THE CARBUTETOR AND ITS IMPORTANCE IN S.I.ENGINES

Spark-ignition engines normally use volatile liquid fuels.Preparation of homogeneous mixture is normally not completed in the inlet manifold.Fuel droplets which remain in suspension continue to evaporate and mix with air even during suction and compression processes.The process f mixture preparation is extremely important for spark-ignition engines.The purpose of carburetion is to provide a combustible mixture of fuel and air in the required quantity and quality for efficient operation of th engine under all conditions.

DEFINITION OF CARBURETION:

The process formation of a combustible fuel-air mixture by mixing the proper amount of fuel with air before admission to engine cylinder is called carburetion and the device which does this job is called a carburetor.

HISTORY ABOUT THE CARBURETOR:

The carburetor is the part of the car combines air and fuel to createcombustion. Before the creation of fuel-injected engines, the carburetor.

Even though Karl Benz invented the carburetor in 1885 and patented the device in 1886, there is some doubt whether he actually created it. Some believe a pair of Hungarian engineers also invented the carburetor.

PRINCIPLE OF CARBURETION:

Both air and gasoline are drawn through the carburetor and into the engine cylinders by the suction created by the downward movement of the pisto.This suction is due to an increase in the volume of the cylinder and consequent decreases in the gas pressure in this chamber.It is the difference in pressure between the atmsphere and cylinder that casuses the air to flow into the chamber.In the carburetor,air passing into the combustion chamber picks up fuel discharged from a tube.This tube has a fine orifice called carburetor jet which is exposed to the air path.The rate at which fuel is between the float chamber and the throat f the venturi and on the area of the outlet of the tube.In order that the fuel drawn from the nozzle may be thoroughly atomized,the suctin effects must be strong and the nozzle outlet comparatively small. In order to produce a strong suction,the pipe in the carburetor carrying air to the engine is made to have a resriction.At this restriction called throat due to increases in velocity of flow,a suction efffect is created.The restiction is made in the form f a venturi as tominimize

THE CARBUTETOR AND ITS IMPORTANCE IN S.I.ENGINESthroattling lasses.The end of the fuel jet is located at the venturi or throat of the carburetor.The geometry of venturi tube is as shown.It has a narrow path at the centre so that the flow area through which the air must pass is considerbly reduced.As the same amount of air must pass through every point in the tube, its velosity will be greatest at the narroest point.The smaller the area,the greater will be the velcity of the air,and thereby the suction is proportionately increases. As mentioned earlier,the opening of the fuel discharge jet is usually located where the suction is maximum.Normally,this is just below the narrowest section of the venturi tube.The spray of gasoline from the nozzle and the air entering through the venturi tube are mixed together in intae manifold into the cylinders.Most of the fuel gets atomized and simultaneously a small part will be vapourized.Increases air velocity at the throat of the venturi helps the rate of evaporation of fuel.The difficulty of obtaining a mixture of sufficiently high fuel vapour-air ratio for efficient starting of the engine and for uniform fuel-air ratio in different cylinders(in case of multicylinder engine)cannot befully met by the increased air velocity alone at the venturi

throat.

THE SIMPLE CABURETOR

Carburetor are highly complex.Let us first understand the working principle of a simple or elementary carburetor which provides an air-fuel mixture for cruising or normal range at a single speed.Later,other mechanisms to provide for the various special requirementslikestarting,idling,variable load and speed operations and acceleration will be included.the simple carburetor is as shown in figure………..

The simple carburetor mainly consists of a float chamber,fuel dicharge and nozzle and a neddle valve system maintains a constant level f gasoline in the float chamber.If the amount of fuel in the float chamver falls below the designed level,the float goes down,thereby opening the fuel supply valve and admitting fuel.when the designed level has been reached,the supply system.float

THE CARBUTETOR AND ITS IMPORTANCE IN S.I.ENGINESchamber is vented either to the atmosphere or to the upstream side of the venturi.

Durng suction stroke air is drawn through the venturi.as already descrived,venturi is a tube f decreaseing cross section with a minimum area at the throat.venturi tube is also known as the choke tube and is so shaped that it offers minimum resistance to the air flow.As the air passes through the venturi the velocity increases reaching a maximum at the venturi throat.Correspondingly,the pressure decreases reaching a minimum.From the float chamber,the fuel is fed to a discharge jet,the tip of which is located in the throat of the venturi,Because of the differential pressure between the float chamber and the throat of the venturi,known as carburetor depression,fuek is discharged into the air stream,The fuel dicharge is affected by the size of the discharge jet and it is chosen to give the required air fuel ratio.The pressure at the throat at the fully open throttle condition lies between 4 to 5cm of Hg,below atmospheric and seldom exceeds 8 cm ofHg below atmospheric.To avoid over flow of fuel through the jet,the level of the liquid in the float chamber is maintained at a level slightly below the tip of the heigh between the top of the nozzle and the float chamber level is marked as h. The gasoline engine is quntity governed,which means that when power output is to be varied at a particular speed,the amount of charge delivered to the cylinder is varied,This is achived by means of a throattle valve usually of the butterfly type which is situated after the venturi tube .As the quantity of air-fuel miture delivered to the cylinder and hence power output is reduced.As the throttle is opened,more air flows through the choke tube resulting in increases

quantity of mixture being delivered to the engine.This increases the engine power output.

A simple carburetor of the type described above suffers from a fundamental drawback in that it provides the required A/F ratio only at one throttle position.At the other throttle positions the mixture is either leaner or richer depending on whether the throttle is opened less or more.As the throttle opening is varied,the air flow varies and creates a certain pressure differential between the float chamber and the venturi throat.The same pressure differential regulates the flow of fuel vary in a simillar manner.At throat decreases with increasing air flow whereas that of the fuel remains unchanged.This results in a simple carburetor producing a progressively rich mixture with increasing throttle opening.

ESSENTIAL PARTS OF THE CARBURETOR:::

1. The Carburetor Systemo A carburetor system

works on three basic principles: metering the correct proportion of fuel and air, atomization of the fuel into a

THE CARBUTETOR AND ITS IMPORTANCE IN S.I.ENGINESvapor, and the distribution of a uniform mixture of both fuel and air into the engine.When fuel reaches the carburetor, it flows through the fuel supply pipe and into a float bowl. Fuel then flows through the fuel jet into the other side of the carburetor. This side includes an air entry point, also known as the barrel or throat, at which there is an air filter; a Venturi pipe, a tube which varies in width and the throttle valve.

The Float Systemo The term "float system"

is used to describe a typical carburetor's operation. Fuel flows through the inlet fitting and through a seat, then past the end of the needle and into the float bowl. The needle is important because as the bowl fills up, the float pushes the needle into the needle seat, cutting off the fuel. The fuel stays constant.

o

The Fuel Jeto The fuel jet is also called

the main nozzle. The metering jet, a calibrated opening on the bottom of the float bowl, determines the amount of fuel that will move into the engine. The metering jet opens into a main well which contains air bleeds and baffles that turn the fuel into a vapor as it moves up the main nozzle in a vacuum process, and into the Venturi pipe.

The Venturi Pipeo The Venturi pipe is so

named because it operates on the Venturi effect. A vacuum is created in the pipe based on how much air is rushing through the carburetor's air

entry. The narrow main nozzle moves the fuel into the Venturi pipe by means of low pressure which pulls it out of the nozzle. This spray is then pushed toward the throttle valve.

The Throttle Valveo The throttle valve is

placed between the spray and the inlet pipe which leads to the engine. There are two types of throttle valves: the butterfly, which is a circular disc and the cylindrical which is as large as the inlet pipe and rotates. The throttle is controlled by the accelerator, with cables or rods attached to the throttle by a lever. Although the accelerator controls the engine speed, variations made to the throttle valve can be made to enhance a car's performance.

Types of Carburetorso There are several

different types, configurations, and manufacturers of carburetors available based on what kind of performance is needed. Different configuration variations include two and four barrel types for more airflow, accelerator pumps, high-flow needles and vacuum secondary diaphragms, just to name a few.Performance carburetor manufacturers include Edelbrock, Holley, AED, Wood and Predator. Small engine carburetor manufacturers include Briggs & Stratton, Bing and Tecumseh, among others. These types include engines for lawnmowers, snow blowers, motorcycle, log splitters and pressure washers.

FACTORS AFFECTING CARBURETION: Of the various factors,the process of carburetin is influenced by

1> the engine speed

THE CARBUTETOR AND ITS IMPORTANCE IN S.I.ENGINES

2> the vapourization charecteristics of the fuel

3> the temperature of the incoming air and

4> the design of the carburetorand they are explanied as fallows………….

ENGINE SPEEDSince modern engines are of high speed type,the time available for mixture formation is very limited.For example,an engine running at 3000 rpm has only about 10 milliseconds(ms) for mixture induction during intake stroke.When the speed becomes 6000 rpm the time available is only 5 ms.

Therefore,in order to havae high quality carburetion(mixture with high vapour content)

THE VAPOURIZATIO CHARECTERISTICS OF THE FUEL:

Other factors which ensure high quality carburetion within a short period are the presence of highly volatile hydrocarbons in the fuel.Therefore suitable evaporation charecteristics of the fuel,indicated by its distillation curve,are necessary for efficient carburetion especially at high engine speeds. TEMPERATURE AND PRESSURE OF THE INCOMING AIR:

The temperature and pressure of surrounding air has a large influence on efficient carburetion.Higher atmospheric air temperature increases the vapourization of fuel(percentage of fuel vapour increases with increases in mixture temperature)and produces a more homogeneous mixture.An increases in atmospheric temperature,however,leads to a decreases in power output of the engine when the air-fuel ratio is constant due to reduced mass flow into the cylinder or,in other words,reduced volumetric efficiency.

THE DESIGN OF THE CARBURETOR:

The design of the carburetor,the intake system and the combustion chamber have considerable influence on uniform distribution of mixture to the various cylinders of the engine.Proper design of carburetor elements alone ensures the supply of desired compositon of the mixture under different operating conditions of the engine.

AIR-FUEL MIXTURE

An engine is generlly operated at different loads and speeds.For this,proper air-fuel mixture should be supplied to the engine cylinder.Fuel and air are mixed to form three different types of mixtures.

1. chemically correct mixture2. rich mixture3. lean mixture

THE CARBUTETOR AND ITS IMPORTANCE IN S.I.ENGINES

and they are explaned as follows……….

CHEICALLY CORRECT MIXTRUE:

Chemically correc or stoichiometric mixture is one in which there is just enough air for complete combustion of the fuel.For example,to burn one kg of octane[C8H8] completely 15.12 kg of air is required. Hence chemically correct air fuel ratio for the C8H8 is 15.12:1;Usually approximated to 15:1.This chemically correct mixture will vary only slightly in numerical value between different hydrocarbon fuels.It is always computed from the chemical equation for complete combustion for a particular fuel.Complete combustion means allcarbon in the fuel is converted to CO2 and all hydrogen to H2O.

RICH MIXTURE;

A mixture which contains less air than the stoichiometric requirements is called a rich mixture.For exmaple: air-fuel raio of 12:1,10:1 LEAN MIXTURE:

A mixture which contains more air than the stoichiometric requirements is called a lean mixtureFor example air-fuel ratio is 17:1,20:1 etc.There is,however,a limited range of A/F ratios is a homogenous miture,ony within which combustion is an si engine will occur.outside this range,the ratio is either too rich or too lean to sustain flame propagation .This range of useful A/F ratio runs from approximately 9:1(rich) to 19:1(lean)as indicated .

MIXTURE REQUIREMENTS AT DIFFERENT LOADS AND SPEEDS The air-fuel ratio at which an engine operates has a considerable influence on its performance.Consider an engine operating at full throttle and constant speed with varying A/F ratio.under these conditions, A/F raio will affect both the power output and the brake specific fuel consumption,as indicated by the typical curves.the mixture corresponding to the maximum output on the curve is called the best power mixture with an A/F ratio of approximately 12:1.The mixture corresponding the minimum point on the bsfc curve is called the best economy mixture.the A/Ftatio is apprximately 16:1 it may be noted that the best power mixture is much richer than the chemically correct mixture and the best economy mixture is slightly leaner than the chemically correct. The different ratios of the air-fuel mixtures are required for the S.I.engine operating at various conditions…….

01) Air-Fuel Ratio for StartingVery rich mixture (10: 1) is required at starting of engine. During starting very small amount of fuel is vaporizes and rest of it stay in the liquid state so as to give an ignitable mixture. 02) Air-Fuel Ratio for IdlingAn idling, engine demands a rich mixture, which can be made leaner as the throttle is gradually opened. During idling, the pressure in the inlet manifold is about 20 to 25% of atmospheric pressure. At suction stroke, inlet valve opens and the product of combustion trapped in the clearance volume, expands in the inlet manifold.

THE CARBUTETOR AND ITS IMPORTANCE IN S.I.ENGINESLatter when the piston moves downwards, the gases along with the fresh charges go into the cylinder. A rich mixture must be supplied during idling, to counteract the tendency of dilution and to get an ignitable mixture. 03) Air-Fuel Ratio for Medium LoadMost of the time, engine is running in medium load condition, therefore, it is desirable that the running should be most economical in this condition. So a lean mixture can be supplied, as engine has low fuel consumption at medium load. Formulti cylinder engine, slightly more fuel is required due to mal distribution of fuel. 04) Air-Fuel Ratio for Maximum Power RangeWhen maximum power is required, the engine must be supplied with rich mixture as the economy is of no consideration. As the engine enters in the power range, the spark must be retarded otherwise knocking would occur. A lean mixture burns at latter part of working stroke. As the exhaust valve expose to high temperaturegases and have very less time to cool down. Moreover, the excess air in the lean mixture may cause an oxidizing action on the hot exhaust valve and leads to failure. 05) Air-Fuel Ratio for AccelerationEven during normal running, sometimes more power is required for a short period such as to accelerate the vehicle for overtaking etc. During this period rich mixture is required.I hope this will help you to understand something about combustion of fuel at different running conditions of the engine.

ADVANTAGES;An engine with a carburetor is said to produce more raw horsepower or more high end power than a fuel injected engine.A carburetor is by far a simple engine when compared to a fuel injected engine. this simplicity would make the carburetor engine easier to adjust, troubleshoot, configure or rebuild.A carburetor engine would be a cheaper engine to purchase and cheaper to install.

DISADVANTGES:CarburetorAn engine with a carburetor is said to produce more raw horsepower or more high end power than a fuel injected engine.A carburetor is by far a simple engine when compared to a fuel injected engine. this simplicity would make the carburetor engine easier to adjust, troubleshoot, configure or rebuild.A carburetor engine would be a cheaper engine to purchase and cheaper to install.

CONCLUSION: The carburetor is essential for the S.I.engine for increasing the efficiency and power output and proper combustion and controlling the emissions.