We were
discussing the Otto cycle and the Diesel cycle in our previous posts and we
have seen there that these cycles are not totally reversible cycles but also
these cycles are internally reversible cycles. Hence, thermal efficiency of an
Otto cycle or Diesel cycle will be less than that of a Carnot cycle working
between similar temperature limits.

Let us consider that we have one heat engine which is operating between heat source having at temperature T

_{H}and heat sink having at temperature T

_{L}. As we have already seen in our previous discussion that for a heat engine to be a complete reversible, difference between the working fluid temperature and heat source temperature or difference between working fluid temperature and heat sink temperature must never go beyond the differential amount of dT during any heat transfer process.

We can also say that heat addition or heat rejection
process must be carried out isothermally and this criterion is fulfilled in
Carnot cycle.

Now we will see here one more important cycle i.e.
Stirling cycle, where heat addition process and heat rejection process will be
carried out isothermally. But we must note it here that Stirling cycle will be
different from Carnot cycle. Because there will be two constant volume
regeneration processes in Stirling cycle instead of isentropic processes of Carnot
cycle.

###
**So,
Let us see here Stirling cycle**

Stirling cycle is one ideal cycle for the operation
of Stirling engine. First we will see here the PV and TS diagram for Stirling
cycle, we will understand here the various processes involved and finally we
will determine the thermal efficiency of the Stirling cycle.

As we can see here from PV and TS diagram, there
will be two reversible isothermal processes and two reversible constant volume
processes.

**Process, 1-2:**Isothermal expansion from state 1 to state 2. Heat energy will be added here from external source. Volume will be increased but pressure will be reduced during this process.

ΔU = 0,

Q

_{1-2}= W_{1-2}= RT_{1}Log (V_{2}/V_{1})

**Process, 2-3:**

Constant volume process, internal heat transfer from the working fluid to regenerator and therefore this process is also termed as constant volume regeneration process.

ΔW = 0,

Q

_{2-3}= dU_{2-3}= C_{V}(T3-T2)

**Process, 3-4:**Isothermal compression from state 3 to state 4. Heat energy will be rejected here to the external sink. Volume will be reduced but pressure will be increased during this process.

ΔU = 0,

Q

_{3-4}= W_{3-4}= RT_{3}Log (V_{4}/V_{3})

**Process, 4-1:**Constant volume process, internal heat transfer from the regenerator back to the working fluid.

ΔW = 0,

Q

_{4-1}= dU_{4-1}= C_{V}(T_{1}-T_{4})
During the process of regeneration, Heat will be
transferred to the regenerator from the working fluid during one part of the
cycle and heat will be transferred back to the working fluid during the second
part of the cycle. Regenerator will be considered as reversible heat transfer
device.

If we think the concept of regeneration where, area
under 2-3 i.e. Q

_{2-3 }and area under 4-1 i.e. Q_{4-1}are equal, Regenerative Stirling cycle will become Carnot cycle because heat energy will be added from an external source at constant temperature and heat will be rejected too to an external sink at constant temperature and hence Regenerative Stirling cycle will become Carnot cycle and therefore it will have similar efficiency as of Carnot cycle.Therefore, efficiency of the Stirling cycle will be written as

Do you have any suggestions? Please write in comment
box.

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**Reference:**

Engineering thermodynamics, By P. K. Nag

Engineering thermodynamics, By S. K. Som

We will see another important topic i.e. Ericsson
cycle in the category of thermal engineering