ic engine carburation

1. MODULE-2
CARBURETION AND FUEL INJECTION
In the SI engine a combustible fuel-air mixture is
prepared outside the engine cylinder. The process of
preparing this mixture is called carburetion. The
carburetor, a device which atomises the fuel and mixes it
with air. The pipe that carries the prepared mixture to
the engine cylinders is called the intake manifold.
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2. A SIMPLE OR ELEMENTARY CARBURETTOR
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3. DRAWBACKS (LIMITATIONS) OF SIMPLE
CARBURETTOR (SINGLE JET)
(1) A single jet carburetor can not provide a very rich mixture as required at
the time of starting the engine
(2) It can not provide very rich mixture required for sudden acceleration of the
engine.
(3) For gradually increasing pressure differences over the jet (at higher speed
of the engine), the weight of the petrol discharged from a single jet
increases at a greater rate than does the air supply. Hence, a single jet
carburetor gives a progressively richer mixture as the air speed increases
when set to give a correct mixture at low air speeds.
(4) It can not reduce the quantity of air flow during starting as required in cold
weather conditions.
(5) The automation control of air and fuel according to the required conditions
is not possible.
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4. PETROL INJECTIONS
• There are two petrol injection systems which are mechanically operated
(1) Continuous port Injection (2) Electronic Fuel Injection system.
• (1) Continuous port Injection system (Lucas Mechanical petrol
Injection System)
In this system, fuel is injected continuously into the inner port as
shown in figure at a varied rate. The petrol is sucked from the tank by
a pump and pressurized petrol at 3 bar is supplied through a
distributor to the fuel injector to a particular cylinder. The relief valve
shown in figure maintains the pressure and allows excess petrol to
return to the tank.
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5. LUCAS MECHANICAL PETROL INJECTION
SYSTEM
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6. ELECTRONIC FUEL INJECTION SYSTEM
• Now-a-days electronically controlled fuel injection system are commonly
used as it functions rapidly and responds automatically to the change in
manifold air pressure, engine speed, crankshaft angle an many other
secondary factors. The electronic control unit assesses data (manifold
pressure, engine speed, crank ange) received from a various sensing
devices and then adjust the A:F supply for the best performance of the
engine.
This system has to contain a means of supplying extra fuel for cold starting,
during warming and enriching the mixture during acceleration.
Ice formation is virtually impossible with this system and the danger of
vaporization is minimized because the petrol is under pressure right upto
the injection point.
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7. FUEL INJECTION SYSTEM-L-JETRONIC WITH
AIR-FLOW METERING
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8. Contd.
(1) Fuel delivery system
(2) Air induction system
(3) Sensors and air flow control system
(4) Electronic control unit
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9. REQUIREMENTS OF DIESEL INJECTION
(1) To supply the correct quantity of fuel to be injected as per the load of the
engine and increase in speed for automobile engines.
(2) To supply the fuel within a precisely defined period of the cycle
(3) To control the rate of fuel injection, such that it should result in the
desired heat release pattern.
(4) To atomize the fuel into very fine particles.
(5) To distribute the fuel uniformly in the combustion chamber of the engine
and results in rapid mixing of fuel and air
(6) Injection starts and stops sharply. There should not be any after-injection.
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10. TYPES OF INJECTION SYSTEMS
Injection system may be divided into two general types, as follows;
(1) Air Injection
(2) Air-less or Solid or Mechanical Injection
(i) Common Rail system (ii) Distributor system (unit Injector)
(iii) Individual pump and nozzle system
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11. AIR INJECTION
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12. Contd.
Advantages:
(1) It provides better automization and distribution of fuel
(2) As the combustion is more complicated, the BMEP is higher than with
other types of injection system.
(3) It allows to use the inferior fuels
Disadvantages:
(1) It requires complicated mechanism to run the compressor
(2) The weights of the engine increases
(3) Part of the power is used to drive the compressor and so the BHP of
the engine is reduced.
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13. AIR-LESS OR SOLID OR MECHANICAL INJECTION
Here, Injection of fuel directly into the combustion chamber without primary
atomisation is termed as solid injection. It is also termed as mechanical
injection.
Theory: In this system ,the fuel is supplied at a very high pressure (150bar)
from the fuel pump to the fuel injector where it is injected to the combustion
chamber with the help of an injector. The main parts of this system are fuel
pump and fuel injector. Depending upon the location of the fuel pumps and
injectors, and upon the method used to meter the fuel, solid injection may
further be classified as follows:
Air-less Injection may further be classified as follows:
(i) Common Rail system (ii) Distributor system (unit Injector)
(iii) Individual pump and nozzle system
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14. COMMON RAIL SYSTEM
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15. Contd.
A single pump supplies high-pressure fuel to header, a relief valve holds
pressure constant. The control wedge adjusts the lift of mechanical
operated valve to set amount and time of injection.
Advantages: (1) It fulfills the requirements of either, (a) the constant load
with variable speed or (b) constant speed with variable load.
(2) Only one pump is sufficient for multi-cylinder engine.
(3) Variation in pump supply pressure will effect all the cylinder uniformly
(4) The arrangement of the system is very simple and maintenance cost is less
Disadvantages:
(1) Very accurate design and workmanship are required
(2) There is tendency to develop leaks in the injection valve.
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16. DISTRIBUTOR SYSTEM (UNIT INJECTOR)
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17. Contd.
In this system, the fuel is metered at a central point; a pump pressurises,
meters
the fuel and times the injection. From here, the fuel is distributed to cylinders
in correct firing order by cam operated poppet valves which open to admit fuel
to the nozzles.
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18. INDIVIDUAL PUMP AND NOZZLE SYSTEM
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19. Contd.
In this system an individual pump or pump cylinder connects directly to each
fuel nozzle. Pump meters charge and control injection timing. Nozzles contain
a delivery valve actuated by the fuel-oil pressure
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20. FUEL PUMP
Fuel Pump: A large number of igneous fuel pump designs have been
developed by the manufacturers, Only one type fuel pump will be
discussed here.
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21. BOSCH FUEL INJECTION PUMP
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22. Contd. (EXPLANATION)
When the plunger is at its bottom stroke the ports SP and Y are uncovered
and
oil from low pressure pump (not shown) after being filtered is forced into the
barrel. When the plunger moves up due to cam and tappet mechanism, a stage
reaches when both the ports SP and Y are closed and with the further upward
movement of the plunger the fuel gets compressed. The high pressure thus
developed lifts the delivery valve off its seats and fuel flows to atomizer
through the passage P. With further rise of the plunger, at a certain moment,
the
port SP is connected to the fuel in the upper part of the plunger through the
rectangular vertical groove by the helical groove, as a result of which a sudden
drop in pressure occurs and the delivery valve falls back and occupies its seat
against the spring force.
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23. Contd.
The plunger is rotated by the rack R which is moved in or out by the governor.
By changing the angular position of the helical groove (by rotating the
plunger) of the plunger to the supply port, the length of stroke during which
the oil is delivered can be varied and thereby quantity of fuel delivered to the
engine is also varied accordingly.
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24. FUEL INJECTOR (ATOMISER)
(1) It consists of a nozzle valve (NV) fitted in the nozzle body (NB). The
nozzle valve is held on its seat by a spring (S) which exerts pressure through
the spindle (E). (AS) is the adjusting screw by which the nozzle valve lift can
be adjusted. Usually the nozzle valve is seat to lift at 135 to 170 bar
pressure.(FP) is the feeling pin which indicates whether valve is working
properly or not.
(2) The fuel under pressure from the fuel pump enters the injector through the
passages (B) and C and lifts the nozzle valve. The fuel travels down nozzle N
and injected into the engine cylinder in the form of fine spray. Then the
pressure of the oil falls, the nozzle valve occupies its seat under the spring
force and fuel supply is cut off. Any leakage of fuel accumulated above the
valve is led to the fuel tank through the passage A. The leakage occurs when
the nozzle valve is worn out.
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25. FUEL ATOMISER OR INJECTOR
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26. TYPES OF NOZZLES AND FUEL SPRAY PATTERNS
Main Requirements of an Injector Nozzle:
(1) To inject fuel at a sufficiently high pressure so that the fuel enters the
cylinder with a high velocity. Higher the velocity of the fuel smaller
will be the droplet size. The momentum of smaller droplets is less,
hence, penetration is also.
(2) Penetration should not be high so as to impinge on cylinder walls; this
may result in poor starting
(3) Fuel supply and cut-off should be rapid; there should be no dribbling.
Types of Nozzles:
The type of nozzle used is greatly dependent on the type of combustion
chamber as open type or pre-combustion chamber. The nozzles are
classified as per the type of orifice nd its number used for injecting the
fuel in the combustion chamber. The nozzles are classified as:
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27. Contd.
The nozzles are classified as :
(1) Single hole nozzle
(2) Multi-hole nozzle
(3) Circumferential nozzle
(4) Pintle nozzle
(5) Pintaux nozzle
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28. SINGLE HOLE NOZZLE
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29. Contd.
(a) This is the simplest type of nozzle and is used in open combustion
chambers
(b) It consists of a single hole bored centrally through the nozzle body and
closed by the needle valve. The size of the hole is usually larger than 0.2
mm
(c) Its sprays cone angle varies from 5 to 150
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30. MULTI-HOLE NOZZLE
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31. Contd.
(1)This type of nozzle finds extensive use in automobile engines,
particularly having open combustion chambers.
(2)It mixes the fuel with air properly even with slow air movement
available with open combustion chambers.
(3) The number of holes varies from 4 to 18; the greater number provides
better fuel distribution. The hole diameter lies between 0.25 to 0.35
mm and hole angle lies between 20 to 450
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32. CIRCUMFERENTIAL ORIFICE (NOZZLE)
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33. Contd.
(1) It sprays characteristics are similar to a plate type opening
(2) The injected fuel particles tend to be projected in the form of plane,
with wide angle cone; the purpose of which is to obtain as large as
possible to come into contact with the air in the combustion chamber.
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34. PINTLE NOZZLE
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35. Contd.
(1) The stream of the nozzle valve is extended to form a pin or pintle which
protrudes through the mouth of the nozzle body. It may be either
cylindrical or conical in shape
(2) The size and shape of the pintle can be varied according to requirement.
The spray core angle is generally 600
(3) The spray obtained by the pintle nozzle is hollow conical spray.
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36. END
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