The document discusses the carburetor and its importance in spark ignition engines. It begins by explaining that carburetors are used to prepare a homogeneous mixture of fuel and air for combustion. It then discusses the basic principles of carburetion, including the venturi system that uses suction to draw fuel into the air stream. The document covers the essential parts of a carburetor like the float system and fuel jet. It also discusses factors that affect carburetion like engine speed, fuel type, air temperature and carburetor design. Different air-fuel mixture ratios are needed for starting, idling and varying loads on the engine. Carburetors are complex devices that precisely meter and mix fuel and air to

1. THE CARBUTETOR AND ITS IMPORTANCE IN
S.I.ENGINES
Spark-ignition engines normally use
volatile liquid fuels.Preparation of
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 f 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 th engine under all
conditions.
DEFINITION OF CARBURETION:
The process 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.
HISTORY ABOUT THE
CARBURETOR:
The carburetor is the part of the car
combines air and fuel to create
combustion. Before the creation of
fuel-injected engines, the carburetor.
Even though Karl Benz invented the
carburetor in 1885 and patented the
device in 1886, there is some doubt
whether he actually created it. Some
believe a pair of Hungarian engineers
also invented 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 pisto.This
suction is due to an increase in the
volume of the cylinder and consequent
decreases in the gas pressure in this
chamber.It is the difference in pressure
between the atmsphere and cylinder that
casuses the air to flow into the
chamber.In the carburetor,air passing
into the combustion chamber picks up
fuel discharged from a tube.This tube
has a fine orifice called carburetor jet
which is exposed to the air path.The rate
at which fuel is between the float
chamber and the throat f 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 suctin
effects 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 resriction.At this
restriction called throat due to increases
in velocity of flow,a suction efffect is
created.The restiction is made in the
form f a venturi as tominimize

2. THE CARBUTETOR AND ITS IMPORTANCE IN
S.I.ENGINES
throattling lasses.The end of the fuel jet
is located at the venturi or throat of the
carburetor.
The geometry of venturi tube is as
shown.It has a narrow path at the centre
so that the flow area through which the
air must pass is considerbly reduced.As
the same amount of air must pass
through every point in the tube, its
velosity will be greatest at the narroest
point.The smaller the area,the greater
will be the velcity of the air,and thereby
the suction is proportionately increases.
As mentioned earlier,the opening of
the fuel discharge jet is usually located
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 intae
manifold into the cylinders.Most of the
fuel gets atomized and simultaneously a
small part will be vapourized.Increases
air velocity at the throat of the venturi
helps the 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 in different
cylinders(in case of multicylinder
engine)cannot befully met by the
increased air velocity alone at the venturi
throat.
THE SIMPLE CABURETOR
Carburetor are highly
complex.Let us first understand the
working principle of a simple or
elementary carburetor which provides an
air-fuel mixture for cruising or normal
range at a single speed.Later,other
mechanisms to provide for the various
special requirementslike
starting,idling,variable load and speed
operations and acceleration will be
included.the simple carburetor is as
shown in figure………..
The simple carburetor mainly consists of
a float chamber,fuel dicharge and nozzle
and a neddle valve system maintains a
constant level f gasoline in the float
chamber.If the amount of fuel in the
float chamver 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 supply system.float

3. THE CARBUTETOR AND ITS IMPORTANCE IN
S.I.ENGINES
chamber is vented either to the
atmosphere or to the upstream side of the
venturi.
Durng suction stroke air is drawn
through the venturi.as already
descrived,venturi is a tube f decreaseing
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,fuek is discharged
into the air stream,The fuel dicharge 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 5cm of Hg,below atmospheric and
seldom exceeds 8 cm ofHg below
atmospheric.To avoid over flow 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 heigh
between the top of the nozzle and the
float chamber level is marked as h.
The gasoline engine is quntity
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 achived by
means of a throattle valve usually of the
butterfly type which is situated after the
venturi tube .As the quantity of air-fuel
miture delivered to the cylinder and
hence power output is reduced.As the
throttle is opened,more air flows through
the choke tube resulting in increases
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
vary in a simillar manner.At throat
decreases with increasing air flow
whereas that of the fuel remains
unchanged.This results in a simple
carburetor producing a progressively
rich mixture with increasing throttle
opening.
ESSENTIAL PARTS OF THE
CARBURETOR:::
1. The Carburetor System
o A carburetor system
works on three basic principles:
metering the correct proportion of fuel
and air, atomization of the fuel into a

4. THE CARBUTETOR AND ITS IMPORTANCE IN
S.I.ENGINES
vapor, and the distribution of a
uniform mixture of both fuel and air
into the engine.
When fuel reaches the carburetor, it
flows through the fuel supply pipe and
into a float bowl. Fuel then flows
through the fuel jet into the other side
of the carburetor. This side includes an
air entry point, also known as the
barrel or throat, at which there is an
air filter; a Venturi pipe, a tube which
varies in width and the throttle valve.
The Float System
o The term "float system"
is used to describe a typical
carburetor's operation. Fuel flows
through the inlet fitting and through a
seat, then past the end of the needle
and into the float bowl. The needle is
important because as the bowl fills up,
the float pushes the needle into the
needle seat, cutting off the fuel. The
fuel stays constant.
o
The Fuel Jet
o The fuel jet is also called
the main nozzle. The metering jet, a
calibrated opening on the bottom of
the float bowl, determines the amount
of fuel that will move into the engine.
The metering jet opens into a main
well which contains air bleeds and
baffles that turn the fuel into a vapor
as it moves up the main nozzle in a
vacuum process, and into the Venturi
pipe.
The Venturi Pipe
o The Venturi pipe is so
named because it operates on the
Venturi effect. A vacuum is created in
the pipe based on how much air is
rushing through the carburetor's air
entry. The narrow main nozzle moves
the fuel into the Venturi pipe by means
of low pressure which pulls it out of
the nozzle. This spray is then pushed
toward the throttle valve.
The Throttle Valve
o The throttle valve is
placed between the spray and the inlet
pipe which leads to the engine. There
are two types of throttle valves: the
butterfly, which is a circular disc and
the cylindrical which is as large as the
inlet pipe and rotates. The throttle is
controlled by the accelerator, with
cables or rods attached to the throttle
by a lever. Although the accelerator
controls the engine speed, variations
made to the throttle valve can be made
to enhance a car's performance.
Types of Carburetors
o There are several
different types, configurations, and
manufacturers of carburetors available
based on what kind of performance is
needed. Different configuration
variations include two and four barrel
types for more airflow, accelerator
pumps, high-flow needles and vacuum
secondary diaphragms, just to name a
few.
Performance carburetor manufacturers
include Edelbrock, Holley, AED, Wood
and Predator. Small engine carburetor
manufacturers include Briggs & Stratton,
Bing and Tecumseh, among others.
These types include engines for
lawnmowers, snow blowers, motorcycle,
log splitters and pressure washers.
FACTORS AFFECTING
CARBURETION:
Of the various factors,the process of
carburetin is influenced by
1> the engine speed

5. THE CARBUTETOR AND ITS IMPORTANCE IN
S.I.ENGINES
2> the vapourization
charecteristics of the fuel
3> the temperature of the
incoming air and
4> the design of the carburetor
and they are explanied as
fallows………….
ENGINE SPEED
Since modern engines are of high
speed type,the time available for
mixture formation is very limited.For
example,an engine running at 3000
rpm has only about 10
milliseconds(ms) for mixture
induction during intake stroke.When
the speed becomes 6000 rpm the time
available is only 5 ms.
Therefore,in order to havae
high quality
carburetion(mixture with high
vapour content)
THE VAPOURIZATIO
CHARECTERISTICS OF THE FUEL:
Other factors which ensure high quality
carburetion within a short period are the
presence of highly volatile hydrocarbons
in the fuel.Therefore suitable
evaporation charecteristics of the
fuel,indicated by its distillation curve,are
necessary for efficient carburetion
especially at high engine speeds.
TEMPERATURE AND PRESSURE
OF THE INCOMING AIR:
The temperature and pressure of
surrounding air has a large influence on
efficient carburetion.Higher atmospheric
air temperature increases the
vapourization of fuel(percentage of fuel
vapour increases with increases in
mixture temperature)and produces a
more homogeneous mixture.An
increases in atmospheric
temperature,however,leads to a
decreases in power output of the engine
when the air-fuel ratio is constant due to
reduced mass flow into the cylinder
or,in other words,reduced volumetric
efficiency.
THE DESIGN OF THE
CARBURETOR:
The design of the carburetor,the intake
system and the combustion chamber
have considerable influence on uniform
distribution of mixture to the various
cylinders of the engine.Proper design of
carburetor elements alone ensures the
supply of desired compositon of the
mixture under different operating
conditions of the engine.
AIR-FUEL MIXTURE
An engine is generlly operated at
different loads and speeds.For
this,proper air-fuel mixture should be
supplied to the engine cylinder.Fuel and
air are mixed to form three different
types of mixtures.
1. chemically correct mixture
2. rich mixture
3. lean mixture

6. THE CARBUTETOR AND ITS IMPORTANCE IN
S.I.ENGINES
and they are explaned as
follows……….
CHEICALLY CORRECT MIXTRUE:
Chemically correc or stoichiometric
mixture is one in which there is just
enough air for complete combustion of
the fuel.For example,to burn one kg of
octane[C8H8] completely 15.12 kg of air
is required. Hence chemically correct air
fuel ratio for the C8H8 is
15.12:1;Usually approximated to
15:1.This chemically correct mixture
will vary only slightly in numerical
value between different hydrocarbon
fuels.It is always computed from the
chemical equation for complete
combustion for a particular
fuel.Complete combustion means
allcarbon in the fuel is converted to CO2
and all hydrogen to H2O.
RICH MIXTURE;
A mixture which contains less air than
the stoichiometric requirements is called
a rich mixture.
For exmaple: air-fuel raio of 12:1,10:1
LEAN MIXTURE:
A mixture which contains more air than
the stoichiometric requirements is called
a lean mixture
For example air-fuel ratio is 17:1,20:1
etc.
There is,however,a limited range of A/F
ratios is a homogenous miture,ony
within which combustion is an si engine
will occur.outside this range,the ratio is
either too rich or too lean to sustain
flame propagation .This range of useful
A/F ratio runs from approximately
9:1(rich) to 19:1(lean)as indicated .
MIXTURE REQUIREMENTS AT
DIFFERENT LOADS AND SPEEDS
The air-fuel ratio at which an engine
operates has a considerable influence on
its performance.Consider an engine
operating at full throttle and constant
speed with varying A/F ratio.under these
conditions, A/F raio will affect both the
power output and the brake specific fuel
consumption,as indicated by the typical
curves.the mixture corresponding to the
maximum output on the curve is called
the best power mixture with an A/F ratio
of approximately 12:1.The mixture
corresponding the minimum point on the
bsfc curve is called the best economy
mixture.the A/Ftatio is apprximately
16:1 it may be noted that the best power
mixture is much richer than the
chemically correct mixture and the best
economy mixture is slightly leaner than
the chemically correct.
The different ratios of the air-fuel
mixtures are required for the S.I.engine
operating at various conditions…….
01) Air-Fuel Ratio for Starting
Very rich mixture (10: 1) is required at
starting of engine. During starting very
small amount of fuel is vaporizes and
rest of it stay in the liquid state so as to
give an ignitable mixture.
02) Air-Fuel Ratio for Idling
An idling, engine demands a rich
mixture, which can be made leaner as
the throttle is gradually opened. During
idling, the pressure in the inlet manifold
is about 20 to 25% of atmospheric
pressure. At suction stroke,
inlet valve opens and the product of
combustion trapped in the clearance
volume, expands in the inlet manifold.

7. THE CARBUTETOR AND ITS IMPORTANCE IN
S.I.ENGINES
Latter when the piston moves
downwards, the gases along with the
fresh charges go into the cylinder. A rich
mixture must be supplied during idling,
to counteract the tendency of dilution
and to get an ignitable mixture.
03) Air-Fuel Ratio for Medium Load
Most of the time, engine is running in
medium load condition, therefore, it is
desirable that the running should be most
economical in this condition. So a lean
mixture can be supplied, as engine has
low fuel consumption at medium load.
Formulti cylinder engine, slightly more
fuel is required due to mal distribution of
fuel.
04) Air-Fuel Ratio for Maximum Power
Range
When maximum power is required, the
engine must be supplied with rich
mixture as the economy is of no
consideration. As the engine enters in the
power range, the spark must be retarded
otherwise knocking would occur. A lean
mixture burns at latter part
of working stroke. As the exhaust valve
expose to high temperaturegases and
have very less time to cool down.
Moreover, the excess air in the lean
mixture may cause an oxidizing action
on the hot exhaust valve and leads to
failure.
05) Air-Fuel Ratio for Acceleration
Even during normal running, sometimes
more power is required for a short period
such as to accelerate the vehicle for
overtaking etc. During this period rich
mixture is required.
I hope this will help you to understand
something about combustion of fuel at
different running conditions of the
engine.
ADVANTAGES;
An engine with a carburetor is said to
produce more raw horsepower or more
high end power than a fuel injected
engine.
A carburetor is by far a simple engine
when compared to a fuel injected engine.
this simplicity would make the
carburetor engine easier to adjust,
troubleshoot, configure or rebuild.
A carburetor engine would be a cheaper
engine to purchase and cheaper to install.
DISADVANTGES:
Carburetor
An engine with a carburetor is said to
produce more raw horsepower or more
high end power than a fuel injected
engine.
A carburetor is by far a simple engine
when compared to a fuel injected engine.
this simplicity would make the
carburetor engine easier to adjust,
troubleshoot, configure or rebuild.
A carburetor engine would be a cheaper
engine to purchase and cheaper to install.
CONCLUSION:
The carburetor is essential for the
S.I.engine for increasing the efficiency
and power output and proper combustion
and controlling the emissions.