chapter 7 - tep-tepe, faculty of...
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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.
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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
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Mass fraction burned with crank angle
17
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
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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
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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 =