We were discussing Otto
cycle, an ideal cycle for internal combustion spark ignition reciprocating
engines or simply petrol engines, in our recent post. We have also discussed
the derivation of efficiency
of Otto cycle and volumetric efficiency of Otto cycle.
Today we will see here another air standard cycle i.e. Diesel cycle. Diesel engines are operated on the principle of Diesel cycle.
Today we will see here another air standard cycle i.e. Diesel cycle. Diesel engines are operated on the principle of Diesel cycle.
Diesel cycle: The ideal cycle for compression ignition engines
Diesel cycle is one type of air standard cycle which
is designated as the ideal cycle for the operation of internal combustion compression
ignition reciprocating engines. Before understanding the diesel cycle, we must
be aware about the operations performed by a reciprocating compression ignition
engines or simply Diesel engine.
Therefore first let us see an overview of internal
combustion compression ignition reciprocating engines. Basic components of internal
combustion compression 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
center (IDC) and at this state piston will make lowest volume in the cylinder.
IDC i.e. inner dead center will also be termed as top dead center (TDC) if
engine is one vertical engine.
Second fixed position will be termed as ODC i.e.
outer dead center and at this state piston will make largest volume in the
cylinder. Outer dead center will also be termed as bottom dead center (BDC) if
engine is one vertical engine.
Piston will also be connected with crank lever
mechanism as shown in above figure 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 Diesel cycle
with step by step and simultaneously we will draw here the PV diagram also for
the Diesel 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, air 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
air will be admitted in to the cylinder through the intake manifold.
We must note it here that this is the basic
difference between Otto cycle and Diesel cycle that in case of Otto cycle,
fresh mixture of air and fuel will be admitted in to the cylinder during the
suction stroke through the intake manifold. There will be one igniter and
igniter will start the burning of mixture of air and fuel.
While for Diesel cycle, fresh air will be admitted
in to the cylinder through the intake manifold. There will be one injector of
fuel and it will inject the fuel during combustion stroke.
Now let us consider this situation clearly, piston
is moving downward and suction pressure created and therefore fresh air is
coming in to the cylinder and therefore pressure will be equalize here due to
admission of fresh air. In simple we can say that admission of air 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 and cylinder
will be filled with fresh air when piston will reach to ODC or BDC.
Now when piston will start to move towards IDC, air
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 air will be compressed. Hence pressure and temperature of working fluid
i.e. air 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 air.
Pressure and temperature developed, due to the
compression of air, at the end of compression stroke will be quite high as
compared to that of petrol engine. This process is displayed in PV diagram by a
process 1 to 2 and this process will be isentropic process.
As we can see above figure, there will be one
injector of fuel and it will inject the fuel during combustion stroke. As we
know that molecular mixing of fuel and air will be required for the burning and
therefore any liquid fuel must be vaporized first so that fuel and air may mix
with each other as desired. Fuel in liquid phase could not mix as desired with
air because molecular mixing of fuel and air will not be possible in different
phase.
Fuel at very high pressure will flow through a fuel
injector and fuel jet will be discharged through the orifice of fuel injector
at very high speed. Fuel i.e. Diesel will be injected by the fuel injector in
the form of atomized spray.
Therefore as we have studied that air will be at
high pressure and high temperature at the end of compression stroke, heterogeneous
combustion will take place as fuel will be injected in the form of atomized
spray.
Hence pressure and temperature of the product of
combustion will be increased due to the heterogeneous combustion process. We
must note it here that this combustion process will be carried out with constant
pressure process. This process is displayed in PV diagram by a process 2 to 3
and this process will be constant pressure process.
This is one more difference between Otto cycle and
Diesel cycle. Combustion process in case of Otto cycle will follow the constant
volume process and if we talk about Diesel cycle then combustion process will
follow the constant pressure 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. Process 3-4 will also be termed as power
stroke of Diesel cycle.
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 above 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 Diesel cycle.
Diesel engines are working on the principle of Diesel cycle. Piston executed
here four complete strokes and crankshaft will rotate by two revolutions for
each thermodynamic cycle.
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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
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