Qip Ice 11 Carburetor

8/10/2019 Qip Ice 11 Carburetor

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Internal Combustion Engines

Lecture-11

Ujjwal KSaha, Ph.D.

Department of Mechanical EngineeringIndian Institute of Technology Guwahati

Prepared under

QIP-CD Cell Project


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Carburetion

The process of mixture preparation in an SIengine is called carburetion. This air-fuel mixtureis prepared outside the cylinder in a device

called CARBURETOR.

The carburetor atomizes the fuel and mixeswith air in different proportions for various LOAD

conditions.

Loads

Starting

Idling

Cruising

Accelerating


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Petrol & Diesel Engines


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Functions

It must run the engine smoothly by

supplying a correct mixture strength.

It must atomize, vaporize and mix thefuel homogeneously with air.

It must supply correct amount of air-fuel mixture in correct proportion under

all load conditions and speed of theengine.


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Factors affecting Carburetion

the quality of the fuel supplied

the time available for mixture preparation

the temperature of the incoming air

the engine speed

the design of the carburetor


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Remark

The temperature affects the vaporization offuel. High temperature leads to high rate of

vaporization. This is achieved by heating theinduction manifold in some cases. However,this causes a reduction in the power outputbecause of decrease in mass flow rate.

For high speed engines (3000 rpm), the timeavailable for mixture preparation is very small(0.02 sec).

The design of carburetor, as such, is verycomplicated because the optimum air-fuelratio varies over its operating range.


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Air-Fuel Mixtures

VaryLoad/speed

Chemically Correct (15:1) Rich Mixture (10:1) Lean Mixture (17:1)


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Variation of power output and sfc with A-F ratio in SI engine(Full thro ttle a nd c onsta nt sp eed )

Maximum Output = 12:1 (Best power mixture)

Minimum Fuel Consumption = 16:1(Best economy mixture)


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Various Loads

Idling/Starting: Engine runs without load.Produces power only to overcome frictionbetween the parts. Rich mixture is required tosustain combustion.

Normal Power/Cruising/Medium Load: Engine

runs for most of the period. Therefore, fueleconomy is maintained. Low fuel consumptionfor maximum economy.Requires a lean mixture.

Maximum power/Acceleration: Overtaking avehicle (short period) or climbing up a hill (extraload). Requires a rich mixture.


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Carburetor Performance


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Simple Carburetor

Fuel

Float

Vent

FloatChamber

Throttle

Fuel dischargenozzle

Fuel meteringjet lip, h

Choke

Air

InletValve

Fuelfromsupply


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A float chamber with a float tostore fuel and to adjust its level

A choke valve to control the air supply in

order to provide a rich or a lean mixture

A round cylinder with a venturifor atomization of fuel.

A fuel nozzle to atomize and produce aspray of fuel

A throttle valve to supply varying quantityof the mixture at different load conditions

Components of a Simple Carburetor

Fuel

Float

Vent

FloatChamber

Throttle

Fuel dischargenozzle

Fuel metering

jet lip, h

Choke

Air

InletValve

Fuelfromsupply


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Venturi-type Carburetor

P+1/2 V2 = Constant

Bernoulli Effect:

Valve StemFuel Inlet

Float

Metering Orifice

Throttle Plate

Air/Fuel Mixture To Engine

Choke Plate

Fuel

Nozzle

Inlet Air

Bowl

Atomized Fuel

Venturi


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The fuel supply to the float chamber iscontrolled by the action of the float and the

attached fuel supply valve. During the intake orthe suction stroke of the engine, the pistonmoves from TDC to BDC, and creates a vacuumin the space above it and in the suction

manifold. Due to this fall in pressure, theatmospheric air rushes into the carburetor. Nearthe venturi, velocity increases, pressure

decreases and the fuel comes out in the form ofa jet. The fuel gets mixed with air and goes intothe cylinder.

Operation


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Because of the narrow passage at the venturithroat, the air velocity increases but its pressure falls.This causes a partial vacuum (ca l led carbure to rdepression)at the venturi throat. This carburetordepression causes fuel to come out as jet in theform of a spray. This fuel spray vaporizes and mixeswith the incoming air, and the mixture goes into thecylinder through the throttle valve.

Operation

Fuel

Higher Pressure

Outside Engine

VenturiChoke Throttle


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A simple carburetor as describedsuffers from the fact that it provides therequired air-fuel ratio only at onethrottle position.

At all other throttle positions, themixture is either leaner or richerdepending on whether the throttle is

opened less or more.

Drawback of Simple Carburetor


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Drawback of Simple Carburetor

Throttle opening changes the velocityof air. The opening changes the

pressure differential between the floatchamber and venturi throat, andregulates the fuel flow through thenozzle.

Increased throttle opening gives arich mixture. Opening of throttle usually

increases engine speed. However, asload is also a factor (e.g., climbing anuphill), opening the throttle may notincrease the speed.


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Calculation of Air-Fuel Ratio

Let the tip ofthe fuel nozzlebe at a height z

from fuel levelin the floatchamber.

i.e., we need tocalculate

mA a

F mf

=


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2 1 2 1

1 2 2( )

2

q w h h c c = +

Applying SFEE between A-A (point 1) and B-B

(point 2) and considering unit mass of airflow

For adiabatic flow,

10, 0, 0q w and c= = We have,

2 1 2

2 1 2

2( )

2 ( )

c h h

c C T T p

= =


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1

211

11 1

1 2

2 2

2

2 2 1

A Cma

vk

kA pm C Ta p

pkRT p

p p

gives =

=

Finally, we have12

2 2

1 1

2 1( ) 2

1

kk kA p p p

m Ca theoretical p p pR T

+

=

( ) ( )m C ma actual d a theoreticalt

=

argwhere C coefficient of disch e of venturi throatdt =


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To find mass flow rate of fuel

1 2 2

2

2

p p C

zg = + +

Assuming fuel to be incompressible, we havefrom Bernoullis theorem

1 2

2

2

f

f f

Cp p

gz = +

1 2

2ff

p p

C gz

=

fbeing the density of fuel, Cfis the fuel velocity

at the nozzle exit and z is the nozzle lip.


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Thus, we have velocity of fuel at the nozzle exit

argwhere C coefficient of disch e of fuel nozzled

f

=

1 2

( )

( ) 2 ( )

ff

f

f f

f f f

m A Ctheoretical

m A p p gztheoretical

=

=

( ) ( )f f

m C mactual d theoreticalf

=

1 22f

f

p pC gz

=


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A simple carburetor is capable to supply acorrect air-fuel mixture to the engine only at aparticular load and speed. In order to meet the

engine demand at various operating conditions,the following additional systems are added to thesimple carburetor.

idling system

auxiliary port system

power enrichment by economizer system accelerating pump system

choke

Complete Carburetor


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During startingor idling, engineruns without load

and the throttlevalve remains inclosed position.

Engine producespower only toovercome frictionbetween the parts,and a rich mixtureis to be fed to theengine to sustain

combustion.

Idling system

Idling jet

Air bleed

Adjustingscrew

Float chamber

Mainjet

Air

Throttle


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The idling system as

shown consists of an idlingfuel passage and an idlingport. When the throttle ispartially closed, adepression past the throttleallows the fuel to go intothe intake through the idle

tube. The depression alsodraws air through the idleair bleed and mixes with

fuel. The fuel flow dependson the location of the idlenozzle and the adjustment

of the idle screw.

Idling system contd.

Idling jet

Air bleed

Adjusting

screw

Float chamber

Mainjet

Air

Throttle


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During normal power orcruising operation, where theengine runs for most of theperiod, the fuel economy hasto be maintained. Thus, it is

necessary to have lower fuelconsumption for maximumeconomy. One sucharrangement used is the

auxiliary port carburetor asshown, where opening ofbutterfly valve allowsadditional air to be admitted,and at the same timedepression at the venturithroat gets reduced, thereby

decreasing the fuel flow rate.

Auxiliary port system

Butterflyvalve

Throttle

Main jet

Air

Air +Fuel


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During sudden

acceleration of anengine (e.g., overtakinga vehicle), an extraamount of fuel is

momentarily required tosupply a rich mixture.This is obtained by anaccelerating pumpsystem. It consists of aspring-loaded plunger,and the necessary

linkage mechanism.

Accelerating pump system

Pump

Open

Plunger

Floatchamber

Accelerating pump system

The rapid opening of the throttle moves theplunger into the cylinder, and an additional

amount of fuel is forced into the venturi.


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During cold starting period, at low crankingspeed and before the engine gets warmed up,

a rich mixture has to be supplied, simplybecause a large fraction of the fuel remains inliquid state in the cylinder, and only the vaporfraction forms the combustible mixture with air.The most common method of obtaining this richmixture is to use a choke valve between theentry to the carburetor and the venturi throat.

Remark


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Up-draught (updraft) carburetor Down-draught (downdraft) carburetor

Cross-draught or horizontal carburetor

Types of Carburetor

based on direction of flow


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A single barrel carburetor has one outletconnected to the intake manifold of engine.

Multi-barrel Carburetor

A multi-barrel barrel carburetor is one with twooutlets connected to two intake manifolds ofengine. Such unit is basically one with twocarburetors.

As such, a multi-barrel barrel carburetor hastwo numbers of idling, power and acceleratingsystems, two chokes, two throttles but withalternate cylinders in the firing order. As forexample, in a six cylinder engine, one barrelsupplies mixture to cylinders 1, 3 and 2; while

the other barrel supplies to 5, 6 and 4.


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Automobile carburetors are calibrated atsea-level conditions

Aircraft Carburetors

Lower altitudes (than sea-level): Lean mixture

Higher altitudes (than sea-level): Rich mixture

(emits hydrocarbon, CO)

At higher altitudes, density decreases andhence, the mass flow rate gets reduced.


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Enrichment(due to variation of air density)

If /0 = 0.84,

0

0

0

0

0

1

1

1

E

p RTERT p

p TE pT

+ =

+ =

+ =

11 1.091

0.84

0.091 9.1 %

E

E

+ = =

= =

Enrichment of mixture over the calibrated ratio


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Altitude Compensation Device

Admit more air and less fuel into the

induction manifold.

METHODS

As density decreases, the mass flow rate also

decreases and hence the Power gets reduced.

Reduction of pressure in float chamber Auxiliary air valve/air port

Supercharger


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Summary

2. Most carbureted (as opposed to fuelinjected) engines have a single carburetor,though some, primarily higher performance

engines, can have multiple carburetors. Mostautomotive carburetors are either downdra f t (flow of air is downwards) or side-draf t(flow ofair is sideways). In the United States, downdraftcarburetors were almost ubiquitous, partlybecause a downdraft unit is ideal for Vengines. In Europe, side-draft carburetors are

much more common in performanceapplications. Small propeller-driven flatairplane engines have the carburetor belowthe engine (updraft).

References


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1.1. Crouse WH,Crouse WH, andandAnglin DLAnglin DL, (1985),Automotive Engines,Tata McGraw Hill.

2.2. Eastop TD,Eastop TD, andand McConkey A,McConkey A, (1993), Applied Thermodynamics for Engg.Technologists, Addison Wisley.

3.3. Fergusan CR,Fergusan CR, andand Kirkpatrick ATKirkpatrick AT,, (2001), Internal Combustion Engines, JohnWiley & Sons.

4.4. Ganesan VGanesan V,, (2003), Internal Combustion Engines,Tata McGraw Hill.

5.5. Gill PW, Smith JH,Gill PW, Smith JH, andandZiurys EJZiurys EJ,, (1959), Fundamentals of I. C. Engines, Oxfordand IBH Pub Ltd.

6.6. Heisler H,Heisler H, (1999), Vehicle and Engine Technology,Arnold Publishers.

7.7. Heywood JB,Heywood JB, (1989), Internal Combustion Engine Fundamentals, McGraw Hill.

8.8. Heywood JB,Heywood JB, andandSher E,Sher E, (1999), The Two-Stroke Cycle Engine,Taylor & Francis.

9.9. Joel R,Joel R, (1996),(1996), Basic Engineering Thermodynamics,Addison-Wesley.

10.10. Mathur ML, and Sharma RP,Mathur ML, and Sharma RP, (1994), A Course in Internal Combustion Engines,Dhanpat Rai & Sons, New Delhi.

11.11. Pulkrabek WW,Pulkrabek WW, (1997),Engineering Fundamentals of the I. C. Engine, Prentice Hall.

12.12. Rogers GFC,Rogers GFC, andand Mayhew YRMayhew YR, (1992), Engineering Thermodynamics, AddisonWisley.

13.13. Srinivasan S,Srinivasan S, (2001),Automotive Engines,Tata McGraw Hill.

14.14. Stone R,Stone R, (1992), Internal Combustion Engines,The Macmillan Press Limited, London.

15.15. Taylor CF,Taylor CF, (1985), The Internal-Combustion Engine in Theory and Practice,Vol.1 & 2,

The MIT Press, Cambridge, Massachusetts.

References

Web Resources


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1. http://www.mne.psu.edu/simpson/courses

2. http://me.queensu.ca/courses3. http://www.eng.fsu.edu4. http://www.personal.utulsa.edu5. http://www.glenroseffa.org/6. http://www.howstuffworks.com7. http://www.me.psu.edu8. http://www.uic.edu/classes/me/ me429/lecture-air-cyc-web%5B1%5D.ppt9. http://www.osti.gov/fcvt/HETE2004/Stable.pdf10. http://www.rmi.org/sitepages/pid457.php11. http://www.tpub.com/content/engine/14081/css12. http://webpages.csus.edu

13. http://www.nebo.edu/misc/learning_resources/ ppt/6-1214. http://netlogo.modelingcomplexity.org/Small_engines.ppt15. http://www.ku.edu/~kunrotc/academics/180/Lesson%2008%20Diesel.ppt16. http://navsci.berkeley.edu/NS10/PPT/17. http://www.career-center.org/ secondary/powerpoint/sge-parts.ppt

18. http://mcdetflw.tecom.usmc.mil19. http://ferl.becta.org.uk/display.cfm20. http://www.eng.fsu.edu/ME_senior_design/2002/folder14/ccd/Combustion21. http://www.me.udel.edu22. http://online.physics.uiuc.edu/courses/phys14023. http://widget.ecn.purdue.edu/~yanchen/ME200/ME200-8.ppt -

Web Resources

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Qip Ice 11 Carburetor

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