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CHAPTER NO 09
MECHANICAL
INJECTION
SYSTEMS
TALHA BIN NADEEM
LECTURER
DEPARTMENT OF MECHANICAL ENGINEERING
NED UNIVERSITY OF ENGINEERING & TECHNOLOGY
INTRODUCTION
 The fuel injection system is the most vital component
in the working of CI engines.
 The engine performance is greatly dependent on the
effectiveness of fuel injection system.
 The injection system has to perform the important
duty of initiating and controlling the combustion
process.
COMPARISON BETWEEN CARBURETOR & MECHANICAL
INJECTION SYSTEMS
 Purpose of Carburetion and Fuel Injection = Preparation of combustible
charge but there is some difference:
Carburetion Fuel injection
Air Speed is greater than fuel
speed at fuel nozzle to atomize
the fuel
Fuel Speed is greater than Air
speed at the delivery point to
atomize the fuel
The amount of fuel drawn into
the engine depends upon
the air velocity in the venturi
The amount of fuel delivered
into the air stream going to
the engine is controlled by a
pump which forces the fuel
under pressure
FUNCTIONAL REQUIREMENTS OF INJECTION SYSTEM
1) Accurate metering of the fuel injected per cycle. This is
very critical due to the fact that very small quantities of
fuel being handled. Metering errors may cause drastic
variation from the desired output. The quantity of the fuel
metered should vary to meet speed and load requirements
of the engine.
2) Timing the injection of the fuel correctly in the cycle to
obtain maximum power; ensuring fuel economy and clean
burning.
3) Proper control of rate of injection so that the desired heat
release pattern is achieved during combustion.
4) Proper atomization of fuel into very fine droplets.
FUNCTIONAL REQUIREMENTS OF INJECTION SYSTEM
5) Proper spray pattern to ensure rapid mixing of fuel and
air
6) Uniform distribution of fuel droplets throughout the
combustion chamber.
7) To supply equal quantities of metered fuel to all
cylinders in case of multi cylinder engines.
8) No lag during beginning and end of injection i.e. to
eliminate dribbling of fuel droplets into the cylinder.
CLASSIFICATIONS OF INJECTION SYSTEMS
 A fuel-injection system is required to inject and atomize fuel into
the cylinder of CI engines.
 For producing the required pressure for atomizing the fuel either
air or a mechanical means is used.
 Thus the injection systems can be classified as:
Air injection system
Solid injection systems
AIR INJECTION SYSTEM
 In this system, fuel is forced into the cylinder by means of compressed
air.
Advantages:
1- Good mixing of fuel with the air
2- Higher MEP
3- It has the ability to utilize high viscosity
(less expensive) fuels
Disadvantages:
1- The requirement of bulky multistage air
compressor making air injection system
obsolete
SOLID (AIRLESS MECHANICAL) INJECTION SYSTEM
 In this system, the liquid fuel is injected directly into the combustion
chamber without the aid of compressed air. Hence, it is also called
airless mechanical injection (or) solid injection system.
 Solid injection systems can be classified into four types:
1) Individual Pump and Nozzle Systems
2) Unit Injector Systems
3) Common Rail Systems
4) Distributor Systems
MAIN COMPONENTS OF MECHANICAL INJECTION SYSTEMS
 All the injection systems (mentioned in slide # 08) mainly comprise of
the following components.
1) Fuel tank
2) Fuel feed pump to supply fuel from the main fuel tank to the
injection system
3) Injection pump to meter and pressurize the fuel for injection
4) Governor to ensure that the amount of fuel injected is in
accordance with the variation in load
5) Injector to take the fuel from the pump and distribute it in the
combustion chamber by atomizing it into fine droplets
6) Fuel filters to prevent dust and abrasive particles from entering the
pump and injectors thereby minimizing the wear and tear of the
components
MAIN COMPONENTS OF MECHANICAL INJECTION SYSTEMS
WORKING OF MECHANICAL INJECTION SYSTEMS
 Fuel from the fuel tank first enters the coarse filter from which it is
drawn into the plunger feed pump where the pressure is raised very
slightly.
 Now the fuel enters the fine filter where all the dust & dirt particles
are removed.
 From the fine filters the fuel enters the fuel feed pump where it is
pressurized to about 200 bar & injected into the engine cylinder by
means of injector.
 Any spill over in the injector is returned to fine filter. A pressure
relief valve is also provided for the safety of system.
INDIVIDUAL PUMP AND NOZZLE SYSTEMS
 In this system, each cylinder is provided with
one pump and one injector.
 In this arrangement a separate metering and
compression pump is provided for each cylinder.
 The pump may be placed close to the cylinder
(fig. a) or they may be arranged in a cluster (fig.
b) .
 The high-pressure pump plunger is actuated by
a cam and produces the fuel pressure necessary
to open the injector valve at the correct time.
 The amount of fuel injected depends on the
effective stroke of the plunger.
UNIT INJECTOR SYSTEM
 The unit injector system is one in which the
pump and the injector nozzle are combined in
one housing.
 Each cylinder is provided with one of these unit
injectors.
 Fuel is brought up to the injector by a low-
pressure feed pump, where at the proper time,
a rocker arm actuates the plunger and thus
injects the fuel into the cylinder.
 The amount of fuel injected is regulated by the
effective stroke of the plunger.
 The pump and the injector can be integrated in
on unit as shown in figure (c).
COMMON RAIL SYSTEM
 In the common rail system, a HP system supplies fuel, under high
pressure, to a fuel header.
 High pressure in the header forces the fuel to each of the nozzles
located in the cylinders.
 At the proper time, a mechanically operated (by means of a push
rod and rocker arm) valve allows the fuel to enter the proper
cylinder through nozzle.
 The pressure in the fuel header must be that, for which the
injector system was designed, i.e. it must be enable to penetrate
and disperse the fuel in the combustion chamber.
 The amount of fuel entering the cylinder is regulated by varying
the length of the push rod stroke
COMMON RAIL SYSTEM
 A high-pressure pump is used for supplying fuel to a header,
from where the fuel is metered by injectors (assigned one per
cylinder).
DISTRIBUTOR SYSTEM
 In this system the pump which pressurizes the fuel also meters
and times it.
 The fuel pump after metering the required amount of fuel supplies
it to a rotating distributor at the correct time for supply to each
cylinder.
 The number of injection strokes per cycle for the pump is equal to
the number of cylinders.
 Since there is one metering element in each pump, a uniform
distribution is automatically ensured. Not only that, the cost of
the fuel injection system also reduces to a value less than two-
thirds of that for individual pump system.
DISTRIBUTOR SYSTEM
COMPARISON OF VARIOUS FUEL-INJECTION SYSTEMS
FUEL FEED PUMP
 It is of spring-loaded plunger type.
 The plunger is actuated through a push
rod from the cam shaft.
 At the minimum lift position of the cam
the spring force on the plunger creates a
suction which causes fuel flow from the
main tank into the pump.
 When the cam is turned to its maximum
lift position, the plunger is lifted upwards.
 At the same time the inlet valve is closed,
and the fuel is forced through the outlet
valves.
INJECTION PUMP
 The main objectives of fuel-injection pump is to deliver accurately
metered quantity of fuel under high pressure (in the range from
120 to 200 bar) at the correct instant to the injector fitted on each
cylinder.
 Injection pumps are of two types:
1) Jerk Type Pumps
2) Distributor Type Pumps
JERK TYPE PUMP
JERK TYPE PUMP
 It consist of the reciprocating plunger
inside a barrel. The plunger is driven by
camshaft.
(a) A sketch of typical plunger is shown.
(b) Near the port A, fuel is always available
under relatively low pressure. While the
axial movement of the plunger is through
camshafts, its rotational movement
about its axis by means of rack D. Port B
is orifice through which fuel is delivered
to injector.
When the plunger is below port A, the
fuel gets filled in the barrel above it. As
the plunger rises & closes the port A the
fuel will flow out through port C.
JERK TYPE PUMP
(c) At this stage, the rack rotates the plunger
& as a result port C also closes. The only
escape route for the fuel is past the check
valve through this orifice B to the injector.
This is the beginning of the injection &
also effective stroke of plunger.
(d) The injection continues till the helical
indentation on the plunger uncovers port
C. Now the fuel will take the easy way out
through C and the check valve will close
the orifice B. The fuel injection stops &
the effective stroke ends. Hence the
effective stroke of the plunger is the axial
distance traversed between the time port
A is closed off & the time port A is
uncovered.
JERK TYPE PUMP
(e) & (f) The plunger is rotated to the
position shown. The same sequence of
events occur. But in this case C is
uncovered sooner. Hence the effective
stroke is shortened.
 It is important to remember here that
though the axial distance traversed by
the plunger is same for every stroke,
the rotation of the plunger by the rack
determining the length of the effective
stroke & thus the quality of fuel
injected.
DISTRIBUTOR TYPE PUMPS
 This pump has only a single pumping
element and the fuel is distributed to each
cylinder by means of a rotor.
 A central longitudinal passage in the rotor
and also two sets of radial holes (each
equal to the number of engine cylinders)
located at different heights.
 One set is connected to pump inlet via
central passage whereas the second set is
connected to delivery lines leading to
injectors of the various cylinders.
DISTRIBUTOR TYPE PUMPS
 The fuel is drawn into the central rotor
passage from the inlet port when the pump
plunger move away from each other.
 Wherever, the radial delivery passage in
the rotor coincides with the delivery port
for any cylinder the fuel is delivered to
each cylinder in turn.
 Main advantages of this type of pump lies
in its small size and light weight.
INJECTION PUMP GOVERNOR
 Fuel delivered by a pump increases with speed whereas the
opposite is true about the air intake.
 This results in over fueling at higher speeds, and at idling speeds
(low speeds) the engine tends to stall due to insufficiency of fuel.
 Quantity of fuel delivered increases with load causing excessive
carbon deposits and high exhaust temperature.
 Drastic reduction in load will cause over speeding to dangerous
values.
 It is the duty of injection pump governor to take care of the above
limitations. Governors are generally of two types:
1) Mechanical Governor
2) Pneumatic Governor
MECHANICAL GOVERNOR
 When the engine speed tends to
exceed the limit the weights fly
apart. This causes the bell crank
levers to raise the sleeve and
operate the control lever in
downward direction.
 This actuates the control rack on
the fuel injection pump in a
direction which reduces the
amount of fuel delivered. Lesser
fuel causes the engine speed to
decrease.
PNEUMATIC GOVERNOR
 The amount of vacuum applied
to the diaphragm is controlled
by the accelerator pedal through
the position of the butterfly
valve in the venturi unit.
 A diaphragm is connected to the
fuel pump control rack.
 Position of the accelerator pedal
also determines the position of
the pump control rack and
hence the amount of fuel
injected.
FUEL INJECTOR ASSEMBLY
 By atomizing the fuel into very fine
droplets, it increases the surface area of the
fuel droplets resulting in better mixing and
subsequent combustion.
 Atomization is done by forcing the fuel
through a small orifice under high
pressures.
 The injector assembly consists of
1) A needle valve
2) A compression spring
3) A nozzle
4) An injector body
WORKING OF FUEL INJECTOR
 When the fuel is supplied by the injection
pump it exerts sufficient force against the
spring to lift the nozzle valve, fuel is sprayed
into the combustion chamber in a finely
atomized particles.
 After, fuel from the delivery pump gets
injected, the spring pressure pushes the nozzle
valve back on its seat.
 For proper lubrication between nozzle valve
and its guide a small quantity of fuel is allowed
to leak through the clearance between them
and then drained back to fuel tank through
leak off connection.
 The spring tension and hence the valve
opening pressure is controlled by adjusting the
screw provided at the top.
NOZZLE
 Nozzle is that part of an injector through which the liquid fuel is sprayed
into the combustion chamber.
 The nozzle should fulfill the following functions:
1) Atomization: This is a very important function since it is the first phase
in obtaining proper mixing of the fuel and air in the combustion chamber.
2) Distribution of Fuel: Distribution of fuel to the required areas within
the combustion chamber. Factors affecting this are:
(a) Higher the injection pressure better the dispersion &
penetration of fuel
(b) If the density of compressed air in the combustion chamber is
high, the resistance to the movement of the droplets is higher and
dispersion of the fuel is better.
(c) The physical properties of fuel like self-ignition temperature,
vapor pressure, viscosity etc play an important role in the
distribution of fuel.
NOZZLE
3) Prevention of impingement on walls: Prevention of the fuel
from impinging directly on the walls of combustion chamber or
piston. This is necessary because fuel striking the walls,
decomposes and produces carbon deposits. This causes smoky
exhaust as well as increase in fuel consumption.
4) Mixing: Mixing the fuel and air in case of non-turbulent type of
combustion chamber should be taken care of by the nozzle.
 Types of Nozzles:
1) Pintle Nozzle
2) Single Hole Nozzle
3) Multi-hole Nozzle
4) Pintaux Nozzle
TYPE OF NOZZLES
1) Pintle Nozzle:
 The stem of the nozzle valve is extended to
form a pin or pintle which protrudes through
the mouth of the nozzle.
 It provides a spray operating at low injection
pressures of 8-10 MPa.
 The spray cone angle is generally 60°.
 Advantage of this nozzle is that:
 It avoids weak injection and dribbling.
 It prevents the carbon deposition on the
nozzle hole.
TYPE OF NOZZLES
2) Single Hole Nozzle:
 At the centre of the nozzle body there is a single
hole which is closed by the nozzle valve.
 The size of the hole is usually of the order of 0.2
mm.
 Injection pressure is of order of 8-10 MPa
 Spray cone angle is about 15°.
 Advantage:
 Simple in construction & operation
 Disadvantage:
 Tendency to dribble
 Their spray angle is too narrow to facilitate good
mixing unless higher velocities are used.
TYPE OF NOZZLES
3) Multi Hole Nozzle
 It consists of a multiple holes bored in the tip of the
nozzle.
 No. of holes varies from 4-18 & size from 35-200μm.
 The hole angle may be from 20° upwards.
 Injection Pressure: 18 MPa
 Advantage:
 Mix fuel with air properly even with lower air
motion available in open combustion chambers.
 Disadvantage:
 Holes are small and liable to clogging.
 Close tolerances (due to small holes) and hence
costly.
TYPE OF NOZZLES
4) Pintaux Nozzle
 It is a type of pintle nozzle with auxiliary hole
drilled in the nozzle body.
 It injects a small amount of fuel through this
additional hole (pilot injection) in the upstream
direction slightly before the main injection.
 The needle valve does not lift fully at low speeds
and most of the fuel is injected through the
auxiliary hole.
 Advantage:
 Better cold starting performance (20 to 25°C
lower than multi hole design).
 Disadvantage:
 Injection characteristics are poorer than the
multi hole nozzle.
SPRAY FORMATION
 At the start of the fuel-injection the pressure difference across the
orifice is low. Therefore single droplets are formed (figure a).
 As the pressure difference increases a stream of fuel emerges from the
nozzle (figure b).
 The stream encounters aerodynamic resistance from the dense air in
the combustion chamber (12 to 14 times the ambient pressure) and
breaks into a spray, say at a distance of l3 (figure c).
SPRAY FORMATION
 With further and further increase in the pressure difference, the
break-up distance decreases and the cone angle increases until the
apex of the cone practically coincides with the orifice.
 Break-up Distance (l3): the length at which a fuel start forming spray.
SPRAY FORMATION
 The fuel jet velocity at the exit of the orifice, Vf, is of the order of 400
m/s. It is given by the following equation.
 Where,
Cd = Coefficient of Discharge
Pinj = Fuel Pressure at the inlet of Injector (N/m^2)
Pcyl = Pressure of charge inside the Cylinder (N/m^2)
ρf = Fuel Density (kg/m^3)
SPRAY FORMATION
 The spray from a circular orifice has a denser and compact core,
surrounded by a cone of fuel droplets of various sizes and vaporized
liquid.
 Larger droplets provide a higher penetration into the chamber but
smaller droplets are required for quick mixing and evaporation of the
fuel. The diameter of most of the droplets in a fuel spray is less than 5
microns.
 The droplet sizes depends on various factors which are listed below:
1) Mean droplet size decreases with increase in injection pressure.
2) Mean droplet size decreases with increase in air density.
3) Mean droplet size increases with increase in fuel viscosity.
4) Size of droplets increases with increase in the size of the orifice.
QUANTITY OF FUEL & SIZE OF NOZZLE ORIFICE
 The quantity of the fuel injected per cycle depends to a great extent
up on the power output of the engine. The volume of the fuel injected
per second, Q, is given by
 Where,
Ni = Number of Injection per minute (N/2 for 4-stroke)
d = Diameter of Orifice in (m)
n = Number of Orifices
θ = Duration of Injection in Crank Angle (degrees)
INJECTION IN SI ENGINES
 Fuel Injection systems are common in CI engines. Presently gasoline
injection system is coming into vogue in SI engines because of the
following drawbacks of the carburetors.
(i) non-uniform distribution of mixture in multi-cylinder engines
(ii) loss of volumetric efficiency due to restrictions for the mixture
flow & the possibility of backfiring.
 A gasoline injection system eliminates all these drawbacks. The
injection of fuel into an SI engine can be done by employing any of the
following methods which are shown in figure on next slide.
INJECTION IN SI ENGINES
INJECTION IN SI ENGINES
 There are two types of gasoline injection systems
1) Continuous Injection: Fuel is continuously injected. It is adopted when
manifold injection is contemplated
2) Timed Injection: Fuel injected only during induction stroke over a limited
period. Injection timing is not a critical factor in SI engines.
 Major advantages of fuel-injection in an SI engine are:
 Increased volumetric efficiency
 Better thermal efficiency
 Lower exhaust emissions
 High quality fuel distribution
 The use of petrol injection is limited by
 high initial cost,
 complex design and
 increased maintenance requirements

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Chapter 9 Mechanical Injection Systems.pdf

  • 1. CHAPTER NO 09 MECHANICAL INJECTION SYSTEMS TALHA BIN NADEEM LECTURER DEPARTMENT OF MECHANICAL ENGINEERING NED UNIVERSITY OF ENGINEERING & TECHNOLOGY
  • 2. INTRODUCTION  The fuel injection system is the most vital component in the working of CI engines.  The engine performance is greatly dependent on the effectiveness of fuel injection system.  The injection system has to perform the important duty of initiating and controlling the combustion process.
  • 3. COMPARISON BETWEEN CARBURETOR & MECHANICAL INJECTION SYSTEMS  Purpose of Carburetion and Fuel Injection = Preparation of combustible charge but there is some difference: Carburetion Fuel injection Air Speed is greater than fuel speed at fuel nozzle to atomize the fuel Fuel Speed is greater than Air speed at the delivery point to atomize the fuel The amount of fuel drawn into the engine depends upon the air velocity in the venturi The amount of fuel delivered into the air stream going to the engine is controlled by a pump which forces the fuel under pressure
  • 4. FUNCTIONAL REQUIREMENTS OF INJECTION SYSTEM 1) Accurate metering of the fuel injected per cycle. This is very critical due to the fact that very small quantities of fuel being handled. Metering errors may cause drastic variation from the desired output. The quantity of the fuel metered should vary to meet speed and load requirements of the engine. 2) Timing the injection of the fuel correctly in the cycle to obtain maximum power; ensuring fuel economy and clean burning. 3) Proper control of rate of injection so that the desired heat release pattern is achieved during combustion. 4) Proper atomization of fuel into very fine droplets.
  • 5. FUNCTIONAL REQUIREMENTS OF INJECTION SYSTEM 5) Proper spray pattern to ensure rapid mixing of fuel and air 6) Uniform distribution of fuel droplets throughout the combustion chamber. 7) To supply equal quantities of metered fuel to all cylinders in case of multi cylinder engines. 8) No lag during beginning and end of injection i.e. to eliminate dribbling of fuel droplets into the cylinder.
  • 6. CLASSIFICATIONS OF INJECTION SYSTEMS  A fuel-injection system is required to inject and atomize fuel into the cylinder of CI engines.  For producing the required pressure for atomizing the fuel either air or a mechanical means is used.  Thus the injection systems can be classified as: Air injection system Solid injection systems
  • 7. AIR INJECTION SYSTEM  In this system, fuel is forced into the cylinder by means of compressed air. Advantages: 1- Good mixing of fuel with the air 2- Higher MEP 3- It has the ability to utilize high viscosity (less expensive) fuels Disadvantages: 1- The requirement of bulky multistage air compressor making air injection system obsolete
  • 8. SOLID (AIRLESS MECHANICAL) INJECTION SYSTEM  In this system, the liquid fuel is injected directly into the combustion chamber without the aid of compressed air. Hence, it is also called airless mechanical injection (or) solid injection system.  Solid injection systems can be classified into four types: 1) Individual Pump and Nozzle Systems 2) Unit Injector Systems 3) Common Rail Systems 4) Distributor Systems
  • 9. MAIN COMPONENTS OF MECHANICAL INJECTION SYSTEMS  All the injection systems (mentioned in slide # 08) mainly comprise of the following components. 1) Fuel tank 2) Fuel feed pump to supply fuel from the main fuel tank to the injection system 3) Injection pump to meter and pressurize the fuel for injection 4) Governor to ensure that the amount of fuel injected is in accordance with the variation in load 5) Injector to take the fuel from the pump and distribute it in the combustion chamber by atomizing it into fine droplets 6) Fuel filters to prevent dust and abrasive particles from entering the pump and injectors thereby minimizing the wear and tear of the components
  • 10. MAIN COMPONENTS OF MECHANICAL INJECTION SYSTEMS
  • 11. WORKING OF MECHANICAL INJECTION SYSTEMS  Fuel from the fuel tank first enters the coarse filter from which it is drawn into the plunger feed pump where the pressure is raised very slightly.  Now the fuel enters the fine filter where all the dust & dirt particles are removed.  From the fine filters the fuel enters the fuel feed pump where it is pressurized to about 200 bar & injected into the engine cylinder by means of injector.  Any spill over in the injector is returned to fine filter. A pressure relief valve is also provided for the safety of system.
  • 12. INDIVIDUAL PUMP AND NOZZLE SYSTEMS  In this system, each cylinder is provided with one pump and one injector.  In this arrangement a separate metering and compression pump is provided for each cylinder.  The pump may be placed close to the cylinder (fig. a) or they may be arranged in a cluster (fig. b) .  The high-pressure pump plunger is actuated by a cam and produces the fuel pressure necessary to open the injector valve at the correct time.  The amount of fuel injected depends on the effective stroke of the plunger.
  • 13. UNIT INJECTOR SYSTEM  The unit injector system is one in which the pump and the injector nozzle are combined in one housing.  Each cylinder is provided with one of these unit injectors.  Fuel is brought up to the injector by a low- pressure feed pump, where at the proper time, a rocker arm actuates the plunger and thus injects the fuel into the cylinder.  The amount of fuel injected is regulated by the effective stroke of the plunger.  The pump and the injector can be integrated in on unit as shown in figure (c).
  • 14. COMMON RAIL SYSTEM  In the common rail system, a HP system supplies fuel, under high pressure, to a fuel header.  High pressure in the header forces the fuel to each of the nozzles located in the cylinders.  At the proper time, a mechanically operated (by means of a push rod and rocker arm) valve allows the fuel to enter the proper cylinder through nozzle.  The pressure in the fuel header must be that, for which the injector system was designed, i.e. it must be enable to penetrate and disperse the fuel in the combustion chamber.  The amount of fuel entering the cylinder is regulated by varying the length of the push rod stroke
  • 15. COMMON RAIL SYSTEM  A high-pressure pump is used for supplying fuel to a header, from where the fuel is metered by injectors (assigned one per cylinder).
  • 16. DISTRIBUTOR SYSTEM  In this system the pump which pressurizes the fuel also meters and times it.  The fuel pump after metering the required amount of fuel supplies it to a rotating distributor at the correct time for supply to each cylinder.  The number of injection strokes per cycle for the pump is equal to the number of cylinders.  Since there is one metering element in each pump, a uniform distribution is automatically ensured. Not only that, the cost of the fuel injection system also reduces to a value less than two- thirds of that for individual pump system.
  • 18. COMPARISON OF VARIOUS FUEL-INJECTION SYSTEMS
  • 19. FUEL FEED PUMP  It is of spring-loaded plunger type.  The plunger is actuated through a push rod from the cam shaft.  At the minimum lift position of the cam the spring force on the plunger creates a suction which causes fuel flow from the main tank into the pump.  When the cam is turned to its maximum lift position, the plunger is lifted upwards.  At the same time the inlet valve is closed, and the fuel is forced through the outlet valves.
  • 20. INJECTION PUMP  The main objectives of fuel-injection pump is to deliver accurately metered quantity of fuel under high pressure (in the range from 120 to 200 bar) at the correct instant to the injector fitted on each cylinder.  Injection pumps are of two types: 1) Jerk Type Pumps 2) Distributor Type Pumps
  • 22. JERK TYPE PUMP  It consist of the reciprocating plunger inside a barrel. The plunger is driven by camshaft. (a) A sketch of typical plunger is shown. (b) Near the port A, fuel is always available under relatively low pressure. While the axial movement of the plunger is through camshafts, its rotational movement about its axis by means of rack D. Port B is orifice through which fuel is delivered to injector. When the plunger is below port A, the fuel gets filled in the barrel above it. As the plunger rises & closes the port A the fuel will flow out through port C.
  • 23. JERK TYPE PUMP (c) At this stage, the rack rotates the plunger & as a result port C also closes. The only escape route for the fuel is past the check valve through this orifice B to the injector. This is the beginning of the injection & also effective stroke of plunger. (d) The injection continues till the helical indentation on the plunger uncovers port C. Now the fuel will take the easy way out through C and the check valve will close the orifice B. The fuel injection stops & the effective stroke ends. Hence the effective stroke of the plunger is the axial distance traversed between the time port A is closed off & the time port A is uncovered.
  • 24. JERK TYPE PUMP (e) & (f) The plunger is rotated to the position shown. The same sequence of events occur. But in this case C is uncovered sooner. Hence the effective stroke is shortened.  It is important to remember here that though the axial distance traversed by the plunger is same for every stroke, the rotation of the plunger by the rack determining the length of the effective stroke & thus the quality of fuel injected.
  • 25. DISTRIBUTOR TYPE PUMPS  This pump has only a single pumping element and the fuel is distributed to each cylinder by means of a rotor.  A central longitudinal passage in the rotor and also two sets of radial holes (each equal to the number of engine cylinders) located at different heights.  One set is connected to pump inlet via central passage whereas the second set is connected to delivery lines leading to injectors of the various cylinders.
  • 26. DISTRIBUTOR TYPE PUMPS  The fuel is drawn into the central rotor passage from the inlet port when the pump plunger move away from each other.  Wherever, the radial delivery passage in the rotor coincides with the delivery port for any cylinder the fuel is delivered to each cylinder in turn.  Main advantages of this type of pump lies in its small size and light weight.
  • 27. INJECTION PUMP GOVERNOR  Fuel delivered by a pump increases with speed whereas the opposite is true about the air intake.  This results in over fueling at higher speeds, and at idling speeds (low speeds) the engine tends to stall due to insufficiency of fuel.  Quantity of fuel delivered increases with load causing excessive carbon deposits and high exhaust temperature.  Drastic reduction in load will cause over speeding to dangerous values.  It is the duty of injection pump governor to take care of the above limitations. Governors are generally of two types: 1) Mechanical Governor 2) Pneumatic Governor
  • 28. MECHANICAL GOVERNOR  When the engine speed tends to exceed the limit the weights fly apart. This causes the bell crank levers to raise the sleeve and operate the control lever in downward direction.  This actuates the control rack on the fuel injection pump in a direction which reduces the amount of fuel delivered. Lesser fuel causes the engine speed to decrease.
  • 29. PNEUMATIC GOVERNOR  The amount of vacuum applied to the diaphragm is controlled by the accelerator pedal through the position of the butterfly valve in the venturi unit.  A diaphragm is connected to the fuel pump control rack.  Position of the accelerator pedal also determines the position of the pump control rack and hence the amount of fuel injected.
  • 30. FUEL INJECTOR ASSEMBLY  By atomizing the fuel into very fine droplets, it increases the surface area of the fuel droplets resulting in better mixing and subsequent combustion.  Atomization is done by forcing the fuel through a small orifice under high pressures.  The injector assembly consists of 1) A needle valve 2) A compression spring 3) A nozzle 4) An injector body
  • 31. WORKING OF FUEL INJECTOR  When the fuel is supplied by the injection pump it exerts sufficient force against the spring to lift the nozzle valve, fuel is sprayed into the combustion chamber in a finely atomized particles.  After, fuel from the delivery pump gets injected, the spring pressure pushes the nozzle valve back on its seat.  For proper lubrication between nozzle valve and its guide a small quantity of fuel is allowed to leak through the clearance between them and then drained back to fuel tank through leak off connection.  The spring tension and hence the valve opening pressure is controlled by adjusting the screw provided at the top.
  • 32. NOZZLE  Nozzle is that part of an injector through which the liquid fuel is sprayed into the combustion chamber.  The nozzle should fulfill the following functions: 1) Atomization: This is a very important function since it is the first phase in obtaining proper mixing of the fuel and air in the combustion chamber. 2) Distribution of Fuel: Distribution of fuel to the required areas within the combustion chamber. Factors affecting this are: (a) Higher the injection pressure better the dispersion & penetration of fuel (b) If the density of compressed air in the combustion chamber is high, the resistance to the movement of the droplets is higher and dispersion of the fuel is better. (c) The physical properties of fuel like self-ignition temperature, vapor pressure, viscosity etc play an important role in the distribution of fuel.
  • 33. NOZZLE 3) Prevention of impingement on walls: Prevention of the fuel from impinging directly on the walls of combustion chamber or piston. This is necessary because fuel striking the walls, decomposes and produces carbon deposits. This causes smoky exhaust as well as increase in fuel consumption. 4) Mixing: Mixing the fuel and air in case of non-turbulent type of combustion chamber should be taken care of by the nozzle.  Types of Nozzles: 1) Pintle Nozzle 2) Single Hole Nozzle 3) Multi-hole Nozzle 4) Pintaux Nozzle
  • 34. TYPE OF NOZZLES 1) Pintle Nozzle:  The stem of the nozzle valve is extended to form a pin or pintle which protrudes through the mouth of the nozzle.  It provides a spray operating at low injection pressures of 8-10 MPa.  The spray cone angle is generally 60°.  Advantage of this nozzle is that:  It avoids weak injection and dribbling.  It prevents the carbon deposition on the nozzle hole.
  • 35. TYPE OF NOZZLES 2) Single Hole Nozzle:  At the centre of the nozzle body there is a single hole which is closed by the nozzle valve.  The size of the hole is usually of the order of 0.2 mm.  Injection pressure is of order of 8-10 MPa  Spray cone angle is about 15°.  Advantage:  Simple in construction & operation  Disadvantage:  Tendency to dribble  Their spray angle is too narrow to facilitate good mixing unless higher velocities are used.
  • 36. TYPE OF NOZZLES 3) Multi Hole Nozzle  It consists of a multiple holes bored in the tip of the nozzle.  No. of holes varies from 4-18 & size from 35-200μm.  The hole angle may be from 20° upwards.  Injection Pressure: 18 MPa  Advantage:  Mix fuel with air properly even with lower air motion available in open combustion chambers.  Disadvantage:  Holes are small and liable to clogging.  Close tolerances (due to small holes) and hence costly.
  • 37. TYPE OF NOZZLES 4) Pintaux Nozzle  It is a type of pintle nozzle with auxiliary hole drilled in the nozzle body.  It injects a small amount of fuel through this additional hole (pilot injection) in the upstream direction slightly before the main injection.  The needle valve does not lift fully at low speeds and most of the fuel is injected through the auxiliary hole.  Advantage:  Better cold starting performance (20 to 25°C lower than multi hole design).  Disadvantage:  Injection characteristics are poorer than the multi hole nozzle.
  • 38. SPRAY FORMATION  At the start of the fuel-injection the pressure difference across the orifice is low. Therefore single droplets are formed (figure a).  As the pressure difference increases a stream of fuel emerges from the nozzle (figure b).  The stream encounters aerodynamic resistance from the dense air in the combustion chamber (12 to 14 times the ambient pressure) and breaks into a spray, say at a distance of l3 (figure c).
  • 39. SPRAY FORMATION  With further and further increase in the pressure difference, the break-up distance decreases and the cone angle increases until the apex of the cone practically coincides with the orifice.  Break-up Distance (l3): the length at which a fuel start forming spray.
  • 40. SPRAY FORMATION  The fuel jet velocity at the exit of the orifice, Vf, is of the order of 400 m/s. It is given by the following equation.  Where, Cd = Coefficient of Discharge Pinj = Fuel Pressure at the inlet of Injector (N/m^2) Pcyl = Pressure of charge inside the Cylinder (N/m^2) ρf = Fuel Density (kg/m^3)
  • 41. SPRAY FORMATION  The spray from a circular orifice has a denser and compact core, surrounded by a cone of fuel droplets of various sizes and vaporized liquid.  Larger droplets provide a higher penetration into the chamber but smaller droplets are required for quick mixing and evaporation of the fuel. The diameter of most of the droplets in a fuel spray is less than 5 microns.  The droplet sizes depends on various factors which are listed below: 1) Mean droplet size decreases with increase in injection pressure. 2) Mean droplet size decreases with increase in air density. 3) Mean droplet size increases with increase in fuel viscosity. 4) Size of droplets increases with increase in the size of the orifice.
  • 42. QUANTITY OF FUEL & SIZE OF NOZZLE ORIFICE  The quantity of the fuel injected per cycle depends to a great extent up on the power output of the engine. The volume of the fuel injected per second, Q, is given by  Where, Ni = Number of Injection per minute (N/2 for 4-stroke) d = Diameter of Orifice in (m) n = Number of Orifices θ = Duration of Injection in Crank Angle (degrees)
  • 43. INJECTION IN SI ENGINES  Fuel Injection systems are common in CI engines. Presently gasoline injection system is coming into vogue in SI engines because of the following drawbacks of the carburetors. (i) non-uniform distribution of mixture in multi-cylinder engines (ii) loss of volumetric efficiency due to restrictions for the mixture flow & the possibility of backfiring.  A gasoline injection system eliminates all these drawbacks. The injection of fuel into an SI engine can be done by employing any of the following methods which are shown in figure on next slide.
  • 44. INJECTION IN SI ENGINES
  • 45. INJECTION IN SI ENGINES  There are two types of gasoline injection systems 1) Continuous Injection: Fuel is continuously injected. It is adopted when manifold injection is contemplated 2) Timed Injection: Fuel injected only during induction stroke over a limited period. Injection timing is not a critical factor in SI engines.  Major advantages of fuel-injection in an SI engine are:  Increased volumetric efficiency  Better thermal efficiency  Lower exhaust emissions  High quality fuel distribution  The use of petrol injection is limited by  high initial cost,  complex design and  increased maintenance requirements