fuel evaporation system. Fuel does not
collect in puddles at the base of the manifold. This means the intake manifold
passages can be tuned or designed for better low-speed power availability. The
port-type systems provide a more accurate and efficient delivery of fuel. Some
engines are now equipped with variable induction intake manifold that have
separate runners for low and high speeds. This technology is only possible with
throttle body in a port fuel injection system controls the amount of air that
enters the engine as well as the amount of air that enters the engine as well as
the amount of vacuum in the manifold. It also houses and controls the idle air
control (IAC) motor and the throttle position sensor (TPS). The TPS enables the
ECU to know where the throttle is positioned at all times.
throttle body is a single, cast aluminum housing with a single throttle blade
attached to the throttle shaft. The TPS and the IAC valve/motor are also
attached to the housing. The throttle shaft is controlled by the accelerator
pedal. The throttle shaft extends the full length of the housing. The throttle
bore controls the amount of incoming air that enters the air induction system. A
small amount of coolant is also routed through a passage in the throttle body to
prevent icing during cold weather.
systems require an additional control system that throttle body injection units
do not require. While throttle body injectors are mounted above the throttle
plates and are not affected by fluctuations in manifold vacuum, port system
injectors have their tips located in the manifold where constant changes in
vacuum would affect the amount of fuel injected (at a given pulse width). To
compensate for these fluctuations, port injection systems are equipped with fuel
pressure regulators that sense manifold vacuum and continually adjust the fuel
pressure to maintain a constant pressure drop across the injector tips at all
Pressure Regulator The pressure regulator in port injection systems is
similar to the regulator used in TBI systems. A diaphragm and valve assembly is
positioned on the center of the regulator, and a diaphragm spring seats the
valve on the fuel outlet.
fuel pressure reaches the setting of the regulator, the diaphragm moves against
the spring tension, and the valve opens. This action allows fuel to flow through
the return line to the fuel tank. The fuel pressure drops slightly when the
pressure regulator valve opens, and the spring closes the regulator valve. In
many systems, the regulator maintains fuel pressure at 39 psi.
vacuum hose is connected from the intake manifold to the vacuum inlet on the
pressure regulator. This hose supplies vacuum to the area where the diaphragm
spring is located. This vacuum works with the fuel pressure to move the
diaphragm and open the valve. When the engine is running at idle speed, high
manifold vacuum is supplied to the regulator. Under this condition, the
specified fuel pressure opens the regulator valve. When the engine is running
under heavy load and/or wide-open throttle, a very low vacuum is supplied to the
regulator. During these times, the vacuum does not help open the regulator valve
and a higher fuel pressure is required to open the valve.
change in fuel pressure allows the fuel to be sprayed into the manifold with
same effect. regardless of the pressure present in the manifold. When there is a
high vacuum in the manifold, a very low pressure exists and the pressure
difference between the fuel spray and the vacuum is the same as when there is a
higher pressure in the manifold (low vacuum) and a higher fuel pressure.
Port Firing Control While all port injection systems operate using an
injector at each cylinder, they do not fire the injectors in the same manner.
This one statement best defines the difference between typical multiport
injection systems (MPI) and sequential fuel injection systems (SFI).
systems control each injector individually so it is opened just before the
intake valve opens. This means the mixture is never static in the intake
manifold and adjustments to the mixture can be made almost instantaneously
between the firing of one injector and the next. Sequential firing is the most
accurate and desirable method of regulating port injection.
MPI systems, the injectors are grouped together in pairs or groups, and these
pairs or groups of injectors are turned on at the same time. When the injectors
are split into two equal groups, the groups are fired alternately, with one
group firing each engine revolution.
only two injectors can be fired relatively close to the time when the intake
valve is about to open, the fuel charge for the remaining cylinders must stand
in the intake manifold for varying periods of time. These periods of time are
very short; therefore, the standing of fuel in the intake manifold is not that
great a disadvantage of MPI systems. At idle speeds, this wait is about 150
milliseconds; at higher speeds, the time is much less. The primary advantage of
SFI is the ability to make instantaneous changes to the mixture.
SFI systems, each injector is connected individually into the computer, and the
computer completes the ground for each injector, one at a time. In MPI systems,
the injectors are grouped and all injectors within the group share the same
common ground wire.
injection systems fire all of the injectors at the same time for every engine
revolution. This type of systems offers easy programming and relatively fast
adjustments to the air/fuel mixture. The injectors are connected in parallel so
the ECU sends out just one signal for all injectors. They all open and close at
the same time. It simplifies the electronics without compromising injection
efficiency. The amount of fuel required for each four-stroke cycle is divided in
half and delivered in two injections, one for every 360 degrees of crankshaft
rotation. The fact that the intake charge must still wait in the manifold for
varying periods of time is the system's major drawback.