16998 Fuel Injection 2
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Transcript of 16998 Fuel Injection 2
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Fuel Injection
The purpose of carburetion and fuel-injection is the same viz,
preparation of the combustible charge.
But in the case of carburetion fuel is atomized by process relying on
the air speed greater than fuel-injection.
The fuel speed at the point of delivery is greater than the air speed
to atomize the fuel.
When the fuel is injected into the combustion chamber towards the
end of compression stroke, it is atomised into very fine droplets.
These droplets vaporise due to the heat transfer from the
compressed air and form a fuel-air mixture.
Due to continued heat transfer from hot air to the fuel, thetemperature reaches a value higher than its self-ignition temp.
This causes the fuel-to ignite spontaneously.
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Functional requirement of an injection
system -For a proper running and good performance from the
engine, the following requirements must be met by the injectionsystem
Accurate metering
Timing the injection
Rate of injection Proper atomization
Proper spray pattern
Uniform distribution of fuel droplets
Equal quantities of metered fuel to all cylinders for multicylinder engines.
No lag thus eliminating dribbling.
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Classification of injection systems -
In a constant-pressure cycle or diesel engine, pure air is compressed in the
cylinder and then fuel is injected into the cylinder by means of a fuel-
injections system. For producing the required pressure for atomizing the
fuel either air or a mechanical means is used. Accordingly the injection
systems can be classified as
1. Air injection system. In this system fuel is forced into the cylinder by
means by compressed air. This system is little used now-a-days, because
it requires a bulky multi-stage air compressor.
2. Solid injection system (Misnomer). 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
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Solid injection systems can be classified into four types-
(a) Individual pump and nozzle system
(b) Unit injector system(c) Common rail system
(d) Distributor system.
All the above systems comprise mainly of the following components
(i) Fuel tank
(ii) Fuel feed pump
(iii) Injection pump
(iv) Governor
(v) Injector
(vi) Fuel fitters.
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Solid Injection System used in CI Engine: A typical arrangement of
various component of solid injection system used in CI Engine is
shown below.
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INJECTION PUMP
The main objectives of fuel-injection pump is to deliver
accurately metered quantity of fuel under high pressure (in the
range of 120-200 bar) at the correct instant to the injector fitted
on each cylinder.
Injection pumps are generally of two types
(I) Jerk type pumps
(ii) Distributor type pumps
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FUEL INJECTOR
Quick and complete combustion is ensured by a well designed fuel injector. By
atomizing the fuel into very fine droplets, it increases the surface area of the fuel
droplets the surface area of the fuel droplets resulting in better mixing and
subsequent combustion. Atomization is done by forcing the fuel through a smallorifice under high pressure.
The injector assembly consists of
(i) A nozzle value
(ii) A compression spring
(iii) A nozzle
(iv) An injector body
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FUEL INJECTOR (BOSCH)
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NOZZLE
Nozzle is that part of an injector through which the liquidfuel is sprayed into the combustion chamber. The nozzleshould fulfill the following functions
(i) Atomization
(ii) Distribution of fuel
factors affecting this are-
(a) Injection pressure
(b) Density of air in the cylinder
(c) Physical properties of fuel
(iii) Prevention of impingement on walls
(iv) Mixing
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Various types of nozzles are used in C I engines.
The most common types are
(i) Pintle nozzle
(ii) Single hole nozzle
(iii) Multi-hole nozzle
(iv) Pintaux nozzle.
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(i) Pintle Nozzle -It provides a spray operating at low injection pressures of 8 -
10 MPa. The spray cone angle is generally 60 0 . Advantage of this nozzle is
that it avoids weak injection and dribbling. It prevents the carbon deposition
on the nozzle hole.
(ii) (ii) Single hole nozzle -The size of the hole is usually of the order of 0.2 mm.
Injection pressure is of order of 8-10 MPa and spray cone angle is about 15 0
. Ma-jor disadvantage with such nozzle is that they tend to brittle Besides,
their spray angle is too narrow to facilitate good mixing unless highervelocities are used.
(iii) Multi-hole nozzle -The number of holes varies from 4 to 18 and the size from
35 to 200 um. The hole angle may be from 20 degree upwards. These nozzles
operate at high injection pressures of the order of 18 MPa. Their advantage
lies in the ability to distribute the fuel properly even with lower air motion
available in open combustion chamber.
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(iv) Pintaux nozzle -It is a type of pintle nozzle which has an auxiliary hole drilled
in the nozzle body. This hole is used for pilot injection. The needle valve does not
lift fully at low speeds and most of the fuel is injected through the auxiliary hole.
Main advantage of this nozzle is better cold starting performance. A major
drawback of this nozzle is that its injection, characteristics are poorer than the multi
hole nozzle.
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SPRAY FORMATION
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At the start of the fuel-injection the pressure difference across the orifice is
low. Therefore single droplets are formed as in the above fig(a). As the
pressure difference increases the following process occur one after the
other.
(i) A stream of fuel emerges from the nozzle fig (b).
(ii) The stream encounters aerodynamic resistance from the dense air present
in the combustion chamber (12 to 14 times the ambient pressure) and
breaks into a spray, say at a distance of l3, fig (c). The distance of this point
where this event occurs from the orifice is called the break-up distance (l3)
(iii) With further and further increase in the pressure difference the break-up
distance decreases and the cone angle increases initial the apex of the cone
practically coincides with the orifice, fig (d) , (e) , (f).
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The droplets sizes depends on various factors which are listed
below
(i) Mean droplet size decreases with increase in injection
pressure
(ii) Mean droplet size decreases with increases in air density.
(iii) Mean droplet size increases with increase in fuel viscosity.
(iv) Size of droplets increases with increase in the size of the
orifice.
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