11 carburetor

8/3/2019 11 Carburetor

http://slidepdf.com/reader/full/11-carburetor 1/40

1



2

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

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

conditions.

Loads

Starting

IdlingCruising

Accelerating



3

Petrol & Diesel Engines



4

Functions

It must run the engine smoothly bysupplying 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 underall load conditions and speed of theengine.



5

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



6

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.



7

Air-Fuel Mixtures

VaryLoad/speed

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



8

Variation of power output and sfc with A-F ratio in SI engine(Full throttle and constant speed)

Maximum Output = 12:1 (Best power mixture)

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



9

Various Loads

Idling/Starting: Engine runs without load.Produces power only to overcome friction

between 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.



10

Starting a Cold Engine

When an engine is cold, a very small % fuel

will vaporize in the intake and compressionprocess. The fuel is also cold, and much moreviscous, creating a lower flow rate. The enginemetal parts are cold and inhibit vaporization.Further, during the compression stroke, coldcylinder walls will absorb heat and reducevaporization. Engine lubrication is cold andmore viscous, making the engine turn moreslowly in the starting process.



11

Carburetor Performance



12

Simple Carburetor

Fuel

Float

Vent

FloatChamber

Throttle

Fuel dischargenozzle

Fuel metering jet lip, h

Choke

Air

InletValve

Fuelfromsupply



13

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

A choke valve to control the air supply inorder 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 quantity

of the mixture at different load conditions

Components of a Simple Carburetor

Fuel

Float

Vent

Float

Chamber

Throttle

Fuel dischargenozzle

Fuel metering jet lip, h

Choke

Air

InletValve

Fuelfromsupply



14

Venturi-type Carburetor

P+1/2 ρV2 = Constant

Bernoulli Effect:

Valve StemFuel Inlet

Float

Metering Orifice

Throttle Plate

Air/Fuel Mixture To Engine

Choke Plate

FuelNozzle

Inlet Air

Bowl

Atomized Fuel

Venturi



15

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, pressuredecreases and the fuel comes out in the form of

a jet. The fuel gets mixed with air and goes intothe cylinder.

Operation



16

Because of the narrow passage at the venturithroat, the air velocity increases but its pressure falls.This causes a partial vacuum (called carburetor

depression) 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



17

A simple carburetor as described

suffers 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 isopened less or more.

Drawback of Simple Carburetor



18

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 usuallyincreases engine speed. However, as

load is also a factor (e.g., climbing anuphill), opening the throttle may notincrease the speed.



19

Calculation of Air-Fuel Ratio

Let the tip ofthe fuel nozzlebe at a height zfrom fuel levelin the floatchamber.

i.e., we need tocalculate

m A a

F m f

=



20

2 1 2 1

1 2 2

( )2q 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

= −

= −



21

1 1 1 2 2 2

1 1 2 2

1 2

m A C A C a A C A C

ma v v

ρ ρ = =

= =

Since mass flow is constant

Also,1 1 2 2

1 2

12

k k p v p v

k v p

k pv

=

=

1

12 1

21

1 12

1 2

k pv v

p

k RT pv

p p

∴ =

=

We have



22

1

2

11 11 1

1 2

2 22

2 2 1

A C ma v

k k A p

m C T a p pk RT p

p p

gives∴ =

−

= −

Finally, we have12

2 2

1 1

2 1( ) 2

1

k k k A p p p

m C a theoretical p p p R T

+

= −

( ) ( )m C ma actual d a theoreticalt

=

argwhere C coefficient of disch e of venturi throat d

t

=



23

To find mass flow rate of fuel

1 2 2

2

2 p p C z

gγ γ = + +

Assuming fuel to be incompressible, we havefrom Bernoulli’s theorem

1 2

2

2

f

f f

C p p

gz ρ ρ ∴ − = +

1 22 f

f

p pC gz

ρ

−∴ = −

ρρρρf being the density of fuel, Cf is the fuel velocityat the nozzle exit and z is the nozzle lip.



24

Thus, we have velocity of fuel at the nozzle exit

argwhere C coefficient of disch e of fuel nozzled f

=

1 2

( )

( ) 2 ( )

f f

f

f f

f f f

m A C theoretical

m A p p gztheoretical

ρ

ρ ρ

=

∴ = − −

( ) ( ) f f m C mactual d theoretical

f

∴ =

1 22 f

f

p pC gz

ρ

−∴ = −



25

A simple carburetor is capable to supply acorrect air-fuel mixture to the engine only at a

particular load and speed. In order to meet theengine 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



26

During startingor idling, engineruns without loadand the throttlevalve remains inclosed position.

Engine producespower only toovercome frictionbetween the parts,

and a rich mixtureis to be fed to theengine to sustaincombustion.

Idling system

Idling jet

Air bleed

Adjustingscrew

Float chamber

Main jet

Air

Throttle



27

The idling system asshown consists of an idlingfuel passage and an idlingport. When the throttle is

partially 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 withfuel. The fuel flow dependson the location of the idlenozzle and the adjustmentof the idle screw.

Idling system – contd.

Idling jet

Air bleed

Adjusting

screw

Float chamber

Main jet

Air

Throttle



28

During normal power or

cruising 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, therebydecreasing the fuel flow rate.

Auxiliary port system

Butterfly

valve

Throttle

Main jet

Air

Air+

Fuel



29

In order to obtainmaximum power, the

carburetor must supplya rich mixture. Thisadditional fuel requiredis supplied by a power

enrichment system thatcontains a meter rodeconomizer thatprovides a larger orifice

opening to the main jetas the throttle is openedbeyond a certain point.

Power enrichment system



30

During suddenacceleration of anengine (e.g., overtakinga vehicle), an extra

amount of fuel ismomentarily required tosupply a rich mixture.This is obtained by anaccelerating pumpsystem. It consists of aspring-loaded plunger,and the necessarylinkage mechanism.

Accelerating pump system

Pump

Open

Plunger

Floatchamber

Accelerating pump system

The rapid opening of the throttle moves theplunger into the cylinder, and an additionalamount of fuel is forced into the venturi.



31

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



32

Up-draught (updraft) carburetor

Down-draught (downdraft) carburetor

Cross-draught or horizontal carburetor

Types of Carburetorbased on direction of flow



33

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 with

alternate cylinders in the firing order. As forexample, in a six cylinder engine, one barrelsupplies mixture to cylinders 1, 3 and 2; whilethe other barrel supplies to 5, 6 and 4.



34

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.



35

Enrichment

(due to variation of air density)

If ρρρρ/ρρρρ0 = 0.84,

0

0

0

0

0

1

1

1

E

p RT E

RT p

p T

E pT

ρ

ρ + =

+ =

+ =

1

1 1.0910.84

0.091 9.1%

E

E

+ = =

= =

Enrichment of mixture over the calibrated ratio



36

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



37

Basic Carburetor

1. The carburetor is adevice which mixes airand fuel in areciprocating internalcombustion engine.Carburetors are still

found in small enginesand in older orspecialized automobilessuch as racing cars.However, most cars

built since the early1980s use fuel injectioninstead of carburetion.

Summary



38

Summary2. Most carbureted (as opposed to fuelinjected) engines have a single carburetor,though some, primarily higher performanceengines, can have multiple carburetors. Mostautomotive carburetors are either downdraft (flow of air is downwards) or side-draft (flow ofair is sideways). In the United States, downdraft

carburetors were almost ubiquitous, partlybecause a downdraft unit is ideal for Vengines. In Europe, side-draft carburetors aremuch more common in performance

applications. Small propeller-driven flatairplane engines have the carburetor belowthe engine (updraft).



39

!" !"

## $%$% &!! &!! ! "#

'' () () ** (+,-.(+,-. /"./". $%&'()*+,

00 )*+!

11 "2.3"2.3

44 "2.3"2.3 ,-,- !" "-

55 .% .%.. # /

66 -,-%+-,-%+ 0 1!23"#41!

+!!+!! 5 )

% ($% ($ "-2 7% "-2 7%

## ,8,,8,

'' ,%,% !6),6,

** "$ "$ $ 7" !')86+9!*



40

-9::222:: -9::;:# -9::222<' -9::222* -9::222<<:0 -9::222-2<<2!

1 -9::2224 -9::222:::'5:"2=*3=*5 -9::2228:<8:66':,< 6 -9::222::'*1- -9::222:::'64: -9::2

# -9::222::>::0' -9::?":,>* -9::222!:@!::46:=664=60 -9::8!":A,6:++:1 -9::222:":2:4 -9::<2

5 -9::<!:"<6 -9::222<:>>:66:<':: -9::222 -9::-"::-"'6# -9::2:@"-:66:664

11 carburetor

Documents