Fuel Pump and Carburetor

FUEL PUMP AND CARBURETOR 2.1 Fuel Pump 1. Mechanical type fuel transfer pump The 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 2.2 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 petrol 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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Fuel Pump and Carburetor 2.2.1 Requirements of air fuel mixture and their properties S/No. Requirements

1

Cold starting

Proportion

Quality

Power

Specific fuel

of petrol air

of

produced

consumption

by weight

mixture

1:7 to 1:8

Very rich

Less than

Very high

optimum 2

starting

1:8 to 1:10

richer

Less

Remarks

Much carbon deposits

higher

More carbon deposits

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

Correct

About 5% less

Much lower

Best

or

than the

compromise,

optimum

maximum

maximum CO2 in

rich 5

Metro driving

1:15

exhaust 6

7

Continuous

1:16 to 1:18

Weaker

About 10% less

driving on

than the

highway

maximum

Quick

Minimum

Maximum CO2 in exhaust

1:7 to 1:9

Very rich

Maximum

Very high

Much smoke

1:18 to 1:21

Very

About 30% less

Lowest

Causes back fire

weak

than the

and erratic

maximum

running

acceleration 8

Continuous driving on express way

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 is because the nozzle and the venturi dimensions are set for a particular speed and the load. But working of carburetor at a single condition is impracticable. 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

Provisions / arrangements incorporated

encountered at 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 devices.

Acceleration Higher altitude

Auxiliary pump with an acceleration jet. 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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Fuel Pump and Carburetor

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 noncold starting, the tickler is pressed ‘to and fro’ a few times to 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.

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The proportion of this small

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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Fuel Pump and Carburetor

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: 9

Fuel Pump and Carburetor 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) jet. 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

nozzle.

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 vehicle’s requirement. A compound jet arrangement has been incorporated in Zenith carburetor.

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Fuel Pump and Carburetor

2.3.4 Provisions to Overcome Acceleration Difficulties:

Quite often we need sudden acceleration of the vehicle for which the throttle is suddenly 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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Fuel Pump and Carburetor 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 mixture’s strength according to engine needs. As the air stream through the carburetor’s 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 supplies petrol 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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Fuel Pump and Carburetor

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 gap for 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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Fuel Pump and Carburetor 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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Fuel Pump and Carburetor

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Fuel Pump and Carburetor

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Fuel Pump and Carburetor 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 carburetor. 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 be raised 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 Carburetor Exercise 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

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