Electronic fuel injection and engine management 217 Basic principles of EFI 218 Types of EFI systems 218 Block diagrams of an EFI system 220 Operation of a multipoint EFI system 222 Components of an EFI system 224 Engine management 229 Electronic concentrated control system 231 Sequential multipoint fuel injection system 234 Other features of engine control systems 235 Throttlebody injection (TBI) 241 Servicing engine control systems 242 Locating basic faults 244 Fault diagnosis 245 Testing equipment 246 Technical terms 248 Review questions 248
Trang 1Electronic fuel injection
and engine management
Chapter 13
Basic principles of EFI
Types of EFI systems
Block diagrams of an EFI system
Operation of a multipoint EFI system
Components of an EFI system
Engine management
Electronic concentrated control system
Sequential multipoint fuel injection system
Other features of engine control systems
Throttle-body injection (TBI)
Servicing engine control systems
Locating basic faults
Fault diagnosis
Testing equipment Technical terms Review questions
Trang 2Electronic fuel injection (EFI) is a fuel system for
petrol engines which uses electronically controlled
injectors to spray fuel into the engine’s intake
mani-fold The system has an electronic control unit (ECU),
or computer, to control injection The ECU receives
input signals from various sensors, which it processes.
It then sends output signals which adjust the quantity
of fuel being injected and, in some cases, when the fuel
is injected.
The computer can be arranged to monitor almost all
engine controls This monitoring and controlling
process is usually referred to as engine management.
The control units of different manufacturers are called
by many different names but the generic term ECU
(electronic control unit) will be used in this chapter.
Engine management systems use electronics to
not only control fuel injection, but they also control
ignition and emissions for improved engine
performance.
Basic principles of EFI
The basic operation of an EFI system hinges around
two main components:
1 Electrically operated injectors, which spray fuel
into the intake manifold (Figure 13.1).
2 An ECU (Figure 13.2), which collects information
from various parts of the engine and controls the
fuel delivered by the injectors.
EFI compared with a carburettor system
Apart from the petrol tank, almost all the components
of an EFI fuel system are different from those of a carburettor fuel system EFI performs the same overall functions as a carburettor, but there are a number of reasons why EFI is used.
One of the main reasons is that the air–fuel ratio can be closely controlled throughout the operating range of the engine This is much harder to do with a carburettor, where various methods of mixture correction have to be used.
Types of EFI systems
There are two main types of EFI The difference between the systems is the location of the injectors and their mode of injection (Figure 13.3) These two systems are as follows:
1 Multipoint injection A separate injector is used for each cylinder, and the fuel is sprayed into the intake ports This is also called port injection.
2 Throttle-body injection One or two injectors are used in a throttle body which is mounted on the intake manifold in a similar manner to a carbu- rettor This is also called single-point injection Very simply, the throttle body and injector form an electronically operated carburettor.
Both EFI systems use a similar type of electronic control and their injectors operate in a similar way.
As well as these two systems, there are mechanical injection systems and also systems that are both
figure 13.1 Fuel injector located in the manifold at the
intake valve port FORD
figure 13.2 ECU and wiring harness HOLDEN LTD
The quantity of fuel delivered by the injectors is
quickly adjusted by the ECU to meet changes in
engine conditions Almost instantaneous changes can
be made to the quantity of fuel being delivered to the
engine.
Trang 3mechanical and electronic EFI systems are the most
commonly used; other systems will not be covered here.
■ Throttle body injection is sometimes called
centre-point or central injection.
Multipoint EFI system
Figure 13.4 shows a basic multipoint EFI system The parts are:
1 Air cleaner and ducts to provide clean air.
2 Airflow meter to measure the flow of air.
3 Throttle valve, which controls the flow of air into the engine.
4 Plenum chamber (surge tank or swirl chamber) to dampen the flow of air.
5 Fuel tank to store fuel.
6 Electric fuel pump to provide pressure in the system.
7 Fuel filter to protect the injectors.
8 Fuel rail to supply the injectors with fuel.
9 Injectors that spray fuel into the intake ports.
10 Pressure regulator to control the pressure in the system.
11 ECU to control injection and other engine requirements.
Throttle-body injection
Figure 13.5 is a basic throttle-body, or single-point, EFI system This has the same type of supply system
as a multipoint system, but it has a throttle body with
an injector In both systems, the ECU is used to control the quantity of fuel delivered by the injectors.
figure 13.3 Diagrams show the difference between
multipoint and throttle-body injection
plenum chamber
intake manifold
special length tubes
injector air
(a) Multipoint injection
(b) Throttle-body injection
air injectors
air
injectors
figure 13.4 Basic multipoint EFI system
electronic control unit (ECU)
Trang 4These provide the mixture of air and fuel, and control a
number of other functions The basic EFI systems
(Figures 13.4 and 13.5) have these three subsystems,
and the parts of each can be identified on the simple
diagrams.
All EFI systems, whether simple or complex, will
have the three subsystems This is the way in which a
system should be looked at, as it is easier to understand
if the subsystems are examined separately.
Block diagrams of an EFI system
An easy way to identify the components of an EFI
system is by using block diagrams Figures 13.6 to
13.9 show block diagrams for the air, fuel and control
subsystems of a multipoint system.
Air-intake system
In the block diagram of an air system shown in Figure
13.6, the air enters at the air cleaner and flows through
the system to the engine.
The parts and their functions are as follows:
1 Air cleaner This has a cellulose fibre element
which provides clean air for the system.
2 Airflow sensor This measures the quantity of air flowing into the system so that the correct amount
of fuel can be provided for the optimum air–fuel ratio.
3 Throttle body The throttle body has a throttle valve that is operated by the accelerator pedal This controls the amount of air that enters the engine.
4 Plenum chamber This acts as a surge tank and distributes the air to the branches of the intake manifold.
5 Intake manifold The branches or pipes of the manifold carry air to the cylinders and also provide
a mounting for the injectors.
figure 13.5 Basic throttle-body injection (TBI) system
electronic control unit (ECU)
figure 13.6 Block diagram of the air system for
multipoint EFI
air cleaner
throttle body
airflow sensor
chamber
intake manifold air
Trang 5line In some installations, a submerged fuel pump
is fitted.
2 Fuel pump An electric pump is used This has the
electric motor and pump in a common housing and
operates whenever the engine is running If the fuel
pump is fitted above the level of the fuel tank, an
additional low-pressure pump is sometimes located
inside the fuel tank to prime the main pump.
3 Fuel filter A paper filter is fitted in the fuel line so
that all the fuel is filtered before it reaches the
injectors.
4 Fuel rail The fuel rail, or distributor pipe, receives
fuel from the filter and supplies it to the injectors.
All the injectors are connected in parallel with the
common fuel rail.
5 Injectors These spray atomised fuel into the intake
ports of the engine For each rotation of the
crank-shaft, each injector valve opens once, regardless of
whether the inlet valve is open or not.
If the valve is closed, the fuel is stored in and around the inlet port until the next time the valve
opens The fuel will then be drawn into the
com-bustion chamber With this arrangement, only half
the fuel requirements are injected each time
injection occurs.
6 Pressure regulator The pressure regulator is fitted
at the end of the fuel rail It maintains the pressure
in the system high enough for injection The
pres-sure regulator also maintains a constant prespres-sure
differential across the injector.
■ This arrangement is referred to as a return-to-tank
system because it has a return fuel line from the
pressure regulator in the engine compartment.
Returnless fuel system
The main parts of a returnless fuel system are shown
in the block diagram in Figure 13.8 This system has
been developed in order to overcome the evaporative emission problems associated with returning hot fuel from the fuel rail back to the fuel tank The main features that are different from a return-to-tank system are:
1 Fuel tank In this system the fuel tank also houses the fuel pump, fuel filter, and pressure regulator assembly There is no return line from the fuel rail.
2 Fuel pump The pump assembly contains the fuel filter, the pressure regulator valve and the fuel gauge sender unit (see Figure 13.33).
3 Fuel filter The filter, as part of the pump assembly,
is mounted in the fuel tank.
4 Pressure regulator The pressure regulator is part
of the pump assembly fitted inside the fuel tank as shown in the diagram It maintains the pressure in the system high enough for injection.
Control system
The electronic control system consists of a central ECU and a number of sensors and other electronic controls The sensors are shown in the block diagram
in Figure 13.9 By various means, these sense or measure the conditions at their locations and this information is transmitted to the ECU.
1 ECU This is an electronic device that processes the information provided by the sensors It then allows the injectors to pulse to earth, which determines their opening period, and therefore the quantity of fuel injected.
2 Timing sensor An engine-driven sensor provides a signal which, after being processed by the ECU, is used to time the injectors Each injector could be
figure 13.7 Block diagram of a fuel system for multipoint
EFI (return-to-tank system)
figure 13.8 Block diagram of a returnless fuel system for
multipoint EFI
fuel pump fuel filter
fuel tank assembly
pressure regulator
Trang 6opened once per engine revolution, or once each
second engine revolution.
3 Airflow sensor Already referred to as part of the air
system, the airflow sensor provides a continuous
measurement of all the air being taken into the
system The ECU directs the injector to provide
the right amount of fuel to suit the air.
■ The relationship of the following should be noted:
the throttle valve, which is controlled by the driver
through the accelerator, determines the amount of
air flow; the airflow sensor measures the air flow;
and the injector provides an appropriate amount
of fuel.
4 Air-temperature sensor This is installed in the
airflow meter or the intake air stream to measure
the temperature of the air being drawn into the
system The density of air (and the amount of
oxygen the air contains) is related to its
tem-perature The ECU adjusts the quantity of fuel to
suit the air density.
5 Coolant-temperature sensor This is a sensor in the
cylinder head that measures the coolant
tempera-ture It enables a slightly richer mixture to be
provided when the engine is cold.
6 Throttle-valve switch This is operated by the
throttle-valve shaft and is used to tell the ECU
when the engine is idling so that the mixture
strength can be slightly adjusted The throttle
sensor is also used to indicate various positions of
throttle opening.
7 Other sensors Other sensing devices are used for particular conditions, such as for cold starting The voltage of the electrical system is also taken into account because the state of the vehicle’s electrical system affects the operation of the injector solenoids Adjustment is made to compensate for changes in voltage.
■ Some engines have a cylinder-head temperature sensor instead of, or in addition to, the coolant- temperature sensor.
Operation of a multipoint EFI system
Figure 13.10 shows the arrangement of a Bosch L-Jetronic fuel system This contains the three subsystems (air-intake, fuel and control) which have been previously described The principles of the L-Jetronic system have been applied to many other EFI systems and it has been developed further to include engine-management functions.
Bosch components, or components of similar design, are used in many other systems The operation
of the system will be described, and then the ponents will be considered in greater detail.
2 The airflow sensor measures the quantity of air as
it flows through This information is sent to the ECU (6).
3 The throttle valve (11) is used by the driver to control the flow of air into the engine and so control the engine’s speed and power The position
of the throttle valve is relayed to the ECU by the throttle-valve switch (10).
4 Air passes through the plenum chamber and then into the intake manifold before entering the engine.
5 Air enters the engine through the intake valve and
is mixed with fuel sprayed from the injector (7) to form a fuel charge.
6 The auxiliary-air device (18) allows extra air past the throttle valve when the engine is cold to speed
up the engine This has a similar effect to opening the throttle slightly.
figure 13.9 Block diagram of a control system for EFI
timing
airflow
air temperature
coolant temperature
throttle position
other
electronic control unit (ECU)
injectors
Trang 7Fuel system
The fuel system pumps fuel from the fuel tank and
sprays it into the intake manifold through the injectors:
1 The fuel is pumped from the fuel tank by an electric
pump (2) which maintains a pressure in the system.
2 Fuel is filtered as it passes through the filter (3) into
the fuel rail or distributor pipe (4).
3 Fuel pressure is held in the fuel rail by the pressure
regulator (5).
4 The injectors are connected to the fuel rail They
spray fuel into the intake manifold when directed
by the ECU.
5 A start valve (8) is used for cold conditions This
supplies additional fuel for starting.
Control system
The control system receives input signals (information)
from a number of sources, processes them, and sends
out signals to the injectors.
When the ignition switch (21) is turned on, the relay (13) is energised and the ECU (6) is activated The ECU can then adjust injection to suit the information that it receives This information comes from a number of sensors:
1 the distributor (17) for engine speed and ignition timing
2 the airflow sensor (12) for the quantity of air (for which the right amount of fuel must be injected)
3 the throttle switch (10) for the throttle-valve opening
4 the temperature sensor (15) for coolant temperature
5 the oxygen sensor (14) in the exhaust manifold, for correct air–fuel mixture
6 the thermo time switch (16) for the length of time that the cold-start valve remains operative.
figure 13.10 Schematic arrangement of the L-Jetronic system
1 fuel tank, 2 electric fuel pump, 3 fuel filter, 4 fuel rail or distributor pipe, 5 pressure regulator, 6 ECU,
7 injector, 8 start valve, 9 idle-speed adjusting screw, 10 throttle-valve switch, 11 throttle valve, 12 airflow sensor, 13 relay,
14 lambda or oxygen sensor, 15 coolant-temperature sensor, 16 thermo time switch, 17 distributor, 18 auxiliary-air device,
19 idle-mixture adjusting screw, 20 battery, 21 ignition starter switch BOSCH
Trang 8Components of an EFI system
The above covers the general operation of a multipoint
EFI system The components will now be considered
in more detail.
Airflow sensor
The airflow sensor in Figure 13.11 consists of a flap or
vane that is deflected by the flow of air through the
sensor The flap is connected to the moving contact of a
potentiometer (variable resistance) so that the resistance
of the potentiometer is varied according to the position
of the flap This provides a voltage signal that is related
to air flow The ECU then adjusts the fuel from the
injectors to suit the amount of air entering the engine.
The flap is V-shaped One part of the flap is used as
an airflow valve; the other part is a compensating flap.
This operates in a damping chamber to dampen flap
Fuel pump
The fuel pump is a roller-cell electric pump (Figure 13.12) It consists of a pumping element and an electric motor in a common housing An electric pump
is used because the system must be full and pressurised before the engine can be started The injectors need pressure to spray starting fuel into the intake manifold The pump has an inlet at one end and an outlet
at the other Fuel flows straight through the pump, and this both lubricates and cools the pump motor.
A permanent-magnet-type motor is used and this drives the roller-cell pump element.
The roller-cell pump consists of a rotor in a housing (Figure 13.13) Slots in the rotor carry the rollers The rotor is offset in its housing to form a pumping chamber so that fuel is carried around between the rollers as the pump rotates.
■ The fuel system must be pressurised before the engine can be started.
Fuel filter
The filter is an in-line filter between the pump and the regulator It has a paper filter element as well as a screen to trap larger particles (Figure 13.14).
Fuel pressure regulator
The pressure regulator (Figure 13.15) is used to maintain a regulated pressure in the system, at a nominal 250 kPa The regulator has a metal housing with a spring-loaded diaphragm When pressure builds
up, the diaphragm lifts the valve to return surplus fuel back to the tank The pressure is set during manu- facture and is determined by the strength of the spring The chamber above the diaphragm is connected to the intake manifold, and so subject to manifold vacuum Changes in manifold pressure will therefore vary the pressure in the fuel system However, with this arrangement, the pressure difference across the injector valves is held constant With the pressure
figure 13.11 Airflow sensor of the flap or vane type FORD
Trang 9constant, the quantity of fuel injected is related only to
the time that the injector is open.
■ This is a return-to-tank fuel system with the
pressure regulator at the end of the fuel rail.
Fuel injector
The location of the injector of a multipoint system was
shown in Figure 13.1 It is fitted in the intake manifold
so that the fuel is sprayed directly into the intake port
in the cylinder head The fuel is directed in a finely atomised form with a spray angle of about 25°.
The atomised fuel is maintained in suspension in the air Wetting the surfaces of the manifold and valve port is avoided because this would leave unmixed fuel This fuel would not be fully burnt in the combustion chamber and would contribute to hydrocarbon emissions in the engine’s exhaust.
The injector is fitted into the manifold in special rubber mouldings which protect it from heat and
figure 13.12 Roller-cell electric fuel pump assembly FORD
figure 13.13 Roller-cell pump element FORD
figure 13.14 Fuel filter for an EFI system BOSCH
figure 13.15 Fuel-pressure regulator for an EFI system
FORD
Trang 10vibration It has an electrical connection and a fuel
connection.
A section through an injector is shown in Figure
13.16 Fuel is supplied through the top of the injector
and retained in the injector by the needle valve, which
is held on its seat by the spring At appropriate times,
electrical pulses from the ECU energise the solenoid
windings and attract the plunger and needle away from
its seat As a result, a spray of fuel is directed into the
intake port of the engine.
■ The needle has a very small lift When fully open,
this is only about 0.1 mm.
As well as these, there are a number of sating signals, such as engine temperature and air temperature, for which the air–fuel mixture is modified from that used for normal operation The ECU provides for a basic amount of fuel to be delivered by the injectors, and this is then varied to suit the different operating conditions.
compen-The distributor is used to signal engine speed and also the crank-angle position for injection timing The pulse signal is passed through a pulse-shaping circuit
in the ECU, which changes it from peaks into a rectangular form (Figure 13.17) The pulse is then used
to operate the injectors.
figure 13.16 Section through a fuel injector for multipoint
EFI TOYOTA
figure 13.17 Pulse signal to the injectors – pulse width
can be varied, as shown by (a) and (b), for different engine conditions
Electronic control unit (ECU)
The ECU receives two main signals:
1 engine speed
2 the amount of air taken in by the engine.
In the diagram, each rectangular pulse represents the opening of an injector The height represents voltage, and the width represents time The ECU widens the pulse for higher loads and increases the frequency of the pulse for higher engine speeds At low engine loads, the pulse is narrow, so only a small quantity of fuel is delivered by the injectors.
Figure 13.18 shows the main inputs and outputs of
an EFI control system, while the diagram in Figure 13.19 illustrates how the ECU adjusts to suit different operating conditions.
Engine-temperature sensor
A rich air–fuel ratio is needed for cold starting and for warming up, so the ECU needs to know the engine tem- perature This signal is supplied by the engine coolant- temperature sensor (Figure 13.20).
The temperature sensor is screwed into the engine block or cylinder head with its end extending into the water-jacket The end of the sensor is a thermistor with a negative temperature coefficient The resistance of the sensor decreases as its temperature increases This change
in resistance is used to measure engine temperature.
Trang 11Cold-starting devices
The cold-start valve injector is operated by a solenoid.
It sprays fuel into the manifold for cold starting and is
operated by the cranking signal via the thermo-time switch.
The thermo-time switch (Figure 13.21) is mounted
figure 13.18 The main inputs and outputs of an EFI control system BOSCH
figure 13.19 The ECU adjusts the basic amount of fuel injected to suit different engine-operating conditions
down and deceleration
Trang 12on the engine where it is affected by both engine
temperature and ambient air temperature It is a
bimetal switch which has its contacts closed at lower
temperatures so that extra fuel is delivered by the start
valve When hot, the switch contacts are opened to
switch off the start valve The bimetal switch is also
electrically heated, so that the contacts automatically
open after a short period of time.
■ The cold-start injector operates only during
cranking.
Idle-speed control
Idle-speed control is shown in Figure 13.22 The air sensor has an adjusting screw which allows air through the bypass in the air sensor The air can be adjusted for idling mixture.
figure 13.20 Engine coolant-temperature sensor
BOSCH
figure 13.21 Thermo-time switch for a cold-starting
device BOSCH
figure 13.22 Idle-speed control and airflow sensor BOSCH
figure 13.23 Throttle-valve (throttle position) switch
For cold idle, the auxiliary-air device allows some air to bypass the throttle valve This air has already been measured by the air sensor and taken into account
by the ECU in determining the amount of fuel to be injected The bypass air therefore has the same effect
as opening the throttle valve.
Throttle-valve switch
The throttle-valve (throttle-position) switch is operated
by the throttle It has a set of contacts for both idle and full-throttle positions (Figure 13.23) The switch signals the throttle opening to the ECU, which pro- vides mixture correction for both idle and full-throttle conditions.
full-load contacts cam
valve shaft
throttle-idle contact
Trang 13Oxygen sensor
The oxygen sensor (Figure 13.24), also referred to as a
lambda sensor, is used to provide a closed-loop
system.
The oxygen sensor is located in the exhaust
manifold and is used to detect the oxygen content of
the exhaust gases as they pass from the engine The
amount of oxygen in the exhaust is a measure of
the air–fuel ratio of the mixture entering the engine.
The oxygen sensor feeds information to the ECU,
which then adjusts the fuel from the injectors This is a
continuous process, so the optimum air–fuel ratio is
always obtained.
■ A closed-loop system provides feedback, whereas
an open-loop system has no feedback.
Engine management
Systems with EFI, in which the ECU is used to control
injection, ignition and engine emissions, are referred to
as engine-management systems Almost all the
functions associated with the engine can be controlled
electronically.
With any engine system, the fuel system and the
ignition system are the two most important items to
be controlled because they affect both engine
perfor-mance and exhaust emissions Data stored in the
memory of the ECU is compared with information that
it receives from the various switches and sensors.
Output signals then adjust the fuel and ignition to give
optimum engine performance.
Engine-management system components
Figure 13.25 shows the input and output components
of one engine-management system, and their ship with the ECU Figure 13.25(a) illustrates the input devices that generate input signals for the ECU Figure 13.25(b) illustrates the output devices that receive signals from the ECU.
relation-Many of the functions carried out by the ECU could
be performed in other ways, but electronic control has been extended beyond injection and ignition Compon- ents such as the air conditioner and turbocharger have been included For example, the air-conditioner compressor can be stopped at idle so that the engine idles better and produces less emission The turbocharger can be controlled and warning signals provided.
Input devices
Starting from the top of Figure 13.25(a) and moving clockwise, the input devices and their functions are described briefly as follows:
1 Air-conditioning switch Signals that the air conditioner is switched on.
2 Airflow meter Measures air flow passing into the engine.
3 Electrical switches These include the headlamp switch, blower switch and electric-fan switch They have an effect on engine load.
4 Power-steering pressure switch Signals that the power steering is functioning.
5 Throttle sensor Senses throttle opening.
6 Coolant thermosensor Senses the coolant temperature.
7 Oxygen sensor Measures the amount of oxygen in the exhaust gases.
8 Air-intake sensor Senses the temperature of the incoming air.
9 Diagnosis connector This is a multiterminal connector to which diagnostic test equipment is connected It is used to check inputs to the ECU.
10 Neutral/clutch switch Signals that the engine has
no load.
11 Stop light switch Signals braking.
12 Ignition switch Supplies power to the ECU and signals starting.
13 Battery This is the general power supply.
figure 13.24 Exhaust-oxygen (lambda) sensor HOLDEN LTD
Trang 14figure 13.25 Components of an engine management system
(a) ECU and input devices (b) ECU and output devices FORD
Trang 1514 Distributor Provides engine-speed signals and
crank-angle signals.
15 Knock sensor and knock control unit Detects
detonation and signals the need for less spark advance.
Output devices
Figure 13.25(b) shows the output devices to which the
ECU sends signals The system shown includes
turbo-charger devices The functions of the output devices of
the system are briefly as follows:
1 Overboost warning buzzer This is a warning
signal that the turbocharger pressure is too high.
2 Air-conditioning circuit relay Opens at idle and
full throttle to stop the air-conditioning pressor and relieve the load on the engine.
com-3 Fuel injector Sprays fuel as directed.
4 Diagnosis connector Checks output signals with
7 Solenoid valve Used to purge the charcoal
canister (emission control).
8 Fuel pump resistor/relay Reduces fuel pump
voltage and reduces pump speed when ments are low.
require-9 Igniter Used for ignition-timing control.
10 Air-conditioning relay Switches the
air-conditioning compressor on and off.
11 ISC valve This is the idle-speed control valve.
12 Turbocharge indicator Lights to show that the
turbocharger is in operation.
■ Output devices are sometimes called actuators.
Different management systems
While engine-management systems generally perform
similar functions, there are many variations in
arrange-ment and design The L-Jetronic, as described earlier, has
engine-management variations such as the Motronic.
Some other systems have been given names, such as the
electronic concentrated control system (ECCS),
electronic engine control (EEC), electronic concentrated
injection (ECI), and central fuel injection (CFI).
The systems that are described in the following sections are representative of those in use, but reference will have to be made to the relevant service manual for information on any particular system.
Electronic concentrated control system
A diagram of an electronic concentrated control system (ECCS) is shown in Figure 13.26 This is an engine-management system As with all systems, there are four factors to be considered: air, fuel, ignition, and control The following are points relating to components of the system.
Air system
Air from the air cleaner passes through the airflow meter and then through ducting to the throttle body and air chamber before entering the engine A number of auxiliary-air devices are used in conjunction with the air chamber for cold starting and idle.
Airflow sensor
An airflow sensor is illustrated in Figure 13.27 In this design, a heated wire in the sensor is located in the path of the air flow and used as a sensing element Air passing over the heated element produces a cooling effect, which reduces its temperature.
Changes in temperature are used as signals to the ECU The signals are used to measure the mass of air flowing through the air sensor and into the engine.
Idle air supply
At engine idle, the throttle valve is closed and allows very little air to pass, so idle air passages are provided
in the air system These bypass the throttle valve to supply air to the air chamber during engine idle.
The fuel for idle is provided by the injectors, the quantity being determined by the ECU to suit the air flow through the airflow sensor The idle control unit has provision for normal idle and also has a fast-idle control device (FICD) The idle unit includes solenoid- operated valves.
The ECU sends signals to the solenoid valves to increase the air supply when needed and also increases the amount of fuel injected This maintains idle speed under varying engine conditions such as increased electrical and mechanical load.
When the idle speed tends to drop due to an increased electrical load on the engine (for example, when the
Trang 16headlamps are switched on), the solenoid of the auxiliary
air control (AAC) valve is energised to provide
additional air (and fuel) to maintain engine idle speed.
When the air conditioner is switched on, an increased
mechanical load is placed on the engine The ECU then
energises the solenoid valve of the fast-idle control
device (FICD) to provide even more air (and fuel) to
maintain engine idle rpm under the additional load.
Air regulator
The air regulator bypasses the throttle valve to control the quantity of air for increasing the engine idling speed during cold starting at lower temperatures.
Fuel system
The fuel tank has a low-pressure priming pump that delivers fuel to the external high-pressure fuel pump The system includes a fuel damper and fuel filter.
A fuel-pressure regulator regulates pressure in the system and returns surplus fuel to the tank through the fuel return line (return-to-tank system) This system employs a swirl pot to ensure adequate fuel feed when the fuel level in the tank is low.
Fuel pump control
The ECU together with the ignition switch controls the operation of the electric fuel pump, so that it operates only when needed and not continuously with the ignition switch turned on and the engine stopped.
If a ‘start’ signal is not received by the ECU after the ignition switch is turned on, the fuel pump will operate for a short period only Therefore, if the engine fails to start or the ignition switch is accidentally left
on, the fuel pump will be stopped.
figure 13.26 Schematic arrangement of an electronic concentrated control system (ECCS) HOLDEN LTD
figure 13.27 Section through a hot-wire-type airflow
sensor FORD
Trang 17The fuel pump will continue to operate as long as
the engine is running, but when the engine stops, the
fuel pump will automatically stop after about one
second.
■ Some systems also use a roll-over switch or air-bag
deployment signal to switch the fuel pump off in an
accident situation.
Carbon canister
A carbon canister is also shown in the system This has
a connection to the engine’s air intake system.
Evaporative emissions are able to be stored in the
canister and later burnt by the engine.
Control system
The function of the ECU is to receive and process
signals from a number of sensors, to compare the
signals with data in its memory, and then to send
signals to the fuel and ignition systems and other
components The electronic components and circuitry
are contained within a metal container (Figure 13.2).
The electrical cables are connected to the ECU by
means of multipin connectors.
Crank-angle sensor
The crank-angle sensor for this system is located in the
distributor This detects both engine rpm and crank
angle, which is the angular position of the crankshaft.
An optical method of sensing is used With this
arrangement a perforated disc is used, together with a
light-emitting diode (LED) as a light source and a
photodiode as a light sensor.
A simplified arrangement is shown in Figure 13.28.
The disc is rotated between the diodes, and light from
the LED passes through the holes in the disc onto the
photodiode which acts as a light sensor The frequency
of the light passing through the holes in the disc is used as a means of sensing speed It can also be used
to sense the angular position of the disc.
Distributor sensor
A sensor fitted in a distributor is shown in Figure 13.29 This consists of two parts: a disc, or rotor plate, and a sensor unit which carries the diodes The rotor has 360 slits spaced at 1° intervals These are used to provide a light signal at each 1° of disc rotation, and this is used for engine rpm.
The disc also has four slots at 90° intervals for a four-cylinder engine, and six slots at 60° for a six- cylinder engine The signal from these slots is used for ignition and injection timing The light signals from both sets of slots are changed to voltage pulses
by the photodiode and are then directed to the ECU.
Oxygen sensor
The exhaust system has a catalytic converter and a muffler An oxygen sensor is fitted in the exhaust system ahead of the catalytic converter.
This is a closed-loop system, and the oxygen sensor provides signals to the ECU to indicate the condition
of the exhaust gases This enables the ECU to make corrections to the fuel being injected so that the optimum air–fuel ratio can be provided.
Coolant-temperature sensor
This is fitted into the water-jackets of the engine It senses coolant temperature and signals this to the ECU, which can adjust the fuel mixture and ignition to suit a hot or cold engine.
Vehicle-speed sensor
Signals from a vehicle-speed sensor are supplied to the ECU so that it can make the necessary adjustments for vehicle speed.
figure 13.28 Simplified optical crank-angle sensor
figure 13.29 Distributor with an optical sensor HOLDEN LTD