Fuel
Injection system for SI engines;
1.
Carburetion
Spark-ignition engines normally
use volatile liquid fuels. Preparation of fuel-air mixture is done outside the
engine cylinder and formation of a homogeneous mixture is normally not
completed in the inlet manifold. Fuel droplets, which remain in suspension,
continue to evaporate and mix with air even during suction and compression
processes. The process of mixture preparation is extremely important for
spark-ignition engines. The purpose of carburetion is to provide a combustible
mixture of fuel and air in the required quantity and quality for efficient
operation of the engine under all conditions.
Definition
of Carburetion;
The process of formation of a
combustible fuel-air mixture by mixing the proper amount of fuel with air
before admission to engine cylinder is called carburetion and the device which
does this job is called a carburetor.
Definition
of Carburetor;
The carburetor is a device used
for atomizing and vaporizing the fuel and mixing it with the air in varying
proportions to suit the changing operating conditions of vehicle engines.
Factors
Affecting Carburetion
Of
the various factors, the process of carburetion is influenced by
i. The
engine speed
ii. The
vaporization characteristics of the fuel
iii. The
temperature of the incoming air and
iv. The
design of the carburetor
Principle of Carburetion
Both air and gasoline are drawn through the carburetor and
into the engine cylinders by the suction created by the downward movement of
the piston. This suction is due to an increase in the volume of the cylinder
and a consequent decrease in the gas pressure in this chamber.
It is the difference in pressure between the atmosphere and
cylinder that causes the air to flow into the chamber. In the carburetor, air
passing into the combustion chamber picks up discharged from a tube. This tube
has a fine orifice called carburetor jet that is exposed to the air path.
The rate at which fuel is discharged into the air depends on
the pressure difference or pressure head between the float chamber and the
throat of the venturi and on the area of the outlet of the tube. In order that
the fuel drawn from the nozzle may be thoroughly atomized, the suction effect
must be strong and the nozzle outlet comparatively small. In order to produce a
strong suction, the pipe in the carburetor carrying air to the engine is made
to have a restriction. At this restriction called throat due to increase in
velocity of flow, a suction effect is created. The restriction is made in the form
of a venturi to minimize throttling losses.
The end of the fuel jet is located at the venturi or throat of
the carburetor. The geometry of venturi tube is as shown in Fig.16.6. It has a
narrower path at the center so that the flow area through which the air must
pass is considerably reduced. As the same amount of air must pass through every
point in the tube, its velocity will be greatest at the narrowest point. The
smaller the area, the greater will be the velocity of the air, and thereby the
suction is proportionately increased
As
mentioned earlier, the opening of the fuel discharge jet is usually loped where
the suction is maximum. Normally, this is just below the narrowest section of
the venturi tube. The spray of gasoline from the nozzle and the air entering
through the venturi tube are mixed together in this region and a combustible
mixture is formed which passes through the intake manifold into the cylinders.
Most of the fuel gets atomized and simultaneously a small part will be
vaporized. Increased air velocity at the throat of the venturi helps he rate of
evaporation of fuel. The difficulty of obtaining a mixture of sufficiently high
fuel vapour-air ratio for efficient starting of the engine and for uniform
fuel-air ratio indifferent cylinders (in case of multi cylinder engine) cannot
be fully met by the increased air velocity alone at the venturi throat.
2. The Simple Carburetor
Carburetors are highly complex. Let us first understand the
working principle bf a simple or elementary carburetor that provides an air
fuel mixture for cruising or normal range at a single speed. Later, other
mechanisms to provide for the various special requirements like starting,
idling, variable load and speed operation and acceleration will be included.
Figure 3. shows the details of a simple carburetor.
The simple carburetor mainly consists of a float chamber, fuel
discharge nozzle and a metering orifice, a venturi, a throttle valve and a
choke. The float and a needle valve system maintain a constant level of
gasoline in the float chamber. If the amount of fuel in the float chamber falls
below the designed level, the float goes down, thereby opening the fuel supply
valve and admitting fuel. When the designed level has been reached, the float
closes the fuel supply valve thus
stopping
additional fuel flow from the supply system. Float chamber is vented either to
the atmosphere
or to the” upstream side of the venturi.During suction stroke air is drawn
through the
venturi.
As
already described, venturi is a tube of decreasing cross-section with a minimum
area at the throat, Venturi tube is also known as the choke tube and is so
shaped that it offers minimum resistance to the air flow. As the air passes
through the venturi the velocity increases reaching a maximum at the venturi
throat. Correspondingly, the pressure decreases reaching a minimum. From the
float chamber, the fuel is fed to a discharge jet, the tip of which is located
in the throat of the venturi. Because of the differential pressure between the
float chamber and the throat of the venturi, known as carburetor depression,
fuel is discharged into the air stream.
The fuel discharge is affected by the size of the discharge
jet and it is chosen to give the required air-fuel ratio. The pressure at the
throat at the fully open throttle condition lies between 4 to 5 cm of Hg, below
atmospheric and seldom exceeds8 cm Hg below atmospheric. To avoid overflow of
fuel through the jet, the level of the liquid in the float chamber is
maintained at a level slightly below the tip of the discharge jet. This is
called the tip of the nozzle. The difference in the height between the top of
the nozzle and the float chamber level is marked h in Fig.3.
The gasoline engine is quantity governed, which means that
when power output is to be varied at a particular speed, the amount of charge
delivered to the cylinder is varied. This is achieved by means of a throttle
valve usually of the butterfly type that is situated after the venturi tube.
As the throttle is closed less air flows through the venturi
tube and less is the quantity of air-fuel mixture delivered to the cylinder
and hence power output is reduced. As the” throttle is opened,
more air
flows through the choke tube resulting in increased quantity of mixture being
delivered to the engine. This increases the engine power output. A simple
carburetor of the type described above suffers from a fundamental drawback in
that it provides the required A/F ratio only at one throttle position.
At the other throttle positions the mixture is either leaner
or richer depending on whether the throttle is opened less or more. As the
throttle opening is varied, the air flow varies and creates a certain pressure
differential between the float chamber and the venturi throat. The same
pressure differential regulates the flow of fuel through the nozzle. Therefore,
the velocity of flow of air II and fuel vary in a similar manner.
The Choke and the Throttle
When the vehicle is kept stationary for a long period during
cool winter seasons, may be overnight, starting becomes more difficult. As
already explained, at low cranking speeds and intake temperatures a very rich
mixture is required to initiate combustion. Some times air-fuel ratio as rich
as 9:1 is required. The main reason is that very large fraction of the fuel may
remain as liquid suspended in air even in the cylinder. For initiating
combustion, fuel-vapour and air in the form of mixture at a ratio that can
sustain combustion is required.
It may be noted that at very low temperature vapour fraction
of the fuel is also very small and this forms combustible mixture to initiate
combustion. Hence, a very rich mixture must be supplied. The most popular
method of providing such mixture is by the use of choke valve. This is simple
butterfly valve located between the entrance to the carburetor and the venturi
throat as shown in Fig.3.
When the
choke is partly closed, large pressure drop occurs at the venturi throat that
would normally result from the quantity of air passing through the venturi
throat. The very large depression at the throat inducts large amount of fuel
from the main nozzle and provides a very rich mixture so that the ratio of the
evaporated fuel to air in the cylinder is within the combustible limits.
Sometimes, the choke valves are spring loaded to ensure that large carburetor
depression and excessive choking does not persist after the engine has started,
and reached a desired speed.
This choke can be made to operate
automatically by means of a thermostat so that the choke is closed when engine
is cold and goes out of operation when engine warms up after starting. The
speed and the output of an engine is controlled by the use of the throttle
valve, which is located on the downstream side of the venturi.
The more the throttle is closed
the greater is the obstruction to the flow of the mixture placed in the
passage and the less is the quantity of mixture delivered to .the
cylinders. The decreased quantity of mixture gives a less powerful
impulse to the pistons and the output of the engine is reduced
accordingly. As the throttle is opened, the output of the engine
increases. Opening the throttle usually increases the speed of the
engine. But this is not always the case as the load on the engine is also a
factor. For example, opening the throttle when the motor vehicle is starting to
climb a hill may or may not increase the vehicle speed, depending upon the
steepness of the hill and the extent of throttle opening. In
short, the throttle is simply a means to regulate the output of the
engine by varying the quantity of charge going into the cylinder.
Compensating
Devices
An automobile on road has to run
on different loads and speeds. The road conditions play a vital role.
Especially on city roads, one may be able to operate the vehicle between 25 to
60% of the throttle only. During such conditions the carburetor must be able to
supply nearly constant air-fuel ratio mixture that is economical
(16:1).However, the tendency of a simple carburetor is to progressively richen
the mixture as the throttle starts opening.
The main metering system alone
will not be sufficient to take care of the needs of the engine. Therefore,
certain compensating devices are usually added in the carburetor along with the
main metering system so as to supply a mixture with the required air-fuel ratio.
A number of compensating devices are in use. The important ones are
i. Air-bleed
jet
ii. Compensating
jet
iii. Emulsion
tube
iv.Back suction control mechanism
v. Auxiliary
air valve
vi.Auxiliary air port
As
already mentioned, in modern carburetors automatic compensating devices are
provided to maintain the desired mixture proportions at the higher speeds. The
type of compensation mechanism used determines the metering system of the
carburetor. The principle of operation of various compensating devices are
discussed briefly in the following sections.
Air-bleed
jet
Figure 4. illustrates a principle of an air-bleed system in
atypical modern downdraught carburetor. As could be seen it contains an
air-bleed into the main nozzle. An orifice restricts the flow of air through
this bleed and therefore it is called restricted air-bleed jet that is very
popular. When the engine is not operating the main jet and the air bleed jet
will be filled with fuel. When the engine starts, initially the fuel starts
coming through the main as well as the air bleed jet (A). As the engine picks
up, only air starts coming through the air bleed and mixes with fuel at B
making a air fuel emulsion.
Thus
the fluid stream that has become an emulsion of air and liquid has negligible
viscosity
and surface tension. Thus the
flow rate of fuel is augmented and more fuel is sucked at low suctions.
‘By
proper design of hole size at B compatible with the entry hole at A, it is
possible to maintain a fairly
uniform mixture ratio for the entire power range of the operation of an engine.
If the fuel flow nozzle of the air-bleed system is placed in the centre of the
venturi, both the air-bleed nozzle and the venturi are subjected to same engine
suction resulting approximately same fuel-air mixture for the entire power
range of operation.
Compensating
Jet
The principle of compensating jet device is to make the
mixture leaner as the throttle opens progressively. In this method, as can be
seen from Fig.5 in addition to the main jet, a compensating jet is
incorporated. The compensating jet is connected to the compensation well. The
compensating well is also vented to atmosphere like the main float chamber.
The compensating well is supplied with fuel from the main
float chamber through a restricting orifice. With the increase in airflow rate,
there is decrease of fuel level in the compensating well, with the result that
fuel supply through the compensating jet decreases. The compensating jet thus
progressively makes the mixture leaner as the main jet progressively makes the
mixture richer. The main jet curve and the compensating jet curve are more or
less reciprocals of each other.
Emulsion Tube
The
mixture correction is attempted by air bleeding in modern carburetor. In one
such arrangement as shown in Fig.6, the main metering jet is kept at a level of
about 25 mm below the fuel level in the float chamber. Therefore, it is also
called submerged jet. The jet is located at the bottom of a well. The sides of
the well have holes. As can be seen from the figure these holes are in
communication with the atmosphere. In the beginning the level of petrol in the
float chamber and the well is the same.
When the
throttle is opened the pressure at the venturi throat decreases and petrol is
drawn into the air stream. This results in progressively uncovering the holes
in the central tube leading to increasing air-fuel ratios or decreasing
richness of mixture when all holes have been uncovered. Normal flow takes place
from the main jet. The air is drawn through these holes in the well, and the
fuel is emulsified and the pressure differential across the column of fuel is
not as high as that in simple carburetor.
Acceleration Pump System
Acceleration is a transient phenomenon. In order to accelerate
the vehicle and consequently its engine, the mixture required is very rich and
the richness of the mixture has to be obtained quickly and very rapidly. In
automobile engines situations arise when it is necessary to accelerate the
vehicle. This requires an increased output from the engine in a very short
time.
If the throttle is suddenly opened there is a corresponding
increase in the air flow. However, because of the inertia of the liquid fuel,
the fuel flow does not increase in proportion to the increase in air flow. This
results in a temporary lean mixture ca11singtheengine to misfire and a
temporary reduction in power output.
To prevent this condition, all modern carburetors are equipped
with an accelerating system. Figure 7. illustrates simplified sketch of one
such device. The pump comprises of a spring loaded plunger that takes care of
the situation with the rapid opening of the throttle valve. The plunger moves
into the cylinder and forces an additional jet of fuel at the venturi throat.
When the throttle is partly open, the spring sets the plunger
back. There is also an arrangement which ensures that fuel in the pump cylinder
is not forced through the jet when valve is slowly opened or leaks past the
plunger or some holes into the float chamber.
Mechanical
linkage system, in some carburetor, is substituted by an arrangement where by
the pump plunger is held up by manifold vacuum. When this vacuum is decreased
by rapid opening of the throttle, a spring forces the plunger down pumping the
fuel through the jet.
3. Types of Carburetors
There are
three general types of carburetors depending on the direction of flow of air.
The first is the up draught type shown in Fig.8(a) in which the air enters at
the bottom and leaves at the top so that the direction of its flow is upwards.
The disadvantage of the up draught carburetor is that it must lift the sprayed
fuel droplet by air friction. Hence, it must be designed for relatively small
mixing tube and throat so that even at low engine speeds the air velocity is
sufficient to lift and carry the fuel particles along. Otherwise, the fuel
droplets tend to separate out providing only a lean mixture to the engine. On
the other hand, the mixing tube is finite and small then it cannot supply
mixture to the engine at a sufficiently rapid rate at high speeds.
In order to overcome this drawback the downdraught carburetor
[Fig.8 (b)] is adopted. It is placed at a level higher than the inlet manifold
and in which the air and mixture generally follow a downward course. Here the
fuel does not have to be lifted by air friction as in the up draught
carburetors but move into the cylinders by gravity even if the air velocity is
low. Hence, the mixing tube and throat can be made large which makes high
engine speeds and high specific outputs possible.
Constant Choke Carburetor:
In the constant choke carburetor, the air and fuel flow areas
are always maintained to be constant. But the pressure difference or
depression, which causes the flow of fuel and air, is being varied as per the
demand on the engine. Solex and Zenith carburetors belong to this class.
Constant Vacuum Carburetor:
In the
constant vacuum carburetor, (sometimes called variable choke carburetor) air
and fuel flow areas are being varied as per the demand on the engine, while the
vacuum is maintained to be always same. The S.U. and Carter carburetors belong
to tills class.
Multiple Venturi Carburetor:
Multiple venturi system uses double or triple venturi. The
boost venturi is located concentrically within the main venturi.The discharge
edge of the boost venturi is located at the throat of the main venturi. The
boost venturi is positioned upstream of the throat of the larger main venturi.
Only a fraction of the total air flows though the boost venturi. Now the
pressure at the boost venturi exit equals the pressure at the main venturi
throat. The fuel nozzle is located at the throat of the boost venturi.
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