DESIGN AND ANALYSIS OF TWO WHEELER CARBURETOR

ABSTRACT

Carburettor is the most important component in fuel feed system of spark ignition engines. It is

connected between the fuel filter and induction manifold. It supplies air-fuel mixture of varying

proportions to suit engine operating conditions. There are two laws which allow the carburettor

to function. They are Vacuum and Venturi Effect.

INTRODUCTION

SI engines generally use volatile liquids. The preparation of the fuel-air mixture is done

outside the engine cylinder. The fuel droplets that remain in suspension also continue to

evaporate and mix with air during suction and compression processes also. So carburetion is

required to provide a combustible mixture of fuel and air in required quantity and quality.

The literature survey reveals a lot of work has been carried out in the area of designing

and analyzing the carburettor using CREO and ANSYS. The analysis of carburetor has been

carried out by applying structural and thermal loads. The present work particularly deals with the

making of carburettor by using Rapid Proto Typing to increase mass production, to achieve cost

reduction, to achieve weight reduction and to prevent the component from getting corroded or

undergoing corrosion.

1.1 CARBURETTOR:

Carburettor is the most important component in fuel feed system of spark ignition

engines. It is connected between the fuel filter and induction manifold. It supplies air-fuel

mixture of varying proportions to suit engine operating conditions.

1.1.1 Definition:

The carburettor is a device for atomizing and vaporizing the fuel and mixing it with the

air in varying proportions to suit the changing conditions of spark ignition engines. The air-fuel

mixture so obtained from the carburettor is called the combustible mixture. The process of

mixing the gasoline fuel with air to obtain the combustible mixture is called carburetion.

Vaporization is the change of state of fuel from liquid to vapour. Atomization is the mechanical

breaking-up of the liquid fuel into small particles (but not actually breaking-up into atoms, as the

name implies) so that every particle of fuel is surrounded by the air. In order to produce very

quick vaporization of the liquid fuel, it is sprayed into the air passing through the carburettor.

Spraying of the liquid turns into many fine particles, so that the vaporization occurs almost

instantly

1.1.2 Working Principle of Carburettor:

There are two laws which allow the carburettor to function. They are Vacuum and

Venturi Effect.

Vacuum is simply suction, or air pressure below that of local atmospheric conditions.

When a piston drops on its intake stroke, the intake valve above it opens and the resulting suction

from the cylinder causes a vacuum to be created in the intake tract. The throttle plate controls

how much of the vacuum created in the intake tract gets to which of the carburettor’s fuel

circuits. While idling the engines intake vacuum is held downstream of the throttle plate,

isolating the main fuel system and giving the pilot circuit the full effect of intake vacuum,

because its fuel delivery downstream of the throttle plate(between the throttle plate and the

cylinder heads).

The Venturi effect means that when a gas (air in this case) passes through a reduction in

diameter of tube, the velocity of the gas increases and its pressure drops. The area of the

carburettor throttle containing the slide is called venturi because it has tapered reduction in

diameter and, with the controlled movement of the slide , it uses the venturi effect to create a

controlled low pressure above the needle jet. Any pressure below ambient conditions acts as a

vacuum, or source of suction. The faster the air flow under the slide the greater the drop in

pressure above the needle jet.

At idle condition, the throttle plate nearly closes off the bore of the carburettor, which

results in a strong vacuum downstream of the throttle plate and with the slide at its lowest

position, nearly closing off the needle jet with the thickest part of the needle, the pilot circuit

regulates the fuel flow. As the throttle is opened, allowing more air into the engine, the needle

and slide begins to rise, intake vacuum weakens (causing a drop in a pilot fuel delivery) and air

flow above the needle jet speeds up. The Venturi effect creates a low pressure causing suction

above the needle jet and fuel is drawn up from the midrange /main circuit. As the needle and

slide continue to rise, they expose an ever-greater amount of needle jet orifice to the air flow

above it, causing a progressive rise in fuel delivery with the increasing air flow.

1.1.3 Air-Fuel ratio: (mixture requirements)

Fig: 1.1. Shows the variation of mixture requirements from no-load to full-load in a S.I Engine.

1. Idling and low speed : (From no-load to about 20% of rated power)

Idling refers to no power demand. During idling air supply ids throttled and

residual gases make up a large fraction of charge at the end of the suction period. In

addition, during overlap period some exhaust gases are drawn back into the cylinder. The

result is chemically correct (stoichiometric) mixture of air and fuel (~ 15: 1) would be so

diluted by residual gases that combustion would be erratic or impossible. A rich mixture

is supplied during idling (say A/F ratio 11: 1 or 12: 1). The richness should gradually

change to slightly lean for the second range as shown in the above figure.

2. Cruising or normal power :(From about 25% to about 75% of rated power)

In the normal power range the main consideration is fuel economy. Because

mixture of fuel and air is never completely homogeneous the stoichiometric mixture of

fuel and air will not burn completely and some fuel will be wasted. For this reason an

excess of air, say 10% above theoretically correct (~ 16.5: 1), is supplied in order to

ensure complete burning of fuel.

3. Maximum power : (From 75% to 100% of rated power)

Maximum power is obtained when all the air supplied is fully utilized. As the

mixture is completely homogeneous a rich mixture must be supplied to assure utilization

of air.

1.2 Factors influencing carburetion:

1. The engine speed; the time available for preparation of mixture

2. The vaporization characteristics of fuel.

3. The temperature of the incoming air.

4. The design of carburettor.

In case of modern high speed engines, the time duration available for formation of

mixture is very small and limited. The time duration for mixture formation and

induction may be of the order of 10 to 5 milliseconds.

Atomization, mixing and vaporization are the processes which require a finite

time to occur. The time available for mixture formation is very small in high

speed engines(For example in an engine running at 3000 r.p.m.., the induction

process lasts for less than 0.02 second). For completion of these processes in such

a small period a great ingenuity is required in designing the carburettor system.

In order to achieve high quality carburetion within such a short time requires good

vaporization characteristics of the fuel which are ensured by presence of high

volatile hydro-carbons in the fuel.

The temperature is a factor which effectively controls vaporisation process of the

fuel. If the temperature of incoming air is high, it results in higher rates of

vaporization. The mixture temperature can be increased by heating the induction

manifold but it will result in reducing power due to reduction in mass flow rate.

For a S.I engine, the design of carburetion system is very complicated owing to

the fact that the air-fuel ratio required by it varies widely over its range of

operation, particularly for an automotive engine. For idling as well as for

maximum power rich mixture is required.

Carburetor is required to provide the best economy mixture. In the peak power operation the

engine requires a much richer mixture

(i) During peak power operation some parts of the cylinder gets heated up. So

enrichening the mixture reduces the flame temperature and the cylinder temperature and

the cooling problem is solved.

(ii) Since high power is required the cruising setting must be transferred to a setting in which

the mixture will deliver maximum power or to a setting in the air-fuel ration lies in the

range of 12:1.

1.3 Compensating Devices of Carburetor:

The automobile has to run on different roads on different loads and conditions.

The main metering system of the carburetor alone will not be able to take care of the needs of the

engines. Therefore, compensating devices are provided. The important compensating devices are

1.3.1 Air-Bleed Jet

The air bleed jet is present in the main nozzle. The flow of air through the orifice is

restricted. Initially, when the engine is not operating both the jets are filled with fuel. When the

engine starts fuel comes out from both the nozzles but gradually t5he engine picks up and after

that only air comes out of the air-bleed jet and mixes with the fuel coming out from the

main nozzle and forms the fuel-air emulsion.

1.3.2Emulsion Tube

The main metering is jet is generally kept 25mm below the fuel level in the float chamber

so as to avoid the overflow of the fuel. A jet is placed at the bottom of a well having holes which

are connected to the atmosphere. When the throttle is opened fuel starts to flow from the

well and the holes get uncovered and the air-fuel ratio increases i.e. the richness of the mixture

decreases when all the holes get uncovered. The air is drawn through these holes and the

fuel gets emulsified and the differential of pressure across the column of fuel is not as high as

that of the simple carburetor.

1.3.3 Compensating Jet

The main purpose of the compensating jet, which is connected to a compensating

well, is to make the mixture leaner as the throttle valve opens gradually. The compensating well

is vented to the atmosphere and is also connected to the main fuel chamber through a

restricting orifice.

With the increase in air flow rate, the fuel level in the compensating well decreases so the

fuel supply rate through the compensating jet also decreases. Thus the compensating jet tends to

lean the mixture whereas the main jet tends to richen the mixture. So the sum of the two jets

tends to keep the mixture to the required ratio.

1.3.4 Back Suction Control Mechanism

In this device, the top of the fuel float chamber is connected to the entry part of the body

of the carburetor by means of a long vent line fitted with a control valve. Another vent

is connected from the top of the chamber to the venturi of the throat. When the control

valve is completely open then pressure at the float chamber is same as that of the air

inlet. So there lies a pressure difference between the float chamber and the venturi and fuel from

the float chamber flows into the venturi. But when the control valve is closed the pressure at the

venturi and the float chamber are same and there is no fuel flow. Thus by proper control of the

control valve a proper differential between the float chamber and the throat can be maintained

and hence the quality of the mixture.

1.3.5 Auxiliary Valve

When the engine is in idle conditions the pressure at the top of the auxiliary valve is

atmospheric. With increase in load, the vacuum at the throat of the venturi increases. So a

pressure differential is created between the throat and the spring and this pressure difference

raises the valve against the spring force. And as a result more air flows and the mixture becomes

leaner.

1.3.6 Auxiliary Port

The auxiliary port connects the air entering part above the throat with the air leaving part

below the throat by means of a long vent containing a butterfly valve. If the butterfly valve is

opened then some additional amount of air passes through this vent and thus the flow of air

across the venturi decreases.

1.4 Survey of Carburetor:

1.4.1 Introduction:

The Design and analyzation of carburetor has been done using Pro- E. The analysis of

carburettor has been carried out by applying structural and thermal loads.

1.4.2 Existing system:

The Existing Carburettor installed in the two-wheeler uses a metallic component made

out of Cast-iron, Aluminum, Zinc. The make of the carburetor is from a 100c.c Hero Honda

splendor bike.

1.4.3 Disadvantages of Existing Carburetor:

1. It weighs more, so assuming /expecting weight reduction is quite un-natural.

2. The initial cost of the component is high.

3. The component probably has a chance to get corroded.

4. The heat conduction capacity is more.

1.5.4 Proposed system:

The proposed system is related with the plastic conversion of carburettor using CAD, CAE

and RPT Technologies. The component is designed , analized and then it is manufactured in RPT

process, the material used in this process is polyamide.