Combustion in compressed ignition engine, pressure data [Autosaved]1.pptx

8/10/2019 Combustion in compressed ignition engine, pressure data [Autosaved]1.pptx

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Combustion in compressionignition engine, analysis ofcylinder pressure data andheat release in combustionPresented By:2011-ME-313

Presented To: Dr.Shahid Imran


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Combustion in compression ignition(CI)engine

Combustion is defined as rapid chemical combination ofhydrogen and carbon in the fuel with oxygen present inair resulting in liberation of energy in the form of heat.

Combustion in CI is unsteady, occurring simultaneously atvarious spot in non homogeneous mixture and controlled

by fuel injection

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Combustion in CI engine

Fuel injected into cylinder at high pressure from nozzleatomizes , vaporize, mix with high pressure and temperatureair

Auto ignition of suitable mixture of air and fuel occur after adelay period of few crank angle degrees

Cylinder pressure increases as the combustion occurs

Consequent compression of unburned portion of charge shortenthe delay before ignition after delay it burn rapidly

Combustion detail depend upon properties of fuel, design ofcombustion chamber and fuel ignition system

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Combustion chamber designMain problem in combustion chamber design is achieving

rapid mixing between the injected fuel and air in cylinderto complete combustion close to top-center

Mixing rate is controlling the fuel burning rate

As engine size decreases more vigorous air air motion isrequired so it leads to different combustion chamberdesign

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Types of diesel combustion or CI system

According to combustion chamber design diesel enginehave two types

1) Direct injection engine which have single opencombustion chamber in which fuel is injected directly

2) Indirect injection engine where chamber is divided into

two region and fuel is injected into pre chamber which isconnected to main chamber via nozzle

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Open combustion chamber

Have a single open combustion chamber into

which fuel is injected directlyUsed for large size engines

This type of chamber requires a higherinjection pressure and a greater degree offuel atomization than other combustion

chamber

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Pre combustion chamber

It is separated into two chambers The smaller chamber occupies about 30 percent oftotal combustion space

As the pre combustion chamber runs hot, delayperiod is very short. This results into small rate of

pressure rise and thus , tendency of Diesel knock isminimum , and as such running is smooth

Products of combustion from pre chamber move tomain chamber in a violent way, which helps in avery rapid combustion in third stage

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Types of diesel fuel injection system

Direct injection system

Indirect injection system

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Direct injection system

Fuel is injected directly into the upper portion ofthe cylinder (i.e. combustion chamber). This typedepends little on turbulence to perform themixing

High injection pressures and multi orificenozzles are required

Use in large size engine where air fuel mixing isnot very necessary

Can be use for small size but air swirl have to begenerated as by bowl in piston confguration

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Indirect ignition system(IDI)

Fuel is not directly injected into thecombustion chamber

Injection is first in pre combustion chamber

IDI systems is used for smallest enginesizes ,It is used to obtain the vigorous air

motion required for high fuel air mixingrates

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Combustion analysis tool

P-q diagram

Needle lift diagram

Injection system fuel line pressure diagram

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Figure shows the data for cylinder pressure p,fuel injector needle lift lN and injectionsystem fuel line pressure Pi as a function ofcrank angle

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In cylinder pressure measurement

Pressure is usually measured with piezoelectric pressuretransducerThis type of transducer contain a quarts crystalHigh-speed electronic transducers, capable of converting thedeflection of a low inertia diaphragm into an electrical signalOne end of crystal is exposed through a diaphragm to cylinderpressure, as cylinder pressure increases the crystal iscompressed and generate an electric charge which isproportional to pressureA charge amplifier is used to produce an output voltageproportional to this charge

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Importance of pressure data

A tool for combustion analysis

Continuous updating of peak pressure, location of peak,and standard deviation of peak pressure

Rate of heat release can be calculated


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Theoretical graph between pressureand crank angle

It is theoretical pressureand crank angle graph inwhich maximum pressureis at top dead center(TDC)and combustion is assumedat constant volume orcombustion is assumed tobe instantaneous. From ato b compression and fromb to c combustion is goingon.

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Actual graph between pressure andcrank angle

Actual pressure and crankangle graph maximumpressure is after top deadcenter( because combustion isnot taking placeinstantaneously. It occurspartially before TDC andpartially after TDC. From b toc combustion is going on.

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Graph between pressureand crank angle

Three phases of combustion for ciengine

Ignition delayPremixed or period of uncontrolledcombustionPeriod of controlled combustionThird phase is followed by afterburning which may be called forthphase of combustion.

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Ignition delay periodIt is defined as the time interval between the start of injectionand the start of combustion.

The delay period is subdivided into physical and chemicaldelay.

The period of physical delay is the time between the beginningof injection and attainment of chemical reaction conditions.

In physical delay fuel is atomized, vaporized, mixed with airand raised in temperature

In chemical delay period reaction start slowly and thenaccelerate until ignition

Pressure reached during second stage will depend upon the

duration of the delay period.

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Longer the delay period , the more rapid and higher thepressure rise because more fuel is present before the rateof burning comes under control, this may cause knock dueto which it is aim to keep the delay period as short aspossible for smooth running and to control over pressure

change

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Period of rapid or uncontrolledcombustion

This period is counted from the end of delay period to thepoint of maximum pressure on the indicator diagramIn this rise of pressure is rapid because during the delayperiod the droplets of fuel had time to spread over widearea and fresh air around them properly mixed with freshairAbout one-third of heat evolve in thisThe rate of pressure rise depends on the amount of fuelpresent at the end of delay period, degree of turbulence,fitness of atomization and spray pattern

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Period of controlled combustion

Temperature and pressure is very high at the end ofsecond stage of combustion so fuel droplets injected inthird stage burn almost as they enter.

Pressure rise is controlled by injection rate

End of this period is assumed to be at maximum cycletemperature

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Rate of heat release and crank angleRate at which the chemicalenergy of the fuel is released by

the combustion processHeat release diagram shownegligible heat transfer untiltoward the end of compressionwhen a slight loss of heat duringthe delay period because of fuelevaporation and heat transfer towalls

During the combustion processthe burning proceeds in threedistinguish able stages. 24


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Rate of heat release

In figure from b to c represent uncontrolled

combustion, first peak occur resulted fromrapid combustion of injected fuel in delayperiod

The period c to d represent controlledcombustion during which heat release reach asecond peak

The heat release during this phase depend uponthe injection duration

As duration increase amount of fuel increasewhich increase the rate of heat release in thisphase

It decrease as the phase progress25


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Rate of heat release

The period d-e represent thelate combustion phase

Heat release rate decreasesfurther and continues at alower rate

Expansion stroke come in thisphase

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Rate of heat release and rate ofinjection

A heat release diagram corresponding tothe rate of fuel injection and cylinderpressure data is shown in figure.

The heat release diagram shows

negligible heat release until toward theend of compression when as light loss ofheat during the delay period is evident

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Combustion in compressed ignition engine, pressure data [Autosaved]1.pptx