1 stages of combustion in ci engine

8/13/2019 1 STAGES of Combustion in CI Engine

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The Combustion in C.I. engine is divided

into four stages

Delay Period(Pre-flame combustion) (A-B)

Uncontrolled combustion (B-C)

Controlled combustion (C-D)

After burning (D-E)


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TDC


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Delay period (A-B)

No noticeable deviation of pressure diagram from pure air

compression curve

This Period is counted from start of injection and start of

combustion

Physical DelaySpray disintegration and small droplets formation (friction)

Heating of liquid fuel and evaporation

Diffusion of vaporized fuel into air to form an ignitable

mixture within the air fuel range

Chemical Delay

Decomposition of heavy hydrocarbons into lightercomponents

Pre-ignition chemical reaction between the decomposed

components and oxygen


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Rapid or Uncontrolled combustion (B-C)

A considerable amount of fuel is accumulated in the

combustion chamber during the delay period.

So, enormous amount of energy produced

Ignition in one place is followed by else where .

The rate and quantity of combustion depends on duration of

delay period .

peak pressure is attained during this stage .


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Controlled combustion (C-D)

The temperature in the cylinder is so high that any fuel

injected after this time will burn as soon as it finds the

oxygen .

Further rise in pressure is controlled by injection rate .

This period is assumed from peak pressure to peak

temperature


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Late Burning phase or Afterburning (D-E)

Occurs in the expansion stroke of Engine.

Combustion does not stop after the completion of injection

process

The Thermal decomposition of the part of fuel takes place

during uncontrolled and controlled combustion.

The decomposed fuel molecules contain enough number of

hydrocarbons and carbon particles left in the combustion

chamber start burning as soon as they contact with oxygen,which have lower reaction rate. This process continues for

certain duration is called late burning

Produces smoky exhaust .


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Combustion in CI Engine

In a CI engine the fuel is sprayed directly into the cylinder and the vaporised

part of the fuel mixes with air and ignites spontaneously.

These photos are taken in a RCM under CI engine conditions with swirl

0.4 ms after ignition 3.2 ms after ignition

3.8ms after ignition Late in combustion process

1cm

Air flow


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In-Cylinder Measurements

This graph shows the fuel injection flow rate, net heat release rate and

cylinder pressure for a direct injection CI engine.

Start of injection

Start of combustion

End of injection


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Combustion in CI Engine

The combustion process proceeds by the following stages:

Ignition delay (ab)- fuel is injected directly into the cylinder towards the end ofthe compression stroke. The liquid fuel atomizes into small drops and

penetrates into the combustion chamber. The fuel vaporizes and mixes with

the high-temperature high-pressure air.

Premixed combustion phase (bc)combustion of the fuel which has mixed

with the air to within the flammability limits (air at high-temperature and high-

pressure) during the ignition delay period occurs rapidly in a few crank angles.

Mixing controlled combustion phase (cd)after premixed gas consumed, the

burning rate is controlled by the rate at which mixture becomes available for

burning. The burning rate is controlled primarily by the fuel-air mixing process.

Late combustion phase (de)heat release may proceed at a lower rate well

into the expansion stroke (no additional fuel injected during this phase).

Combustion of any unburned liquid fuel and soot is responsible for this.


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Four Stages of Combustion in CI Engines

Start of

injection

End of

injecction

-10 TC-20 10 20 30


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CI Engine Types

Two basic categories of CI engines:

i) Direct-injectionhave a single open combustion chamber into which fuel

is injected directly

ii) Indirect-injectionchamber is divided into two regions and the fuel is

injected into the prechamber which is connected to the main chamber via a

nozzle, or one or more orifices.


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Direct Injection

quiescent chamber

Direct Injection

multi-hole nozzle

swirl in chamber

Direct Injection

single-hole nozzle

swirl in chamber

Indirect injection

swirl pre-chamber


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Combustion Characteristics

Combustion occurs throughout the chamber over a range of equivalence

ratios dictated by the fuel-air mixing before and during the combustion phase.

In general most of the combustion occurs under very rich conditions within the

head of the jet, this produces a considerable amount of solid carbon (soot).

1o

ASI5o

ASI

Diffusion flame

Fuel vapourHigh soot

Liquid fuel


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Ignition Delay

Ignition delay is defined as the time (or crank angle interval) from when

the fuel injection starts to the onset of combustion.

Both physical and chemical processes must take place before a significant

fraction of the fuel chemical energy is released.

Physical processesare fuel spray atomization, evaporation and mixing of

fuel vapour with cylinder air.

Good atomization requires high fuel pressure, small injector hole diameter,

optimum fuel viscosity, high cylinder pressure (large divergence angle).

Rate of vaporization of the fuel droplets depends on droplet diameter,

velocity, fuel volatility, pressure and temperature of the air.

Chemical processessimilar to that described for autoignition phenomenon

in premixed fuel-air, only more complex since heterogeneous reactions

(reactions occurring on the liquid fuel drop surface) also occur.


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Fuel Ignition Quality

The ignition characteristics of the fuel affect the ignition delay.

The ignition quality of a fuel is defined by its cetane numberCN.

For lowcetane fuels the ignition delay is long and most of the fuel is

injected before autoignition and rapid combustion, under extreme cases

this produces an audible knocking sound referred to as dieselknock.

For highcetane fuels the ignition delay is short and very little fuel is injected

before autoignition, the heat release rate is controlled by the rate of fuel

injection and fuel-air mixingsmoother engine operation.


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Cetane Number

The method used to determine the ignition quality in terms of CN is analogous

to that used for determining the antiknock quality via the ON.

The cetane number scale is defined by blends of two pure hydrocarbon

reference fuels.

By definition, isocetane (heptamethylnonane, HMN) has a cetane number of

15 and cetane (n-hexadecane, C16H34) has a value of 100.

The higher the CN the better the ignition quality, i.e., shorter ignition delay.

The cetane number is given by:

CN = (%hexadecane)+ 0.15 (% HMN)


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Factors Affecting Ignition Delay Time

Injection timingAt normal engine conditions the minimum delay occurs

with the start of injection at about 10-15 BTC.

Earlier or later injection timing results in a lower air temperature and

pressure during the delay periodincrease in the ignition delay time

Injection quantityFor a CI engine the air is not throttled so the load is

varied by changing the amount of fuel injected.

Increasing the load (bmep) increases the residual gas and wall temperature

which results in a higher charge temperature at injectiondecrease in the

ignition delay.

Intake air temperature and pressurean increase in ether will result in a

decrease in the ignition delay, an increase in the compression ratio has the

same effect.

ff f l l


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Effect of Multiple Injections on

combustion processInjection

System

Start of Injection

deg, bTDC

Fuel injection

Nomenclaturea(b)-c-d(e)

Injection

Profile

Single step 12 0(0)-0-100(20)

Split 1 12 10(2)-6-90(18)

Split 2 12 20(4)-6-80(16)

Split 3 12 40(8)-6-60(12)

Split 4 12 50(10)-6-50(10)

a - Percentage mass injected during pre-injection pulse

b - Duration of pre-injection pulse (in degrees)

c - Dwell period between pre and main injection pulses (in degrees)

d - Percentage mass injected during main injection pulse

e - Duration of main injection pulse (in degrees)

Eff t f M lti l I j ti H t


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-5

5

15

25

35

45

55

65

75

8595

105

348 358 368 378 388 398 408

Crankangle,deg

HRR,J

/de

Single

Split1

Split2

Split3

Split4

Injection rate

shape

Ignition delay,

deg

Peak Heat Release

Rate, J/deg

Combustion

Duration, deg

Single step 6.6 97.1 55.5

Split1 6.1 47.9 61.3

Split2 6.3 74.3 60.1

Split3 6.4 90.2 57.2

Split4 6.5 95.2 56.4

Effect of Multiple Injections on Heat

Release Rate(HRR)


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Heat release rate

1 stages of combustion in ci engine

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