# Thermodynamics cycle & Classification

Thermodynamics cycle::-

A thermodynamics cycle consists of a series of thermodynamics operations ( process), which take place in a certain order, and the initial condition is restored at the end of the process. When the question for the process of the cycle is plotted on the p-v diagram, day from a closed figure each operation being represented by its won curve. Since the area under each curve gives the work done to some scale, during one cycle will be given the enclosed area of the diagram as shown below…

Notes-
1) a cycle, which requires 4 Piston stock and to complete regulation of the crank is known as four stock cycle. But is the cycle, which requires only two Piston strokes and one revolution of the crank is known as two Stroke cycle.

2) when air is assumed to be the working substance inside the engine cylinder, the cycle is called an air cycle.

Classification of thermodynamics cycles::-
The thermodynamic cycles are classified into the following groups.

Reversible cycle::- a process in which some change in the reverse direction, reverse the process completely is known as a reversible process. In a reversible process, there should not be a loss of heat due to friction, radiation, or conduction. A cycle will be reversible all the processes constituting the cycle are reversible. thus in a reversible cycle, the initial conditions are restored at the end of the cycle.

A little consideration will show the operation is performed in the revised order the cycle draws heat of the cold body and rejects it to the hot body. This operation requires an external power to drive the mechanism according to the second law of thermodynamics. Which operates on a river cycle is regarded as a ” heat pump” such as a refrigerator, because I eat palms heat from the cold body to the hot body. Following are the conditions for the possibility of a cycle.

1) the pressure and temperature of the working substance must not differ appreciatively from those of the surrounding at any stage in the process.
2) all the processes taking place in the cycle of operation must be extremely slow.
3) the working parts of the engine must be fiction-free.
4) there should be no loss of energy during the cycle of operation.

Note- a reversible cycle should not be confused with a mathematically reversible engine. Steam engine crank may be made to revolve a reverse direction by mathematically Alter in the valve settings. But this does not reverse the cycle on which it works. A two Stroke Petrol engine may be made to revolve in the reverse direction by altering the timing of the ignition. But this also does not reverse the actual cycle.

Irreversible cycle::-   a process in which change in the reverse direction does not reverse the process is called Irreversible process. In an Irreversible process, there is a loss of heat due to friction, radiation or conduction.

In actual practice, most of the processes are Irreversible to some degree. The main causes for the irreversibility are…
i) mechanical and fluid friction
ii)  unrestricted expansion
iii) heat transfer a finite temperature difference. Moreover, friction converts the mechanical work into heat. This heat cannot supply back the sum amount of mechanical work which was consumed for its production. Thus that there is some friction involved in the process it becomes Irreversible. A cycle will be well if any of the processes constituting the cycle is Irreversible. Thus in an Irreversible cycle, the initial conditions are not restored at the end of the cycle.

Notes-
A) we have discussed the various thermodynamics process. The process such as constant volume, constant pressure, isothermal or constant temperature ( i.e pv = C) , adiabatic( i.e, pv = C) and polytropic( i.e, pvn = C) are all reversible process.
B) the throttling is an Irreversible process.