16998 Fuel Injection 2

8/13/2019 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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16998 Fuel Injection 2

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