Carnot Cycle
REVERSIBLE & IREVERSIBE PROCESSES
Irreversible Process: A process that can not be reversed.
Irreversibilities (Factors which cause irreversible processes):
- Friction
-Unrestricted Expansion
-Mixing of fluids
-Electric Resistance
-Heat transfer across a finite temperature difference
Reversible Process: A process that can be returned to the original state without leaving a trace on the surroundings.
Reversible processes are helpful as they are easy to analyse and provide ideal models next to which real processes can be judged.
Reversible processes can be:
-Internally Reversible: When no irreversibilities happen within system boundaries during the process.
-Externally Reversible: When no irreversibilities happen outside system boundaries during the process.
-Totally Reversible: No irreversibilities occur outside or within the system.
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
THE CARNOT CYCLE
The Carnot Cycle is a well known reversible cycle used for heat engines. It is made up of four reversible processes and can operate either in a steady-flow or closed system.
The following outlines the 4 steps the Carnot cycle using a gas within a frictionless piston.
Step
A
B
C
D
Process
Reversible Isothermal Expansion
1 to 2
Reversible Adiabatic Expansion
2 to 3
Reversible Isothermal Compression
3 to 4
Reversible Adiabatic Compression
4 to 1
Description
The gas in the cylinder expands, doing work on the surroundings. Heat is transferred from a heat source to the gas to keep it at a constant temperature (TH).
QH is the heat transferred from the heat source to the gas.
The cylinder becomes completely insulated, making it an adiabatic system. The gas cools (from TH to TL) and expands, doing more work on the surroundings.
Now in contact with a heat sink, work is done on the cylinder. This compresses the gas but the heat produced is transferred to the heat sink so the temperature remains constant (TL). QL is the heat transferred from the gas to the heat sink.
The cylinder is once again made adiabatic with the use of insulation. Work is done on the cylinder and the temperature increases (from TL to TH).
Diagram
P-V Diagram of the Carnot Cycle
Carnot Principles
Violating either of the following principles violates the second law of thermodynamics:
1. All reversible heat engines using the same reservoirs have the same efficiencies.
2. An irreversible heat engine is less efficient than a reversible heat engine using the same reservoirs.