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Chapter 2 Carburetor

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  • 8/7/2019 Chapter 2 Carburetor


    Fuel Pump and Carburetor



    2.1 Fuel Pump

    1. Mechanical type fuel transfer pumpThe fuel system of a vehicle is operated by an eccentric, mounted on a

    camshaft of an engine. The pump consists of a spring loaded flexible diaphragm

    actuated by a rocker arm. The rocker arm is actuated by the eccentric. Spring loaded

    valves are there in the inlet and outlet of the pump. These valves ensure flow of fuel

    in the proper direction.

    As the rocker arm is moved by the eccentric, the diaphragm is pulled down against

    the spring force. This movement causes a partial vacuum in the pump chamber. Now

    the delivery valve remains closed and the suction valve opens. This admits fuel into

    the pump chamber. At the maximum position of the eccentric, the diaphragm is

    flexed to the maximum extent. After this, further rotation of the eccentric will release

    the rocker arm. Now the rocker arm will simply follow the eccentric by the action of

    the return spring. The diaphragm spring will now push the diaphragm upwards and

    force the fuel to flow out, opening the delivery valve, into the delivery tube. Now the

    suction valve remains closed.

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    Fuel Pump and Carburetor


    2. Electromagnetic fuel pump

    An electromagnetically operated fuel pump is shown in figure below. The

    valves diaphragm and push pull rod are similar to the mechanical pump but a

    solenoid, an armature, a toggle switch and electrical contacts are new features.

    In this arrangement, a diaphragm is operated by means of a solenoid

    armature assembly. A contact breaker toggle switch is connected to the electrical

    system of the vehicle. A break in the contact causes solenoid to attract the armature

    and induces intake stroke. During this stroke the diaphragm is pulled by the push pull

    rod, a vacuum is created inside the pump chamber, intake valve opens and the fuel

    flows into it from the tank. There after, when the toggle switch makes contact, the

    solenoid releases the armature, and fuel is delivered under the influence of

    diaphragm spring pressure. The sequence of opening and closing of inlet and outlet

    valves is similar to that in the mechanical pump.

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    Fuel Pump and Carburetor



    Functions of a carburetor:

    a. It maintains a small quantity of petrol in the float chamber at constant head

    (height) to ensure uninterrupted supply for vaporization.

    b. It vaporizes (atomizes) petrol, i.e. it converts liquid petrol to vapor form for

    convenient mixing with the air.

    c. It does carburetion i.e. prepares a homogeneous mixture of air and vapor


    d. It delivers correct air-fuel mixture to the engine through the manifold under

    varying conditions of load and speed of the engine.

    A simple carburetor consists of a float chamber, float, needle valve, jet nozzle,

    mixing chamber, venturi, throttle and a choke. The float is a hollow and light weight

    part made of thin metal sheet. The float chamber maintains the fuel at a constant

    level which is necessary for normal operation of a carburetor. The float chamber is

    vented through a hole to communicate with the atmosphere. When the fuel level

    sinks, the float goes down, opens the needle valve and admits fuel into the chamber.

    And when the fuel level reaches its normal level, the float goes up, closes the needle

    valve and stops inflow of the fuel. A normal level is reached when the fuel in the

    chamber is 1-2 mm below the edge of the nozzle. This level ensures easy suction of

    fuel from the nozzle and prevents leakage when the carburetor is inoperative.

    The jet tube - with a calibrated hole of definite diameter meters out the amount of

    fuel to be supplied. The pulverizer which takes the form of a pin tube communicates

    with float chamber through the jet. Mixing chamber is straight or bent tube one of

    whose ends is connected to the engine inlet pipe and the other to the air cleaner.

    The fuel is mixed with air precisely in this chamber. Venturi mounted in the mixing

    chamber at the end of the nozzle increases the velocity of the air stream in the

    mixing chamber and there by provides a more intensive atomization of fuel. Throttle

    changes the cross section presented to the combustible mixture. The throttle is

    controlled by the driver from the cab. The degree to which the throttle is opened

    determines the amount of mixture passed and accordingly changes the power of the

    engine. A simple carburetor is shown in the figure below.

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    Fuel Pump and Carburetor


    The figure above shows the different types of mixing chamber. The most commonly

    used mixing chamber is the down draught type mixing chamber.

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    2.2.1 Requirements of air fuel mixture and their properties

    S/No. Requirements Proportion

    of petrol air

    by weight






    Specific fuel



    1 Cold starting 1:7 to 1:8 Very rich Less than


    Very high Much carbon


    2 starting 1:8 to 1:10 richer Less higher More carbon


    3 Idling 1:10 to 1:12 rich Less high Less smoke

    4. City driving 1:11 to 1:14 Less


    Maximum power Lower Good driving

    5 Metro driving 1:15 Correct



    About 5% less

    than the


    Much lower Best


    maximum CO2 in


    6 Continuous

    driving on


    1:16 to 1:18 Weaker About 10% less

    than the


    Minimum Maximum CO2 in


    7 Quick


    1:7 to 1:9 Very rich Maximum Very high Much smoke

    8 Continuousdriving on

    express way

    1:18 to 1:21 Veryweak

    About 30% lessthan the


    Lowest Causes back fireand erratic


    Limitations of simple

    carburetor and methods to

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    Fuel Pump and Carburetor


    overcome them

    A simple carburetor works well when the engine runs at a preset speed and

    the stipulated load. It isbecause thenozzle and the venturi dimensions are set for a

    particular speed and the load. But working of carburetor at a single condition is


    The construction of a simple carburetor cannot fulfill the fuel demand at different

    loads, changing speeds and varying weather conditions. Due to these limitations, a

    simple carburetor cannot be used in actual practice. To make it suitable for

    commercial applications, its limitations are to be overcome. For that various

    arrangements are incorporated in its construction. These arrangements and

    difficulties (limitations) overcome by them are given as below.

    Difficulties to be

    encountered at

    Provisions / arrangements incorporated

    Cold start Ticker, Choke, Adjustable jet area,

    Auxiliary air bypass

    Idling Auxiliary (Idling) passage

    Low speeds Compensator jet

    High speeds Extra air valve, compensatory (compound) jet,

    Air bleeder compensation, Multi jet compensation

    Changing the venturi area by Suction controlled


    Acceleration Auxiliary pump with an acceleration jet.

    Higher altitude An automatic valve regulation to help in maintaining

    less than atmospheric pressure in the float chamber

    Varying weather condition Weather control devices, by varying petrol jet area, by

    varying air intake area.

    Icing condition Heating of carburetor port and valve by engine

    exhaust gasses

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    2.3.1 Provisions to Overcome Starting Difficulties:

    Tickler: (Fig.1) is a spring controlled

    pin used to strike-on the float which in

    turn, pushes the additional quantity of

    petrol to the jet. Invariably during cold

    starting and many a times during non-

    cold starting, the tickler is pressed to

    and fro a fewtimesto cause flooding of

    the carburetor. Thus the mixture

    becomes very rich, ignites easily, and

    the engine starts quickly.

    Choke: (Fig.2)

    Choke is the name given to a butterfly valve (or choke valve) which normally remains

    in the dotted position when the choke is not used, but turns about its pivot when the

    choke is operated. In doing so, it chokes the air passage and allows a very small

    quantity of air to flow past itself. A small quantity of fuel issues out of the jet due to

    the vacuum produced in the cylinder on cranking. The proportion of this small

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    Fuel Pump and Carburetor


    quantity of petrol forms a rich to very rich mixture with a very small quantity of air.

    Hence the mixture ignites without trouble and the engine starts.

    The choke valve is fitted on the top of the air horn with its pivot located eccentrically

    (fig.2a) or in the centre (fig.2b). The eccentrically mounted choke opens easily and

    immediately due to a twisting moment produced by unequal forces on AB and BC

    sides of the choke. If a centrally mounted choke is used, a strangler valve is also

    employed on it (Fig. 3). This helps in avoiding the mixture from becoming over-rich

    by allowing air to pass through it when the engine is started.

    Adjustable Jet Area: In some designs, an adjusting screw having a long

    tapered end is used to vary the area of fuel (nozzle) jet. When the engine is to

    started, the screw is loosened so as to increase the jet area. This helps in supply of

    increased quantity of petrol, thereby offering a rich mixture for easy start.

    Auxiliary Air By-pass: An auxiliary air passage is also provided in some

    carburetors so as to reduce the quantity of air mixing with the atomized petrol. The

    extra air flows down the throttle valve.

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    2.3.2 Provisions to over come difficulties at Idling and Low speeds:

    Idling Passage: During idling of an engine, the suction developed inside the

    engine is not sufficient to draw fuel from the main nozzle. Since engine remains

    operative and throttle valve remains slightly opened in this condition, therefore fuel is

    supplied through an idling hole, which is provided above the air screw hole.

    Compensator Jet: When engine operates at low speeds at the throttle is wide

    open, the suction produced within engine is not very strong. Hence petrol is lifted

    from compensator nozzle 1 and also from the main nozzle 2. The main nozzle is

    being fed from open well 3 as shown in figure above. The action of compensator

    nozzle holds down the supply of petrol and so the open well is emptied.

    2.3.3 Provisions to Overcome Difficulties at Higher speeds:

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    Compound Jet. In this case a compensatory jet is also provided for supply of

    petrol in addition to a main jet (nozzle) the provision also involves an open well which

    is vented to atmosphere. Its use is required when the engine is not running. In that

    case the level of petrol in it remains the same as that in the float chamber, and petrol

    does not flow via the compensatory jet. The arrangement is known as compound jet

    because it involves use of two jets: main jet and the compensatory (or submerged)


    While operating at high speeds, the suction is greatly increased as indicated by

    several arrows in Figure. Now supply of petrol is maintained mainly through the

    main nozzle, however the auxiliary nozzle still gives the same measured quantity of

    petrol because of the suction of compensatory


    The quantity of petrol supplied by the two jets is a function of throttle opening. At

    lesser throttle opening the supply is mainly through the compensating jet which

    diminishes with increase in the throttle opening. The supply from main jet is just

    opposite to it. It increases with an increase in the throttle opening. Since flow of air

    increases at higher speeds but the flow of petrol from compensatory jet remains

    constant, therefore a weaker mixture is the outcome for vehicles requirement. A

    compound jet arrangement has been incorporated in Zenith carburetor.

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    2.3.4 Provisions to Overcome Acceleration Difficulties:

    Quite often we need sudden acceleration of the vehicle for which the throttle issuddenly opened. This causes a large quantity of air to rush into the carburetor at

    once. However, the petrol through the main jet does not flow immediately. Its effect

    is a weak mixture due to lagging petrol supply which causes spluttering and so the

    acceleration to the vehicle is hampered.

    To overcome this difficulty and to avoid spluttering, a reciprocating pump is installed

    to supply extra petrol for a short while until the enhanced petrol supply starts through

    the main nozzle. This pump consists of a piston-cylinder assembly and a spring,Refer to the figure above. It sucks petrol from the float chamber via a ball valve and

    delivers beyond the throttle valve through an acceleration jet. Its piston is connected

    to the accelerator pedal through the piston rod and/or linkages.

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    In normal state (released pedal position as shown in figure, the piston sucks the

    petrol from the float chamber, and delivers the same when the accelerator pedal is

    pressed. The stiffness of the spring is such that it gets compressed only when a

    sudden force is applied on the accelerator pedal. Otherwise, it does not influence

    normal working of the accelerator or pedal.

    2.4 Types of carburetor in commercial use:

    1. Constant jet or constant choke carburetor Solex carburetor,

    2. Constant vacuum carburetor SU carburetor,

    1. Constant jet carburetor:

    The fixed jet or fixed choke carburetor incorporates various jets and an accelerator

    pump to alter the mixtures strength according to engine needs. As the air stream

    through the carburetors venture speeds up, the air becomes thinner and without

    some compensating device the mixture would become progressively richer until it

    becomes too rich to burn.

    The fixed jet carburetor solves this problem by air correction, mixing some air with

    petrol before the petrol is drawn into the venture. On most carburetors air correction

    is by means of perforated tube which emulsifies the mixture. The main jet suppliespetrol to a spraying well which contains the perforated tube, closed at the top by a

    calibrated air correction jet. As the engine speed rises and the petrol level in the well

    falls, an increased amount of air is drawn through the series of holes in the tube,

    automatically weakening the mixture.

    The alternative method is to put in a compensative jet in addition to the main jet. As

    the fuel level drops in the well along side the float chamber, air is drawn into the

    compensating jet so that a mixture of air and petrol, instead of petrol alone reaches

    the main discharge point. The weakness of the mixture from the compensating jet

    cancels out the richness of the mixture by the main jet. The size of the main jet is

    usually designed to give the relatively weak mixture necessary for economic cruising.

    To give the richer mixture necessary for full throttle, the fixed jet carburetor has an

    additional jet feeding the main discharge well. This supplements the main jet which

    can be kept small for economy.

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    Fuel Pump and Carburetor


    Figure shows the sectional view of the Solex carburetor

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    Fuel Pump and Carburetor


    Figure above shows approaching full throttle: Fuel is drawn from the main discharge system and fed

    into the venturi.

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    Figure shows the acceleration module: The extra fuel required is supplied by the accelerator pump,

    which also delivers into the venturi

    2.Constant Vacuum SU Carburetor

    The variable-Jet or constant-depression carburetor, like the fixed jet type, has a

    constant level fuel supply, a throttle valve and a venture. The main difference is

    that the size of the venture throat can be varied to maintain an almost constant

    partial vaccum at the fuel-discharge jet.

    A sliding piston controls the area of the venture throat, and the position of the

    piston is determined by the degree of throttle opening. If the throttle is almost

    closed, as when the engine is idling, the flow of air through the venture drops. The

    weight of the piston and its spring causes the piston to fall, leaving only a small gapfor the passage of air.

    When the throttle is opened by depressing the accelerator pedal, the swifter

    passage of air through the venture increases the partial vacuum above the piston.

    This causes the piston to rise, and further increases the flow of air into the engine.

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    The flow of fuel is controlled by a tapered needle attached to the piston and passing

    into the fuel jet. As the piston rises, the needle rises too, allowing more petrol to be

    drawn from the jet. The position of the jet and shape of the needle ensure the

    correct proportion of petrol and air.

    Enrichment of the mixture when accelerating is provided by a damper, which

    slows the rate of rise of the piston when the throttle is opened. This increases the

    partial vacuum at the fuel jet and so provides a temporary enrichment.

    Since the air pressure in the venture remains reasonably constant at any

    given engine speed, there is no need to provide a separate fuel circuit for idling, as in

    the fixed-jet carburetor. The fuel is fed into the air stream at the point of maximum

    velocity, ensuring efficient atomization (breaking up into droplets) of the fuel.

    The idling mixture strength can be altered by an adjusting nut, which controls the jet

    position, and the idling speed is controlled by a throttle-stop screw.

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    Operational details of SU

    Constant depression carburetor

    G Air valve, P Piston,

    R Piston rod, S Suction chamber,

    V Pair of holes A hole,

    J Jet sleeve, N Needle,

    B Flanged bush.

    The SU carburetor type HS, without its

    extra emission control features, is

    illustrated in the figure. An air valve G is

    integral with the piston P and securely

    attached to the piston rod R, which slides

    in the close-fitting bore of the suction

    chamber S. The depression just

    downstream of the air valve is transmitted

    through a pair of holes, V, to the suction chamber, above the piston. Because of the very low rate of

    the return spring, the load to be supported by the vacuum is practically constant, being mainly the

    weight of the piston. The underside of the piston is vented through hole A to the entry to the


    Also rigidly attached to the piston assembly is the tapered needle N which therefore, as the piston

    moves, simultaneously varies the annular orifice between itself and the jet. The jet sleeve J can beraised or lowered for overriding the setting, for idling or starting. This is done by means of a cam and

    link connection to the throttle and cold start controls. Petrol is delivered through a nylon tube to the

    lower end of the jet sleeve, from the float chamber which can be bolted on to either side of the

    carburetor body, to suit the particular installation.

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    Fuel Pump and CarburetorExercise 2

    1. Explain with a neat sketch Working of a) Mechanical pump

    b) Electromagnetic Pump

    2. What are the functions of carburetor?

    3. What are the requirements of air fuel mixture and their properties?

    4. What are the Provisions to Overcome Starting Difficulties? Explain

    5. What are the Provisions to Overcome Acceleration Difficulties? Explain

    6. What are the Provisions to Overcome Difficulties at Higher speeds? Explain

    7. Draw and explain working of Constant jet or constant choke carburetor

    8. Explain with a neat sketch working of Constant vacuum carburetor