Piston Engine Fuel Systems: Carburetion & Injection Guide

0 PISTON ENGINE FUEL SYSTEM
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

CONTENTS
1. INTRODUCTION TO CARBUKRETION ………………..7
2. MEANING AND DEFINITIONS ………………………….9
3. OBJECTIVES ……………………………………………..12
4. EVOLUTION OF CARBURETORS……………………...13
5. PRINCIPLES OF CARBURETION ………………………16
6. VENTURI PRINCIPLES …………………………………17
7. FLOAT-TYPE CARBURETORS…………………………25
8. FLOAT MECHANISM WORKING………………………26
9. MAIN METERING SYSTEM ……………………………30
10. IDLING SYSTEM……………………………………32
11. ACCELERATING SYSTEM…………………………38
12. ECONOMIZER SYSTEM……………………………41
13. MIXTURE CONTROL SYSTEM……………………46
14. AUTOMATIC MIXTURE CONTROL ……………..52
15. DOWNDRAFT CARBURETORS…………………..56
16. PRESSURE INJECTION CARBURETORS…………57
17. PRINCIPLE OF OPERATION………………………58
18. ADVANTAGES AND DISADVANTAGES OF
CARBURETORS…………………………………………77
19. INTRODUCTION TO FUEL INJECTION
SYSTEMS…………………………………………………79
20. HISTORY OF FUEL INJECTION……………………81
21. FUEL INJECTION SYSTEM TYPES………………..82
22. TYPICAL FUEL INJECTION SYSTEM …………….84
23. ADVANTAGE AND DISADVANTAGE FUEL
INJECTION SYSTEM……………………………………..100

24. COMPARATIVE STUDY OF EFI AND MFI ………..104
25. FINDINGS AND SUGGESTIONS……………………..108
26. CONCULSION ………………………………………....110
27. BIBLIOGRAPHY……………………………………….112
APPENDIX……………………

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 BREAKINGUP 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 METERINGSYSTEM…………………………………………………….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 ACCELERATINGPUMP IN OPERATION……………………………………...40
FIGURE 12.1 NEEDLE VALVE ECONOMIZER………………………………………………43

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 MEASURINGSECTION………………….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

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…………………

PISTON ENGINE FUEL SYSTEM
SECTION 1
INTRODUCTION
TO
CARBURETION

INTRODUCTION
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
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
form an explosive mixture especiallyfor 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 UniversityPress.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

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.
 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.I
t was apparently also invented by theHungarian engineers JanosCsonka and
DonáBánki in 1893.FrederickWilliamLanchester of Birmingham, England ex
perimentedearly on with the wick carburetor in cars.
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.Unh
appy 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 successfullyincor
porating 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 las
t carbureted light truck was the 1994suzu. Elsewhere, Lada cars used carburet
ors until 1996.

Amajority of motorcycles still utilize carburetors due to lower costand throttl
e response problems with early injection set ups, but asof 2005, many new m
odels are now being introduced with fuel
injection Carburetors are still found in small engines and in olderor specialize
d automobiles, such as those designed for stock car racing
FIGURE 4.1SIDE DRAFT HORIZONTAL CARBURETOR
FIGURE 4.2 CUT VIEW OF BASIC CARBURETOR

PISTON ENGINE FUEL SYSTEM
SECTION 3
IMPORTANCE

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.

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 calledthe 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.

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

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 airspeedin 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 deliveryfalls
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

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

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

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

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

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 closelyagainst 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
deliveredto 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

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 deliveredto 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.

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
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
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

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.

9.MAIN METERING SYSTEM
The 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
.

The airflow through an opening of fixed size and the fuel flow through
an air-bleedjet system respond to variations of pressure in approximately
equal proportions. if the discharge nozzle of the air-bleedsystem is located
in the center of the venturi, so that both the air-bleednozzle 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 desiredfor 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 relativelylow manifold vacuum. This results in high volumetric
efficiencies.

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.

FIGURE 10.1 THREEPIECE MAIN DISCHARGE ASSEMBLY
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
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
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-bleedinlet. 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.

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

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.
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.
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 raisedand 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.
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 SYSTEM
An 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
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 deliveryat
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

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.

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.

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.

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 describedas a mechanism or device
through which the richness of the mixture entering engine during flight can

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 widelyused. 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.

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.

FIGURE13.1 NEEDLE TYPE MIXTURE CONTROL

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.

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.

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.

FIGURE14.1AUTOMATICMIXTURECONTROLMECHANISM

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 provisionfor manual control if the
automatic control fails.

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.

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
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
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 systemwhich 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
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 loweredbecause 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.
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.
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
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.
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
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.
If the vapor vent valve sticks open or the vapor vent float becomes filled with
fuel and sinks, a continuous flow of fuel and vapor occurs through the vent
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 airfollowedby 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-parallelhookups
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
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
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.
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.

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 slowlyfrom chamber A
to chamber B through the small bleed (often referred to as the backsuction
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 basicallyof 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.

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

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.

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.
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.
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.

PISTON ENGINE FUEL SYTEM
SECTION 4
CHARACTERISTIC
FEATURES

18. ADVANTAGES AND DISADVANTAGES OF
CARBURETORS.
Carburetors work amazingly well, considering that they are just basicallya
"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.

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.

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 possibilityin

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.

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

Miglia. The engine had six electricallyoperated 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.

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 deliveryoperates 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
(possiblyall) 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.

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 describedto 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.

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

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
lowereddue 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 the
pressure 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.

FIGURE22.2 AIRFLOW SECTION OF A FUEL INJECTOR
FIGURE22.3 IMPACT TUBES FORINLET 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.

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

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 to
meet individual engine requirements.
Flow Divider:
The metered fuel is deliveredfrom 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

At idle speed, the fuel pressure from the regulator must build up to overcome
the spring force appliedto 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.

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 swirlingmotion so that only liquid fuel is deliveredto 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.

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.

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.

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.

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 properlyproportioned 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

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 drilledfor 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

Piston Engine Fuel Systems: Carburetion & Injection Guide

Piston Engine Fuel Systems: Carburetion & Injection Guide

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