We were discussing Rankine cycle and its various terminologies in our recent post, where we have discussed the basic concept
of ideal cycle for steam power plants i.e. Rankine cycle and also we have seen
there the basic operation and arrangements of various components of steam power
cycle or Rankine cycle.
Today we will be focused here to
understand the basic concept of Otto cycle.
Otto cycle: The ideal cycle for internal combustion spark ignition reciprocating engines
Otto cycle is one type of air standard
cycle which is designated as the ideal cycle for the operation of internal
combustion spark ignition reciprocating engines. Before understanding the Otto
cycle, we must be aware about the operations performed by a reciprocating spark
ignition internal combustion engine or simply petrol engine.
Therefore first let us see an overview
of internal combustion spark ignition engine. Basic components of internal
combustion spark ignition reciprocating engines are displayed here with the
help of following figure.
We can see here the arrangement of
cylinder and piston with two valves i.e. inlet valve and exhaust valve. Piston
will reciprocate within the cylinder between two fixed positions i.e. IDC and
ODC
The inner most position will be termed
as inner dead centre (IDC) and at this state piston will make lowest volume in
the cylinder. IDC i.e. inner dead centre will also be termed as top dead centre
(TDC) if engine is one vertical engine.
Second fixed position will be termed as
ODC i.e. outer dead centre and at this state piston will make largest volume in
the cylinder. Outer dead centre will also be termed as bottom dead centre (BDC)
if engine is one vertical engine.
Piston will also be connected with
crank-connecting rod mechanism and therefore reciprocating motion of piston
will be converted in to rotary motion and vice versa. Piston will move within
the cylinder and when piston will move from one dead centre to another dead
centre such as IDC to ODC then that movement of piston will be termed as
stroke.
Now we will start to understand the Otto
cycle with step by step and simultaneously we will draw here the PV diagram
also for the Otto cycle.
So let us start our cycle with IDC i.e.
from inner dead centre. When piston will be at IDC, intake valve will be in
open condition and exhaust valve will be closed. In this situation, pressure
inside the cylinder above the piston will be approximately equivalent to atmospheric
pressure. When piston will start to move towards ODC or BDC, gas above the
piston will start to expand as piston will be moving downward towards ODC.
Therefore pressure will be reduced below
the atmospheric pressure and hence suction pressure will be created and hence
fresh mixture of air and fuel will be admitted in to the cylinder through the
intake manifold.
Now let us consider this situation clearly,
piston is moving downward and suction pressure created and therefore fresh
mixture of air and fuel is coming in to the cylinder and therefore pressure
will be equalise here due to admission of fresh mixture of air and fuel. In simple
we can say that admission of mixture of air and fuel in to the cylinder will be
done at approximately constant pressure. This process is displayed in PV
diagram by process 0 to1.
When piston will reach to ODC, intake
valve will be closed and hence we can say that in this situation both valve
will be closed. Intake valve and exhaust valve both will be in closed position
when piston will reach to ODC or BDC.
Now when piston will start to move
towards IDC, mixture of air and fuel will be compressed within the cylinder due
to movement of piston towards IDC. When piston will reach IDC, piston will made
lowest volume in the cylinder and hence mixture of air and fuel will be
compressed. Hence pressure and temperature of working fluid i.e. mixture of air
and fuel will be high and we must note it here that cylinder will be insulated
and hence there will not be any transaction of heat energy during this process
of compression of mixture of air and fuel. This process is displayed in PV
diagram by a process 1to 2 and this process will be isentropic process.
As we can see above figure, there will
be one igniter and this igniter will start the burning of mixture of air and
fuel. Hence pressure and temperature of the product of combustion will be
increased due to the exothermic reaction. We must note it here that this
combustion process of charge (mixture of air and fuel) will be so instantly
that we can assume that this process will be constant volume process. This
process is displayed in PV diagram by a process 2to 3 and this process will be constant
volume process.
Now due to high pressure and temperature
of the combustion product, piston will start to move towards ODC. We can say
that piston will move from IDC to ODC and we must note it here that for this
process also entropy will be constant because there will not be any heat energy
transfer during this process 3-4.
As piston will move from IDC to ODC due
to high pressure and temperature of the combustion product, crankshaft will
start to rotate and will develop the useful work.
Now piston will be at ODC and in this
case exhaust valve will be opened and therefore combustion product will leave
the cylinder through the exhaust valve. This process will be so instantly that
we can assume that this process will be constant volume process. This process
is displayed in PV diagram by a process 4 to 1 and this process will be constant
volume process.
So let us see the PV diagram, Heat
energy will be added to the system during the process 2-3 and heat energy will
be rejected during the process 4 to 1.
Work energy will be done over the system
during the compression stroke or during the process 1 to 2 and work energy will
be produced as useful work during the expansion process 3 to 4.
Now piston will start again to move
towards IDC in order to remove the rest combustion product from the cylinder,
piston will be moving towards IDC and compressing the combustion product but
same time combustion product leaving the cylinder through the exhaust valve and
hence we can say that pressure will be constant. In simple way, we can say that
exhaust stroke will be done at constant pressure process and this process is
displayed by the process 1 to 5.
Cycle 1-2-3-4-1 will be termed as Otto
cycle. Petrol engine or internal combustion spark ignition reciprocating engines
are working on the principle of Otto cycle. Piston executed here four complete
strokes and crankshaft will rotate by two revolutions for each thermodynamic
cycle.
Do you have any suggestions? Please
write in comment box.
We will see another topic in our next
post in the category of thermal engineering.
Reference:
Engineering thermodynamics by P. K. Nag
Engineering thermodynamics by Prof S. K.
Som
Image courtesy: Google
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