Good afternoon. We will continue our discussion on Combustion
in internal combustion engines. So, if you try to recall in the last lecture
we have discuss about the SI engine combustion and we have seen several issues related to
the SI engine combustion. So, today in continuation of that, we will
try to see the you know pressure rise associated with the combustion in SI engines. And then, we will try to discuss several issues
of the you know CI engine combustion and then we will try to discuss about two important
parameters, which are you know essential. Rather which are in I can say the indicator
of the you know two important. In fact, the you know desirable undesirable
phenomenon that we have discussed in the context of SI engine combustion that is detonation. So, knowing the magnitude of these two parameters,
we can say whether the undesirable phenomenon related to SI engine as well as CI engine
combustion will there or not. So, before we go to discuss about those parameters,
first I would like to discuss about the pressure rise because of these combustion in SI engines. So, I can say pressure rise in SI engines. Of course, I can write pressure rise during
combustion in SI engines. So, again we need to draw a schematic and one external engine that will be there
for the SI engine combustion. So, we have discussed several times that,
the movement of the piston is restricted between these two locations; that is top dead centre
and the bottom dead centre. Now, if you try to draw you know the rise
in pressure and. So, say this point is top dead centre and
this is bottom dead centre and this is also bottom dead centre. So, if you try to draw this is crank angle
theta and so, if you try to draw that the crank is rotating through this circular path. And so, if we say this is BDC location, towards
this and again we will be coming towards BDC. So, this is top dead centre and this will
be the bottom dead centre. So, piston movement is continuous and the
piston is travelling between these two centers. What is happening? So, what is done, if you try to draw a curve
so, say compression stroke. Compression stroke piston travels from BDC
to TDC and then say this is pressure and say this is atmospheric pressure. Now, compression stroke cylinder is filled
up with you know charge or air and then piston travels from BDC to TDC; and pressure inside
the cylinder will be increased. And after reaching at TDC pressure will be
high and again when piston is travelling back from TDC to BDC; because when piston travels
from BDC to TDC during compression stroke both the valves closed. Now, when again piston is coming from TDC
to BDC we will have fall in pressure. So, this is the diagram and this curve is
known as monitoring curve; monitoring curve. So, this curve is known as monitoring curve
and this monitoring curve is produced. So, this curve is generated by a virtual external
motor, which is used to rotate the crank. So, this is the monitoring curve. As if we are we are using an external motor
and the motor is in a rotating the crank and the with the change in crank angle what will
be the pressure inside the cylinder that is what we have plotted. What is done? So, a similar curve will be there even during
normal condition, but what is done before piston reaches at TDC towards the end of the
compression stroke, we switch on the spark plug. And that is what is used to ignite the charge
in the SI engines. So, we will see now, that depending upon the
location of switching on this spark plug whether it is whether the spark plug switched is on,
exactly are the whole piston is at TDC or if you switch on the spark plug when piston
is 10 degree before TDC. And if you switch on the spark plug when piston
is you know 20 degree before TDC, then what will be the rise in pressure inside the cylinder. And of course, the you know impact of the
pressure that is developed because of this combustion on the overall performance. So, if we now, switch on the spark plug say
at 3 different points; 1, this is 1, this is 2 and this is 3. So, I have identified these 3 different points,
on the monitoring curve because monitoring curve will be there what is done say, if we
consider point 1, then if we switch on the spark plug at point 1, what will happen? The compress charge will be ignited using
spark plug combustion will take place. And because of this combustion, as we have
discussed in the last class there will be you know generation of heat and pressure and
temperature of the you know combustion of the gases will increase. So, if I now, try to plot the rising pressure
on the same curve the curve will be like this right so, this is the case. If we consider another point, so say this
is 1, 1 prime 1 double prime. So, the pressure rise
because of the combustion is shown by 1, 1 prime 1 double prime. Now, if we are if we consider point 2, then
so, this is the peak pressure right and if I write peak pressure is 1 prime. Now, if you consider point 2, in that case
the nature of the curve will be same, but say this is 2 prime and this is 2 double prime. So, pressure rise will be 2, 2 prime 2 double
prime with peak pressure at 2 prime. And if we consider another one that is at
point 3, then what will happen? Say so, this is 1 double prime, this is 3
double prime, this is 3 prime, this is 2 prime and this is 3. So, if we consider point 3 then peak pressure
will be at 3 prime, what we can see from the figure is that, if we now, you know shift
the point at which spark plug switched is on, from 1 to 2; the peak pressure develop
in the cylinder will be less. If peak pressure which is being developed
because of these combustion becomes less, then work output of the engine will be less. On the other hand, if we you know advances
the you know switching on point from 1, 2, 3; that means, if we switch on the spark plug,
when piston is sufficiently higher from the TDC. The rise in peak pressure will be high that
is seen from this diagram not only that because of these high peak pressure, the output power
from the engines will high. So, what we can you know conclude from this
diagram is that, if we you know switch on the spark plug which is say 25 degree or 30
degree before TDC, then the rise in pressure inside the cylinder because of these combustion
will be high. And as a result of which the total thrust
total force that will get during the expansion stroke, you know that is the because of this
high pressure will be more. So, we will get more work output. So, from this you know figure we can say the
you know, if we can you know rather if we advances the switching on switching on point
of the spark plug. Then the power developed by the engine power
output from the engine will be high, but this is not desirable that is what we should discuss
now. So, if we see that when; that means, if we
you know delay the switching on time; that means, if we somehow can make sure that the
spark plug switch will be on when piston you know reaches at TDC. Then what will happen? In that case pressure rise will be so small
inside the cylinder, that the power output from the cylinder will be very less. But although, the power output from the cylinder
are developed by the engine will be less, but we can prevent another undesirable phenomenon
associated with the engine operation. What is that? See, if we now, you know delay the I mean
if we advances the switching on point. That means, if you switch on the spark plug
sufficiently, when piston is you know sufficiently higher from the BDC that is we are advancing
the switching on point. Then, what will happen we can see that the
pressure developed by the pressure developed inside the cylinder because of the combustion
is really high and of course, this high pressure will create high thrust, we will get high
work output. But at the same time we are inviting another
problem. What is that problem? Problem is if we now, switch on the spark
plug at point 3, what will happen? That means, the piston during reach at TDC,
we are switching on the spark plug. So, because of the combustion the pressure
that will be developed that will now, try to create resistance on the piston face ; that
is different that is an important issue. So, the resistance that will be created. So, if you switch on the spark plug at point
3, the entire combustion will take part into two different you know parts. So, what will happen? The first part of the combustion pressure
that will be developed that pressure will try to create resistance on the piston face
for its movement towards TDC. And, that pressure rise will try to create
a very jerking motion of the piston cylinder. So, the engine the engine operator, here see
we will feel that the piston movement is not smooth. Rather, the pressure rise developed because
of this first part of the combustion will create, a resistance to the movement of the
piston towards TDC, during the compression stroke itself. So, although we may get higher power output,
if we advances the spark plug you know switching switch on mechanism. But, at the same time we are inviting another
problem and that is not desirable at all. So, keeping that in mind; that means, if we
delay the switching on time, rather if we delay the spark plug switch on then the power
output power developed by engine will be very small and that is not also economic you know
consideration. So; that means, what I would like to say the
you know spark plugs should not be switched on, when piston is very close to TDC. On the other hand, spark plugs should not
be switched on when piston is sufficiently higher from the TDC. Keeping these two you know important points
in mind; that means, in if we you know delay the switching on time, we will get lesser
output if we advances the switching on time we will get jerking motion. So, these two important points we need to
keep in mind and considering these two important points what is done is that. The you know it is ensured that, for the
operation of SI engines that is very important we should know that 50 percent you know the. Rather I can say 50 percent of you know total
pressure rise; pressure rise required to achieve before TDC and 50 percent after TDC. So, this is the important point you should
keep in mind. That means, if we need to achieve 50 rather
we should achieve 50 percent the total pressure rise before TDC while 50 percent will be after
TDC. So, if we do like this then we can optimize
the engine performance, if we do like this then we can optimize
the engine performance. So, optimize means that we are not going to
have very jerky operation. On the other hand rather the same time, we
are getting desirable amount of power output from the engine so; that means, to keep this
two aspects in mind this is done. So, that so, this is done by controlling the
you know switching spark plug switching on time. So, that the total engine operation can be
optimized. So, next we will try to see, you know the
combustion of the CI engines. So, before we go to discuss about the you
know combustion of CI engines in fact, we will try briefly discuss about what do mean
by that. In fact, if you try to recall it in the last
class, we have discussed about what is combustion. And to know that to know the same what is
happening inside the CI engine cylinders we should discuss a few important points. And to understand the different aspects different
issues associated with the combustion of CI engines, we need to know the rather we need
to recall what we have studied in our previous lectures. You know that, if we try to recall for the
CI engines we do not have rather engines are you know ignited, rather I can say the combustion
the engines are not equipped with special instruments special device that is spark plug. So, we have seen that the SI engines are having
you know a special, I can say instrument device which is an external agent I can say to ignite
the combustion. But for the combustion you know for this compression
ignition engines the I mean, we have seen that there is no such you know you know device
or instrument to ignite the fuel. Instead what is done? The thermodynamic straight of the compressed
air for the CI engine instead of air fuel mixture only air is drawn into the cylinder
during the intake stroke. So, the high pressure and temperature at the
end of the compression process itself, is used to initiate the combustion the moment
when the fuel is spread into the cylinder by the fuel nozzle. So, that is what we have understood from the
from our previous classes. So, now, what we will do? I will try to draw one draw the schematic
of the engine cylinder for the compression ignition engines. I am drawing the schematic because the schematic
depiction will help us to understand, the you know mechanism in a rather convenient
way. So, instead of you know spark plug we have
fuel nozzle and which is used to supply fuel inside the cylinder. So, we have exhaust valve intake valve only
air is taken into the cylinder during intake stroke and this is fuel and this is the combustion
gas. So, this is this is top dead centre this part
is bottom dead centre and this is engine cylinder. So, although the you know combustion is as
I said chemical reaction of hydrogen and carbon with the oxygen, which is present in the air. So, we cannot trivially ignore the chemical
reaction, which is important for the combustion. So, when we talk about SI and CI engines the
combustions you know process is you know undoubtedly similar. But, there are a few issues which make the
combustion process completely different; I mean combustion process of CI engines is completely
different. Then what is you know rather what are happening
you know inside the SI engine cylinder; SI engine cylinder. So; that means, what I would like to say,
although the chemical reaction that is we have discussed in the last class. That is that chemical reactions are basically
exothermic reaction and because of what we are getting energy rather that in the form
of heat. So, the chemical reactions will be definitely
similarly, but there are a few issues few aspects, which are I mean from you know from
you know from those perspective. I mean if we look at those critical issues
critical aspects, we can differentiate CI engine combustions rather you know CI engines
combustion from the SI engine combustion. So, what are the difficult rather critical
issues? So, now, we need to know what are the critical
issues, which make the CI engines combustion different than the SI engine combustion. So, what are the different issues? I will write in details that will help you,
but for the time being I will try to discuss pointwise. So, now, and to discuss to have a better understanding
on this aspect I have drawn this schematic. Now, if you try to recall that for the SI
engine combustion, we had one external agent spark plug; which was used to ignite the compress
charge. And if you try to recall that we have explained,
the combustion which is manifested by the visible flame or pressure rise. Rise in pressure we can we cannot we can measure,
but the flame we can sense. So, I can say the combustion which is manifested
by the visible flame or by the rise in pressure. So, rise in pressure we can measure, but at
least we can sense the flame, which is being produced because of this combustion because
of this combustion process. Now, if you try to recall that for the SI
engines, the combustion starts from the charge element which is very close to the spark plug
and because of which we will have visible flame. The flame will now, try to propagate; that
means, the energy which is being generated because of the chemical reaction exothermic
reaction, that energy will be transferred will be percolated into the next or adjacent
charge layer. So, the next charge layer will be now, combusted
and because of these combustion process again we will get another flame. So, in this way the flame propagates across
the combustion chamber and we are getting total combustion that is for the SI engines
scenario. What is happening inside the CI engines? We do not have any spark plug, instead we
are using the high pressure and relatively high temperature of the compressed air to
ignite the that thermodynamic state itself is use to ignite the fuel the movement, when
it is sprayed inside the cylinder by a fuel nozzle. So; that means, if we now, compare that SI
engine combustion is localized; that means, we can start combustion for a particular point. And because of this combustion the energy
which is being generated that is responsible to you know to complete the total combustion
because of these travel of the flame. And we have discussed many such critical issues
in the context of that you know movement of the flame movement of the flame fine. But in case of the SI CI engines it is not
localized; that means, I can say the compressed charge sorry. The compressed air rather all the compressed
air inside the combustion chamber in a is in a position to initiate combustion the movement
when, fuel is injected in to the cylinder. So, this is not a localized phenomenon. So, if we now, differentiate that CI engine
combustion is not a localized phenomenon is not localized,
instead we can initiate combustion I mean across the combustion chamber. In fact, it can you know combustion can be
started from every you know points every elementary compressed charge within the compression chamber. But in case of a SI engine the combustion
will start only from the elementary charge which is there in the facility of the spark
plug. So, this from that point of view from that
perspective, I am telling the CI engine combustion is not localized. Number 2 is what? So, this is the a part we have discussed very
important point. Number 2 is that for the SI engine we have
seen that the main flame front, which is coming from the spark plug and the main flame front
you know transferred energy to the adjacent charge layer. And in doing so, the entire combustion is
completed. But in case of a CI engine, again I am telling
that combustion it is possible that all the elementary compressed you know air, which
is present inside the inside the combustion chamber can participate in the combustion
process. So, the combustion in I can say for the SI
engine to some extend it is you know uniform the movement of the movement of the flame
front which is coming from the spark. In the in the in the region you know, which
is close to the spark plug the flame traverses across the combustion chamber. And we can say if we somehow can suppress
the secondary flames, which are being developed because of the hot spots, we can say the combustion
is fairly uniform. But in case of the CI engines, as all the
elementary you know charge all the elementary compressed air are I mean all you know compressed
air which is present towards the end of the compression stroke. So, from every corner every location, the
compressed air can take part in the combustion. So, keeping that in mind it is very you know
it is not rather I can say, it is not possible to say the combustion will be uniform rather
it will be erratic in nature. So, this is number 2. So, these two important points we should keep
in mind that, the CI engine combustion is localized not localized that is what I have
explained. And also because of since this is not localized
I can say the combustion is not expected to have rather I can say uniform behavior. But, if we can somehow suppress the you know
secondary flames which are being produced because of the hotspot in the context of a
SI engine combustion, we can say the SI engine combustion will be fairly uniform. So, these two points I will now, elaborate
and that will help you to you know to understand what I have explained. I can say in a better way. So, now, I will try to write that CI engine
combustion, can be you know we have seen that the SI engine combustion, we have you know
categorized into two different parts; one is normal other is abnormal. Again abnormal combustion can be sub classified
into two different parts; that is you know pre ignition or second one is detonation;
that is what is we have discussed and we have understood in the last lecture. In a similar way, CI engine combustion can
be you know divided into three different parts. Number 1 is known as delay period, number
2 is period of rapid combustion and number 3 is third phase of the combustion. So, these three important parts rather three
the three this I mean, this three important you know classifications can be done in the
context for the I mean can be done to describe the CI engine combustion. So, that is first one is delay period, second
one is period of rapid combustion and third one is the third phase of combustion. So, now, we will try to elaborate what do
mean by delay period then period of rapid combustion and finally, the third phase of
the combustion. So, now, when we will be talking about delay
period that is again another important point we should know about the CI engine combustion. So, if we try to recall that for the CI engines,
fuel is sprayed into the engine cylinder towards the end of the compression stroke. When the pressure and temperature of the compressed
air is beyond a threshold value and that pressure and temperature is you know sufficient to
ignite the fuel, which is being injected sprayed into the cylinder and then total combustion
will take part. And as I said that it is not like the case,
what we have seen in the context of SI engine operation. Instead it is the point of local condition;
that means, I mean it is the you know situation, where the compressed air elementary volume
of air which is I mean having pressure and temperature beyond the threshold value can
ignite the fuel element, which is in contact with that particular you know air volume. So, the fuel is sprayed inside the cylinder,
we can assume that the entire combustion you know combustion chamber is now, having compressed
air. The moment when, fuel is in contact with the
compressed air depending upon the local condition of the pressure and temperature of that particular
air combustion may initiate. It may so happens that, the air which is there
inside the you know combustion chamber at the end of the compression stroke, rather
or towards the end of the compression stroke we will have a gradient in pressure and temperature. So, if you try to plot the pressure across
the combustion chamber, temperature across the combustion chamber. I deliver we are suppose to get an uniform
temperature uniform pressure across the combustion chamber, but it is not happening in reality. So, there are a few issues we need to consider. Now, because the air which is which is in
contact with the cylinder valve. So, if we now consider, say engine is started
and you know at latest you know stage; I mean may be after a few cycles of operation what
will happen? The cylinder valve temperature will be higher. So, maybe we are now compressing and we are
suppose to get an uniform temperature of you know air of the you know of the of the compressed
air inside the compression chamber, but that won’t be the case. That is the air which is in contact with the
cylinder valve will have higher temperature, then the air which is there middle of the
combustion chamber. So, when the fuel is injected in to that regions,
where temperature of the air because of these hot spot is higher the combustion will start
initially. So, we can say the and that too also depends
upon the you know fuel, the you know fuel the function of the nozzle is to atomize the
fuel into number of smaller droplets. Depending upon the chemical structure of the
fuel, it is you know you know very difficult to say that the moment when fuel is injected
fuel is sprayed the entire combustion will take part. So that means, what I can say that, depending
upon the local pressure and temperature of the air that is there inside the combustion
chamber. And also depending upon the molecular structure
whether we can we have been able to fragment the fuel particle using the nozzle; nozzle
is mechanical device it will start malfunctioning may be after certain period of time. So, with time the performance of the nozzle
will be poorer. So, what we can say depending upon the local
condition of the pressure local pressure and temperature of the air which is of the compressed
air which is there inside the cylinder, together with the you know chemical structure and,
I can say volume of the fuel which is being sprayed into the cylinder the combustion will
be combustion will takes place. So, not only that again the injection of fuel
though the fuel nozzle will take some finite time. It is very difficult to say that may be we
need to supply certain amount of fuel for a particular cycle of operation. So, to for the injection of that certain amount
of fuel we have designed the nozzle. But as I said with time the nozzle will start
malfunctioning. So, it is very difficult to say that within
the certain amount of time. I mean within that time the amount which you
have rather the nozzle will be able to supply designed amount of I mean fuel, which was
you know design I mean to obtain during the during fuel injection. So, what we can say that, nozzle will take
some time for the injection, together with the together with different other conditions. Like, the local condition local pressure and
temperature of the air of the compressed air, along with the volume of the fuel whether
we get you know required amount of fuel which was designed to which was design. Or the amount of fuel which is being injected
by the nozzle whether the value is the design value or which is or that is the lesser than
the design value. So, depending upon these you know considerations,
we can say that the combustion which you are supposed to get we will not get that means,
we require some amount of time. So, the moment if we have discussed today,
that the fuel injection will take part when piston is slightly before TDC; to ensure that
the first part of the fuel which is being injected, will take part in the combustion
when piston is at TDC. Considering the fact that, combustion will
take some you know some took combustion will takes certain amount of finite time to you
know takes place. So, this is known as delay period. So, this delay period is the period; so delay
period is the period I am writing now, delay period. So, the delay period in diesel engine often called, the ignition
lag. This is also known as the ignition lag apparently,
corresponds to period you know of preliminary reaction which occur prior to the appearance
of the flame. So that means, combustion will combustion
started as I said, we will come to know combustion is manifested by a visible flame or you know
by in appreciable rise in pressure. So, we can measure only the rise in pressure,
but we can see the flame which is produced. So, if we say the rather if we can see the
flame we can say combustion is started. So, to get the appearance of flame, we need
certain amount of time and that time is known as delay period or sometimes it is known as
ignition lag. This delay period can be further classify
into two categories; one is known as physical delay other one is known as chemical delay. So, if we now, go to the next slide, that
physical delay and the chemical delay. So, total ignition lag total physical delay
is the summation of the physical delay plus chemical delay; total ignition lag and total
delay period is the summation of the physical delay and the chemical delay. So, what is physical delay? So, physical delay, I mean that is time needed
for evaporation of fuel droplets and also mixing with air. While, chemical delay that is solely
dependent upon the characteristics of the of the fuel. So, that means, physical delay which is associated
with the fuel injection, which is associated with the performance of the nozzle not only
that, then fuel which is supplied that will take certain amount of time to get mixed with
the air. If it is I mean if nozzle is performing well,
then the fuel will be fragmented into a number of smaller droplets. So, it will take small amount of time to mix
with the air. So, physical delay is essentially related
to the nozzle performance. Chemical delay solely dependent upon the chemical
characteristics chemical compositions molecular rate of the fuel. So, some if it is heavy hydrocarbon, it may
take I mean it depends upon the you know fuel chemistry. So, these two delay, I mean physical delay
and chemical delay constitute together to you know define the ignition lag or delay
period for the CI engine operation. So, we have understood what is the delay period. So, this is so that means, the CI engine combustion
will be you know started into three different parts. First we need to consider the delay period
which is essential, which is important considering these two important you know aspects that
is the physical delay which is associated with nozzle performance and the chemical delay
that depends upon the chemical characteristics of the fuel itself. So, the first of you know first part the moment
we inject fuel combustion own start immediately and it will take some amount of time and this
is what is the physical delay. Next is the period of rapid combustion. So, once we I mean once this delay period
is over then now, entire fuel air mixture is now, in a straight to ignite. So, then the combustion will be accelerated,
that is why it is known it is known the period of rapid combustion. So, if I write combustion in this phase
is due to is mainly due to burning of such fuel, nots only fuel, burning of such fuel
as had time to be evaporated and mixed with the air during delay period. The rate and extent of burning during this
period are closely associated with the you know with the lengths associated the length
of the delay period and the injection process. So, this is what we have written. That means, the delay period we needs after
you know some finite time. And by that time the you know thermodynamic
state of the you know combustion chamber will be you know chased and the chased straight
chased thermodynamic state will be responsible for you know to burn the amount of fuel that
will be accumulated by that time. So, the fuel injection is continuous process. So, fuel injection is you know we have a certain
amount of fuel injection time the first part of the fuel that will be injected that fuel
will go through the delay period and chemical delay. Now, because of these combustion of the first
part of the you know fuel which is being injected thermodynamic straight will be chased inside
the combustion chamber. But now, the thermodynamic you know chased,
straight chased thermodynamic straight will be in a position to ignite the amount of fuel
which is being supplied by that time. And there will be rapid combustion and that
is why it is known as period of rapid combustion. And last one is you know third phase of combustion. So, this is
the third phase of combustion is the period from maximum pressure to the
point, where combustion is you know measurably complete; measurably complete. So, this is the third phase again I am reiterating
that combustion is nothing but; nothing but the appearance of the visible flame. If we would like to see it, if we would like
to measure it then we can say their pressure will be increased. So, the third phase of the combustion is the
period from maximum pressure. So, the maximum pressure that is developed
to the point where combustion is measurably complete. Maximum pressure that is developed because
of these rapid combustion, but it will take again small amount of time to complete the
combustion and that is known as third phase of the combustion. So, I can say that, to summarize today’s
discussion we have discussed about the you know peak pressure rise, we have discussed
you know in the context of SI engines that fuel is ignited using spark plug. So, now, depending upon the time at which
spark switch is on, we have discussed that there will be an important inter play between
the power output by the engine and the engine operation; there are I can say jerky operation
of the engine. And taking three different points, we have
critically discussed that what will be the consequences. If we switch on the spark plug which is very
you know when piston is very close to TDC; also when piston is you know sufficiently
away from the TDC during compression stroke. And from there we have concluded that it is
desirable to have maximum you know 50 percent the total pressure rise before TDC. Rather piston when is when piston is before
TDC and remaining 50 percent of the pressure rise will be when piston is travelling back
from the TDC to BDC and that is happening. Because this is the you know internal combustion
engine operation process. Next we have discuss about the CI engine combustion. We have tried to understand you know what
are the differences between the CI and SI engine combustion processes. And we have identified although, the combustion
process is you know undoubtedly similar for both the you know classes of engines. But the CI engine combustion is distinctly
different than the SI engine combustion from two different perspective. And that is what we have discussed in a in
a you know in a greater detail today. And finally, we have seen that the CI engine
combustion can be divided into rather entire combustion can be divided into three different
parts; delay period we have discussed which is also known as the ignition lag. And then finally, we have discuss about the
period of rapid combustion and the third phase of combustion, which is also there in the
context of CI engine operation. So, with this I stop my discussion today and
we will continue our discussion in the next class. Thank you.
