piston engine fuel system

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ACKNOWLEDGEMENTS I wish to acknowledge the higher authorities and especially thankful to the honorary secretary captain S.N.Reddy of TELANGANA STATE AVIATION ACADEMY Hyderabad for granting the permission to do the scheduled project successfully. I thank my project guide Mr. SOMPAL SINGH and chief instructor Wg.Cdr,A.Appa Rao (Retd) and other instructors of Telangana State Aviation Academy for giving their assistance and for their interest in helping me to complete the project. 0 PISTON ENGINE FUEL SYSTEM

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ACKNOWLEDGEMENTS

I wish to acknowledge the higher authorities and especially thankful to the honorary secretary captain S.N.Reddy of TELANGANA STATE AVIATION ACADEMY Hyderabad for granting the permission to do the scheduled project successfully.

I thank my project guide Mr. SOMPAL SINGH and chief instructor Wg.Cdr,A.Appa Rao (Retd) and other instructors of Telangana State Aviation Academy for giving their assistance and for their interest in helping me to complete the project.

I gratefully acknowledge the co-operation and encouragement received from Mr. Chandra Shekar, Head of the Department and staff of the M.Sc. section.

K. RAKESH

0 PISTON ENGINE FUEL SYSTEM

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CONTENTS

1. INTRODUCTION TO CARBUKRETION ………………..72. MEANING AND DEFINITIONS ………………………….93. OBJECTIVES ……………………………………………..124. EVOLUTION OF CARBURETORS……………………...135. PRINCIPLES OF CARBURETION ………………………166. VENTURI PRINCIPLES …………………………………177. FLOAT-TYPE CARBURETORS…………………………258. FLOAT MECHANISM WORKING………………………269. MAIN METERING SYSTEM ……………………………3010. IDLING SYSTEM……………………………………3211. ACCELERATING SYSTEM…………………………3812. ECONOMIZER SYSTEM……………………………4113. MIXTURE CONTROL SYSTEM……………………4614. AUTOMATIC MIXTURE CONTROL ……………..5215. DOWNDRAFT CARBURETORS…………………..5616. PRESSURE INJECTION CARBURETORS…………5717. PRINCIPLE OF OPERATION………………………5818. ADVANTAGES AND DISADVANTAGES OF

CARBURETORS…………………………………………7719. INTRODUCTION TO FUEL INJECTION

SYSTEMS…………………………………………………7920. HISTORY OF FUEL INJECTION……………………8121. FUEL INJECTION SYSTEM TYPES………………..8222. TYPICAL FUEL INJECTION SYSTEM …………….8423. ADVANTAGE AND DISADVANTAGE FUEL

INJECTION SYSTEM……………………………………..100

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24. COMPARATIVE STUDY OF EFI AND MFI ………..10425. FINDINGS AND SUGGESTIONS……………………..10826. CONCULSION ………………………………………....11027. BIBLIOGRAPHY……………………………………….112

APPENDIX……………………

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LIST OF FIGURES

FIGURE 4.1 SIDE DRAFT HORIZONTAL CARBURETOR…………………………..….14

FIGURE 4.2 CUT VIEW OF BASIC CARBURETOR……………………………………...14

FIGURE 5.1 OPERATION OF A VENTURI TUBE………………………………………...17

FIGURE 5.2 VENTURI PRINCIPLE APPLIED TO A CARBURETOR…………………...18

FIGURE 5.3 BASIC VENTURI TYPE CARBURETOR…………………………………….20

FIGURE 5.4 SUCTION LIFTS A LIQID…………………………………………………….21

FIGURE 5.5 EFFECTS OF AIR BLEED……………………………………………………..21

FIGURE 5.6 AIR BLEED BREAKING UP A LIQUID…………………………………….…22

FIGURE 5.7 THROTTLE VALVE……………………………………………………………24

FIGURE 5.8 THROTTLE VALVE IN OPEN POSITION……………………………………24

FIGURE 8.1 FUEL AND NEEDLE VALVE MECHANISM IN A CARBURETOR……………26

FIGURE 8.2 FLOAT AND NEEDLE VALVE MECHANISM IN A CARBURETOR…………..27

FIGURE 8.3 CONCENTRIC FLOAT AND VALVE………………………………………….…28

FIGURE 8.4 ECCENTRIC FLOAT AND VALVE……………………………………………….28

FIGURE 8.5 TYPICAL CARBURETAOR STAINER………………………………………..….29

FIGURE 9.1 MAIN METERING SYSTEM…………………………………………………….30

FIGURE 10.1 THREEPIECE MAIN DISCHARGE ASSEMBLY………………………………33

FIGURE 10.2 CONVENTIONAL IDLE SYSTEM…………………………………………….34

FIGURE 10.3 FLOAT TYPE CARBURETOR AT DIFFERENT ENGINE SPEEDS………………35

FIGURE 10.4 CUT VIEW……………………………………………………………………….37

FIGURE 11.1 MOVABLE PISTON TYPE ACCELERATING PUMP………………………..39

FIGURE 11.2 ACCELERATING PUMP IN OPERATION……………………………………...40

FIGURE 12.1 NEEDLE VALVE ECONOMIZER………………………………………………43

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FIGURE 12.2(A) PISTON TYPE ECONOMIZER……………………………………………….44

FIGURE 12.3(B) PISTON TYPE ECONOMIZER………………………………………………44

FIGURE 12.4 NAP OPEATED ECONOMIZER…………………………………………………45

FIGURE 13.1 NEEDLE TYPE MIXTURE CONTROL…………………………………………49

FIGURE 13.2 AIR-PORT TYPE MIXTURE CONTROL……………………………………….50.

FIGURE 14.1 AUTOMATIC MIXTURE CONTROL MECHANISM…………………………...53

FIGURE 14.2 AMC IN OPERATION……………………………………………………………54

FIGURE 15.1 DOWN DRAFT CARBURETOR………………………………………………..56

FIGURE 17.1 PRESSURE TYPE CARBURETOR………………………………………………58

FIGURE 17.2 REGULATOR UNIT……………………………………………………………..61

FIGURE 17.3 FUEL CONTROL UNIT…………………………………………………………65

FIGURE 17.4 MANUAL MIXTURE CONTROL VALVE PLATE POSITIONS…………….67

FIGURE 17.5 AUTOMATIC MIXTURE CONTROL AND THROTTLE BODY…………….69

FIGURE 17.6 SCHEMATIC OF THE PS SERIES CARBURETOR………………………….71

FIGURE 17.7 AIRFLOW POWER ENRICHMENT VALVE…………………………………74

FIGURE 19.1 BASIC FUEL INJECTION SYSTEM…………………………………………..80

FIGURE 22.1 CUTAWAY VIEW OF AIRFLOW MEASURING SECTION………………….85

FIGURE 22.2 AIRFLOW SECTION OF A FUEL INJECTION….............................................87

FIGURE 22.3 IMPACT TUBES FOR INLET AIRPRESSURE…………………………….....87

FIGURE 22.4 FUEL DIAPHRAM WITH BALL VALVE ATTACHED………………………87

FIGURE 22.5 FUEL METERING SECTION OF INJECTION………………………………..88

FIGURE 22.6 FUEL INLET AND METERING………………………………………………..88

FIGURE 22.7 FLOW DIVIDER………………………………………………………………..89

FIGURE 22.8 FUEL PUMP……………………………………………………………………..92

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FIGURE 22.9 FUEL INJECTION PUMP……………………………………………………….93

FIGURE 22.10 FUEL AIR CONTROL UNIT………………………………………………….94

FIGURE 22.11 DUAL FUEL CONTROL ASSEMBLY………………………………………94

FIGURE 22.12 FUEL MAINFOLD ASSEMBLY……………………………………………..95

FIGURE 22.13 TYPICAL AE FUEL MANIFOLD ASSEMBLY…………………………….96

FIGURE 22.14 FUEL DISCHARGE NOZZLES……………………………………………..97

FIGURE 23.1 GRAPH…………………

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PISTON ENGINE FUEL SYSTEM

SECTION 1

INTRODUCTION

TO

CARBURETION

INTRODUCTION

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Fuel is used to power everything. It is the life blood of transportation system. As our technology advances, society is able to use several natural and man-made resources to power our vehicles.

The heart of any fuel system of an internal combustion engine is the fuel metering device. Because internal combustion engines require fuel and air to be supplied in proper proportions into the cylinders at all altitudes , at all atmospheric pressures and at different flight attitudes.

The efficiency of fuel metering device is paramount for a reciprocating engine, because a slight change in air/fuel ratio would result in abnormal response by engine.

Two types of fuel metering devices employed for delivering fuel into the cylinders of the reciprocating engines are.

Carburetor system Fuel injection system.

Carburetor is the earliest form of fuel supply mechanism of automobile. The primary function of carburetor is to provide the air-fuel mixture to the engine in the required proportion.The goal of a carburetor is to mix just the right amount of gasoline with air so that the engine runs properly.

Fuel injection technology is used to eliminate the need for carburetors. The technology helps the engine to supply fuel directly to the cylinder in the intake manifold, eliminating the use of carburetor to much extent.

Overall, the fuel injection is required to supply fuel directly to the engine. It is one wherein the fuel is directly supplied to the cylinder in the intake chamber. Sensors located in such engines will regulate the flow of fuel

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injected and maintains it to appropriate levels. This system is developed so as to improve fuel efficiency

Basic requirement of a fuel metering device is it must meter fuel proportionately to air to establish the proper fuel/air mixture ratio for engine.

Second requirement of the metering device is to atomize and distribute the fuel into the mass airflow in such a manner that the air charges going to all cylinders will hold the similar amounts of fuel so that the fuel/air mixture

reaching each cylinder is of the same ratio.

2.MEANING AND DEFINITONS.MEANING OF CARBURETION:The process of mixing (as in a carburetor) the vapor of a flammable hydrocarbon (as gasoline) with air to

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form an explosive mixture especially for use in an internal-combustion engine.

The word carburetor comes from the French carburetor meaning "carbide".Carburer  means to combine with carbon. In fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fluid by mixing it with a volatile hydrocarbon.From Wikipedia

Carburetormeans to combine with carbon. In fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fluid by mixing it with a volatile hydrocarbon.

A carburetor is a device that blends air and fuel for an internal combustion engine.From Wikipedia

Carburetor: part of a gasoline engine in which liquid fuel is converted into a vapor and mixed with a regulated amount of air for combustion in the cylinder.The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights

reserved.

An apparatus in which coal gas, hydrogen, or air is passed through or over a

volatile hydrocarbon, in order to confer or increase illuminating power.

Meaning of fuel injection: the spraying of liquid fuel into the cylinders or combustion chambers of an engine.Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Info please.

Fuel injection: a system for introducing atomized liquid fuel under pressure directly into the combustion chambers of an internal-combustion engine without the use of a carburetor.From Wikipedia

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PISTON ENGINE FUEL SYSTEM

SECTION 2

OBJECTIVES

3. OBJECTIVES. To study and understand in detail about piston engine carburetion and fuel

injection systems. To have an idea of the evolution and development of carburetors and fuel

injection systems.

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To understand various types of carburetor’s used on piston engine. To have an accurate understanding of the application of venture principle.

To study in detail the working of float type carburetors. To study in detail the working of various systems of float type carburetors.

To understand the working principle of pressure injection carburetor. To understand the working of pressure injection carburetors. To study in detail the parts of pressure injection carburetor. To study in detail the working of Stromberg PS carburetor. To study the advantages and disadvantages of carburetors.

To have an understanding of the evolution of fuel injection systems. To understand the working principle of fuel injection system.

To have a brief understanding of different types of fuel injection systems. To study in detail the working of bendix and continental fuel injection

system. To study in detail the parts of a typical fuel injection system.

To study the advantages and disadvantages of the fuel injection system.

4. EVOLUTION OF CARBURETORS

The carburetor was invented by Karl Benz before 1885 andpatented in 1886.It was apparently also invented by theHungarian engineers JanosCsonka and DonáBánki in 1893.FrederickWilliamLanchester of Birmingham, England experimentedearly on with the wick carburetor in cars.

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In 1896 Frederick andhis brother built the first petrol driven car in England, a singlecylinder 5 hp (4 kW) internal combustion engine with chain drive.Unhappy with the performance and power, they re --built theengine the next year into a two cylinder horizontally opposedversion using his new wick carburetor design.

This versioncompleted a 1,000 mile (1600 km) tour in 1900 successfullyincorporating the carburetor as an important step forward in automotive engineering.

Carburetors were the usual fuel delivery method for almost allgasoline (petrol)-fuelled engines up until the late 1980s, when fuel injection became the preferred method of automotive fuel delivery.

In the US market, the last carbureted car was the 1991 Ford Crown Victoria.PoliceInterceptor equipped with the 351 in(5.8 L) engine, and the last carbureted light truck was the 1994suzu. Elsewhere, Lada cars used carburetors until 1996.

Amajority of motorcycles still utilize carburetors due to lower costand throttle response problems with early injection set ups, but asof 2005, many new models are now being introduced with fuel injection Carburetors are still found in small engines and in olderor specialized automobiles, such as those designed for stock car racing

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FIGURE 4.1SIDE DRAFT HORIZONTAL CARBURETOR

FIGURE 4.2 CUT VIEW OF BASIC CARBURETOR

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PISTON ENGINE FUEL SYSTEM

SECTION 3

IMPORTANCE

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5. PRINCIPLES OF CARBURETION

To obtain heat from fuel, the fuel must be burned, and the burning of fuel requires a combustible mixture. The purpose of carburetion, or fuel metering, is to provide the combustible mixture of fuel and air necessary for the operation of an engine.

Since gasoline and other petroleum fuels consist of carbon (c) and hydrogen (H) chemically combined to form hydrocarbon molecules (CH), it is possible to burn these fuels by adding oxygen (O) to form a gaseous mixture. The carburetor mixes the fuel with the oxygen of air to provide a combustible mixture which is supplied to the engine through the induction system. The mixture is ignited in the cylinder; then the heat energy of the fuel is released and the fuel-air mixture is converted to carbon dioxide (co2),water(h2o), and possibly some carbon monoxide(co).

The carburetors used in aircraft engines are comparatively complicated because they play an extremely important part in engine performance, mechanical life, and the general efficiency of the airplane, due to the widely diverse conditions under which airplane engines are operated. The carburetor must deliver an accurately metered fuel-air mixture for engine loads and speeds between wide limits and must provide for automatic or manual correction under changing conditions of temperature and altitude, all the while being subjected to a continuous vibration that tends to upset the calibration and adjustment. Sturdy construction is essential for all parts of an aircraft carburetor, to provide durability and resistance to the effects of vibration.Knowledge of the function of these parts is essential to an understanding of carburetor operation.

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6. VENTURI PRINCIPLES

The carburetor must measure the airflow through the induction system and use this measurement to regulate the amount of fuel discharged into the airstream. The air measuring unit is the venturi,which makes use of a basic law of physics: as the velocity of a gas or liquid increases, the pressure decreases.

As shown in the diagram of the simple venture, it is a passageway or tube in which there is a narrow portion called the throat. As the air speeds up to get through the narrow portion. Its pressure drops. Note that the pressure in the throat is lower than that in any other part of the venture. This pressure drop is proportional to the velocity and is therefore a measure of the airflow.The basic operating principle of most carburetors depends on the differential pressure between the inlet and the venture throat.

FIGURE5.1: OPERATION OF A VENTURI TUBE.

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Application Of Venturi Principle To Carburetor:The carburetor is mounted on the engine so that air to the cylinder passes through the barrel, the part of the carburetor which contains the venture.

The size and shape of the venture depends on the requirements of the engine for which the carburetor is designed. A carburetor for a high-powered engine may have one large venturi or several small ones. The air may flow either up or down the venturi,depending on the design of the engine and the carburetor. Those in which the air passes downward are known as the downdraft carburetors. And those in which the air passes upward are called updraft carburetors.

FIGURE5.2:VENTURI PRINCIPLE APPLIED TO A CARBURETOR.

The figure5.2 shows the venture principle applied to a simplified carburetor. The amount of fluid which flows through a given passage in any

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unit of time is directly proportional to the velocity at which the fluid is moving. The velocity is directly proportional to the difference in the applied

forces. If a fuel discharge nozzle is placed in the venture throat of a carburetor, the effective force applied to the fuel will depend on the velocity

of air going through the venture. The rate of flow of the fuel through the discharge nozzle will be proportional to the amount of air passing through the

venture, and this will determine the supply of the required fuel-air mixture delivered to the engine. The ratio of the fuel to air should be varied within

certain limits; therefore, a mixture control system is provided for the venture-type carburetor.

Pressure Differential In A Simple Carburetor:The rapid flow of air through the venture reduces the pressure at the discharge nozzle so that the pressure in the fuel chamber can force the fuel out into the airstream. Since the airspeed in the tube is comparatively high and there is a relatively great reduction in pressure at the nozzle during medium and high engine speeds, there is a reasonably uniform fuel supply at such speeds.

When the engine speed is low and the pressure drop in the venture tube is slight, the situation is different. This simple nozzle, otherwise known as a fuel discharge nozzle, in a carburetor of a fixed size does not deliver a continuously richer mixture as the engine suction and airflow increase. Instead, a plain discharge nozzle will give a fairly uniform mixture at medium and high speeds; but at low speeds and low suction, the delivery falls off greatly in relation to the airflow.

This occurs partly because some of the suction force is consumed in raising the fuel from the float level to the nozzle outlet, which is slightly higher than the fuel level in the fuel chamber to prevent the fuel from overflowing when

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the engine is not operating. It is also caused by the tendency of the fuel to adhere to the metal of the discharge nozzle and to break off intermittently in large drops instead of forming a fine spray. The discharge from the plain fuel nozzle is therefore retarded by an almost constant force, which is not important at high speeds with high suction but which definitely reduces the flow when the suction is low because of reduced speed.

FIGURE5.3: BASIC VENTURI TYPE CARBURETOR.

Figure5.3 shows how the problem in the design and construction of the venture-type carburetor. Air is bled from behind the venture and passed into the main discharge nozzle at a point slightly below the level of the fluid, causing the formation of a finely divided f/a mixture which is fed into the airstream at the venture. A metering jet between the fuel chamber and the main discharge nozzle controls the amount of fuel supplied to the nozzle. A metering jet is an orifice, or an opening, which is carefully dimensioned to meter (measure) fuel flow accurately in accordance with the pressure

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differential between the float chamber and discharge nozzle. The metering jet is an essential part of the main metering system.

Air Bleed :The air bleed in a carburetor lifts an emulsion of air and liquid to a higher level above the liquid level in the float chamber than would be possible with unmixed fuel. The below figure shows a person sucking on a straw placed in a glass of water. The suction is great enough to lift the water above the level in the glass without that person drawing any into the mouth. In fig 5.4 a tiny hole has been pricked in the side of the straw above the surface of the water in the glass, and the same suction is applied as before. The hole causes bubbles of air to enter the straw and liquid is drawn up in a series of air to enter the straw, and liquid is drawn up in a series of small drops or slugs.

FIGURE 5.4: SUCTION LIFTS A LIQUID.FIGURE 5.5: EFFECTS OF AIR BLEED

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In fig no-5.6 the air is taken into a tube through a smaller tube which enters the main tube below the level of the water. There is a restricting orifice at the bottom of the main tube; that is, the size of the main tube is reduced at the bottom. Instead of a continuous series of small drops or slugs being drawn up through the tube when the person sucks on it, a finely divided mixture of air and water is formed in the tube.

FIGURE 5.6: AIR BLEED BREAKING UP A LIQUID

Since there is a distance through which the water must be lifted from its level in the glass before the air begins to pick it up, the free opening of the main tube at the bottom prevents a very great suction from being exerted on the air bleed hole or vent. If the air openings were too large in proportion to the size of the main tube.

The carburetor nozzle in fig no-5.3 has an air bleed, as explained previously. We can summarize our discussion by stating that the purpose of

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this air bleed in the discharge nozzle is to assist in the production of a more uniform mixture of fuel and air throughout all opening speeds of the engine

Vaporization Of Fuel:The fuel leaves the discharge nozzle of the carburetor in a stream which breaks up into drops of various sizes suspended in the airstream, where they become even more finely divided. Vaporization occurs on the surface of each drop, causing the very fine particles to disappear and the large particles to decrease in size. The problem of properly distributing the particles would be simple if all the particles in each drop vaporized completely before the mixture left the intake pipe. But some particles of the fuel enter the engine cylinders while they are still in a liquid state and thus must be vaporized and mixed in the cylinder during the compression stroke.

The completeness of vaporization depends on the volatility of the fuel, the temperature of air, and the degree of atomization. Volatile means readily vaporized; therefore, the more volatile fuels evaporate more readily. Higher temperatures increase the rate of vaporation; therefore, carburetor air intake heaters are sometimes provided. Some engines are equipped with “hot-spot” heaters which utilize the heat of exhaust gases to heat the intake manifold between the carburetor and the cylinders. This is usually accomplished by routing a portion of the engine exhaust through a jacket surrounding the intake manifold. In another type of hot spot heater, the intake manifold is passed through the oil reservoir of the engine. The hot oil supplies heat to the intake manifold walls, and the heat is transferred to the F/A mixture.

The degree of atomization is the extent to which fine spray is produced; the more fully the mixture is reduced to fine spray and vaporized, the greater is

the efficiency of the combustion process. The air bleed in the main discharge nozzle passage aids in the atomization and vaporization of the fuel. If the fuel

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is not fully vaporized, the mixture may run lean even though an abundance of fuel is present.

Throttle Valve:A throttle valve, usually a butterfly-type valve, is incorporated in the fuel-air duct to regulate the fuel-air output. The throttle valve is usually an oval-shaped metal disk mounted on the throttle shaft in such a manner that it can completely close the throttle bore. The edges of the throttle disk are shaped to fit closely against the sides of the fuel-air passage. The arrangement of such a valve is shown in fig 5.7. The amount of air flowing

through the venture tube is reduced when the valve is turned toward its closed position. This reduces the suction in the venture tube, so that less fuel is delivered to the engine. When the throttle valve is opened, the flow of the F/A mixture to the engine is increased. Opening or closing the throttle valve thus regulates the power output of the engine.

FIGURE5. 7: THROTTLE VALVE. FIGURE5. 8: THROTTLE VALVE IN OPEN POSITION

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7. FLOAT-TYPE CARBURETORS

Essential Parts Of A Carburetor: The carburetor consists essentially of a main air passage through the engine draws its supply of air, mechanisms to control the quantity of fuel discharged in relation to the flow of air, and a means for regulating the quantity of F/A mixture delivered to the engine cylinders.

The essential parts and systems of a float-type carburetor are (1) the float mechanism and its chamber, (2) the strainer, (3) the main metering system, (4) the idling system, (5) the economizer system, (6) the accelerating system, and(7) the mixture control system.

In the float-type carburetor, atmospheric pressure in the fuel chamber forces fuel from the discharge nozzle when the pressure is reduced at the venture tube. The intake stroke of the piston reduces the pressure in the engine

cylinder, thus causing air to flow through the intake manifold to the cylinder. This flow of air passes through the venture of the carburetor and causes the reduction of pressure in the venture which, in turn, causes the fuel to be sprayed from the discharge nozzle.

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8. FLOATMECHANISM WORKING

The float in a carburetor is designed to control the level of fuel in the float chamber. This fuel level must be maintained slightlybelowthe discharge-nozzle outlet holes to provide the correct amount of fuel flow and to prevent leakage of fuel from the nozzle when the engine is not running. The arrangement of a float mechanism in relation to the discharge nozzle is shown in fig 8.1

FIGURE 8.1: FUEL AND NEEDLE VALVE MECHANISM IN A CARBURETOR

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Note that the float is attached to a lever which is pivoted and that one end of the lever is engaged with the float needle valve. When the float rises, the needle valve closes and stops the flow of fuel into the chamber. At this point, the fuel level is correct for proper operation of the carburetor, provided that the needle seat is at the correct level.

FIGURE 8.2 FLOAT AND NEEDLE VALVE MECHANISM IN A CARBURETOR

As shown in fig 8.2, the float valve mechanism includes a needle and a seat. The needle valve is constructed of hardened steel, or it may have a synthetic-rubber section which fits the seat. The needle seat is usually made of bronze. There must be a good fit between the needle and the seat to prevent fuel leakage and overflow from the discharge nozzle. During operation of the carburetor, the float assumes a position slightly below its highest level, to

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allow a valve opening sufficient for replacement of fuel as it is drawn out through the discharge nozzle. If the fuel level is too low, the mixture will be lean. To adjust the fuel level washers are placed under the float needle seat. If the fuel level needs to be raised, washers are removed from under the seat. If the level needs to be lowered, washers are added. For some carburetors, the float level is adjusted by bending the float arm.

Additional types of float mechanisms

Concentric float and valve, having a common center Eccentric float and valve, having off center.

FIGURE 8.3 FIGURE 8.4

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Fuel Strainer:In most carburetors, the fuel supply must first enter a strainer chamber, where it passes through a strainer screen. The strainer screen consists of a fine wire mesh or other type of filtering device, cone shaped or cylindrically shaped, located so that it will intercept any dirt particles which might clog the needle valve opening or, later, the metering jets. The strainer is usually removable so that it can be taken out and thoroughly drained and flushed.

FIGURE 8.5: TYPICAL CARBURETOR STRAINER.

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9.MAIN METERING SYSTEMThe main metering controls the fuel feed in the upper half of the engine speed range as used for cruising and full-throttle operations. It consists of three principal divisions or units: (1) Themain metering jet, through which fuel is drawn from the float chamber; (2) Themain discharge nozzle, which may be any one of several types; and (3) Thepassage leading to the idling system.

Although the previous statement is correct some authorities state that the purpose of the main metering system is to maintain a constant Fuel/Air mixture at all throttle openings throughout the power range of engine operation. The same authorities divide the main metering system into four parts (1) the venture, (2) a metering jet which measures the fuel drawn from the float chamber, (3) a main discharge nozzle, including the main air bleed,

and (4) a passage leading to the idling system.

The three functions of the main metering system are (1) to proportion the fuel/air mixture, (2) to

decrease the pressure at the discharge

nozzle, and (3) to control the airflow at full throttle

FIGURE 9.1: MAIN FIGURE METERING SYSTEM

.

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The airflow through an opening of fixed size and the fuel flow through an air-bleed jet system respond to variations of pressure in approximately equal proportions. if the discharge nozzle of the air-bleed system is located in the center of the venturi, so that both the air-bleed nozzle and the venturi are exposed to suction of the engine in the same degree, it is possible to maintain an approximately uniform mixture of fuel and air throughout the power range of engine operations.

This is illustrated in figure no 9.1,which shows the air –bleed principle and the fuel level of the float chamber in a typical carburetor. If the main air bleed ofa carburetor should become restricted or clogged, the fuel/air mixture would be excessively rich because more of the available suction would be acting on the fuel in the discharge nozzle and less air would be introduced with the fuel.

The full power output from engine makes it necessary to have, above the throttle valve, a manifold suction which is between 0.4 and 0.8 psi at full engine speed. However, more suction is desired for metering and spraying the fuel and is obtained from the venturi. When a discharge nozzle is located in the central position of the venturi, the suction obtained is several times as great the as the suction found in the intake manifold. Thus it is possible to maintain a relatively low manifold vacuum. This results in high volumetric efficiencies.

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10. IDLING SYSTEM

At idling speeds, the airflow through the venture of the carburetor is too low to draw sufficient fuel from the discharge nozzle, so the carburetor cannot deliver enough fuel to keep the engine running . At the same time, with the throttle nearly closed, the air velocity is high and the pressure is low between the edges of the throttle valve and the walls of the air passages. There is very high suction on the intake side of the throttle valve. Because of this situation, an idling system with an outlet at the throttle valve s added. This idling system with an outlet at the throttle valve is added.

This idling system delivers fuel only when the throttle valve is nearly closed and the engine is running slowly. An idle cutoff valve stops the flow of fuel through this idling system on some carburetors, and this is used for stopping the engine. An increased amount of fuel is used in the idle range because at idling speeds there may not be enough air flowing around its cylinders to provide cooling.

Fig 10.1 shows a three-piece main discharge assembly, with a main discharge nozzle, main air bleed, main-discharge-nozzle stud , idle feed passage, main metering jet, and accelerating well screw. This is one of the two types ofmain discharge nozzle assemblies used in updraft, float-typecarburetors.

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FIGURE 10.1 THREEPIECE MAIN DISCHARGE ASSEMBLY

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An updraft carburetor is one in which the air flows upward through the carburetor to the engine. In other type, the main discharge nozzle and the

discharge-nozzle stud are combined in one piece that is screwed directly into the discharge-nozzle boss, which is part of the main body casting, there by

eliminating the necessity of having a discharge-nozzle screw.

FIGURE 10.2 CONVENTIONAL IDLE SYSTEM.

Figure 10.2 is a drawing of a conventional idle system, showing the idling discharge nozzles, the mixture adjustment, the idle air speed, the idle metering jet, and the idle metering tube. Note that the fuel for the idling system is taken from the fuel passage for the main discharge nozzle and that the idle air-bleed air is take from a chamber outside the venture section. Thus the idle air is at air inlet pressure. The idle discharge is divided between two

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discharge nozzles, and the relative quantities of fuel flowing through these nozzles are dependent on the position of the throttle valve. At very low idle, all the fuel passes through the upper orifice, since the throttle valve covers the lower orifice. In this case the lower orifice acts as an additional air bleed for the upper orifice. As the throttle is opened further, exposing the lower orifice, additional fuel passes through this opening.

Since the idle-mixture requirements vary with climatic conditions and altitude, a needle valve type of mixture adjustment is provided to vary the orifice in the upper idle discharge hole. Moving this needle in or out of the orifice varies the idle fuel flow accordingly, to supply the correct Fuel/Air ratio to the engine.

FIGURE 10.3 FLOAT TYPE CARBURETOR AT DIFFERENT ENGINE SPEEDS.

The idling system described above is used in the bendix–Stromberg NA-S3A1 carburetor and is not necessarily employed in other carburetors. The principles involved are similar in all carburetors, however.Fig10.3, A, B,C show a typical float-type carburetor at idling speed, medium speed, and full speed, respectively. The greatest suction in the intake manifold above the throttle is at the lowest speeds, when the smallest amount of air is received, which is also the condition requiring the smallest amount of fuel. When the engine speed increases, more fuel is needed, but the suction in the manifold

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decreases. For this reason, the metering of the idling system is not accomplished by the suction existing in the intake manifold. Instead, the metering iscontrolled by the suction existing in a tiny intermediate chamber, or slot, formed by the idling discharge nozzle and the wall of the carburetor at the edge of the throttle valve. This chamber has openings into the barrel of the carburetor, both above and below the throttle.

In10.3, note that there is a small chamber surrounding the main discharge nozzle passage just below the main air-bleed inlet. This chamber serves as an accelerating well to store extra fuel that is drawn out when the throttle is suddenly opened. If this extra supply of fuel were not immediately available, the fuel flow from the discharge nozzle would be momentarily decreased and the mixture entering the combustion chamber would be too lean ,thereby causing the engine to misfire.

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In the carburetor shown in fig 10.3, when the engine is operating at intermediate speed, the accelerating well still holds some fuel. However, when the throttle is wide open, all the fuel from the well is drawn out. At full power, all fuel is supplied through the main discharge and economizer system, and the idling system then acts as an auxiliary air bleed to the main metering system. The main metering jet provides an approximately constant mixture ratio for all speeds above idling, but it has no effect during idling.

The purpose of the accelerating well is to prevent a power lag when the throttle is opened suddenly.in many carburetors. An accelerating pump is used to force an extra supply of fuel from the discharge nozzle when the

throttle is opened quickly.

FIGURE 10.4

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11. ACCELERATING SYSTEM

When throttle controlling an engine is suddenly opened. There is a corresponding increase in the airflow; but because of the inertia of the fuel, the fuel flow does not accelerate in proportion to the airflow increase. Instead the fuel lags behind, which results in a temporary reduction in power. To prevent this condition, all carburetors are equipped with an accelerating pump or an accelerating system. This is either an accelerating pump or an accelerating well

The function of the accelerating system is to discharge an additional quantity of fuel into the carburetor airstream when the throttle is opened suddenly, thus causing a temporary enrichment of the mixture and producing a smooth and positive acceleration of the engine.

The accelerating well is a space around the discharge nozzle and is connected by holes to the fuel passage leading to the discharge nozzle. The upper holes are located near the fuel level and are uncovered at the lowest pressure that will draw fuel from the main discharge nozzle; therefore they receive air during the entire time that the main discharge nozzle operates.

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Very little throttle opening is required at idling speeds. When the throttle is opened suddenly, air is drawn in to fill the intake manifold and whichever cylinder is on the intake stroke. This sudden rush of air temporarily creates a high suction at the main discharge nozzle, brings into operation the main metering system, and draws additional fuel from the accelerating well. Because of the throttle opening, the engine increases and the main metering system continues to function.

FIGURE11.1 MOVABLE PISTON TYPE ACCELERATING PUMP.

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The accelerating pump illustrated in fig 11.1 is a sleeve-type piston operated by the throttle. The piston is mounted on a stationary hollow stem screwed into the body of the carburetor. The hollow stem opens into the main fuel passage leading to the discharge nozzle. Mounted over the stem and piston is a movable cylinder, or sleeve, which is connected by the pump shaft to the throttle linkage. When the throttle is closed, the cylinder is raised and the space within the cylinder fills with the fuel through the clearance between the piston and the cylinder.

If the throttle is quickly moved to the open position, the cylinder is forced down as shown in fig 11.2, and the increased fuel pressures also forces the piston partway down along the stem. As the piston moves down, it opens the pump valve and permits the fuel to flow through the hollow stem into the main fuel passage. With the throttle fully open and the accelerating pump cylinder all the way down, the spring pushes the piston up and forces most of the fuel out of the cylinder. When the piston reaches itshighest position, it closes the valve and no more fuel flows toward the main passage.

FIGURE 11.2ACCELERATING PUMP IN OPERATION.

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There are several types of accelerating pumps, but each serves the purpose of providing extra fuel during rapid throttle opening and acceleration of the engine.

When a throttle is moved slowly toward the open position, the accelerating pump does not force extra fuel into the discharge system, because the spring in the pump holds the valve closed unless the fuel pressure is great enough to overcome the spring pressure. When the throttle is moved slowly, the trapped fuel seeps out through the clearance between the piston and the cylinder, and the pressure does not build up enough to open the valve.

12.ECONOMIZER SYSTEMAn economizer, or power enrichment system, is essentially a valve which is closed at low engine and cruising speeds but is opened at high speeds to provide an enriched mixture to reduce burning temperatures and prevent detonation .in other words, this system supplies and regulates the additional fuel required for all speeds above the cruising range. An economizer is also a device for enriching the mixture at increased throttle settings. It is important, however, that the economizer close properly at cruising speed; otherwise, the engine may operate satisfactorily at full throttle but will “load up” at and below cruising speed because of the extra fuel being fed into the system. The extra-rich condition is indicated by rough running and by black smoke emanating from the exhaust.

The economizer gets its name from the fact that it enables the pilot to obtain maximum economy in fuel consumption by providing for a lean

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mixture during cruising operation and a rich mixture for full-power setting. Most economizers in their modern form are merely enriching devices. The carburetors equipped with economizers are normally set for their modern form are merely enriching devices. The carburetors equipped with economizer are normally set for their leanest practical mixture delivery at cruising speeds, and enrichment takes place as required for high power settings.

Three types of economizers for float type carburetors are

Needle valve type economizer The Piston type economizer

The MAP-operated economizer

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Needle Valve Type:

This mechanism utilizes a needle valve which is opened by the throttle linkage at a pre-determined throttle position. This permits a quantity of fuel, in addition to the fuel from the main metering jet, to enter the discharge-nozzle passage.as shown in , the economizer needle valve permits fuel to bypass the cruise-valve metering jet.

FIGURE 12.1 NEEDLE VALVE ECONOMIZER.

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The Piston-Type Economizer:

FIGURE12.2(A)PISTONTYPE ECONOMIZER FIGURE12.3(B)PISTONTYPE ECONOMIZER

Illustrated in fig 12.1, is also operated by the throttle. The lower piston serves as a fuel valve, preventing any flow of fuel through the system at cruising speeds. In view A the upper piston functions as an air valve, allowing air to flow through the separate economizer discharge nozzle at part throttle. As the throttle is opened to higher power positions, the lower piston uncovers the fuel port leading from the economizer metering valve and the upper piston closes the air ports(view b).

fuel fills the economizer well and is discharged into the carburetor venturi where it adds to the fuel from the main discharge nozzle. The upper piston of the economizer permits a small amount of air to bleed into the fuel, thus assisting in the atomization of the fuel from the economizer system. The space below the lower piston of the economizer acts as an accelerating well when the throttle is opened.

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The MAP Operated Economizer:

FIGURE 12.4 MAP OPERATED ECONOMIZER.

Has a bellows which is compressed when the pressure from the engine blower rim produces a force greater than the resistance of the compression spring in the bellows chamber. as the engine speed increases, the blower pressure also increases .the this pressure collapses the bellows and causes the economizer valve to open. Fuel then flows through the economizer metering jet to the main discharge system. The operation of the bellows and spring is stabilized by means of a dashpot, as shown in figure 12.4.

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13. MIXTURE CONTROL SYSTEM.

At higher altitudes, the air is under less pressure, has less density, and is at a lower temperature. The weight of the air taken into unsupercharged engine decreases with the decrease in the density, and the power is reduced in approximately the same proportions. Since the quantity of oxygen taken into the engine decreases, the Fuel/Air mixture becomes too rich for normal operations. The mixture proportion delivered by the carburetor becomes richer at a rate inversely proportional to the square root of the increase in air density.

Remember that the density of the air changes with the temperature and pressure. If the pressure remains constant, the density of air will vary according to temperature, increasing as the temperature drops. This will cause leaning of the Fuel/Air mixture in the carburetor because the denser air contains more oxygen. The change in air pressure due to altitude is considerably more a problem than the change in density due to temperature changes. At an altitude of 18,000feet, the air pressure is approximately one-half what it would be at sea level. Adjustment of fuel flow to compensate for changes in air pressure and temperature is a principal function of the mixture control.

Briefly, a mixture control system can be described as a mechanism or device through which the richness of the mixture entering engine during flight can

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be controlled to a reasonable extent. This control should be maintainable at all normal altitudes of operation.

The FunctionsOf The Mixture Control System:1. To prevent the mixture from becoming too rich at high altitudes.

2. To economize on fuel during engine operation in the low the power range where cylinder temperature will not become excessive with the

use of the leaner mixture

Mixture Control Systems

Can be classified as per their principle of operation as:

1. The back suction type: which reduces the effective suction on the metering system.

2. The needle type: which restricts the flow of fuel through the metering system.

3. The air port type: which allows additional air to enter the carburetor between the main discharge and the throttle.

The back-suction-type: mixture control system is the most widely used. In this system, a certain amount ofventuri low pressure acts upon the fuel in the float chamber sothat it opposes the low pressure existing at the main dischargenozzle.

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An atmospheric line, incorporating an adjustablevalve, opens into the float chamber. When the valve iscompletely closed, pressures onthe fuel in the float chamberand at the discharge nozzle are almost equal, and fuel flowis reduced to maximum lean.With the valve wide open,pressure on the fuel in the float chamber is greatest and fuelmixture is richest. Adjusting the valve to positions between these two extremes controls the mixture. The quadrant in the cockpit is usually marked “lean” near the back end and “rich “ at the forward end. The extreme back position is marked “idle cutoff” and is used when stopping the engine.

Theneedle type mixture control: in this control , the needle is used to restrict the fuel passage through the main metering jet. When the mixture control is in the full rich position and the fuel is accurately measured by the main metering jet. The needle valve is lowered into the needle valve seat to lean the mixture, thus reducing the supply of fuel to the main discharge nozzle, even though the needle valve is completely closed, a small bypass hole from the float chamber to the fuel passage allows some of the fuel to flow; therefore, the size of this bypass hole determines the control range.

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FIGURE13.1 NEEDLE TYPE MIXTURE CONTROL

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The air port type mixture control: has an air passage leading from the region between the venturi tube and the throttle valve to atmospheric pressure. In the air passage is a butterfly valve which is manually controlled by the pilot in the cockpit.

FIGURE 13.2 AIR-PORT TYPE MIXTURE CONTROL

When the pilot opens the butterfly valve in the air passage, air which has not been mixed with fuel will be injected into the Fuel/Air mixture. At the same time, the suction in the intake manifold will be reduced, thereby reducing the velocity of air coming through the venturi tube. This will further reduce the amount of fuel being drawn into the intake manifold.

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Idle Cut Off: The term “idle cutoff” describes the position of certain mixture controls in the control is enabled to stop the flow of fuel into the intake airstream. Some float-type carburetors and the majority of pressure-type carburetors incorporate the idle cutoff position in the mixture control system.

Essentially, the idle cutoff system stops the flow of fuel from the discharge nozzle and is therefore used to stop the engine. This provides an important safety factor, because it eliminates the combustible mixture in the intake manifold and prevents the engine from firing as a result of a hot spot in one or more cylinders.

In some, engines which have been stopped by turning off the ignition switch have one who may move the propeller. In an engine equipped with idle cutoff feature, the engine ignition switch is turned off after the engine is first stopped by means of moving the mixture control to the idle cutoff position. This procedure also eliminates the possibility of unburned fuel entering the cylinder and washing the oil film from the cylinder walls.

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14. AUTOMATIC MIXTURE CONTROL.

Some of the more complex aircraft carburetors are often equipped with a device for automatically controlling the mixture as altitude changes. Automatic mixture control system may be operated on the back-suction principle and the needle valve principle or by throttling the air intake to the carburetor.

In the latter type of AMC, the control regulates power output within certain limits in addition to exercising its function as a mixture control.In AMC systems operating on the back-suction and needle valve principles, the control may be directly operated by the expansion and contraction of a pressure-sensitive evacuated bellows through a system of mechanical linkage.

This is the simplest form of AMC and is generally found to be accurate, reliable, and easy to maintain. Some mixture control valves such as one in the figure 14.1 are operated by a sealed bellows in a compartment vented to the atmosphere; therefore, the fuel flow is proportional to the atmospheric pressure. Fig14.2 shows the bellows type of mixture control valve installed on a carburetor as a back-suction control device.

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FIGURE14.1AUTOMATICMIXTURECONTROLMECHANISM

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FIGURE 14.2 AMC IN OPERATON

As atmospheric pressure decreases, the bellows will expand and begin to close the opening chamber into the fuel chamber. this will cause a reduction of pressure in the chamber, resulting in a decreased flow of fuel from the discharge nozzle. In some systems equipped with external superchargers, both the fuel chamber and the bellows may be vented to the carburetor intake to obtain the correct mixtures of fuel and air.

Automatic controls often have more than one setting in order to obtain the correct mixtures for cruising and high speed operation. In addition to the automatic control feature, here is usually a provision for manual control if the automatic control fails.

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When the engine is equipped with a fixed-pitch propeller which allows the engine speed to change as the mixture changes, a manually operated mixture control can be adjusted by observing the change in engine rpm as the constant-speed propeller.

If a constant-speed propeller cannot be locked into fixed-pitch position, and if the extreme-pitch positions cause engine speeds outside the normal flight operating range, the following expressions may be employed to describe the manual adjustments of the mixture control.

Full rich: the mixture control setting in the position for maximum fuel flow.

Rich best power: the mixture control setting which, at a given throttle setting, permits maximum engine rpm with the mixture control as far toward full rich as possible without reducing the rpm.

Lean best power: the mixture control setting which, at a given throttle setting, permits maximum engine rpm with the mixture control as far toward lean as possible without reducing the rpm.

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15. DOWNDRAFT CARBURETORS

A downdraft carburetor takes air from above the engine and causes the air to flow down through the carburetor. Those who favor downdraft carburetors claim that they reduce fire hazard, provide better distribution of mixture to the cylinders of an upright engine, and have less tendency to pick up sand and dirt from the ground.

Downdraft carburetors are very similar in function and systems to updraft carburetors used for aircraft. Figure 15.1 illustrates a portion of a downdraft carburetor and its idling system, emphasizing the path of the fuel leaving the float chamber and the position of the idle air bleed. When the engine is not operating, this air bleed, in addition to its other functions prevents the siphoning of fuel. An average intake pressure to the mixture control system is supplied by the series of vents at the entrance to the venturi

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FIGURE 15.1

16. PRESSURE INJECTION CARBURETORS.

Pressure injection carburetors are distinctly different from float-type carburetors as they do not incorporate a vented float chamber or suctionpickup from a discharge nozzle located in the venturi tube. Instead, they provide a pressurized fuel system that is closed from the engine fuel pump to the discharge nozzle.

The venturi serves only to create pressure differentials for controlling the quantity of fuel to themetering jet in proportion to airflow to the engine.The pressure injection carburetor is a radical departure from float type carburetor designs and takes an entirely different approach to aircraft engine fuel metering. It employs the simple method of metering the fuel through fixed

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orifices, combined with the additional function of atomizing the fuel spray under positive pump pressure.

Although pressure carburetors are not used on modern aircraft, Many older aircraft still incorporate various types of pressure carburetors. Pressure carburetors do have some advantages over float-type carburetors; they operate during all types of flight maneuvers and carburetors icing is less of a problem.

17. PRINCIPLE OF OPERATION

The basic principle of the pressure injection carburetor can be explained by stating that mass airflow is utilized to regulate the pressure of fuel to a metering system which in turn governs the fuel flow. The carburetor therefore increases fuel flow in proportion to mass airflow and maintains a correct fuel/air ration in accordance with the throttle and mixture settings of the carburetor.

The injection carburetor is a hydro mechanical device employing a closed feed system from the fuel pump to the discharge nozzle. It meters fuel through fixed jets according to the mass airflow through the throttle body and discharges it under a positive pressure.

The illustration in figure17.1represents a pressure-typecarburetor simplified so that only the basic parts are shown.Note the two small passages, one

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leading from the carburetor air inlet to the left side of the flexible diaphragm and the other from the venturithroat to the right side of the diaphragm.

FIGURE 17.1 PRESSURE TYPE CARBURETOR

When air passes through the carburetor to the engine, the pressure on the right of the diaphragm is lowered because of the drop in pressure at the venturi throat. As a result, the diaphragm moves to the right, opening the fuel valve. Pressure from the engine-driven pump then forces fuel through the open valve to the discharge nozzle, where it sprays into the airstream. The distance the fuel valve opens is determined by the difference between the two pressures acting on the diaphragm.

This difference in pressure is proportional to theairflow through the carburetor. Thus, the volume of airflowdetermines the rate of fuel discharge.

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The Pressure Injection Carburetor Is An Assembly Of The Following Units:

1. Throttle body

2. Automatic mixture control

3. Regulator unit

4. Fuel control unit (some are equipped with an adapter)

Throttle Body:The throttle body contains the throttle valves, main venturi, boost venturi, and the impact tubes. All air entering thecylinders must flow through the throttle body; therefore, it is the air control and measuring device. The airflow is measured by volume and by weight so that the proper amount of fuel canbe added to meet the engine demands under all conditions.

As air flows through the venturi, its velocity is increased and its pressure is decreased (Bernoulli’s principle). This low pressure is vented to the low pressure side of the air diaphragm (figure 17.1chamber)in the regulator assembly. The impact tubes sense carburetor inlet air pressure and direct it to the automatic mixture control, which measuresthe air density.

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From the automatic mixture control, the air is directed to the high pressure side of the air diaphragm (chamber A). The pressure differential of the two chambers acting upon the air diaphragm is known as the air metering force which opens the fuel poppet valve. The throttle body controls the airflow with the throttle valves.

The throttle valves may be either rectangular or disk shaped, depending on the design of the carburetor. The valves are mounted on a shaft, which is connected by linkage to the idle valve and to the throttle control in thecockpit. A throttle stop limits the travel of the throttle valve and has an adjustmentwhich sets engine idle speed

Regulator Unit:The regulator is a diaphragm-controlled unit divided into five chambers and contains two regulating diaphragms and a poppet valve assembly. Figure 17.2Chamber A is regulated air-inlet pressure from the air intake. Chamber B is boost venturi pressure. Chamber C contains metered fuel pressure controlled by the discharge nozzle or fuel feed valve.Chamber D contains unmetered fuel pressure controlled by the opening of the poppet valve. Chamber E isfuel pump pressure controlled by the fuel pump pressure relief

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valve.

FIGURE 17.2 REGULATOR UNIT

The poppet valve assembly is connected by a stem to the two main control diaphragms. The purpose of the regulator unit is to regulate the fuel pressure to the inlet side of the metering jets in the fuel control unit. This pressure is automatically regulated according to the mass airflow to the engine.

The carburetor fuel strainer, located in the inlet to chamber E, is a fine mesh screen through which all the fuel must pass as it enters chamber D. The strainer must be removed and cleaned at scheduled intervals.

Referring to figure 17.2, assume that for a given airflow in lb./hr. through the throttle body and venturi, a negative pressure of 1⁄4 psi is established in chamber B. This tends to move the diaphragm assembly and the poppet valve in a direction to open the poppet valve permitting more fuel toenter chamber D.

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The pressure in chamber C is held constant at 5 psi (10 psi on some installations) by the dischargenozzle or impeller fuel feed valve. Therefore, the diaphragm assembly and poppet valve moves in the open direction until the pressure in chamber D is 51⁄4 psi. Under these pressures, there is a balanced condition of the diaphragm assembly with a pressure drop of 1⁄4 psi across the jets in the fuel control unit(auto-rich or auto-lean).

If nozzle pressure (chamber C pressure) rises to 51⁄2 psi, the diaphragm assembly balance is upset, and the diaphragm assembly moves to open the poppet valve to establish the necessary 53⁄4 psi pressure in chamber D. Thus, the 1⁄4 psi differential between chamber C and chamber D is reestablished,and the pressure drop across the metering jets remains the same.

If the fuel inlet pressure is increased or decreased, the fuel flow into chamber D tends to increase or decrease with the pressure change causing the chamber D pressure to dolikewise.

This upsets the balanced condition previously established, and the poppet valve and diaphragm assembly respond by moving to increase or decrease the flow to reestablishthe pressure at the 1⁄4 psi differential. The fuel flow changes when the mixture control platesare moved from auto-lean to auto-rich, thereby selecting a different set of jets or cutting one or two in or out of the system. When the mixture position is altered, the diaphragm and poppet valve assembly repositions to maintain the established pressure differential of 1⁄4 psi between chambers C and D, maintaining the established differential across thejets. Under low power settings (low airflows), the difference in pressure created by the boost venturi is not sufficient to accomplish consistent regulation of the fuel. Therefore, an idle spring, shown in figure, is incorporated in the regulator.

As the poppet valve moves toward the closed position, it contacts the idle spring. The spring holds the poppet valve off its seat far enough to provide

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more fuel than is needed for idling. This potentially overrich mixture is regulated by the idle valve. At idling speed, the idle valve restricts the fuel flow to the proper amount. At higher speeds, it is withdrawn from the fuel passage and has no metering effect.

Vapor vent systems are provided in these carburetors to eliminate fuel vapor created by the fuel pump, heat in the engine compartment, and the pressure drop across the poppet valve. The vapor vent is located in the fuel inlet (chamber E) or, on some models of carburetors, in both chambers D and E.

The vapor vent system operates in the following way. When air enters the chamber in which the vapor vent is installed, the air rises to the top of the chamber, displacing the fuel and lowering its level. When the fuel level has reached a predetermined position, the float (which floats in the fuel) pulls the vapor vent valve off its seat, permitting the vapor in the chamber to escape through the vapor vent seat, its connecting line, and back to the fuel tank.

If the vapor vent valve sticks in a closed position or the vent line from the vapor vent to the fuel tank becomes clogged, the vapor-eliminating action is stopped. This causes the vapor tobuild up within the carburetor to the extent that vapor passes through the metering jets with the fuel.

With a given size carburetor metering jet, the metering of vapor reduces the quantity of fuel metered. This causes the fuel/air mixture to lean out, usually intermittently.

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line. It is importantto detect this condition, as the fuel flow from the carburetor to the fuel supply tank may cause an overflowing tank with resultant increased fuel consumption.

To check the vent system, disconnect the vapor vent line where it attaches to the carburetor, and turn the fuel booster pump on while observing the vapor vent connection at the carburetor. Move the carburetor mixture control to auto-rich; then return it to idle cutoff. When the fuel booster pump is turned on, there should be an initial ejection of fuel and airfollowed by a cutoff with not more than a steady drip from the vent connection. Installations with a fixed bleed from the D chamber connected to the vapor vent in the fuel inlet by a short external line should show an initial ejection of fuel and air followed by a continuing small stream of fuel. If thereis no flow, the valve is sticking closed; if there is a steady flow, it is sticking open.

Fuel Control Unit:The fuel control unit is attached to the regulator assembly and contains all metering jets and valves. (Fig 17.3)The idle and power enrichment valves, together with the mixture control plates, select the jet combinations for the various settings (i.e., auto-rich, auto-lean, and idle cutoff).

The purpose of the fuel control unit is to meter and control the fuel flow to the discharge nozzle. The basic unit consists of three jets and four valves arranged in series, parallel, and series-parallel hookups

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FIGURE17.3 FUEL CONTROL UNIT

.

(FIG 17.3)These jets and valves receive fuel under pressure from the regulator unit and then meter the fuel as it flows to the discharge nozzle. The manual mixture control valve controls the fuel flow. By using proper size jets and regulating the pressure differential across the jets, the right amount of fuel is delivered to the discharge nozzle, giving the desired fuel/air ratio in the various power settings. It should be remembered that the inlet pressure to the jets is regulated by the regulator unit and the outlet pressureis controlled by the discharge nozzle.

The jets in the basic fuel control unit are the auto-lean jet, the auto-rich jet, and power enrichment jet. The basic fuel flow is the fuel required to run the engine with a lean mixture and is metered by the auto-lean jet. The auto-rich jet adds enough fuel to the basic flow to give a slightly richer mixture than

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best power mixture when the manual mixture control is in the auto-rich position.

The four valves in the basic fuel control unit are:

1. Idle needle valve

2. Power enrichment valve

3. Regulator fill valve

4. Manual mixture control

The functions of these valves are:

1. The idle needle valve meters the fuel in the idle range only. It is a round, contoured needle valve, or acylinder valve placed in series with all other metering devices of the basic fuel control unit. The idle needle valve is

connected by linkage to the throttle shaft so that it restricts the fuel flowing at low power settings (idle range).

2. The manual mixture control is a rotary disk valve consisting of a round stationary disk with ports leading from the auto-lean jet, the auto-rich jet, and two smaller ventholes. Another rotating part, resemblinga cloverleaf, is held against the stationary disk by spring tension and rotated over the ports in that diskby the manual mixture control lever. All ports and vents are closed in the idle cutoff position. In the autolean position, the ports from the auto-lean jet

and the two ventholes are open.The port from the auto-rich jet remains closed in this position.

In the auto-rich position, all ports are open. The valve plate position are illustrated in fig 17.4. The three positions of the manual mixture control lever make it possible to select a lean mixture a rich mixture, or to stop fuel flow

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entirely. The idle cutoff position is used for starting or stopping the engine. During starting, fuel is supplied by the primer.

FIGURE 17.4 MANUAL MIXTURE CONTROL VALVE PLATE POSITIONS

3. The regulator fill valve is a small poppet-type valve located in a fuel passage which supplies chamber C of the regulator unit with metered fuel pressure. In idle cutoff, the flat portion of the cam lines up withthe valve

stem, and a spring closes the valve. This provides a means of shutting off the fuel flow tochamber C and thus provides for a positive idle cutoff.

4. The power enrichment valve is another poppet-type valve. It is in parallel with the auto-lean and auto-rich jets, but it is in series with the power

enrichment jet. This valve starts to open at the beginning of the power range.

It is opened by the unmetered fuel pressure overcoming metered fuel pressure and spring tension.

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The power enrichment valve continues to open wider during the power range until the combined flowthrough the valve and the auto-rich jet exceeds that of the power enrichment jet. At this point the power enrichment jet takes over the metering and meters fuel throughout the power range.

5. Carburetors equipped for water injection are modified by the addition of a derichment valve and a derichment jet. The derichment valve and derichment

jet are in series with each other and parallel with the power enrichment jet.

The carburetor controls fuel flow by varying two basic factors. The fuel control unit, acting as a pressure-reducing valve, determines the metering pressure in response to the metering forces. The regulator unit, in effect, varies the size of the orifice through which the metering pressure forces the fuel. It is a basic law of hydraulics that the amount of fluid that passes through an orifice varies with the size of the orifice and the pressure drop across it. The internal automatic devices and mixture control act together to determine the effective size of the metering passage through which the fuel passes. The internal devices, fixed jets, and variable power enrichment valve are not subject to direct external control.

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Automatic Mixture Control (AMC):The automatic mixture control unit consists of a bellows assembly, calibrated needle, and seat. Figure 17.5The purpose of the automatic mixture control to compensate for changes in air density due to temperature and altitude

changes.

FIGURE 17.5 AUTOMATIC MIXTURE CONTROL AND THROTTLE BODY

The automatic mixture control contains a metallic bellows, which is sealed at 28 "Hg absolute pressure. This bellows responds to changes in pressure and temperature. In the illustration, the automatic mixture control is located at the carburetor air inlet. As the density of the air changes, the expansion and contraction of the bellows moves the tapered needle in the atmospheric line. At sea level, the bellows iscontracted and the needle is not in the atmospheric passage. As the aircraft climbs and the atmospheric pressure decreases, the bellows expands, inserting the tapered needle farther and farther into the atmospheric passage and restricting the flow of air to chamber A of the regulator unit. Figure 17.2Atthe same time, air leaks slowly from chamber A to chamber B through the small bleed (often referred to as the backsuction

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bleed or mixture control bleed). The rate at which air leaks through this bleed is about the same at high altitude as it is at sea level.

As the tapered needle restricts the flow of air into chamber A, the pressure on the left side of the air diaphragm decreases. As a result, the poppet valve movestoward its seat, reducing the fuel flow to compensate for the decrease in air density. The automatic mixture controlcan be removed and cleaned if the lead seal at the point of adjustment is not disturbed.

Stromberg PS Carburetor: The PS series carburetor is a low-pressure, single-barrel, injection-type carburetor. The carburetor consists basically of the air section, the fuel section, and the discharge nozzle mounted together to form a complete fuel metering system.

This carburetor is similar to the pressure-injection carburetor; therefore, its operating principles are the same. In this type carburetor, metering is accomplished on a mass airflow basis. Figure 17.6Air flowing through the main venturi creates suction at the throat of the venturi, which is transmitted to the B chamber in the main regulating part of the carburetor and to the vent side of the fuel discharge nozzle diaphragm. The incoming air pressure is transmitted to a chamber A of the regulating part of the carburetor and to the main discharge bleed in the main fuel discharge jet.

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The discharge nozzle consists of a spring-loaded diaphragm connected to the discharge nozzle valve, which controls theflow of fuel injected into the main discharge jet. Here, it is mixed with air to accomplish distribution and atomization into the airstream entering the engine.

FIGURE 17.6 SCHEMATIC OF THE PS SERIES CARBURETOR.

In the PS series carburetor, as in the pressure-injection carburetor, the regulator spring has a fixed tension, which tends to hold the poppet valve

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open during idling speeds or until the D chamber pressure equals approximately 4psi.

The discharge nozzle spring has a variable adjustment which, when tailored to maintain 4 psi, results in a balanced pressure condition of 4 psi in chamber C of the discharge nozzle assembly and 4 psi in chamber D. This produces a zero drop across the main jets at zero fuel flow. At a given airflow, if the suction created by the venturi is equivalent to 1⁄4 pound, the pressure decrease is transmitted to chamber B and to the vent side of the discharge nozzle.

Since the area of the air diaphragm between chambers A and B is twice as great as that between chambers B and D, the 1⁄4 pound decrease in pressure in chamber B moves the diaphragm assembly to the right to open the poppet valve

Meanwhile, the decreased pressure on the vent side of the discharge nozzle assembly causes a lowering of the total pressure from 4 pounds to 33⁄4 pounds. The greater pressure of the metered fuel (41⁄4 pounds) results in a differential across the metering head of 1⁄4 pound (for the 1⁄4 pound pressure differential created by the venturi).

The same ratio of pressure drop across the jet to venturi suction applies throughout the range. Any increase or decrease in fuel inlet pressure tends to upset the balance in the various chambers in the manner already described. When this occurs, the main fuel regulator diaphragm assemblyrepositions to restore the balance.

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The mixture control, whether operated manually or automatically, compensates for enrichment at altitude by bleeding impact air pressure into chamber B, thereby increasing the pressure (decreasing the suction) in chamber B. Increasing the pressure in chamber B tends to move the diaphragm and poppet valve more toward the closed position, restricting fuel flow to correspond proportionately to thedecrease in air density at altitude.

The idle valve and economizer jet can be combined in one assembly. The unit is controlled manually by the movement of the valve assembly. At low airflow positions, the tapered section of the valve becomes the predominant jet in thesystem, controlling the fuel flow for the idle range. As the valve moves to the cruise position, a straight section on the valve establishes a fixed orifice effect which controls thecruise mixture. When the valve is pulled full-open by the throttle valve, the jet is pulled completely out of the seat, and the seat side becomes the controlling jet. This jet is calibrated for takeoff power mixtures.

An airflow-controlled power enrichment valve can also be used with this carburetor. It consists of a spring-loaded, diaphragm-operated metering valve. Refer to figure 17.7 for a schematic view of an airflow power enrichment valve. One side of the diaphragm is exposed to unmetered fuel pressure and the other side to venturi suction plus spring tension.

When the pressure differential across the diaphragm establishes a force strong enough to compress the spring, the valve opens and supplies an additional amount of fuel to the metered fuel circuit in addition to the fuel supplied by the main metering jet.

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FIGURE 17.7 AIRFLOW POWER ENRICHMENT VALVE

Accelerating Pump:

The accelerating pump of the Stromberg PS carburetor is a spring-loaded diaphragm assembly located in the metered fuel channel with the opposite side of the diaphragm vented B to the engine side of the throttle valve. With this arrangement, opening the throttle results in a rapid decrease in suction. This decrease in suction permits the spring to extend and move the accelerating pump diaphragm. The diaphragm and spring action displace the fuel in the accelerating pump and force it out the discharge nozzle. Vapor is eliminated from the top of the main fuel chamberD through a bleed hole, then through a vent line back to the main fuel tank in the aircraft.

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Manual Mixture Control:A manual mixture control provides a means of correcting for enrichment at altitude. It consists of a needle valve and seat that form an adjustable bleed between chamber A and chamber B. The valve can be adjusted to bleed off the venturi suction to maintain the correct fuel/air ratio as the aircraft gains altitude.

When the mixture control lever is moved to the idle cutoff position, a cam on the linkage actuates a rocker arm which moves the idle cutoff plunger inward against the release lever in chamber A. The lever compresses the regulator diaphragm spring to relieve all tension on the diaphragmbetween chambers A and B. This permits fuel pressure plus poppet valve spring force to close the poppet valve, stopping the fuel flow.

Placing the mixture control lever in idle cutoff also positions the mixture control needle valve off its seat and allows metering suction within the carburetor to bleed off.

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PISTON ENGINE FUEL SYTEM

SECTION 4

CHARACTERISTIC FEATURES

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18. ADVANTAGES AND DISADVANTAGES OF CARBURETORS.

Carburetors work amazingly well, considering that they are just basically a "scent spray" device!  They have had many years of continual refinement. And they evolved into really quite efficient accurate devices

Float type carburetors have been improved steadily but they have important disadvantages or limitations.

Disadvantages:1. The fuel flow disturbances in aircraft maneuvers may interfere with the

functions of the float mechanism, resulting inerratic fuel delivery.

2. When icing conditions are present, the discharge of fuel into the airstream ahead of The throttle causes a drop of temperature and a

resulting formation of ice at the throttle valve.

3. They do not have as accurate control of fuel / air mixture as modern fuel injection so  can easily kill catalytic Converters..

4. Fuel can vaporize or boil in summer making hot restarts difficult.

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5. The essential for fuel draw "venturi" always restricts airflow slightly reducing max possible power

6. Fuel drop out occurs in the manifolds causing higher emissions and fuel mixture dependent on manifold temperature. Fuel sat in the manifold as

condensed droplets goes through the engine largely untouched.

Advantages:

1. More instant throttle response is possible than with Fuel Injection as we don't have to wait for sensors to tell a computer what's happening and

wait for it to calculate a result!

2. Easy to swap some jets to change fuelling characteristics.

3. Works trouble free with just minor maintenance.

4. Cost of maintenance is comparative very less.

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19.INTRODUCTION

TO

FUEL INJECTION SYSTEMS.

In a fuel injection system, the fuel is injected directly into the cylinders, or just ahead of the intake valve.The primary difference between carburetors and fuel injection is that fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, while a carburetor relies on suction created by intake air accelerated through a Venturi tube to draw the fuel into the airstream. The air intake for the fuel injection system is similar to that used in a carburetor system, with an alternate air source located within the engine cowling.

This source is used if the external air source is obstructed. The alternate air source is usually operated automatically, with a backup manual system that can be used if the automatic feature malfunctions.

A fuel injection system usually incorporates six basic components: an engine-driven fuel pump, a fuel/air control unit, fuel manifold (fuel distributor), discharge nozzles, an auxiliary fuel pump, and fuel pressure/flow indicators.

A fuel injection system is considered to be less susceptible to icing than the carburetor system, but impact icing on the air intake is a possibility in

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either system. Impact icing occurs when ice forms on the exterior of the aircraft, and blocks openings such as the air intake for the injection system.

FIGURE 19.1 BASIC FUEL INJECTION SYSTEM.

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20.HISTORY

OF

FUEL INJECTION.Herbert Akroyd Stuart developed the first device with a design similar

to modern fuel injection, using a 'jerk pump' to meter out fuel oil at high pressure to an injector. This system was used on the hot bulb engine and was adapted and improved by Bosch and Clessie Cummins for use on diesel engines (Rudolf Diesel's original system employed a cumbersome 'air-blast' system using highly compressed air. Fuel injection was in widespread commercial use in diesel engines by the mid-1920s.

The first use of gasoline direct injection was on the Hesselman engine invented by Swedish engineer Jonas Hesselman in 1925. Hesselman engines use the ultra-lean principle; fuel is injected toward the end of the compression stroke, then ignited with a spark plug. They are often started on gasoline and then switched to diesel or kerosene.

Direct fuel injection was used in notable WWII aero-engines such as the JunkersJumo210, the Daimler-Benz DB 601, the BMW 801, the Shvetsov ASh-82FN. German direct injection petrol engines used injection systems developed by Bosch from their diesel injection systems. Later versions of the Rolls-Royce Merlin and Wright R-3350 used single point fuel injection. Due to the wartime relationship between Germany and Japan, Mitsubishi also had two radial aircraft engines utilizing fuel injection, the Mitsubishi Kinsei  and the Mitsubishi Kasei .Alfa Romeo tested one of the very first electronic injection systems in Alfa Romeo 6C2500 in 1940 Mille

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Miglia. The engine had six electrically operated injectors and was fed by a semi-high pressure circulating fuel pump system.

21.FUEL INJECTION SYSTEM TYPES

The three types are variants of fuel injected petrol engines are Throttle BodyInjection, Port Fuel Injection,Direct Fuel Injection. They are all improvements over carburetor engines but you will see that fuel injection has also improved over the years as the various fuel injected types have been introduced.

ThrottleBodyInjection: The throttle body injection is the most basic type of fuel injection that can be

found on vehicles today. The throttle body was the improvement on the carburetor and while it may look different, it is similar in design to the

carburetor. The fuel injectors are located in a central housing and the fuel travels with the air throughout the intake manifold on its way to the cylinders.  This type of fuel injection is like a computerized carburetor so it will benefit from the precise delivery of the sensors, but there are disadvantages as all of

the fuel may not enter the cylinder.

PortFuelInjection:

Port fuel or multi point injection is an improvement to the throttle body injection as injectors are placed directly above each intake valve resulting in a more precise fuel delivery. The air and fuel meet at a later stage than the

throttle body so condensation of the fuel is reduced.

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This may be a complicated compared to the throttle body but it offers more power with better fuel economy as more of the fuel can be utilized on each power stroke.

DirectInjection: As good as port fuel may sound, engineers have improved on this type of

fuel injection and this improvement is known as direct fuel injection or gasoline direct injection (GDI).

This type of fuel delivery operates similar to the diesel cycle as the fuel is sprayed directly into the cylinder and mixes with the air. With the use of common rail technology and advanced injectors the fuel is sprayed at a much higher pressure causing the fuel to enter the cylinder as a very fine mist which is ideal for a better ignition of the fuel.

This method decreases the surface area in and around the cylinder where the fuel can come in contact with before it ignites so a higher percentage (possibly all) of the fuel will be utilized for combustion resulting in more power per stroke.

A direct injected engine can be identified by a clicking sound if you listen to the engine idle with the bonnet open but this clicking sound is not heard from the interior of the car.

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GDI results in greater fuel economy and power as less fuel may be wasted. Direct Injection maximizes the benefits of forced induction as the fuel will not mix with the heated compressed air before combustion making the direct injection the most efficient of the fuel injection types.

22. TYPICAL FUEL INJECTION SYSTEM.

The fuel-injection system has many advantages over a conventional carburetor system. There is less danger of induction system icing, since the drop in temperature due to fuel vaporization takes place in or near the cylinder.

Acceleration is also improved because of the positive action of the injection system. In addition, fuel injection improves fuel distribution. This reduces the overheating of individual cylinders often caused by variation in mixture due to uneven distribution. The fuel-injection system also gives better fuel economy than a system in which the mixture to most cylinders must be richer than necessary so that the cylinder with the leanest mixture operates properly.

Fuel-injection systems vary in their details of construction, arrangement, and operation. The Bendix and Continental fuel-injection systems are discussed in this section. They are described to provide an understanding of the operating Principles involved.

The Bendix inline stem-type regulator injection system (RSA) series consists of an injector, flow divider, and fuel discharge nozzle.

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It is a continuous-flow system which measures engine air consumption and uses airflow forces to control fuel flow to the engine. The fuel distribution system to the individual cylinders is obtained by the use of a fuel flow divider and air bleed nozzles.

Fuel Injector:The fuel injector assembly consists of:

1. An airflow section,2. A regulator section, and

3. A fuel metering section. Some fuel injectors are equipped with an automatic mixture control unit.

Airflow Section:The airflow consumption of the engine is measured by sensing impact pressure and venturi throat pressure in the

These pressures are vented to the two sides of an air diaphragm. A cutaway view of the airflow measuring section is shown in figure 22.1.

FIGURE22.1 CUTAWAY VIEW OF AIRFLOW MEASURING SECTION

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Movement of the throttle valve causes a change in engine air consumption. This results in a change in the air velocity in the venturi. When airflow through the engine increases, the pressure on the left of the diaphragm is lowered due to the drop in pressure at theventuri throat.Figure 22.2As a result, the diaphragm moves to the left, opening the ball valve. Contributing to this force is the impact pressure that is picked up by the impact tubes.Figure22.3This pressure differential is referred to as the “air metering force.”

This force is accomplished by channeling the impact and venturi suction pressures to opposite sides of a diaphragm. The difference between these two

pressures becomes a usable force that is equal to the area of the diaphragm times the pressure difference.

Regulator Section:

The regulator section consists of a fuel diaphragm that opposes the air metering force. Fuel inlet pressure is applied to on diaphragm and metered fuel pressure is applied to the other side. The differential pressure across the fuel diaphragm is called the fuel metering force.

The fuel pressure shown on the ball side of the fuel diaphragm is thepressure after the fuel has passed through the fuel strainer and the manual

mixture control rotary plate and is referred to as metered fuel pressure. Fuel inlet pressure is applied to the opposite side of the fuel diaphragm. The ball valve attached to the fuel diaphragm controls the orifice opening and fuel

flow through the forces placed on it. Figure 22.4

The distance the ball valve opens is determined by the difference between the pressures acting on the diaphragms. This difference in pressure is proportional to the airflow through the injector. Thus, the volume of airflow determines the rate of fuel flow.

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FIGURE22.2 AIRFLOW SECTION OF A FUEL INJECTOR

FIGURE22.3 IMPACT TUBES FOR INLET AIRPRESSURE FIGURE22.4 FUEL DIAPHRAGM WITH BALLVALVE ATTACHED

Under low power settings, the difference in pressure created by the venturi is insufficient to accomplish consistent regulation of the fuel. A constant-head idle spring is incorporated to provide a constant fuel differential pressure.

This allows an adequate final flow in the idle range.

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Fuel Metering Section:The fuel metering section is attached to the air metering section and

contains an inlet fuel strainer, a manual mixture control valve, an idle valve, and the main metering jet.

FIGURE 22.5 FUEL METERING SECTION OF INJECTOR

Figure 21.5The idle valve is connected to the throttle valve by means of an external adjustable link. In some injector models, a power enrichment jet is

also located in this section.

FIGURE 22.6 FUEL INLET AND METERING

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The purpose of the fuel metering section is to meter and control the fuel flow to the flow divider. Figure22.6The manual mixture control valve produces full rich condition when the lever is against the rich stop, and a progressively leaner mixture as the lever is moved toward idle cutoff.

Both idle speed and idle mixture may be adjusted externally tomeet individual engine requirements.

Flow Divider:The metered fuel is delivered from the fuel control unit to a pressurized flow divider. This unit keeps metered fuel under pressure, divides fuel to the various cylinders at all enginespeeds, and shuts off the individual nozzle lines when the control is placed in idle cutoff.

Referring to the diagram in figure 22.7, metered fuel pressure enters the flow divider through a channel that permits fuel to pass through the inside diameter of the flow divider needle.

FIGURE 22.7 FLOW DIVIDER

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At idle speed, the fuel pressure from the regulator must build up to overcome the spring force applied to the diaphragm and valve assembly. This moves the valve upward until fuel can pass out through the annulus of the valve to the fuel nozzle.Since the regulator meters and delivers a fixed amount of fuel to the flow divider, the valve opens only as far as necessary to pass this amount to the nozzles. At idle,the opening required is very small; the fuel for the individual cylinders is divided at idle by the flow divider.

As fuel flow through the regulator is increased above idle requirements, fuel pressure builds up in the nozzle lines. This pressure fully opens the flow divider valve, and fuel distribution to the engine becomes a function of the discharge nozzles.

A fuel pressure gauge, calibrated in pounds per hour fuel flow, can be used as a fuel flow meter with the Bendix RSA injection system. This gauge is connected to the flow divider and senses the pressure being applied to the discharge nozzle. This pressure is in direct proportion to the fuel flow and

indicates the engine power output and fuel consumption.

Fuel Discharge Nozzles:The fuel discharge nozzles are of the air bleed configuration. There is one nozzle for each cylinder located in the cylinder head. The nozzle outlet is directed into the intake port. Each nozzle incorporates a calibrated jet.The jet size is determined by the available fuel inlet pressure and the maximum fuel flow required by the engine. The fuel is discharged through this jet into an ambient air pressure chamber within the nozzle assembly. Before entering the individual intake valve chambers, the fuel is mixed with air to aid in atomizing the fuel.

Fuel pressure, before the individual nozzles, is in direct proportion to fuel flow; therefore, a simple pressure gauge can be calibrated in fuel flow in gallons perhour and be employed as a flow meter.

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Engines modified with turbo superchargers must use shrouded nozzles. By the use of an air manifold, these nozzles are vented to the injector

air inlet pressure.

Continental/TCM Fuel-Injection System:The Continental fuel-injection system injects fuel into the intake valve port in each cylinder head. The system consists of a fuel injector pump, a control unit, a fuel manifold, and a fuel discharge nozzle. It is a continuous-flow type, which controls fuel flow to match engine airflow. The continuous-flow system permits the use of a rotary vane pump which does not require timing to the engine.

Fuel-Injection Pump:The fuel pump is a positive-displacement, rotary-vane type with a splined shaft for connection to the accessory drive system of the engine. Figure 22.8 spring-loaded, diaphragm-type relief valve is provided. The relief valve diaphragm chamber is vented to atmospheric pressure. A sectional view of a fuel injection pump is shown in figure 22.9.Fuel enters at the swirl well of the vapor separator. Here, vapor is separated by a swirling motion so that only liquid fuel is delivered to the pump. The vapor is drawn from the top center of the swirl well by a small pressure jet of fuel and is directed into the vapor return line. This line carries

the vapor back to the fuel tank.

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FIGURE22.8 FUEL PUMP

Ignoring the effect of altitude or ambient air conditions, the use of a positive-displacement, engine-driven pump means that changes in engine speed affect total pump flow proportionally. Since the pump provides greater capacity than is required by the engine, a recirculation path is required.

By arranging a calibrated orifice and relief valve in this path, the pump delivery pressure is also maintained in proportion to engine speed. These provisions assure proper pump pressure and fuel delivery for all engine

operating speeds.

A check valve is provided so that boost pump pressure to the system can bypass the engine-driven pump for starting. This feature also suppresses vapor formation under high ambient temperatures of the fuel, and permits use of the auxiliary pump as a source of fuel pressure in the event of engine driven pump failure.

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FIGURE 22.9 FUEL INJECTION PUMP

Fuel/Air Control Unit:

The function of the fuel/air control assembly is to control engine air intake and to set the metered fuel pressure for proper fuel/air ratio. The air throttle is mounted at the manifold inlet and its butterfly valve, positioned by the throttle control in the aircraft, controls the flow of air to the engine. Figure 22.10

The air throttle assembly is an aluminum casting which contains the shaft and butterfly-valve assembly. The casting bore size is tailored to the engine size, and no venturi or other restriction is used.

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FIGURE 22.10 FUEL AIR CONTROL UNIT.

Fuel Control Assembly:The fuel control body is made of bronze for best bearing action with the stainless steel valves. Its central bore contains a metering valve at one end and a mixture control valve at the other end. Each stainless steel rotary valve includes a groove which forms a fuel chamber.

FIGURE 22. 11 DUAL FUEL CONTROL ASSEMBLY

Fuel enters the control unit through a strainer and passes to the metering valve. Figure 22.11This rotary valve has a camshaped edge on the outer part of the end face. The position of the cam at the fuel delivery port controls the fuel passed to the manifold valve and the nozzles. The fuel return port Connects to the return passage of the center metering plug.

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The alignment of the mixture control valve with this passage determines the amount of fuel returned to the fuel pump. By connecting the metering valve to the air throttle, the fuel flow is properly proportioned to airflow for the correct fuel/ air ratio. A control level is mounted on the mixture control

valve shaft and connected to the cockpit mixture control.

Fuel Manifold Valve:The fuel manifold valve contains a fuel inlet, a diaphragm chamber, and outlet ports for the lines to the individual nozzles. Figure 22.12The spring-loaded diaphragm operates a valve in the central bore of the body. Fuel pressure provides the force for moving the diaphragm.

FIGURE22.12 FUEL MANIFOLD ASSEMBLY

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The diaphragm is enclosed by a cover that retains the diaphragm loading spring. When the valve is down against the lapped seat in the body, the fuel lines to the cylinders are closed off. The valve is drilled for passage of fuel from the diaphragm chamber to its base, and a ball valve is installed within the valve. All incoming fuel must pass through a fine screen installed in the diaphragm chamber.

From the fuel-injection control valve, fuel is delivered to the fuel manifold valve, which provides a central point fordividing fuel flow to the individual cylinders. In the fuel manifold valve, a diaphragm raises or lowers a plunger valve to open or close the individual cylinder fuel supply ports simultaneously.

FIGURE22.11 TYPICAL FUEL MANIFOLD ASSEMBLY

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Fuel Discharge Nozzle:The fuel discharge nozzle is located in the cylinder head with its outlet

directed into the intake port. The nozzle body contains a drilled centralPassage with a counterbore at each end.(figure 22.14)The lower end is used

as a chamber for fuel/air mixing before the spray leaves the nozzle. The upper bore contains a removable orifice for calibrating the nozzles.

FIGURE 22.14 FUEL DISCHARGE NOZZLES

Nozzles are calibrated in several ranges, and all nozzles furnished for one engine are of the same range and are identified by a letter stamped on the hex

of the nozzle body.

Drilled radial holes connect the upper counter bore with the Outside of the nozzle body. These holes enter the counter bore above the orifice and draw

air through a cylindrical screen fitted over the nozzle body. A shield is press-fitted on the nozzle body and extends over the greater part of the filter screen,

leaving an opening near the bottom. This provides both mechanical

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protection and an abrupt change in the direction of airflow which keeps dirt and foreign material out of the nozzle interior.

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PISTON ENGINE FUEL SYSTEM

SECTION 5

ANALYTICAL STUDY

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23. ADVANTAGES AND DISADVANTAGES

OF

FUEL INJECTION SYSTEM.

Advantages:1. Reliability

2. Higher peak power possible due to no restriction caused by carb venturi.

3. A fuel injected engine will function better in any environment as the sensors in the engine will quickly adjust the air/fuel mixture in order to

adapt to various environmental conditions.

4. Due to the precise mixing of the fuel and air by the system, fuel injected engines releases a smaller amount of emissions into the environment

providing that you keep your engine in good condition.

5. The biggest advantage fuel injection has over the other systems is the flexible fuel capability that has been added in recent years.

Disadvantages:1. The primary disadvantages of fuel injection system isComplexity and

cost2. fuel injection systems are more expensive to build because their

components must be more rugged –3. These handle fuel at significantly higher pressures than indirect

injection systems and the injectors themselves must be able to withstand the heat and pressure of combustion inside the cylinder.

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4. Slight loss of power, this disadvantage is only a problem if extreme power is what you want.

The below table gives the details of differences between the two fuel metering systems applied to a Cessna aircraft.

CARBURETORFUEL INJECTION

Gravity fed system Engine driven fuel pump

Gravity Never Fails Standby electric pump for use when engine driven pump fails

No airframe modification required. No fuel tanks in

cockpit

Extensive costly modification required.

Holes cut in firewall, fuel tanksInstalled in cockpit.

Carb Heat available if needed Induction Heat or alternate airAvailable if needed.

Cessna built the airplane to utilize a carbureted engine.

Additional instrumentation andControls required for injected

engine.

Very, very unlikely that the large fuel jet in the carburetor

can ever get blocked.One nozzle feeds six cylinders.

It has a “big hole”.

A fuel injector nozzle feeding only one cylinder is easily

blocked causing one cylinder to stop runningor a fuel nozzle

Partially blocked causing extreme high EGT and possible

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engine failure.

An O-360 engine produces 180 hp

IO360 engine produces 200hp, and the only real difference between them is the fueling

method.Less maintenance and can be maintained by lesser skilled

personnel.

Very sophisticated system requires specialized testing equipment for service and

adjustment.

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PISTON ENGINE FUEL SYSTEM

SECTION 6

COMPARITIVE STUDY

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24. COMPARATIVE STUDY

OF

EFI AND MECHANICAL FUEL INJECTION SYSTEM.

Mechanical fuel injection is used to indicate the metering functions of the fuel injection system. Carburetor fuel injection is an advancement of the mechanical fuel injection method. The aim of all types of fuel injections systems is to achieve a delivery of the exact air/fuel mix into the combustion engine. All the types of fuel injections try to force the fuel into the combustion engine under a great pressure through injectors. Fuel injectors provide good mileage and power improvements over RPM range of the engine.

Features of Mechanical Fuel Injection: All the fuel injection systems which may be a mechanical system or an EFI system follow only mechanical operations in order to force fuel into an engine. The difference is that the EFI system performs various on and off operations electronically, which are due to a variety of electronic controls present within the EFI system. These tell how and when the fuel must be injected into the engine. This facility is not available in the mechanical fuel injection system.

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The optimum ratio of air to fuel varies with the changes occurring in the atmospheric temperature, engine temperature, altitude, engine load and speed, ignition timings, and the gasoline engines. So the fuel injections must be designed in such a way that it must adjust to the engine requirements. Hence the fuel injection system must be able to sense the variations in these parameters. Microprocessors are used in order to serve this purpose. These microprocessors compute whether the correct amount of fuel is injected when it is required or not adjusting to the requirements of the engine.

Usage of Mechanical Fuel Injection: Carburetors were used until 1980 because they are inexpensive, and were consumer friendly. Being frequently used in petrol or gasoline engines. Fuel injection and precision pumps were also used at that time, but were very costly. Hence carburetors became the alternative. Also, they were incorporated into the engines such as Mopar and GM vehicles. But these became obsolete by the emergence of electronic fuel injection systems. Many American systems were not fully understood and proved to be very unreliable. Consumers switched quickly for the added value, reliability, and merely a more culturally appropriate device enjoyed in carburetors. Then a number of European systems came into existence.

Historically, mechanical injection systems were used only for racing cars and land speed records during 1950 and 1960. Hilborn and Enderle were 2 of the early innovators. This technology was utilized because EFI systems were

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not culturally relevant and difficult for consumer adaptation. It involved numerous electronic interventions.

EFI and Mechanical Fuel Injection: An EFI injector has special valves which open and close for fuel to enter the engine. They are electronically controlled. EFI sprays the fuel into the open valves in the form of a mist which is very fine. The valves are opened and closed alternatively in order to push the fuel inside. The injector also checks how much fuel is injected into it by a fuel track.

The special feature of EFI fuel injection is that it operates through mechanical sensors within the system. Sensors are included to ensure that the exact amount of fuel is injected at the specific time.

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PISTON ENGINE FUEL SYSTEM

SECTION 7

FINDINGS AND SUGGESTIONS

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25.FINDINGS AND SUGGESTIONS

Study of carburetors in details revealed that,in spite of very good performance the problem of icing perists to be there in

carburetors,which limits its usage to certains extent through carb heat is employed to overcome this problem.

The use of float mechanism in carburetor causes certains problems during inverted flight and aerobatic maneuver’s which results in sloshing of fuel,which may affect fuel air mixture due to formation of fumes

The carburetors also have the problem of vapor lock as continuous flow is not present by the use of carburetor.

If possible I world like to suggest that comburetors must employ electronic sensors which may caution the pilot of ice formation so that

he may limits his speed and altitude,and avoid ice formation. Study of injection system revealed that they work well in all aspects

compared to carburetors and also all the problems aroused in carburetors are dealt and have been eliminated in injection system.

What I found out in injection system is that,the cost and complexity of equipment’s employed in order to meter fuel accurately,which demands

high maintenance cost if something goes wrong in the system. Hence what I suggest is,if economical and cost effective injection is

can be designed by reducing the complex electronics without compromising the safety it would be a great hit.

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PISTON ENGINE FUEL SYSTEM

SECTION 8

CONCLUSION

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Conclusion

Aviation is a sector that grows year on year, the increasing number of budget airlines and routes inspiring passenger numbers to hit record levels. These increases have necessitated radical infrastructural developments, and the need to get aircraft into the air is more crucial than ever. Planes sitting on the tarmac are not profitable, cause queues and stop the smooth flow of passengers. Nowadays magneto-resistance type fuel level sensors are being commonly used in small aircraft applications because of the potential alternative they offer for automotive use. These are highly accurate, and the electronics are completely outside the fuel. The non-contact nature of these sensors address the fire and explosion hazard, and also the issues related to any fuel combinations or additives to gasoline or to any alcohol fuel mixtures. Magneto resistive sensors are suitable for all fuel or fluid combinations, including LPG and LNG. The fuel level output for these sensors can be ratio-metric voltage or preferable CAN bus digital. These sensors also fail-safe in that they either provide a level output or nothing.

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BIBLIOGRAPHY.

Aircraft power plants -12A. Aircraft power plants by kroes and wild.

History of carburetors and fuel injection systems –Wikipedia. Advantages and disadvantages from – various sites.

http://news.carjunky.com http://www.txskyways.com

www.whyhighend.com www.more-power.info

http://www.superchevy.com http://www.carsdirect.com

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