9.1 ESSENTIAL FEATURES OF PROCESS In a conventional spark-ignition engine the fuel and air are mixed together in the intake system, inducted through the intake valve into the cylinder, where mixing with residual gas takes place, and then compressed. Under normal operating conditions, combustion is initiated towards the end of the compression stroke at the spark plug by an electric discharge. Following inflammation, a turbulent flame develops, propagates through this essentially premixed fuel, air, burned gas mixture until it reaches the combustion chamber walls, and then extinguishes. Photographs of this process taken in operating engines illustrate its essential features. Figure 9-1 (color plate) shows a sequence of frames from a high-speed color movie of the combustion process in a special single-cylinder engine with a glass piston crown.' The spark discharge is at - 30". The flame first becomes visible in the photos at about -24'. the flame, approximately circular in outline in this
view through the piston, then propagates outward from the spark plug locatioe
The blue light from the flame is emitted most strongly from the front. The irregular
shape of the turbulent flame front is apparent. At TC the flame diameter is
about two-thirds of the cylinder bore. The flame reaches the cylinder wall farthat
from the spark plug about 15" ATC, but combustion continues around parts oi
the chamber periphery for another 10". At about 10" ATC, additional radiation--
initially white, turning to pinky-orange-centered at the spark plug location is
evident. This afterglow comes from the gases behind the flame which burned
earlier in the combustion process, as these are compressed to the highest ternperatures
attained within the cylinder (at about 15" ATC) while the rest of the
charge burns.'.
Additional features of the combustion process are evident from the data in
Fig. 9-2, taken from several consecutive cycles of an operating spark-ignition
engine. The cylinder pressure, fraction of the charge mass which has burned
(determined from the pressure data, see Sec. 9.2), and fraction of the cylinder
volume enflamed by the front (determined from photographs like Fig. 9-1) are
shown, all as a function of crank angle.4 Following spark discharge, there is a
period during which the energy release from the developing flame is too small for
the pressure rise due to combustion to be discerned. As the flame continues to
grow and propagate across the combustion chamber, the pressure then steadily
rises above the value it would have in the absence of combustion. The pressure
reaches a maximum after TC but before the cylinder charge is fully burned, and
then decreases as the cylinder volume continues to increase during the remainder
of the expansion stroke.
The flame development and subsequent propagation obviously vary, cycleby-
cycle, since the shape of the pressure, volume fraction enflamed, and mass
fraction burned curves for each cycle differ significantly. This is because flame
growth depends on local mixture motion and composition. These quantities vary
in successive cycles in any given cylinder and may vary cylinder-to-cylinder.
Especially significant are mixture motion and composition in the vicinity of the
spark plug at the time of spark discharge since these govern the early stages of
flame development. Cycle-by-cycle and cylinder-to-cylinder variations in combustion
are important because the extreme cycles limit the operating regime of
the engine (see Sec. 9.4.1).
Note that the volume fraction enflamed curves rise more steeply than the
mass fraction burned curves. In large part, this is because the density of the
unburned mixture ahead of the flame is about four times the density of the
burned gases behind the flame. Also, there is some unburned mixture behind the
visible front to the flame: even when the entire combustion chamber is fully
enflamed, some 25 percent of the mass has still to bum. From this description it
is plausible to divide the combustion process into four distinct phases: (1) spark
ignition; (2) early flame development; (3) flame propagation; and (4) flame termination.
Our understanding of each of these phases will be developed in the
remainder of this chapter.
The combustion event must be properly located relative to top-center to
Crank angle, deg
FIGURE 9-2
Cylinder pressure, mass fraction b u d , a nd volume fraction enflamed for five vcnsccutive c ~ l inn a
sPark-imition engine as a function of crank angle. IgnXon timing 30' BTC, widaopn throtUe, IOU
rev/min, q5 = 0.98.4
obtain the maximum power or torque. The combined duration of the flame
development and propagation process is typically between 30 and 90 crank angk
degrees. Combustion starts before the end of the compression stroke, continues
through the early part of the expansion stroke, and ends after the point in the
cycle at which the peak cylinder pressure occurs. The pressure versus crank angk
curves shown in Fig. 9-3a allow us to understand why engine torque (at given
engine speed and intake manifold conditions) varies as spark timing is varied
relative to TC. If the start of the combustion process is progressively advanced
before TC, the compression stroke work transfer (which is from the piston to the
cylinder gases) increases. If the end of the combustion process is progressively
delayed by retarding the spark timing the peak cylinder pressure occurs later in
the expansion stroke and is reduced in magnitude. These changes reduce the
expansion stroke work transfer from the cylinder gases to the piston. The
optimum timing which gives the maximum brake t o r q u ~ a l l e dm aximum brake
torque, or MBT, timing--occurs when the magnitudes of these two opposing
trends just offset each other. Timing which is advanced or retarded from this
optimum gives lower torque. The optimum spark setting will depend on the rate
of flame development and propagation, the length of the flame travel path across
the combustion chamber, and the details of the flame termination process after it
reaches the wall. These depend on engine design and operating conditions, and
the properties of the fuel, air, burned gas mixture. Figure 9-3b shows the effect of
variations in spark timing on brake torque for a typical spark-ignition engine.
The maximumis quite flat.
Empirical rules for relating the mass burning profile and maximum cylinder
pressure to crank angle at MBT timing are often used. For example, with
optimum spark timing: (1) the maximum pressure occurs at about 16" after TC;
(2) half the charge is burned at about 10" after TC. In practice, the spark is often
Rtarded to give a 1 or 2 percent reduction in brake torque from the maximum
value, to permit a more precise definition of timing relative to the optimum.
SO far we have described normal combustion in which the spark-ignited
flame moves steadily across the combustion chamber until the charge is fully
consumed. However, several factors-e.g., fuel composition, certain engine design
and operating parameters, and combustion chamber deposits-may prevent this
normal combustion process from occurring. Two types of abnormal combustion
have been identified: knock and surface ignition.
Knock is the most important abnormal combustion phenomenon. Its name
comes from the noise that results from the autoignition of a portion of the fuel,
air, residual gas mixture ahead of the advancing flame. As the flame propagates
across the combustion chamber, the unburned mixture ahead of the flamethe
end gas-is compressed, causing its pressure, temperature, and density
10 increase. Some of the end-gas fuel-air mixture may undergo chemical reactions
prior to normal combustion. The products of these reactions may then autoignite:
i.e., spontaneously and rapidly release a large part or all of their chemical
energy. When this happens, the end gas burns very rapidly, releasing its energy at
a rate 5 to 25 times that characteristic of normal combustion. This causes highfrequency
pressure oscillations inside the cylinder that produce the sharp metallic
noise called knock.
The presence or absence of knock reflects the outcome of a race between
the advancing flame front and the precombustion reactions in the unburned end
gas. Knock will not occur if the flame front consumes the end gas before these
reactions have time to cause the fuel-air mixture to autoignite. Knock will occur
if the precombustion reactions produce autoignition before the flame front
arrives.
The other important abnormal combustion phenomenon is surface ignition.
Surface ignition is ignition of the fuel-air charge by overheated valves or spark
plugs, by glowing combustion-chamber deposits, or by any other hot spot in the
engine combustion chamber: it is ignition by any source other than normal spark .
ignition. It may occur before the spark plug ignites the charge (preignition) or
after normal ignition (postignition). It may produce a single flame or many
flames. Uncontrolled combustion is most evident and its effects most severe when
it results from preignition. However, even when surface ignition occurs after the
spark plug fires (postignition), the spark discharge no longer has complete
control of the combustion process.
Surface ignition may result in knock. Knock which occurs following normal
Spark ignition is called spark knock to distinguish it from knock which has been
Preceded by surface ignition. Abnormal combustion phenomena are reviewed in
more detail in Sec. 9.6.
9.2 THERMODYNAMIC ANALYSIS OF
SI ENGINE COMBUSTION
9.2.1 Burned and Unburned Mixture States
-
Because combustion occurs through a flame propagation process, the changes in
state and the motion of the unburned and burned gas are much more complex
than the ideal cycle analysis in Chapter 5 suggests. The gas pressure, temperature,
and density change as a result of changes in volume due to piston motion.
During combustion, the cylinder pressure increases due to the release of the fuel's
chemical energy. As each element of fuel-air mixture bums, its density decreases
by about a factor of four. This combustion-produced gas expansion compresses
the unburned mixture ahead of the flame and displaces it toward the combustion
chamber walls. The combustion-produced gas expansion also compresses those
parts of the charge which have already burned, and displaces them back toward
the spark plug. During the combustion process, the unburned gas elements move
away from the spark plug; following combustion, individual gas elements move
back toward the spark plug. Further, elements of the unburned mixture which
burn at different times have different pressures and temperatures just prior to 4
combustion, and therefore end up at different states after combustion. The ther- :%
modynamic state and composition of the burned gas is, therefore, non-uniform. A 5
first law analysis of the spark-ignition engine combustion process enables us to
quantify these gas states.
Consider the schematic of the engine cylinder while combustion is in
progress, shown in Fig. 9-4. Work transfer occurs between the cylinder gases and
the piston (to the gas before TC; to the piston after TC). Heat transfer occurs to
the chamber walls, primarily from the burned gases. At the temperatures and
pressures typical of spark-ignition engines it is a reasonable approximation to
assume that the volume of the reaction zone where combustion is actually
occurring is a negligible fraction of the chamber volume even though the thickness
of-the turbulent flame may not be negligible compared with the chamber
dimensions (see Sec. 9.3.2). With normal engine operation, at any point in time or
crank angle, the pressure throughout the cylinder is close to uniform. The condi-
FIGURE 9-4
Schematic of flame in the engine cylinder during
combustion: unburned gas (U) to left of
burned gas to right. A denotes adiabatic burned-gaJ
core, BL denotes thermal boundary layer in burned
gas, is work-transfer rate to piston, is heat-
II W 11 transfer rate to chamber walls.
tions in the burned and unburned gas are then determined by conservation of
mass :
and conservation of energy:
where V is the cylinder volume, m is the mass of the cylinder contents, o is the
specific volume, xb is the mass fraction burned, Uo is the internal energy of the
cylinder contents at some reference point 80, u is the specific internal energy, W
is the work done on the piston, and Q is the heat transfer to the walls. The
subscripts u and b denote unburned and burned gas properties, respectively. The
work and heat transfers are
where 0 is the instantaneous heat-transfer rate to the chamber walls.
To proceed further, models for the thermodynamic properties of the burned
and unburned gases are required. Several categories of models are described in
Chap. 4. Accurate calculations of the state of the cylinder gases require an equilibrium
model (or good approximation to it) for the burned gas and an ideal gas
mixture model (of frozen composition) for the unburned gas (see Table 4.2).
However, useful illustrative results can be obtained by assuming that the burned
and unburned gases are different ideal gases, each with constant specific heats;6
l.e.,
Combining Eqs. (9.1) to (9.5) gives
where
are the mean temperatures of the burned and unburned gases. Equations
and (9.7) may now be solved to obtain
R, - pV - mRuTu
and T --- RTb, + mRb xb
If we now assume the unburned gas is initially uniform and undergoes
tropic compression, then
This equation, with Eqs. (9.8) and (9.9) enables determination of both xb and ?,
from the thermodynamic properties of the burned and unburned gases, and
known values of p, V, m, and 0. Alternatively, if xb is known then p can k
determined. Mass fraction burned and cylinder gas pressure are uniquely related.
While Eq. (9.9) defines a mean burned gas temperature, the burned gas is
not uniform. Mixture which bums early in the combustion process-is further
compressed after combustion as the remainder of the charge is burned. Mixture
which burns late in the combustion process is compressed prior to combustion
and, therefore, ends up at a different final state. A temperature gradient exists
across the burned gas with the earlier burning portions at the higher tern.
perat~re.~. Two limiting models bracket what occurs in practice: (1) a fully
mixed model, where it is assumed that each element of mixture which burns
mixes instantaneously with the already burned gases (which therefore have a
uniform temperature), and (2) an unmixed model, where it is assumed that no
mixing occurs between gas elements which burn at different times.
In the fully mixed model the burned gas is uniform, T, = z,an d the equations
given above fully define the state of the cylinder contents. In the unmixed
model, the assumption is made that no mixing occurs between gas elements that
burn at different times, and each burned gas element is therefore isentropically
compressed (and eventually expanded) after combustion.t Thus:
t
FIGURE 9-5
Cylinder pressure, mass fraction burned,
and gas temperatures as functions of
crank angle during combustion. T, is
unburned gas temperature, T, is burned
gas temperature, the subscripts e and I
denote early and late burning gas elements,
and is the mean burned gas
temperature.' (Reprinted with per- '
mission. Copyright 1973, American
Chemical Society.)
where &(x;, xb) is the temperature of the element which burned at the pressure
p(.r;) when the pressure is p(x,), and
is the temperature resulting from isenthalpic combustion of the unburned gas at
T&(xb), p(xb). An example of the temperature distribution computed with this
model is shwn in Fig. 9-5. A mixture element that burns right at the start of the
combustion process reaches, in the absence of mixing, a peak temperature after
combustion about 400 K higher than an element that burns toward the end of
the combustion process. The mean buroed gas temperature is closer to the lower
of these temperatures. These two models approximate respectively to situations
where the time scale that characterizes the turbulent mixing process in the
burned gases is (1) much less than the overall burning time (for the fully mixed
model) or (2) much longer than the overall burning time (for the unmixed model).
The real situation lies in between.
Measurements of burned gas temperatures have been made in engines using
spectroscopic techniques through quartz windows in the cylinder head. Examples
of measured temperatures are shown in Fig. 9-6. The solid lines marked A, B, and
C are the burned gas temperatures measured by Rassweiler and Withrow7 using
'he sodium line reversal technique in an L-head engine, for the spark plug end
(4, the middle (B), and the opposite end (C) of the chamber, respectively. Curves
labeled W2 and W, were measured by Lavoie8 through two different windows, W,
a d W3 (with W, closer to the spark), again in an L-head engine. Each set of
exPerimental temperatures shows a temperature gradient across the burned gas
to that predicted, and the two sets have similar shapes.
FIGURE 9-6
Burned gas temperatures measured using spectroscopic
techniques through windows in the
cylinder head, as a function of cylinder pressure
Temperatures measured closer to spark plug
have higher values. Dashed lines show isentropic
behavior.'.
In the unmixed model, the temperature of each burned gas element follows
a different isentropic line as it is first compressed as p increases to p,., and then
expanded as the pressure falls after p,,. The measured temperature curves in
Fig. 9-6 do not follow the calculated isentropes because of gas motion past the
observation ports. As has already been mentioned, the expansion of a gas g element which occurs during combustion compresses the gas ahead of the flame ,
and moves it away from the spark plug. At the same time, previously burned gas -1
is compressed and moved back toward the spark plug. Defining this motion in an <
engine requires sophisticated tiow models, because the combustion chamber ::
shape is rarely symmetrical, the spark plug is not usually centrally located, and
often there is a bulk gas motion at the time combustion is initiated. However, the
gas motion in a spherical or cylindrical combustion bomb with central ignition
which can readily be computed illustrates the features of the combustion-induced
motion in an engine. Figure 9-7 shows calculated particle trajectories for a stoichiometric
methane-air mixture, initially at ambient conditions, as a laminar
FIGURE 9-7
Particle trajectories in unburned and burned Bar
as flame propagates outward at constant vebc1r)l
from the center of a spherical combustion bomb
Stoichiometric methamair mixture initially at 1
N o d 4 radius atm and 300 K.
m e with a constant burning velocity propagates outward from the center of a
spherical container. Applying this gas motion model to an engine, it can be concluded
that a window in the cylinder head initially views earlier burned gas (of
higher temperature and entropy) and that as more of the charge burns, the
,&jow views later burned gas of progressively lower entropy. The experimental
fit this description: they cross the constant entropy lines toward lower
Note that the gradient in temperature persists well into the expansion
indicating that the "unmixed" model is closer to reality than the "fully
" model.
More accurate calculations relating the mass fraction burned, gas pressure,
and gas temperature distribution are often required. Note that the accuracy of
calculatio~ld~e pends on the accuracy with which the time-varying heat loss
to the chamber walls can be estimated (see Sec. 12.4.3) and whether flows into
and out of crevice regions are significant (see Sec. 8.6), as well as the accuracy of
the models used to describe the thermodynamic properties of the gases. Appropriate
more accurate models for the thermodynamic properties are: an equilibrium
model for the burned gas, and specific heat models which vary with
temperature for each of the components of the unburned mixture (see Secs. 4.1
and 4.7). In the absence of significant crevice effects, Eqs. (9.1) and (9.2) can be
written as
where
1 xb Eb = - ub dx and E,, = - I' 0,. dx
Xb 0 - X b xb
and similar definitions hold for iib and ii,. For a given equivalence ratio, fuel and
burned gas fraction :
and - -
ii, = h, - pfi, fib = hb - ptb (9.1 74 b) '
TO simplify the calculations, it is convenient to assume that, for the burned gas,
6, = ub(pbp,) and Eb = vb(Tbp),. This corresponds to the fully mixed assumption
described above. The effect of neglecting the temperature distribution in the calculation
of mass fraction burned is small. In addition, the heat losses from the
unburned gas can usually be neglected; the unburned gas is then compressed
isentropically. 'F, is specified for some initial state of the unburned gas (where
isentr
process
Tu can be determined.
Equations (9.13) to (9.18) constitute a Set of nine equations for the
unknowns Ir,, Irb, ii,, iib, &, 5, T,, Tb, and x, or p. One convenient sol
method is to eliminate xb from Eqs. (9.13) and (9.14) to obtain
fib - i),, fib - ii,
where U = UO - W - Q.
can then be solved using a
obtained from Eq. (9.13). An
of pressure dp/dO and equat
found in Ref. 10. Some exa
measured pressure data, wi
With accurate pressure versus crank angle rec
burned should be close to but lower than unity, usually in the range 0.93
the difference from unity is the combustion inefftcency for lean mixtures (
3-9) and incomplete oxygen utilization for rich mixtures (see Fig. 4-20).
More accurate burned gas temperature c
presence of a thermal boundary layer (of order 1 mm thick) around th
bustion chamber walls (see Sec. 12.6.5). The burned gas region in Fig. 9-4
Degrees after spark
(4
FIGURE 9-8
Mass fraction burned curves determined from measured cylinder pressure data uing two-#
bustion model: (a) gasoline; (b) methanol. 4 = fuelfair equivalence ratio."
COMBUSTION IN SPARK-IGNITION ENGINES 383
FIGURE 9-9
Calculated temperature distribution in
the adiabatic core of the burned .g,a s
zone for the unmixed model assuming
thermodynamic equilibrium. 4 = 1.0.
5 10 15 20 Dashed line is temperature of each
Pressure, am element just aIter it bums.
divided into an adiabatic core and a boundary layer that grows in thickness with
tune. In the adiabatic core, in the absence of mixing between gas elements that
burn at different times, burned gas is compressed and then expanded isentropially.
The burned gas temperature distribution can be calculated as follows.
Gwen the pressure versus crank angle data, the unburned mixture state can be
determined using Eq. (9.18) above. Each small element of unburned mixture
burns in a constant-enthalpy constant-pressure process. So the burned state of an
clcment of unburned charge, which burns at p = pi, can be obtained from the
&lation
After combustion, this element which burned at p =pi is compressed and
txpanded along the isentropic:
An example of the temperature distribution computed in this manner for this
mixed model in the burned gas adiabatic core is shown in Fig. 9-9. The
&men1 ignited by the spark is compressed to the highest peak temperature at
L,. The temperature difference
و امروز یک داستان عبرت آموز از محققان EA
"A son and his father were walking on the mountains.
Suddenly, his son falls, hurts himself and screams: "AAAhhhhhhhhhhh!!!
To his surprise, he hears the voice repeating, somewhere in the mountain: "AAAhhhhhhhhhhh!!
Curious, he yells: "Who are you?"
He receives the answer: "Who are you?"
And then he screams to the mountain: "I admire you!"
The voice answers: "I admire you!"
Angered at the response, he screams: "Coward!"
He receives the answer: "Coward!"
He looks to his father and asks: "What’s going on?"
The father smiles and says: "My son, pay attention."
Again the man screams: "You are a champion!"
The voice answers: "You are a champion!"
The boy is surprised, but does not understand.
Then the father explains: "People call this ECHO, but really this is LIFE.
It gives you back everything you say or do.
Our life is simply a reflection of our actions.
If you want more love in the world, create more love in your heart.
If you want more competence in your team, improve your competence.
This relationship applies to everything, in all aspects of life;
Life will give you back everything you have given to it."
YOUR LIFE IS NOT A COINCIDENCE. IT’S A REFLECTION OF YOU!"
داستان سوم رو نقدا داشته باشید تا پست بعدی
A Scotsman's trunk....
ادامه مطلب ...با عرض پوزش این هم از داستان دوم با یک روز تاخیر
The heavy bag
An Englishman was.........
ادامه مطلب ...با فرا رسیدن ماه مهر و آغاز مدارس و کودکستانها ودانشگاه و................محققان سخت کوشEa
تصمیم گرفتند که هر روز یک داستان طنز به زبان انگلیسی به عنوان یک کار فرهنگی آموزشی منتشر بشه
خوشحالا میشیم که با ما همراه باشید
·Mary was an english girl, but she lived in Rome. she was 6 years old.last year her mother said to her you are six years old now ,Mary,and you are going to begin going to a school here.you're going to like it very much, because it's a nice school.
ادامه مطلب ...