19-09-2014, 03:45 PM
CFD ANALYSIS OF FLOW THROUGH VENTURI OF A CARBURETOR
CFD ANALYSIS.pdf (Size: 1.39 MB / Downloads: 280)
ABSTRACT
Modern passenger vehicles with gasoline engines are provided with different
compensating devices for fuel air mixture supply. Even then there is a high fuel consumption
because of many factors. One of the important factors that affect the fuel consumption is that
design of carburetor. The venturi of the carburetor is important that provides a necessary
pressure drop in the carburetor device. Since different SI engine alternative fuels such as LPG,
CNG are used in the present day vehicles to reduce the pollution and fuel consumption. Still for
a better economy and uniform fuel air supply there is a need to design the carburetor with an
effective analytical tool or software. In this work three parameters namely pressure drop and fuel
discharge nozzle angle of the carburetor will be analyzed using computational fluid dynamics.
For this analysis CFD will be done using 2 softwares namely GAMBIT and FLUENT. The
results obtained from the softwares will be analyzed for optimum design of a carburetor.
Introduction: Internal Combustion Engines
Engine is a device that transforms one form of energy into another form. Heat energy is a device
that transforms the chemical energy contained in a fuel to another form of energy and utilizes
that energy for some useful work. Internal combustion engine is a device in which the
combustion of the working fluid takes place inside the engine e.g. gasoline or diesel engine.
SI Engine
SI engine is known as spark ignition engine. In case of such engines the cycle is completed in 4
strokes of the piston namely suction, compression, power and exhaust.
Suction: Suction strokes starts when the piston is at the top dead center. At this time the intake
valve is open where as the exhaust valve is closed. When the piston moves towards the bottom
dead center, suction is created and fuel-air mixture is drawn into the cylinder.
Compression: During the return of the piston from the bottom dead center towards the top dead
center, the charge sucked during the intake stroke gets compressed. During this stroke both
valves are in open condition. At the end the mixture is ignited with the help of a spark plug. Due
to the ignition the chemical energy of the fuel is converted into heat energy and the temperature
rises to about 20000C.
Expansion: During this stroke both the valves remain in closed position and power is also
produced.
Exhaust: During this stroke the inlet valve remains in closed position whereas the exhaust valve
remains open. The piston moves from bottom dead center to the top dead center and sweeps the
burnt gases out of the cylinder
Definition of Carburetion
The process of forming a combustible fuel-air mixture by mixing the right amount of fuel with
air before admission to the cylinder of the engine is called carburetion and the device doing this
job is called carburetor.
[b]Factors Affecting Carburetion
The various factors affecting the process of carburetion are
1. Engine speed
2. Vaporization characteristics of the fuel
3. Temperature of incoming air
4. Design of the carburetor
Since the engines are of high speed type there is very little time available for mixture
preparation. So to have a high quality carburetion the velocity of the air at point of injection of
fuel has to be increased. To achieve this, a venturi is provided in the path of air.
Presence of highly volatile hydrocarbons in the fuel also ensures high quality carburetion.
The pressure and temperature of the surrounding air also affects the process of carburetion.
Higher atmospheric air temperature increases the vaporization of the fuel and hence a more
homogeneous mixture is produced.
Design of the carburetor, its intake system and the combustion chamber also affect the uniform
distribution of mixture to various cylinders of the engine.
Air-Fuel Mixtures
For proper running of the engine under different loads and speeds a proper mixture of air and
fuel is required. Generally 3 types of fuel mixtures are used like lean, rich and chemically correct
mixture. The mixture in which there is just enough air for the complete combustion of the fuel is
called stoichiometric mixture. The mixture which contains less air than the air required for
stoichiometric mixture is called rich mixture and the mixture containing more air than the
stoichiometric mixture is called lean mixture. The carburetor should provide the air-fuel mixture
according to engine requirement and that must be under combustible range
Principle of Carburetion
Both air and gasoline are drawn into the cylinder due to suction pressure created by the
downward movement of the piston. In the carburetor, the air passing into the combustion
chamber picks up the fuel discharged by a fine orifice in a tube called the carburetor jet. The rate
of discharge of the fuel depends on the pressure difference between the float chamber and the
throat of the venturi of the carburetor and the area of the outlet of the tube. In order that the fuel
is strongly atomized the suction effect must be strong and the nozzle outlet must be
comparatively small. To produce a strong suction, a restriction is generally provided in the pipe
in the carburetor carrying air to the engine. This restriction is called throat. In this throat due to
increase in the velocity of the air the pressure is decreased and suction is created.
The venturi tube has a narrower path at the center so that the path through air is going to travel is
reduced. As same amount of air must travel must travel through the path of the tube so the
velocity of the air at the venturi is increased and suction is created.
Usually the fuel discharge jet is located at the point where the suction is maximum. So this is
positioned just below the throat of the venturi. The spray of the fuel from the fuel discharge jet
and the air are mixed at this point of the throat and a combustible mixture is formed. Maximum
amount of fuel gets atomized and some part gets vaporized. Due to increase in the velocity of the
air at the throat the vaporization of the fuel becomes easier
Emulsion Tube
The main metering is jet is generally kept 25mm below the fuel level in the float chamber so as
to avoid the overflow of the fuel. A jet is placed at the bottom of a well having holes which are
connected to the atmosphere. When the throttle is opened fuel starts to flow from the well and
the holes get uncovered and the air-fuel ratio increases i.e. the richness of the mixture decreases
when all the holes get uncovered. The air is drawn through these holes and the fuel gets
emulsified and the differential of pressure across the column of fuel is not as high as that of the
simple carburetor
Compensating Jet
The main purpose of the compensating jet, which is connected to a compensating well, is to
make the mixture leaner as the throttle valve opens gradually. The compensating well is vented
to the atmosphere and is also connected to the main fuel chamber through a restricting orifice.
With the increase in air flow rate, the fuel level in the compensating well decreases so the fuel
supply rate through the compensating jet also decreases. Thus the compensating jet tends to lean
the mixture whereas the main jet tends to richen the mixture. So the sum of the two jets tends to
keep the mixture to the required ratio.
Compressed Natural Gas (CNG)
By drilling wells petroleum and natural gas are obtained. Crude petroleum contains
hydrocarbons, some amount of water, sulphur and some other impurities also. Mixing of
petroleum with the natural gas gives a highly volatile liquid which is known as natural gasoline.
The natural gas can be compressed and can renamed as Compressed Natural Gas. Just like LPG,
CNG is also used to run the automobiles. Bothe the LPG and CNG fuel feed systems are same.
Petrol driven cars can be converted into CNG driven cars by using the CNG conversion kit. The
kits contain certain auxiliary parts like mixer and converter etc required for conversion.
The following table summarizes the different properties of the fuels.
Literature Review
Diego Alejandro Arias [2] studied and conducted an experiment to validate the steady state
model of a carburetor by measuring the fuel and air flows in a commercial (Nikki) carburetor. He
used a flow-amplifier to create a low pressure zone downstream the carburetor. He compared the
results obtained from the experiment and prediction of the steady state model. The uncertainty in
the measurement was found to be ±2 cm3
/min. These results indicated that the model was
successful in showing the effects of the pressure drop and the metering elements in the emulsion
tube. He also studied the quasi steady state and dynamic model.
1. Both the steady and dynamic models were used to study the effect of different geometry
and physical properties of fuel and air flow.
2. He also used the models to calculate the gravitational and frictional pressure drop across
the carburetor.
3. He developed an experimental set up to access the validity of the two phase flow models
for both horizontal and vertical pipes.
4. He studied the effect of various parameters on the discharge coefficient. The parameters
include the mesh sizes in case of small orifices and chamfered inlet and outlet etc.
5. He studied the effect of mesh size on the velocity profile of the square edged orifices.
6. He studied the effect of inlet and outlet chamfers on the static pressure.
Conclusion
From the above analysis the conclusions obtained are
1. When the flow inside the carburetor was analyzed for different angles of throttle plate
opening, it was found that the pressure at the throat of the venturi decreased with the
increase in opening of the throttle plate. Because when the throttle plate opening
increases then the flow of air through the carburetor increases but the fuel flow remains
constant. So the mixture becomes leaner. But as obtained from the analysis above the
pressure at the throat the throat also decreases with increase in opening of the throttle
plate so the flow of fuel from the float chamber into the throat increases and hence the
quality of the mixture tends to remain constant.
2. When analyzed for fuel discharge nozzle angle of 300
, it was observed that the pressure
distribution inside the body of the carburetor is quite uniform which leads to a better
atomization and vaporization of the fuel inside the carburetor body. But in other cases
like where the fuel discharge nozzle angle was 350
, 400
or 450
, the pressure distribution is
quite non-uniform inside the body of the carburetor. So it is concluded that for gasoline
operated engine the optimum fuel discharge nozzle angle is 300