Fuel Injection system for SI engines; Carburetion
ENGINE AUXILIARY SYSTEMS IGNITION SYSTEM
The fuel feed system for the Spark
ignition engines and Compression ignition engines are clearly discussed below.
Fuel Injection system for SI engines;
Carburetion
1.
The fuel starts moving from the fuel tank
because of the pump working to suck in the fuel from the fuel tank
2.
After that the fuel travels through the
filter , which filters out the unwanted materials from the liquid .
3.
after that it travels to the injectors ,
giving fuel to the combustion chamber
4. we can see there is an accumulator
present in-between the fuel filter and fuel pump , henceforth the fuel travels
to the distributor
5. from there it goes through the inlet
pipe and then the inlet manifold mixing with the air
Definition of
Carburetion;
Definition of
Carburetor;
The carburetor is a
device used for atomizing and vaporizing the fuel and mixing it with the air in
varying proportions to suit the changing operating conditions of vehicle
engines.
Factors Affecting
Carburetion
ii. The
vaporization characteristics of the fuel
iii. The
temperature of the incoming air and
iv. The design of
the carburetor
The rate at which
fuel is discharged into the air depends on the pressure difference or pressure
head between the float chamber and the throat of the venturi and on the area of
the outlet of the tube. In order that the fuel drawn from the nozzle may be thoroughly
atomized, the suction effect must be strong and the nozzle outlet comparatively
small. In order to produce a strong suction, the pipe in the carburetor
carrying air to the engine is made to have a restriction. At this restriction
called throat due to increase in velocity of flow, a suction effect is created.
The restriction is made in the form of a venturi to minimize throttling losses.
The end of the fuel
jet is located at the venturi or throat of the carburetor. The geometry of
venturi tube is as shown in Fig.16.6. It has a narrower path at the center so
that the flow area through which the air must pass is considerably reduced. As
the same amount of air must pass through every point in the tube, its velocity
will be greatest at the narrowest point. The smaller the area, the greater will
be the velocity of the air, and thereby the suction is proportionately
increased
As
mentioned earlier, the opening of the fuel discharge jet is usually loped where
the suction is maximum. Normally, this is just below the narrowest section of
the venturi tube. The spray of gasoline from the nozzle and the air entering
through the venturi tube are mixed together in this region and a combustible
mixture is formed which passes through the intake manifold into the cylinders.
Most of the fuel gets atomized and simultaneously a small part will be
vaporized. Increased air velocity at the throat of the venturi helps he rate of
evaporation of fuel. The difficulty of obtaining a mixture of sufficiently high
fuel vapour-air ratio for efficient starting of the engine and for uniform
fuel-air ratio indifferent cylinders (in case of multi cylinder engine) cannot
be fully met by the increased air velocity alone at the venturi throat.
The Simple
Carburetor
Carburetors are
highly complex. Let us first understand the working principle bf a simple or
elementary carburetor that provides an air fuel mixture for cruising or normal
range at a single speed. Later, other mechanisms to provide for the various
special requirements like starting, idling, variable load and speed operation
and acceleration will be included. Figure 3. shows the details of a simple
carburetor.
The simple
carburetor mainly consists of a float chamber, fuel discharge nozzle and a
metering orifice, a venturi, a throttle valve and a choke. The float and a
needle valve system maintain a constant level of gasoline in the float chamber.
If the amount of fuel in the float chamber falls below the designed level, the
float goes down, thereby opening the fuel supply valve and admitting fuel. When
the designed level has been reached, the float closes the fuel supply valve
thus stopping additional fuel flow from the supply system. Float chamber is
vented either to the atmosphere or to the” upstream side of the venturi.During
suction stroke air is drawn through the venturi.
As already
described, venturi is a tube of decreasing cross-section with a minimum area at
the throat, Venturi tube is also known as the choke tube and is so shaped that
it offers minimum resistance to the air flow. As the air passes through the
venturi the velocity increases reaching a maximum at the venturi throat.
Correspondingly, the pressure decreases reaching a minimum. From the float
chamber, the fuel is fed to a discharge jet, the tip of which is located in the
throat of the venturi. Because of the differential pressure between the float
chamber and the throat of the venturi, known as carburetor depression, fuel
The fuel discharge
is affected by the size of the discharge jet and it is chosen to give the
required air-fuel ratio. The pressure at the throat at the fully open throttle
condition lies between 4 to 5 cm of Hg, below atmospheric and seldom exceeds8
cm Hg below atmospheric. To avoid overflow of fuel through the jet, the level
of the liquid in the float chamber is maintained at a level slightly below the
tip of the discharge jet. This is called the tip of the nozzle. The difference
in the height between the top of the nozzle and the float chamber level is
marked h in Fig.3.
The gasoline engine
is quantity governed, which means that when power output is to be varied at a
particular speed, the amount of charge delivered to the cylinder is varied.
This is achieved by means of a throttle valve usually of the butterfly type
that is situated after the venturi tube.
As the throttle is
closed less air flows through the venturi tube and less is the quantity of air-
fuel mixture delivered to the cylinder and hence power output is reduced. As
the” throttle is opened, more air flows through the choke tube resulting in increased
quantity of mixture being delivered to the engine. This increases the engine
power output. A simple carburetor of the type described above suffers from a
fundamental drawback in that it provides the required A/F ratio only at one
throttle position.
At the other
throttle positions the mixture is either leaner or richer depending on whether
the throttle is opened less or more. As the throttle opening is varied, the air
flow varies and creates a certain pressure differential between the float
chamber and the venturi throat. The same pressure differential regulates the
flow of fuel through the nozzle. Therefore, the velocity of flow of air II and
fuel vary in a similar manner.
The Choke and the
Throttle
When the vehicle is
kept stationary for a long period during cool winter seasons, may be overnight,
starting becomes more difficult. As already explained, at low cranking
speeds and intake temperatures a very rich mixture is required to initiate
combustion. Some times air-fuel ratio as rich as 9:1 is required. The main
reason is that very large fraction of the fuel may remain as liquid
suspended
in air even in the cylinder. For initiating combustion, fuel-vapour and air in
the form of mixture at a ratio that can sustain combustion is required.
It may be noted that
at very low temperature vapour fraction of the fuel is also very small and this
forms combustible mixture to initiate combustion. Hence, a very rich mixture
must be supplied. The most popular method of providing such mixture is by the
use of choke valve. This is simple butterfly valve located between the entrance
to the carburetor and the venturi throat as
When the choke is
partly closed, large pressure drop occurs at the venturi throat that would
normally
result from the
quantity of air passing through the venturi throat. The very large depression
at the throat inducts large amount of fuel from the main nozzle and provides a
very rich mixture so that the ratio of the evaporated fuel to air in the
cylinder is within the combustible limits. Sometimes, the choke valves are
spring loaded to ensure that large carburetor depression and excessive choking
does not persist after the engine has started, and reached a desired speed.
This choke can be
made to operate automatically by means of a thermostat so that the choke is
closed when engine is cold and goes out of operation when engine warms up after
starting. The speed and the output of an engine is controlled by the use of the
throttle valve, which is located on the downstream side of the venturi.
The more the
throttle is closed the greater is the obstruction to the flow of the
mixture placed in the passage and the less is the quantity of mixture
delivered to .the cylinders. The decreased quantity of mixture gives a
less powerful impulse to the pistons and the output of the engine is
reduced accordingly. As the throttle is opened, the output of the engine
increases. Opening the throttle usually increases the speed of the
engine. But this is not always the case as the load on the engine is also a
factor. For example, opening the throttle when the motor vehicle is starting to
climb a hill may or may not increase the vehicle speed, depending upon the
steepness of the hill and the extent of throttle opening. In
short, the throttle is simply a means to regulate the output of the
engine by varying the quantity of charge going into the cylinder.
Compensating Devices
An automobile on
road has to run on different loads and speeds. The road conditions play a vital
role. Especially on city roads, one may be able to operate the vehicle between
25 to 60% of the throttle only. During such conditions the carburetor must be
able to supply nearly constant air-fuel ratio mixture that is economical
(16:1).However, the tendency of a simple carburetor is to progressively richen
the mixture as the throttle starts opening.
The main metering
system alone will not be sufficient to take care of the needs of the engine.
Therefore, certain compensating devices are usually added in the carburetor
along with the main metering system so as to supply a mixture with the required
air-fuel ratio. A number of compensating devices are in use.
The important ones
are
i.
Air-bleed jet
ii.
Compensating jet
iii.
Emulsion tube
iv. Back
suction control mechanism
v.
Auxiliary air valve
vi. Auxiliary air
port
As already
mentioned, in modern carburetors automatic compensating devices are provided to
maintain the desired mixture proportions at the higher speeds. The type of
compensation mechanism used determines the metering system of the carburetor.
The principle of operation of various compensating devices are discussed
briefly in the following sections.
Carburetor
features schematic diagram
Schematic diagram shows the carburetor features of a simple Y
and an ideal X carburetor.
Carburetor Features
They show the change in the composition of the
combustible mixture of the carburetor depending on the load (on the position of
the throttle valve). As the throttle valve opens in the simplest carburetor,
the combustible mixture becomes more enriched, and in only two cases (points C
and D) does the composition of the mixture coincide with the composition of the
combustible mixture prepared by the ideal carburetor (with the throttle fully
open and at some intermediate position). Thus, the main disadvantage of the
simplest carburetor is the inability to prepare a
combustible mixture of the desired composition.
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