carburetor by nite sh
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MALAVIYA NATIONAL INSTITUTE OFTECHNOLOGY JAIPUR
Nitesh Kumar Jatav
2009UME445
M-2
Carburetor
Experiment of Carburetor Design on a HONDAG200 engine
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Carburetor
Experiment of Carburetor Design on a HONDAG200 engine
ABSTRACT:
Experiment was done on Honda gen -200 at different-2 loads to find out the
design of carburetor. When suction stroke takes place in cylinder of the engine
due to pressure drop at the venturi pressure difference occur b/w main jet and
fuel surface in float chamber so fuel come out of jet and mix with air. Fuel takes
heat from the air and vaporize. This mixture goes into the cylinder and
combustion takes places. In SI engines air-fuel ratio is b/w 8:1 to20:1.Stoichiometric air fuel ratio is about 15.4:1 and best power and best economy
conditions takes place at 12:1 and 16:1 air-fuel ratios respectively. We will
calculate the air fuel ratio and by using that one will find the area of venturi and
main jet. As we know as speed increases suction at venture increases so fuel
consumptions increases. As load increases fuel requirement increases. We
assume that difference b/w the heights of main jet and fuel surface in float
chamber is zero. And we use both actual and approximation formula in which we
assume that air is incompressible.
INTRODUCTION:
A carburetor is a device that blends air and fuel for an internal combustion
engine. It is sometimes shortened to carb in North America and the United
Kingdom [1]. Carburetors commonly used for supplying a combustible mixture of
air and liquid fuel to internal combustion engines, comprise a bowl in which a
supply of the fuel is maintained in the liquid phase and a fuel jet which extends
from the liquid fuel into a passage through which air is drawn by the suction of
the engine cylinders. The pressure difference set up between the carburetor inlet
and the throat of the nozzle is used to meter the appropriate fuel flow for that air
flow. On the suction, or intake stroke of the cylinders, air is drawn over and
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around the fuel jet and a charge of liquid fuel is drawn in, broken up and partially
vaporized during its passage to the engine cylinders. Fuel evaporation starts
within the carburetor and continues in the manifold as fuel droplets move with
the air flow and as liquid fuel flows over the throttle and along the manifold walls.
When such a charge is ignited in the engine cylinder, only that portion of the
liquid fuel which has been converted into the vaporous (molecular) state
combines with the air to give an explosive mixture. The remaining portion of the
liquid fuel which is drawn into the engine cylinders and remains in the form of
small droplets does not explode and impart power to the engine, but burns with a
flame and raises the temperature of the engine above that at which the engine
operates most efficiently.
PARTS OF A CARBURETOR:
Fuel strainer- The strainer consists of a fine wire mesh or other type of filtering
device, cone shaped or cylindrical shaped, function of which is to prevent the
possible blockage of the nozzle by dust particles during the fuel flow.
Nozzle-It is a device designed to control the direction or characteristics ofa fluid flow (especially to increase velocity) as it exits (or enters) an enclosed
chamber or pipe via an orifice.
Float chamber- The function of a float chamber in a carburetor is to supply the
fuel to the nozzle at a constant pressure head. This is possible by maintaining a
constant level of the fuel in the float bowl.
Throttle- The speed and the output of an engine are controlled by the use of the
throttle valve. The more the throttle is closed the greater is the obstruction to the
flow of the mixture in the passage and less is the quantity of mixture delivered to
the cylinders.
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Choke- It is a device that restricts the flow of air at the entrance to the
carburetor, before the venturi. It is added to enrich the mixture during engine
starting and warm-up to ensure a combustible mixture within each cylinder at the
time of ignition.
A Simple Carburetor [2]
Metering system- It is installed
To proportionate the fuel-air mixtureto decrease the pressure at the discharge nozzle
to limit the air flow at full throttle.
Idle system- It is added to meter the fuel flow at idle and light loads. It goes out
of action when the throttle is opened beyond about 20%.
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APPARATUS:
To conduct above experiment following apparatus are required:
a) A stationary petrol engine.b) Manometer (to calculate mass flow rate of air)c) Burette ( to calculate fuel consumption)d) Voltmetere) Ammeterf) Bulbs for applying load.
PROCEDURE:
A.Start the engine and wait for some duration for it to attain steadycondition.
B. Vary the load by lightening bulbs of different wattage as per therequirement.
C. Record water head by manometer to calculate mass flow rate of air andalso record fuel consumption.
D.Note voltage and current by voltmeter and ammeter and record enginespeed by rpm meter.
E. Now calculate the fuel consumption.
BASIC FORMULAE DERIVATION [3]
Applying the steady flow energy equation to sections A-A and B-B per unit mass
flow of air: (1) 21
2
2122
1CChhwq
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Here, q and w are the heat and work transfers from the entrance to the throat
and h and C stand for enthalpy and velocity respectively.
If we assume reversible adiabatic conditions, and there is no work transfer, q=0,
w=0, and if approach velocity C10 we get
(2)
Assuming air to be a perfect gas, we get
Tch p Then
(3)
If we assume that the distance from the inlet to the venture throat is short, we
can consider it to be isentropic in the ideal case,
(4)
(5)
Substituting for T1 T2 from Eq. 5 in Eq. 3, we get
)6(12
1
1
2
12
p
pTcC p
By the continuity equation we can write down the theoretical mass flow rate of
air )7(222111.
CACAm a
Where A1 and A2 are the cross-sectional areas at the air inlet (point 1) and venturithroat (point 2).
To calculate the mass flow rate of air at the throat, we have assumed the flow to
be isentropic till the throat so the equation relating p and v (or ) can be used.
)8(2211
Avpvp
212 2 hhC
212
2 TTcC p
1
1
2
121
1
1
2
1
2
1p
pTTT
p
p
T
Tthen
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)8(2
2
1
1 Bpp
)9(12
1
1
2
12
1
1
2
1
.
p
pTcA
p
pm pa
For a perfect gas we have )9(1
1
1A
RT
p
Thus )10(12
1
1
2
12
1
1
1
1
2. Ap
pTcART
p
p
pm pa
And rearranging the above equation we have
)10(2
1
1
2
2
1
2
1
12.
Bp
p
p
pc
TR
pAm pa
Since the fluid flowing in the intake is air, we can put in the approximate values ofR = 287 J/kgK, cp= 1005 J/kgK at 300K, and = 1.4 at 300K.
)11(1562.0
1562.0
1
12
71.1
1
2
43.1
1
2
1
12.
T
pA
p
p
p
p
T
pAma
Where71.1
1
2
43.1
1
2
p
p
p
p
1
1
2
12
p
p
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Equation 11 gives the theoretical mass flow rate of air. The actual mass flow rate,
am.
can be obtained by multiplying the equation by the coefficient of discharge
for the venturi, Cd,a. Thus
)13(1562.0
1
12
,
.
T
pACm ada
Where )14(.
.
,
a
a
ad
m
mC
The coefficient of discharge and area are both constant for a given venturi, thus
)15(
1
1.
T
pma
Since we have to determine the air-fuel ratio, we have to now calculate the fuel
flow rate. Since the fuel is a liquid before mixing with the air, it can be taken to be
incompressible. We can apply Bernoullis equation between the atmospheric
conditions prevailing at the top of the fuel surface in the float bowl, which
corresponds to point 1 and the point where the fuel will flow out, at the venturi,
which corresponds to point 2. Fuel flow will take place because of the drop in
pressure at point 1 due to the venturi effect. Thus
)16(2
2
21 gzCpp f
ff
wheref is the density of the fuel in kg/m3, Cf is the velocity of the fuel at the exit
of the fuel nozzle (fuel jet), and z is the depth of the jet exit below the level of fuel
in the float bowl. This quantity must always be above zero otherwise fuel will
flow out of the jet at all times. The value of z is usually of the order of 10 mm.
From Eq. 16 we can obtain an expression for the fuel velocity at the jet exit as
)17(2 21
gz
ppC
f
f
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Applying the continuity equation for the fuel, we can obtain the theoretical mass
flow rate,.
fm from
)18(2
21
.
gzppA
CAm
fff
ffff
whereAf is the exit area of the fuel jet in m2.If Cd,fis the coefficient of discharge of
the fuel nozzle (jet)
Then .
21,)20(2 gzppACm ffffdf
Since )21(.
.
f
a
m
m
F
A
Fuel
Air
)22(
21562.0
211
12
,
,
gzppT
p
A
A
C
C
F
A
ffffd
ad
If we put 21 pppa , we get the following equation for the air-fuel ratio
)23(2
,
,
gzp
p
A
A
C
C
F
A
fa
a
f
a
ffd
ad
Where )24(
11
2
1
1
2
1
1
2
2
1
2
p
p
p
p
p
p
Air-fuel ratio neglecting compressibility of air
If we assume air to be incompressible, then we can apply Bernoullis equation to
air flow also. Since initial velocity is assumed zero, we have
)29(2
2
221Cpp
aa
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Thus )30(2 212
a
ppC
Applying the continuity equation for the fuel, we can obtain the theoretical
mass flow rate,.
am , from
)31(2212
22
.
ppA
CAm
a
aa
Where A2 is the venturi in m2.If Cd,a is the coefficient of discharge of the venturi
Then .
212,
.
)33(2 ppACm aada
Since
)34(.
.
f
a
m
m
F
A
Fuel
Air
)35(21
212
,
,
gzpp
pp
A
A
C
C
F
A
ff
a
ffd
ad
If we assume z = 0, then [2]
)36(2
,
,
f
a
ffd
ad
A
A
C
C
F
A
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LINE DIAGRAM OF SETUP:
1) Air inlet orifice2) Carburetor3) Water head measurement through orifice meter.4) Fuel consumption measurement.5) Gasoline engine ( G-200, HONDA 5.0 )6) Generator and bulb setup {for brake power (b.p.) measurement}7)
Rotating disc {for rpm measurement}
Calculation:
Area of main jet and venturi
Measured data:-
S. no. Load
(Watt.)
Fuel
flow
rate
(Ml/s)
Water
head
(cm)
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1 500 .465 11.5
2 1000 .513 12.5
Air head=water head*(pw/pa)
=99.13m
Velocity of air = (2gh)
=44.10 m/s
Ma= cd*paAV
=3.277*10-3kg/s
Mf= .465*740*10-6
=3.44*10-4
A/F ratio=Ma/Mf
=9.5:1
By using the approximation formula
f
av
fd
ad
Aj
A
C
C
F
A
,
,
Av/Aj =186.30
By using actual formula
211
1
,
,
21562.0
ppT
p
Aj
Av
C
C
F
A
ffd
ad
71.1
1
2
43.1
1
2
p
p
p
p
P2= pressure at venturi
P1= atmospheric pressure=1 bar
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From calculation
We get pressure ratio
X=.97
P2=.97 bar
By putting this value in
71.1
1
2
43.1
1
2
1
12.
1562.0
p
p
p
p
T
pAMa
Area of venturi
= 4.03*10-5
m2
Diameter of venturi = 7.163 mm
Area of jet = 2.613*10-7m2
Diameter of jet
= .524mm
DATA USED FOR CALCULATION:
Initial temperature and pressure 300K, 1.013 barP 36.4 mm of HgOrifice diameter 12 mmDensity of fuel 740 kg/m3Density of air 1.12 kg/m3Cd,f 0.66Cd,a 0.63 (orifice) and 0.85 (venturi)
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RESULTS AND DISCUSSIONS:
The average diameter of Venture throat -7.163 mm. The average diameter of fuel jet- .524 mm. Untilfgz pa, (z is nozzle tip) no fuel shall come out of the nozzle to mix
with air. As speed increasespa increases therefore fuel flow rate
increases, more fuel is sucked in.
A choke must be added to enrich the mixture during cold starting andwarm up to ensure that a combustible mixture is provided to each cylinder
at the time of ignition.
At low loads, or 25% to 75% throttle opening, the air-fuel mixture is leaneror near stoichiometric (equivalence ratio 0.9). At higher loads, wide open
throttle, the air fuel ratio is rich (equivalence ratio ~1.1 for best power).
Conclusions:At the time of starting of engine we need a rich mixture therefore we
open the chalk to allow air enter in the carburetor.
While accelerating a rich mixture, throttle opening also increases duringacceleration, but a temporary lean mixture forms, to compensate thisacceleration should be installed.
During throttle opening from 25%to 75% (cruising stage) lean mixture issupplied. Max. Efficiency we get.
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Chalk is needed for rich mixture in cold.Reducing the A/F ratio to 9 to 11, causes unsteady and non-smooth,
combustion of most of the mixture occurs in the exhaust system.
In order to reduce the compensation of fuel, multiple venturi systemcan also be installed.
Mixture-control systems like back suction type, needle type, air-porttype can also be installed.
REFERENCES:
www.wikipedia.org, Carburetor. Ganesan, V., Text book of I.C ENGINES. Prof. Poonia, M.P, Class notes I.C. Engines, 2011 www.ieee.org
http://www.wikipedia.org/http://www.wikipedia.org/http://www.ieee.org/http://www.ieee.org/http://www.ieee.org/http://www.wikipedia.org/