Adiabatic Compression

Adiabatic Process

A process in which there is no heat transfer or mass from one system to another is called the adiabatic process. The process can either be reversible or irreversible. It is a process of thermodynamics in which Q = 0. 

This system is insulated. In these processes, the only heat transfer is in the form of work. The adiabatic process supports and explains the theory of the first law of thermodynamics. 

The following conditions are needed for the adiabatic process to take place:

• The process should be carried out very quickly so that there is time for heat to get transferred.
• The system must be completely isolated from the rest of the environment. 

The Equation for the Adiabatic Process

The equation for the adiabatic process is:

PV^y = constant

Where P = the pressure of the system

V = the volume of the system

y = C_p/C_v = adiabatic index

The Adiabatic Processes

U = U₂ – U₁

W = -U = U₁ – U₂

W = -U

Since there is no heat transfer, the entropy (S) =0

Adiabatic Compression

Adiabatic compression is when no heat is added or subtracted from the air. The internal energy of the air increased is equal to the external work done on the air. The pressure of the air is more than the volume when the temperature, entropy and internal energy increases due to compression.

P > V

The compression stroke of a gasoline engine occurs rapidly without losing heat to the surroundings, and hence, it is an example of an adiabatic process.

Adiabatic Expansion

Adiabatic expansion obeys the first law of thermodynamics.  Here the work is carried out by expanding the volume due to which the temperature decreases.

Let’s take an example here. When the gas molecules are firmly bound together, the membrane is compressed. The volume expands to dV, and the temperature drops to dT.

Here, work done is W=PdV and the heat transfer dq=0.

For an adiabatic process  for an Ideal gas is PV^y = constant

Uses of the adiabatic processes

• In gas turbines, the adiabatic processes are applied in the Otto and Brayton cycles (where the piston works on the gasoline).
• Diesel engines (somewhat) use adiabatic compression to ignite the fuel. Adiabats are also useful for calculating the Carnot efficiency (a thermodynamic system’s maximum thermal efficiency) based on two adiabatic processes.

Applications of adiabatic assumption

From the first law of thermodynamics,

ΔU = Q – W

Where ΔU is that change of the internal energy of the system

Q is the quantity of energy

W is the work done by the system

1. If W = 0, the walls are not adiabatic, and the energy will be added to Q > 0. So the system’s temperature will rise.
2. There will be no phase change when the walls are adiabatic, Q = 0 and W < 0, and the temperature will also rise.
3. Suppose the walls of the systems are adiabatic U = 0 and not rigid, which means, W = 0, the system will be frictionless, no phase change will occur, and the temperature will rise. This process is known as the isentropic process. They are reversible processes if the process is isentropic, then dQ=0. 

i.e S(entropy) =dQ/dt =0

4. Now, suppose the walls of the systems are not adiabatic, the energy is transferring, and entropy is also transferred into the system. These processes are neither adiabatic or isentropic. They have Q> 0. It follows the second law of thermodynamics.

Conclusion

A process of thermodynamics where there is no heat transfer from one system to another is called an adiabatic process. This process must be carried out rapidly, so there is no heat transfer. There is adiabatic compression in which there is an increase in the temperature of the gas, and there is no additional subtraction of heat. Adiabatic expansion is the process in which the temperature decreases but the pressure remains constant. The adiabatic process has many applications in the process of thermodynamics.