Carburettor fuel systems 195 Carburettor fuel system 196 Carburettors 198 Air and fuel flow in a carburettor 199 Carburettor operation 199 Carburettor systems 201 Carburettor construction 201 General carburettor design 203 Carburettor external construction 204 Carburettor service and checks 205 Basic carburettor problems 207 Servicing fuel pumps 207 Testing mechanical fuel pumps 208 Checking electric pumps 210 Fuel system problems 210 Technical terms 211 Review questions 211
Trang 1Carburettor fuel systems
Chapter 13
Carburettor fuel system
Carburettors
Air and fuel flow in a carburettor
Carburettor operation
Carburettor systems
Carburettor construction
General carburettor design
Carburettor external construction
Carburettor service and checks
Basic carburettor problems
Servicing fuel pumps
Testing mechanical fuel pumps
Checking electric pumps
Fuel system problems Technical terms Review questions
Trang 2This chapter is about fuel systems that have a
carbu-rettor While electronic fuel injection (EFI) is now
used, there are still many vehicles which have
carburettors Carburettor fuel systems and EFI systems
are designed to deliver the fuel mixture to the engine in
a combustible form, but each system does it in a
different way One basic difference is that carburettors
atomise the fuel, and injectors spray the fuel.
■ Basic carburettors are covered as part of this
chapter, but there is a separate chapter on
carburettors in Volume 2.
Carburettor fuel system
A basic fuel system with a carburettor is shown in
Figure 13.1 The system consists of:
1 The fuel tank to store the fuel.
2 A pump to supply fuel from the tank to the engine.
3 A filter to trap foreign matter in the fuel.
4 A carburettor to atomise the fuel and provide the correct mixture of air and fuel.
5 The engine intake manifold to deliver the air–fuel mixture to the engine.
6 The air cleaner to supply clean air.
These parts form the fuel supply system As well as these, the exhaust system is sometimes considered to
be a part of the fuel system.
Figure 13.2 is a schematic diagram of a carburettor fuel system This shows the layout of the fuel tank, fuel lines and other parts in relation to the body and engine.
figure 13.1 Basic fuel system with a carburettor
figure 13.2 Schematic fuel system shows the fuel lines (pipes) between the fuel tank at the rear of the vehicle and the
engine compartment
Trang 3Fuel tank and fuel lines
There are three pipes (or lines) between the fuel tank at
the rear of the vehicle and the components in the
engine compartment There are also pipes or hoses
between some components and the engine.
The pipes connected to the fuel tank are:
1 The fuel supply line, which is used to carry fuel
from the tank to the filter and then to the fuel pump.
2 The return line, which returns surplus fuel from the
pump.
3 The vapour line, which vents the fuel tank to the
charcoal canister.
Other lines connect the fuel pump to the carburettor,
and the charcoal canister to the engine.
■ For more information, refer to ‘Fuel-supply system
components’ in Chapter 12.
Fuel filters
A fine gauze filter is fitted to the suction pipe in the
fuel tank, and a line filter is fitted in the supply line
between the fuel tank and the fuel pump, or between
the fuel pump and the carburettor (Figure 13.3).
The filter traps dirt and water that may have entered
the fuel tank It is not serviceable and should be
renewed periodically The amount of dirt or water in
the filter can usually be seen through the transparent
plastic body.
A blocked filter on the suction side of the fuel
pump will restrict the fuel supply to the pump and
cause a shortage of fuel at the carburettor.
Fuel pumps
A simplified mechanical fuel pump is shown in Figure 13.4 This has an inlet and an outlet valve and a flexible diaphragm which is moved up and down by the action of the rocker arm The end of the rocker arm
is operated by an eccentric on the camshaft A spring under the diaphragm pushes it upwards, and a spring against the rocker arm makes sure that it follows the eccentric.
The pump operates as follows:
1 As the camshaft rotates, the eccentric moves the rocker arm backwards and forwards.
2 This movement is transferred through the pullrod to the diaphragm.
3 As the diaphragm is pulled down, fuel is drawn through the inlet valve into the pumping chamber above the diaphragm.
4 As the diaphragm is moved up by the action of the diaphragm spring, fuel is forced from the pumping chamber through the outlet valve and into the bowl
of the carburettor.
figure 13.3 Fuel filters fitted into the fuel line
figure 13.4 Simple mechanical fuel pump
1 inlet valve, 2 diaphragm, 3 pullrod,
4 diaphragm link, 5 rocker arm, 6 cam on engine camshaft,
7 outlet valve, 8 diaphragm return spring, 9 spring
Fuel Pump operation
The fuel pump has the capacity to deliver much more fuel than is actually needed by the engine and a return line takes surplus fuel back to the tank The circulating fuel cools the fuel pump and prevents vapour locks from forming It also keeps the fuel at a fairly uniform temperature, which helps with mixture control.
The stroke of the pump is automatically adjusted to suit the volume of fuel required When pressure builds
up between the pump and the carburettor, pressure in
Trang 4the pumping chamber prevents the spring from
forcing the diaphragm fully upwards, and so the
pumping stroke is reduced The eccentric continues to
move the rocker arm through its full movement, but
the design of the pump allows free movement at the
pullrod end.
Electric fuel pumps
Electric fuel pumps are sometimes used with
carburettor systems They are usually plunger pumps
where a small plunger in a cylinder is moved up and
down electro-magnetically Some electric pumps
operate continuously, others cut out under pressure.
Generally, these pumps are not repairable.
Carburettors
The carburettor provides a mixture of air and fuel to
the engine This is not just a matter of mixing fuel and
air, but of mixing them together in the right
pro-portions to suit all the different operating conditions of
the engine.
Too much fuel for the air would be wasteful and
would pollute the atmosphere; not enough fuel for the
air would cause loss of engine power and poor
performance.
As well as providing the correct air–fuel mixture,
the carburettor controls the speed and power of the
engine.
The carburettor has to perform the following
functions:
1 Atomise the fuel and mix it with air.
2 Provide the correct air–fuel mixture for good
combustion.
3 Control the amount of air–fuel mixture delivered to
the engine.
Atomisation
Atomisation means breaking down a liquid to a spray.
Figure 13.5 shows how a carburettor does this by
immersing a nozzle in a bowl of fuel.
1 In Figure 13.5(a), the nozzle is discharging
atomised fuel Fuel from the bowl has collected air
from the air bleed and this has atomised the fuel.
2 In Figure 13.5(b), the nozzle has no air bleed and
this supplies large drops of liquid fuel.
The atomised fuel from the nozzle has less density
than liquid fuel, and the flow of atomised fuel in the
carburettor can be stopped, started or varied much more quickly than if the fuel was a liquid The atomised fuel from the nozzle is further atomised by the air flowing down through the carburettor.
Air–fuel mixture
The ideal ratio of air to fuel is 14.7:1 (15 kg of air to
1 kg of fuel) The carburettor must supply this mixture within very close limits, although the ratio is varied to suit engine operating conditions A slightly richer mixture is provided to the engine for starting, during acceleration, and while operating under heavy load The carburettor has a number of devices which enable this to occur The main ones will be covered later.
Controlling the amount of air–fuel mixture
The amount of air–fuel mixture delivered by the carburettor is controlled by the carburettor throttle valve (sometimes referred to as a butterfly valve) This
is located in the throttle body at the base of the carburettor The throttle valve is connected by a cable
or linkage to the driver’s accelerator pedal.
Pressing the accelerator to open the throttle valve allows more atomised fuel to pass This increases the speed, or power of the engine Closing the throttle valve reduces the amount of atomised fuel passing from the carburettor to a minimum This reduces engine power and, if the vehicle is stationary, allows it
to idle.
figure 13.5 Principle of the air bleed
(a) atomised fuel from the discharge nozzle (b) large drops of liquid fuel because there is no air bleed
Trang 5Air and fuel flow in a carburettor
The simple carburettor shown in Figure 13.6 has a
float bowl for the fuel, a fuel discharge nozzle, an air
venturi, and a throttle valve When the engine is
running, an air flow is created through the venturi.
This is the result of the pumping action of the pistons
during the intake strokes.
The venturi has a particular shape, with the inside
diameter having a constriction A low pressure is
created at the constriction and this is where the end of
the nozzle protrudes into the air flow.
Figure 13.7 shows fuel in the float bowl and the
nozzle discharging fuel into the air stream.
Atmospheric pressure on the fuel in the float bowl is
greater than the pressure at the end of the nozzle, and
this causes fuel to flow from the nozzle.
Venturi action
To understand venturi action, air can be considered to consist of a number of molecules High pressure occurs when the molecules are crowded together, and low pressure occurs when the molecules are spaced apart The venturi works like this: air flows into the top
of the carburettor and the molecules are moving at
a certain speed However, when they reach the constriction in the venturi, they have to speed up so that they can all pass through the narrower space.
At this higher air speed, the molecules tend to separate from each other and so a low pressure occurs where they reach their highest speed The faster the molecules move, the lower the pressure.
Carburettor operation
A basic downdraft carburettor is shown in Figure 13.8.
In addition to the parts in previous figures, this has a float and needle valve in the float bowl, an idle mixture adjusting screw, and a choke valve.
The diagram shows the throttle valve partly open, which would be its position with the engine running at
a reasonable speed Opening and closing the throttle would change the speed of the engine.
Operation at normal driving speeds
Carburettor operation for normal driving speeds, as shown in Figure 13.8, is as follows:
figure 13.6 Simple carburettor with the parts identified
figure 13.7 The venturi causes a low-pressure area in
the air stream and atmospheric pressure forces fuel from the nozzle
figure 13.8 Basic downdraft carburettor at normal
driving speeds – the throttle valve controls air and fuel flow
Trang 61 The fuel pump provides fuel to the float bowl, and
the float and the needle valve control the fuel level.
2 The throttle valve is partly open to allow the
air–fuel mixture from the carburettor to pass into
the intake manifold.
3 Air entering the top of the carburettor flows down
through the venturi to create a low pressure (below
atmospheric) in the area around the end of the
discharge nozzle.
4 Atmospheric pressure acting on the surface of the
fuel in the float bowl causes fuel to be forced
through the main jet and up the discharge nozzle
into the venturi.
5 Fuel from the nozzle discharges into the air stream
passing through the venturi The fuel is atomised by
the air and carried down past the throttle valve to
the intake manifold and the engine.
Operation of the throttle valve
As the throttle valve is being opened, the air speed
through the carburettor increases and the air pressure at
the nozzle decreases This is due to venturi action.
However, because atmospheric pressure on the fuel in
the float bowl remains constant, more fuel flows
through the nozzle into the venturi to mix with the
increased flow of air.
The flow of fuel from the nozzle is related to the
flow of air through the venturi, so that a nearly
constant air–fuel ratio can be maintained through a
range of throttle openings.
The position of the throttle valve also controls the
quantity of air–fuel mixture that enters the engine If
the throttle is opened wider, more air and more fuel
will be delivered to the engine and this will increase
engine power or speed If the throttle is closed, less
air–fuel mixture will enter the engine and the engine
power or speed will be reduced.
Operation at idle
Figure 13.9 shows the carburettor operating with the
engine at idle.
1 The throttle valve is closed and only a small
amount of air flows through the carburettor.
2 Because the throttle valve has blocked off the main
air flow, there will be no venturi action, but there
will be low pressure below the throttle valve.
3 Fuel flows from the float bowl through the idle
passages to be discharged through the idle port
below the throttle valve.
4 The air bleed allows air to enter the idle fuel on its way through the passage from the float bowl This helps to atomise the fuel before it is discharged through the idle port.
5 The idle adjustment screw is used to adjust the idle mixture The screw has a tapered end which fits into the idle port This is screwed in, or out, to vary the amount of fuel that passes through the idle port.
6 A small amount of air passes the throttle valve Fuel from the idle port mixes with this to provide the air–fuel mixture for idling.
Operation at low speeds
Figure 13.10 shows what occurs at lower engine speeds, but above idle.
1 The throttle valve is slightly open and, as a result, only a small amount of air flows in through the top
of the carburettor The air flow through the venturi
is too slow to provide a low pressure, so there is no fuel from the discharge nozzle.
2 The throttle valve is opened so that it is just past the low-speed port, which is located directly above the idle port.
3 Fuel, which already has some air mixed with it from the air bleed, flows from both the idle port and the low-speed port This mixes with the air passing the throttle valve to provide the air–fuel mixture needed for low-speed, low-power operation.
figure 13.9 Idle system of a carburettor – the throttle
valve is almost fully closed and the fuel, with some air from the air bleed above the venturi, is supplied past the idle adjustment screw
Trang 7Operation when starting (choke)
When the engine is being cranked for starting, there is
very little air flow through the carburettor, whether the
throttle valve is open or not The choke valve is needed
for cold starting as shown in Figure 13.11.
1 With the choke valve closed, the top of the
carbu-rettor is almost closed off and there is no air flow.
2 The pumping action of the pistons, as the engine is
cranked, creates a low pressure in the carburettor,
even though there is no venturi action.
3 The low pressure causes fuel to flow from the
discharge nozzle and also from the idle and
slow-speed ports This provides a rich mixture for starting.
Carburettor systems
The carburettor has a number of different systems, which are also referred to as circuits, stages or phases With the exception of the float system, the power system and the accelerating system, these have already been explained as part of carburettor operation.
The various systems are:
1 float system
2 idle system
3 fast-idle or low-speed system
4 main or high-speed system
5 choke system
6 power system
7 accelerating system.
The float system includes the float bowl and the float, also the needle valve and seat Fuel from the fuel pump flows into the carburettor bowl, and is maintained at a set level by the float which opens and closes the needle valve.
The power system provides extra fuel for the discharge nozzle when the engine is operating under heavy load conditions.
The accelerating system squirts extra fuel into the air stream whenever the accelerator is pressed This prevents any hesitation by the engine that could occur due to a temporary shortage of fuel.
■ The term flat spot is used when a weak mixture causes a brief loss of engine power.
Carburettor construction
Figure 13.12 is a diagrammatic view of a complete carburettor that can be used to identify its various parts This is a single-barrel downdraft carburettor Following is a summary of the parts of the carburettor and their functions.
1 Venturi This increases the air velocity to create a depression at the main discharge nozzle There is
a primary venturi, and a secondary venturi, which increases efficiency.
2 Main discharge nozzle This nozzle (or jet) dis-charges atomised fuel into the air stream during normal engine operation.
figure 13.10 Low-speed system of a carburettor – the
throttle valve is slightly open and fuel is supplied through the low-speed port and the idle port
figure 13.11 Choke operation – the choke valve has closed
off the air supply to create a vacuum (negative pressure) in the carburettor, which causes fuel to
flow from the main nozzle and also from the idle ports
Trang 83 Main metering jet Meters the fuel that goes
through the main discharge nozzle.
4 High-speed air bleed Bleeds air into the main
discharge system to atomise the fuel in the
discharge nozzle.
5 Throttle valve Controls the air flow through the
carburettor and so controls the quantity of air–fuel
mixture that goes to the intake manifold.
6 Choke valve During starting and during the
warm-up period, the choke restricts the air supply
to provide a rich mixture.
7 Idle mixture adjustment screw This is used to
adjust the quantity of atomised fuel that is
delivered from the idle port.
8 Idle discharge ports Discharge atomised fuel
during idle and during the slow-speed range.
9 Idle air bleed Air is bled into the idle system to
atomise the fuel.
10 Idle tube A jet on the lower end of the tube
meters the fuel for the idle system.
11 Float and needle valve These control the level of
the fuel in the float bowl.
12 Power jet This supplies fuel for high-speed
driving or for operating under heavy conditions.
This fuel is additional to the fuel supplied by the
main jet It is operated by the vacuum piston.
13 Vacuum piston Controlled by intake vacuum, this
automatically opens the power jet when the engine
is operating under load at low-speeds.
14 Accelerator pump Sprays additional fuel into the venturi area for a short period during acceleration.
15 Accelerator pump inlet valve Opens on the pump upstroke to admit fuel into the pump.
16 Accelerator pump bypass jet Meters the fuel being discharged from the pump It also includes the pump outlet valve.
17 Accelerator pump discharge nozzle This is a small nozzle which discharges the fuel from the accelerator pump when the accelerator is pressed.
■ Some parts of the idle system are not identified in Figure 13.12, but they are shown in Figure 13.10.
Mixture correction
When the carburettor is in operation, the amount of fuel discharged from the main nozzle is related to the air flowing through the venturi These combine to provide the air–fuel mixture for the engine If the throttle is opened or closed, the amount of air and fuel will change for the new throttle position.
Unfortunately, the air–fuel ratio does not remain constant, but becomes richer at higher engine speeds, and has to be corrected.
The most common method of correction is known
as air-bleed correction This adjusts the air–fuel mixture by bleeding more air into the fuel in the discharge nozzle Without this correction, the mixture would be too rich at higher engine speeds.
The reason for the mixture becoming richer is that the density of the air becomes less as its speed through
figure 13.12 Diagrammatic view of a single-barrel carburettor with its internal parts identified BENDIX
Trang 9the venturi increases On the other hand, because petrol
is a liquid, its density does not change This means that
the air in the air–fuel ratio will become less and cause
a richer mixture.
The main discharge nozzle in Figure 13.12 is
designed with air-bleed correction.
General carburettor design
There are three general designs of carburettors:
downdraft, updraft and sidedraft (Figure 13.13) The
names relate to the direction in which the air flows into
the carburettor The downdraft type is the one most
commonly used for motor-vehicle engines.
1 Downdraft The air flows downwards through the
carburettor and the force of gravity on the air and
fuel assists the downward movement.
The term semidowndraft is also used and this refers to a carburettor in which the venturi and air
intake are inclined upwards This is a compromise
between downdraft and sidedraft.
2 Updraft The air enters a horizontal intake and is
then directed upwards The air has to be raised and
a certain amount of effort is required This makes it
less efficient than a downdraft type.
3 Sidedraft The air flows in a horizontal direction.
This arrangement, with the air intake at the side, is
used for certain types of carburettors and also for
carburettors on small engines.
Large quantities of air pass through the carburettor into the engine, so that the positioning
of the venturi to assist the air flow is of some
consideration in carburettor design.
Other design features
As well as the three previous types, carburettors can
also be classed as single-barrel, twin- or dual-barrel,
and four-barrel The terms relate to downdraft carburettors Engines can also be fitted with twin or triple carburettors.
Where multibarrel carburettors are used, their operation is similar to two (or four) single-barrel carburettors, combined into one larger unit, but using some common parts.
SU carburettor
A basic SU carburettor is illustrated in Figure 13.14 This is referred to as a variable venturi carburettor The variable venturi avoids much of the design detail of other types of carburettors which have fixed venturis.
In this design of carburettor, the size of the venturi is adjusted to suit any throttle position and the amount of fuel is also adjusted The size of the fuel jet is varied by the use of a tapered needle passing through its centre.
If it is possible to visualise a carburettor that has a different-sized venturi for each operating condition,
figure 13.13 General designs of carburettors
figure 13.14 SU carburettor in simple form
Trang 10and also a different-sized fuel jet for that condition,
then that is the basic principle of the SU carburettor.
Because the venturi size changes to admit more air,
the velocity of the air does not change, and the low
pressure, or depression, at the jet will remain constant.
This is where the term constant depression comes from.
■ By comparison, the pressure (depression) in the
venturi of a downdraft carburettor is not constant,
but varies with the throttle opening and the rate of
air flow.
SU operation
A basic SU carburettor operates as follows:
1 The piston is in a suction chamber on top of the
carburettor, but the bottom of the piston forms part
of the venturi Air flow through the venturi causes a
low pressure (depression) at the jet, which draws
fuel from the float bowl.
2 The piston can move up and down to alter the size
of the venturi The size of the jet is altered by a
needle attached to the piston, which moves up and
down in the jet.
3 The piston is moved upwards by suction on top and
downwards by its weight, although a spring is
sometimes fitted on top of the piston.
4 A suction passage connects the suction chamber to the main air passage on the throttle side of the venturi Any change in pressure at the end of the suction passage will alter the position of the piston.
5 The pressure at the end of the suction passage is related to the throttle valve opening, so the piston will alter its position to suit changes in the throttle opening.
6 The position of the piston will alter the amount of air that flows, but the air will always flow at the same velocity This keeps the low pressure (depression) at the jet constant.
7 To maintain the correct air–fuel mixture, the metering needle moves with the piston to increase
or decrease the size of the jet to suit the amount of air With this arrangement of a moving piston to vary the venturi size, with a needle attached to vary the jet size, both air and fuel are metered in the correct ratio for almost all conditions.
Carburettor external construction
Figure 13.15 is an external view of a typical dual (two-barrel) downdraft carburettor with its parts identified There are three main parts:
figure 13.15 External view of a dual downdraft carburettor with the parts identified TOYOTA