Air standard cycles – Overview, Defination

Air standard cycles :-

                       A thermodynamic cycle consists of series of thermodynamic processes, which takes place in a certain order and the initial condition is restored at the end of process.

There are many types of thermodynamic Air standard cycles:

1. Carnot cycle
2. Otto Cycle
3. Diesel cycle
4. Dual Cycle
5. Sterling Cycle
6. Ericson cycle
7. Bray ton cycle

a. Carnot cycle- :-

                     Carnot cycle is an ideal cycle as adopted for an ideal heat engine. It consists of two isothermal process (expansion and compression) and two adiabatic process (expansion and compression).The cylinder and piston of the engine are considered as perfect non-conductor of heat but the cylinder cover head is a good conductor of heat. The hot body at a higher temperature is brought in contact with the bottom of the cylinder. The cylinder is fitted with a weightless and a frictionless piston.

From the above two p-v and T-S graph, the horizontal axis represents volume ‘V’ and entropy ‘S’ and the vertical axis represents pressure ‘P’ and temperature ‘T’. Let, engine cylinder contains m kg of air at its original condition represented by point 1 on the p-v and T-S diagrams. At this point, let P1,V1 and T1 be the pressure, temperature and volume.

Process 1- isothermal Expansion-

Let ,unit mass of perfect gas is admitted into the cylinder at the beginning of the outward moment of the piston and the pressure, temperature, volume of the gas at a point 1 is P1,V1 and T1 respectively. The bottom of the cylinder can be covered by an insulating cap. During the movement of the piston, the heat Q1 is supplied to the perfect gas and the gas expands isothermally keeping temperature T1 constant until the volume V2 and pressure P2.

The heat supplied by the hot body is fully absorbed by the air and is utilized by doing external work. So, heat is supplied during this process is equal to the work done during this process. This isothermal expansion is represented by the curve 1-2 on P-V and T-S diagram.

Process 2- Isentropic Expansion- As the piston moves outward,the gas expands adiabatically till the pressure P3,volume V3 and temperature T2 and the hot body is removed from the bottom of the cylinder ‘B’ and the insulating cap is brought in contact.

 In this process, there is no interchange of heat of the surrounding gasses (Q = 0) The reversible adiabatic expansion is represented by the curve 2-3 on Pv diagram.

b. Otto Cycle ( constant Volume Cycle)-

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 PV and TS diagram of Otto cycle is shown in figure 1.6. Otto cycle consist of two isentropic or adiabatic processes and two constant volume processes. We will determine the various properties for unit mass of working fluid.

The efficiency of Otto cycle depends on compression Ratio ( r ),As compression ratio increases the efficiency of Otto cycle increases

c. Diesel Cycle ( Constant Pressure Cycle)

Diesel cycle was invented by Rudolph Diesel in 1893. He put forward an idea by which we can attain higher thermal efficiency, with a high compression ratio. All diesel engine works on this cycle. Diesel is used as fuel in this cycle as it can be compressed at higher compression ratio. It is also known as constant pressure cycle because heat is added in it at constant pressure. It has high thermal efficiency and compression ratio (11:1 to 22:1) It consists of Two isentropic processes , One heat addition constant pressure process and one constant volume heat rejection process.

Process 1-2 – Isentropic Compression process-

In this process the piston moves from BDC to TDC and compression of air takes place isentropically. It means that during compression the entropy remains constant and there is no flow of heat out of the cylinder walls (non-conductors) happens. Here the air is compressed so the pressure increases from P1 to P2, volume decreases from V1 to V2, Temperature increases from T1 to T2 and entropy remains constant ( i.e. S1 = S2). In this process no heat is added or rejected.

Process 2-3 – Constant Pressure Heat addition Process-

During this process, the piston rest for a moment at TDC. The pressure remains constant (i.e. P2 = P3), volume increases from V2 to V3, temperature increases from T2 to T3, entropy increases from S2 to S3. Heat supplied to air = Q=mx Cp ×(T3 – T2)

Process 3-4 – Isentropic Expansion process-

In this process, after heat addition, the expansion of air takes place isentropically and work is obtained from the system. The piston moves downward during this process and reaches to BDC. The pressure falls from P3 to P4, Volume increases from V3 to V4, temperature falls from T3 to T4 and entropy remains constant (i.e. S3=S4). In this process no heat is added or rejected.

Process 4-1- Heat rejection at constant Volume-

 In this process,the pressure decreases from P4 to P1, temperature decreases from T4 to T1, entropy decreases from S4 to S1 and volume remains constant (i.e.V4 = V1)

d. Duel Cycle-

This cycle is combination Of Otto and Diesel cycle . It is also called semi-diesel cycle.

In this cycle heat is absorbed partly at a constant volume and partly at constant pressure . It consists of following processes

i. Process1-2 – Heat addition at constant Pressure

ii. Process 2-3- Isentropic expansion process

iii. Process 3-4 – Constant Volume heat rejection process

iv. Process 4-5 – Isentropic compression Process

v. Process 5-1 – Heat addition at constant Volume

e. Brayton cycle-

It consists of two constant pressure and two isentropic processes as shown in PV and T-S diagram. It is most commonly used in gas turbine

It consists of following processes

i. Process 1-2 – Isentropic compression process

ii. Process 2-3 – heat addition at Constant pressure

iii. Process 3-4 – Isentropic expansion process

iv. Process 4-1 – heat rejection at constant pressure

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