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CHAPTER 7COMBUSTION IN SPARK-IGNITION ENGINES

7.1 ESSENTIAL FEATURES OF PROCESS

7.2 THERMODYNAMIC ANALYSIS OF SI ENGINE COMBUSTION

7.2.1 Burned and Unburned Mixture states

7.2.2 Analysis of Cylinder Pressure Data

7.2.3 Combustion Process Characterization

7.3 FLAME STRUCTURE AND SPEED

7.4 ABNORMAL COMBUSTION

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7.1 Essential Features of ProcessIn a conventional spark-ignition engine, fuel and air are mixed

together in the intake system. Combustion is initiated towards the end of compression stroke at the spark plug by an electric discharge. Important features of combustion process are evident from the data in figure 7.1

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Cylinder pressure

Mass fraction burned

Volume fraction enflamed

Figure 7.1 Different quantities as a function of crank angle, taken from several consecutive cycles

Without combustion(motored)

Cycle-by-cycle variations can be observed.

Spark

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(a) Cylinder pressure vs. crank angle for overadvanced spark timming (50), MBT timing (30) and retarded timing (10).

(b) Effect of spark advance on brake torque at constant speed and (A/F), at wide-open

throttle. MBT is maximum brake torque timing.

Cyl

ind

er

pre

ssu

re

Re

lativ

e to

rqu

e

Ignition

Spark advance=50o

Crank angle, deg Spark advance, deg

(Maximum Brake Torque)

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If the start of combustion process is progressively advanced before TC, the compression stroke work transfer increases. If the end of combustion process is progressively delayed by retarding the spark timing, the peak pressure occurs later in the expansion stroke and is reduced resulting in a reduction of expansion work transfer.

The optimal timing which gives the maximum brake torquecalled maximum brake torque, or MBT, timingoccurs when the magnitudes of the opposing trends just offset each other.

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7.2 THERMODYNAMIC ANALYSIS OF SI ENGINE COMBUSTION

7.2.1 Burned and Unburned Mixture states

Schematic of flame in the engine cylinder during combustion:unburned gas ( )to left of flame, burned

is work-transfer rate to is heat-transfer rate to

gas to right. “A”

chamber walls.denotes adiabatic burned-gas core, piston,

Flame front

Spark PlugUnburned

Burned

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The conditions in the burned and unburned gas are determined by Conservation of mass:

Conservation of energy:

cylinder volumemass fraction burned

internal energy of the cylinder contents

work done on the pistonheat transfer to the walls

at some reference point

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Work and heat transfers

instantaneous heat-transfer rate to chamber walls

► Models used for thermodynamic analysis

1) Ideal modelAssuming that the burned and unburned gases are different ideal gas, each with constant Cp and Cv.

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By solving the previous equations, we obtain

(A)

(B)

where and

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and

Assuming that the unburned gas is undergone isentropic compression with uniform properties

If p, V, m and . (unknowns)we can findare known

From equations(A) and (B)

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Cylinder pressure, mass fraction burned, and gas temperatures as functions of crank angle during combustion.

is unburned gas temperature,is burned gas temperature, the subscript e and l denote early and late burning gas elements, is the mean burned gas temperature.

Crank angle

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Mass fraction burned curves determined from measured cylinder pressure data using two-zone combustion model: (a) gasoline; (b) methanol

(a) (b)

Ma

ss

fra

ctio

n b

urn

ed

Ma

ss

fra

ctio

n b

urn

ed

Degree after spark Degree after spark

Gasoline Methanol

leaner leaner

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7.2.2 Analysis of Cylinder Pressure Data

Cylinder pressure data for propane-fueled SI engine at 1500 rev/min, MBT timing, imep = 513 kPa, = 0.8, and rc = 8.72

- plot (a) Pressure-volume diagram (b)

Pre

ssure

Pre

ssure

V/Vmax V/Vmax

Since compression and expansion processes are isentropic, the observed behavior is as expected.

Polytropic relation (approximately isentropic)

Pressure change

Due to combustion Due to volume change

n = 1.3 (± 0.05)

Assuming the mass of charge burned in the time interval is proportional to pressure rise due to combustion

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7.2.3 Combustion Process Characterization

The following definitions are used to characterize energy-release aspects of combustion:

1)Flame-development angle

Between the spark discharge and the time when a small fraction of mass has burned or fuel chemical has been released.

2)Rapid-burning angle

The bulk of the charge is burned in the interval between the end of flame-development stage and the end of flame-propagation process.

3)Overall burning angle The duration of overall burning process. It is the sum of

the above angle

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Flame-development angle Rapid burning angle

Overall burning angle =

Spark discharge

pe

rcen

tag

e

Mass fraction burned with crank angle

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is the start of combustion

Actual mass fraction burned curves have been fitted with a = 5 and m =2

total combustion duration

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7.3 Flame Structure and Speed

The photographs show how the flame, roughly spherical in shape, grows steadily from the time of spark discharge. Effect of turbulence is already visible in the convoluted flame surface. The volume enflamed behind the front continues to grow.

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Sequence of movie frames from one engine cycle in a square-cross-sectioncylinder, single-cylinder engine with two glass walls, and correspondingpressure and mass fraction burned curves.1400 rev/min, 0.5 atm inlet

pressure.

pxb

p

xb

Cyl

ind

er

pre

ssu

re

Mass

fra

ctio

n b

urn

ed

Crank angle

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A useful relationship between mass fraction burned,

( ) and volume fraction occupied by the burned gas,

( ) can be obtained from the identities

,

And from the above definitions

where

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Relation between mass fraction burned ( )and volume

is residual mass fraction.fraction burned ( ).

mas

s fr

acti

on

bu

rne

d

Volume fraction burned

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No swirl

No swirl

No swirl

Normal swirl

High swirl

Crank angle

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Laser shadowgraphs of engine combustion process taken in single-cylinder engine with transparent cylinder head. From top to bottom: side plug without swirl; side plug with normal swirl; side plug with high swirl; central plug without swirl; two plugs without swirl.

Laminar Burning SpeedsThe burning velocity is defined as the velocity, relative to and normal to the flame front. The flame front is a region of space resulting from the self-sustaining chemical reaction.

The flame front consists of two regions

- Frame front preheat zonereaction zone

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The thickness of the preheat zone is

Flame zone

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Laminar Burning VelocityThe laminar burning velocity is measured in spherical closed vessels by propagating a laminar flame radially outward from the vessel center.

= flame area

,

The burning velocity at temperature and pressure deviated from the reference values can be computed by

= 298 K = 1 atm are the reference temperature and pressure

, and are constants which are a function of

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- For propane, isooctane and methanol

is at which

is maximum with value

Parameters , and

fuel (cm/s) (cm/s)

Methanol 1.11 36.9 -140.5

Propane 1.08 34.2 -138.7

Isooctane 1.13 26.3 -84.7

Gasoline 1.21 30.5 -54.9

Laminar burning velocity for several fuels at 1 atm, 300 K

27Lines are least-squre polynomial fits to data

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Laminar burning velocity for several fuels as function of equivalence ratio, 1 atm and 300 K. Lines are least-squared

polynomial fits to data.Laminar burning velocity as a function of mole fraction of burned gas diluent,

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Effect of burned gas mole fraction ( ) in unburned mixture on

laminar burning velocity.

Fuel: gasoline

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Flame Propagation Relations

Flame photographs effectively define the position of the front of the engine flame. The shadow of the enflamed zone, under normal engine conditions, is close to circle. Thus, to a good approximation, the surface which defines the leading edge of the turbulent flame is a portion of a sphere.

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Schematic of spherical flame front in engine combustion chamber identifying parameters which define flame geometry.

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Spherical volume containing all the burned gas behind:

Spherical burning area is the area of spherical surface:

Laminar burning area

the surface area of flame if it is burned at the laminar speed

(1)

where is the laminar flame speed

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Mean expansion speed of the front,

(2)

where the area enclosed by the “best-fit” circle through the leading edge of flame

and is arc length within the chamber of this “best-fit” circle

(3)

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Mean expansion speed of the burned gas,

(4)

= volume of burned gas

= spherical burning area

Burning speed,

(5)

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The mean gas speed, ug

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Cycle by cyclevariation rf > rb

Sb > SL

AL > Ab

ug=ub-SbLaminar area exceedsflame area due to existence of substantial pockets of behide the flame front

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Example 7.1 At spark timing (18o BTC) in an automobile spark-ignitionengine (bore = 84 mm) at 1800 rev/min, the flame development angled is 8o. The flame propagation ends at 12o ATC. The spark plug islocated 8 mm away from the center. Calculate the mean expansionspeed of the flame front.

Duration of flame expansion covers 30o crank angle.

Time of flame expansion

The expansion distance = 0.084/2 + 0.008 =0 .05 m

Thus the mean expansion speed of the flame front uf

Ans

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7.4 ABNORMAL COMBUSTIONSI engine combustion chamber schematically visualized as long hollow cylinder with the spark plug located at left end.

Mass of air-fuel is equally distributed as spark plug is fired to start combustion

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As flame front moves across chamber, unburned mixture in front of flame is compressed into smaller volume.

Flame front continues to compress unburned mixture into smaller volume, which increases its temperature and pressure. If compression raises temperature of end gas above SIT, self-ignition and knock can occur.

Mass of air-fuel is equally distributed as spark plug is fired to start combustion

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SI engine combustion chamber schematically visualized as long hollow cylinder with the spark plug located at left end.

(a) Mass of air-fuel is equally distributed as spark plug is fired to start combustion.

(b) As flame front moves across chamber, unburned mixture in front of flame is compressed into smaller volume.

(c) Flame front continues to compress unburned mixture into smaller volume, which increases its temperature and pressure. If compression raises temperature of end gas above SIT, self-ignition and knock can occur.

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Cylinder pressure versus crank angle traces of cycles with (a) normal combustion, (b) light knock, and (c) heavy knock. 4000 rev/min, wide-open throttle, 381-cm3 displacement single-cylinder engine.

light knockheavy knockNormal combustion

pre

ssu

re

Ignition timing Ignition timing Ignition timing

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Knock occurs

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Schlieren photographs from high-speed movies of

(a)normal flame propagation through the end gas and

(b) knocking combustion (autoignition occurs in photograph 4), with corresponding cylinder pressure versus crank angle traces

Octane Number (ON)

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Octane number is a practical measure of a fuel’s resistance to knock. It determines whether or not a fuel will knock in a given engine under given operating conditions: the higherthe octane number, the higher the resistance to knock.

The ON scale is based on two hydrocarbons:

n-C7H16 normal heptane

C8H18 Isooctane

ON = 0

ON = 100

• Blends of these two define the knock resistance of intermediate ON.

• The ON is determined by measuring what blend of these matches the fuel’s knock resistance.

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Operating conditions for research and motor methods

Research method

Motor method

Inlet temperature 52C 149C

Inlet pressure Atmospheric

Humidity 0.0036 – 0.0072 kg/kg dry air

Coolant temperature 100C

Engine speed 600 rev/min 900 rev/min

Spark advance 13 BTC (constant)

19 - 26 BTC (varies with

compression ratio)

Air/fuel ratio Adjusted for maximum knock

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Road octane number, Road ON can be related to motor and research ratings by

Road ON = a(RON) + b(MON) + c

where a, b and c are experimentally derived constants.

An antiknock index which the mean of the research and motor octane numbers is used to characterize antiknock quality.

Antiknock index =