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Part A

Unit 1 Carburetion

Carburettor is a device which atomizes the fuel and mixes it with air. The process of preparing the mixture is called carburetion.

Principle:

During the suction stroke vacuum is created in the cylinder which causes the air to

flow through the carburettor and the fuel to be sprayed from the fuel jets. Because of the

volatility of the fuel, most of the fuel vaporizes and forms a combustible fuel air mixture.

Some of the larger droplets may reach the cylinder in the liquid form and is vaporized andmixed with air during the compression stroke before ignition by the electric spark.

Factors which affect the process of carburetion are:

The time available for the preparation of the mixture. The temperature of the incoming air of the intake manifold. The quality of the fuel supplied.

The design of the combustion chamber.

Properties of the air petrol mixtures:

There is a limited range of air fuel ratios, in a homogenous mixture, which can be

ignited in a SI engine. These limits are about 7:1 air fuel ratio by mass on the rich side and

about 20:1 on the lean side in a single cylinder engines.

Maximum energy is released when slightly excess fuel is introduced so that all the oxygen

present in the cylinder is utilized

Maximum efficiency occurs at a point slightly leaner than the chemically correct air fuel

ratio because excess air requires complete combustion when mixing is not perfect.

Mixture requirement for steady state condition:

For automotive engines there are three main areas of steady state operation requiring

different air fuel ratios.

1. Idling and low load.

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2. Normal power range or cruising range.

3. Maximum power range.

Idling range:

The no load running mode of the engine is called idling. During idling, the air supply

is restricted by the nearly closed throttle and the suction pressure is low. This condition

causes backflow of exhaust gases and air leakage from the various parts of the engine intake

system, which dilutes the mixture.

The above phenomena requires that the air fuel ratios used for idling and low loads,

should be rich for smooth engine operation. Up to this point the amount of fuel burned is

quite small and hence fuel economy is not important. The richening of mixture increases the probability of contact between fuel and air particles and thus improves combustion.

Normal power range or cruising range:

As the load is increased the dilution by the residual gases as well as leakage decreases

and therefore in the normal power range the prime consideration is usually the fuel economy.

The maximum fuel economy is obtained at air fuel ratio up to 17:1.

Maximum power range:

Requirement for maximum power range is a rich mixture. A rich mixture also

prevents over heating of exhaust valve at high load and inhibits knocking. At high load there

is greater heat transfer to engine parts. Enriching the mixture reduces the flame temperature

and the cylinder temperature. Thereby, reducing the cooling problem and lessening the

chances of damaging the exhaust valves.

Mixture requirement for transient condition:

The transient mixture requirements are different from steady state running mixture

because in the transient condition the evaporation of the fuel is incomplete, the quantity of

liquid fuel in the inlet manifold may be increasing or decreasing, and the distribution of the

fuel to various cylinders is different. The principle transient conditions of operation are

staring, warming up, acceleration and deceleration.

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NOTE: fuel which as high vapour pressures and low boiling points are called light ends, and

those which are less volatile are called heavy ends. The temperature of the vapour when 10%

of fuel is vaporized is called the 10% point.

Starting and warm up:

While staring from cold the speed as well as engine temperature is low, hence much

of the heavy ends supplied by the carburettor does not vaporize and remain in liquid form.

The vaporized fuel may recondense on coming in contact with cold cylinder walls and piston

head. Therefore even when the fuel air ratio at the carburettor is well within the normal

combustion limits, the ratio of evaporated fuel to air in the cylinder is too lean to ignite.

Therefore during starting a very rich mixture must be supplied as much as 5 to 10 times the

normal amount of petrol, so that enough light ends are available for proper ignition. As the

engine warms up the amount of evaporated fuel increases and the mixture ratio progressively

gets leaner to avoid too rich evaporated mixture.

Acceleration requirements:

Acceleration is increase in engine speed resulting from opening the throttle. The main

purpose of opening the throttle is to provide an increase in torque and whether or not an

increase in speed follows depends on the nature of the load. During acceleration, the liquid

fuel lags behind and, temporarily, the cylinder receives a lean mixture while a rich mixture is

needed to produce more instantaneous power of acceleration. To compensate for the

temporary leaning of mixture a accelerating pump is introduced.

Carburettor for 4 stroke engine :

This kind of carburettor is called "constant vacuum" but that does not mean that the

absolute vacuum is really constant. In a traditional carburettor, when the throttle opens wide

quickly the venture area increases suddenly. At the same time, the rate of flow induced by the

engine has not increased proportionally, since the engine rpm does not increase as quickly.

By increasing the area exposed to a virtually constant rate of flow, the flow speed decreases

and therefore the pressure increases. The result is that this vacuum signal is weak or is

missing so that we must often return to part throttle to get a decent progression. With the

vacuum carburettor we have two elements to adjust the rate of flow: the throttle valve, of

automotive type, driven by the driver, and the traditional piston valve, with conical needle

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actuated by the vacuum system. This valve is connected to a vacuum chamber by means of a

flexible diaphragm. The vacuum chamber is connected by one or more passages with the

narrow section of the venture, under the piston valve.

The lower part of the chamber is exposed to atmospheric pressure because it's

connected to the air intake of the carburettor. The venture vacuum pulls the valve towards the

top by overcoming the contrast spring. This spring becomes an adjustment component, just as

the diameter of the holes of the valve's vacuum intake which influence the transient response

of the piston valve. As the vacuum increases, the piston valve will be lifted higher.

At partial throttle and closed throttle, the vacuum under the piston valve is low and

therefore the valve is lifted only slightly. When the throttle opens wide, the speed of the

inducted flow increases and the valve starts to lift proportionally. If the throttle is suddenly

wide open, the guillotine doesn't lift equally, but follows on its own the effective progression

of the engine, making it independent of the driver's action. With this device the engine is

always fed always with an optimum rate of flow, because the same aspiration signal actuates

the fuel circuit and modulates the power. If we wish to think of this in a simplified analytical

approach, we can demonstrate that the height (h) of the valve (that we have to distinguish

from the throttle) in a vacuum carburettor is dependent on just a couple of variables.

One variable is the rotation angle of the throttle (a) and the other is the engine speed

(n). This means that the lifting of the valve, and therefore the action of the main circuit, is a

function of the same parameters that determine the delivery in an electronic injection device

(a-n). Depending on these two parameters, the passage areas both of the air (venture) and of

the fuel (conical needle) are managed, by letting the mixture ratio change according to the

operating condition. It is then clear how the vacuum carburettor operates independently from

the throttle opening set by the driver. The fuel delivery and the air passage are not only

functions of the throttle opening, but of the engine speed, while in a traditional carburettor the

only control parameter is the throttle stroke and the engine speed has no effect.

Carburettor for 2 stroke engine :

The throttle late controls the amount of air that flow through the carburettor and mixes

with gasoline. Venture creates vacuum that draws in gasoline from a float chamber, which is

supplied from the gas tank by fuel pump, which keeps gasoline to near atmospheric pressure.

When the throttle plate opens the air flows into the carburettor. The venturi creates the

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vacuum needed to draw in the air and a small amount of gasoline which then mixes with air

which is then further into the combustion chamber of the engine.

To enhance the control of air and gasoline fuel a choke is used. A choke restricts the

flow of air at the entrance of the carburettor. This creates a stronger vacuum which draws in

less air but more gasoline to create a rich mixture which is easier to ignite when the engine is

cold. When engine warms up, less fuel is needed.

Design of a simple carburettor:

The float and a needle valve system maintain a constant height of petrol in the float

chamber. If amount of fuel in the float chamber falls below the desired level the float lowers

there by opening the needle of fuel supply valve when the desired level has been reached, the

float closes the needle valve thus stopping additional fuel from the supply system. Float

chamber is vented to the atmosphere.

During suction stroke air is drawn through the venture. Venture is a tube of

decreasing cross section which reaches a minimum at the throat. The air passing through the

venture increases in velocity and the pressure in the throat decreases. Form the float chamber,

the fuel is fed to the discharge jet, the tip of which is located in the throat of the venture.

Because the pressure in the float chamber is atmospheric and that at the discharge jet below

atmospheric, a pressure differential called carburettor depression exists. This causes

discharge of fuel into the air stream and the rate of flow is controlled or metered by the size

of the smallest section in the fuel passage. This is provided by the discharge jet and size of

this jet is chosen empirically to give the required engine performance. To avoid wastage of

fuel, the level of the liquid in the jet is adjusted by the float chamber needle valve to maintain

the level a short distance below the tip of the discharge jet.

Petrol engine is quantity governed which means that when less power is required at a

particular speed the amount of charge delivered to the cylinder is reduced. This is achieved by means of a throttle valve of the butterfly type which is situated after venture tube. As the

throttle is opened, more air flows through the choke tube, and the power of the engine

increases.

The throttle regulates the amount of air flowing up the venture tube; it also checks the

quantity of fuel issuing from the nozzle by regulating the vacuum at the throat. At low engine

speeds the vacuum at the throat is high and hence too rich mixture is supplied.

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Complete Carburettor:

In order to satisfy the demands of an engine under all operating conditions, the

following additional systems are added to the simple carburettor:

1.

Main metering system.2. Idling system.

3. Power enrichment by economiser system.

4. Acceleration pump system.

5. Choke.

Main Metering System:

It is designed to supply a nearly constant air fuel ratio over a wide range of speeds and

loads, this mixture corresponds approximately to best economy at full throttle. A simple

carburettor tends to enrich the mixture at higher speeds automatic compensating device are

incorporated in the main metering devices are:

1. Use of compensating jet: allows an increasing flow of air through a fuel passage as

the mixture flow increases.

2. Use of emulsion tube for air bleeding. In this device the emphasis is on air bleeding

alone.

3. Use of tapered metering pin that is arranged to be moved in and out of the main or

auxiliary fuel orifice either manually or by means of some automatic mechanism

changing the quantity of fuel drawn into the air charge.

4. Back suction control or pressure reduction in float chamber.

5. Changing the position or jet in the venture. The suction action is highest at the venture

throat, therefore by raising the venture the nozzle relatively moves to points with

smaller suction and the flow of fuel is decreased.

6. Use of an auxiliary air valve or port that automatically admits additional air as

mixture flow increases.

Idling System:

At idling and low load the engine requires a rich mixture. During this the main meter

system fails to send the enriched mixture. For this reason a separate idling jet is added to the

basic carburettor. It consists of a small fuel line from the float chamber to a point a on the

engine side of the throttle. This line contains a fixed fuel orifice. When the throttle is closed,

the full manifold suction operates on the outlet to this jet. In addition, the very high velocity

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past the throttle plate increases the suction locally. Fuel can therefore be lifted by the

additional height up to the discharge point but this occurs only at very low rates of air flow.

As the throttle is opened, the main jet gradually takes over while the jet becomes ineffective.

Power Enrichment or Economizer system:

As the maximum power range of operation is approached, some device must allow

richer mixture to be supplied despite the compensating leanness. Such a device is a meter rod

economiser. It provides a large orifice opening to the main jet as the throttle is opened

beyond a certain point. The rod may be tapered or stepped.

Acceleration Pump System:

The pump consists of a spring loaded plunger. A linkage mechanism is provided so

that an additional jet of fuel fed into the venture. The plunger is raised again against the

spring force when the throttle is partly closed. Arrangement is made in such a way that when

throttle is opened slowly, the fuel in the pump cylinder is not forced into the venture but leaks

past plunger or some holes into the float chamber.

Choke:

At low cranking speed and before the engine has warmed up, a mixture much richer

than usual mixtures must be supplied simply because a large fraction of the fuel will remain

liquid even in the cylinder, and only the vapour fraction can provide a combustible mixture

with the air. The most common means of obtaining this rich mixture is by the use of a choke,

which is a butterfly type of valve placed between the entrance to the carburettor and the

venture throat. By closing the choke, a large pressure drop can be produced at the venture

throat that would normally result from the amount of air flowing through the venturi. This

strong suction at the throat will draw large quantities of fuel from the main nozzle and supply

a sufficiently rich mixture so that the ratio of evaporated fuel to air in the cylinders is within

combustible limits.

Effects of Altitude on Carburettor:

Carburettors utilize a delicate balance of fuel and air mixture to power an internal

combustion engine. The distance at or above the sea level that a carburettor runs affects the

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fuel to air mixture level. Because of this level of the carburettor must be calibrated for

specific distance above sea level at which it runs.

As the vehicle climbs higher up a mountain, the density of air decreases. A carburettor

is affected by decrease in density of air because the air molecules are spread out, providing

less oxygen for combustion. Because oxygen is required to combust fossil fuel, the higher the

elevation in which an engine operates, the more difficult it will become to burn the gasoline

or diesel fuel.

The weight of the air decreases as altitude increases. Most automobile carburettors are

designed at altitudes near sea level. If the vehicle is driven at an altitude lower than the

calibrated altitude a lean mixture is obtained which results in poor driveability. At altitudes

higher than the calibration altitude a rich mixture is supplied which causes increase in

hydrocarbons and carbon mono oxide emissions.

Carburettor system for emission control :

A key part of better controlling emissions from a vehicle's tailpipe is to better control

the fuel/air mixture. A rich fuel mixture will increase the amount of CO 2 in the exhaust, so

efforts were made to improve the carburettors ability to maintain proper fuel/air mixtures.

In older vehicles with carburettors, automakers introduced a heated air intake system to

vaporize fuel more quickly and help maintain the fuel/air mixture. Some vehicles also used a

heat riser valve which, when the engine was cold, would close off the flow of exhaust so that

it would travel to the intake manifold, accelerating the vaporization of fuel.

Design of a carburettor:

Let the height of the nozzle tip be Z meter higher than the float chamber level. Which

means the fuel surface is below top of the jet by Z meter. It is provided to prevent spilling of

petrol when the vehicle is stationary.

If the compressibility of air is considered, the air flow will change but the fuel flow

will remain the same. Applying steady flow energy equation (SFEE).

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To engine

D

d

N

air

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