Carnot Cycle
In a heat engine, the net work, thus the cycle efficiency, can be maximised by using processes that are fully reversible. This makes reversible cycles the most efficient, however they cannot really ever be achieved in practice due to the irreversibilities associated with each process being unable to be eliminated. That being said, they can serve purpose as acting as models to which actual heat engines and refrigerators.
One of the best known reversible cycles is the Carnot Cycle, and the heat engine that runs using this cycle is the Carnot Heat Engine. It is made up of four reversible processes - an isothermal expansion and compression, and an adiabatic expansion and compression - and can be performed in either a closed or steady-flow system.
The conditions assumed in the Cornot Cycle are that an adiabatic piston-cylinder device is containing a gas; the insulation of the cylinder head can be removed to let the cylinder come into contact with reservoirs of the gas and thus provide a heat transfer. The four previously mentioned reversible processes that take place are described in more detail below:
Reversible Isothermal Expansion
Initially at state 1, the temperature of the gas is TH, and the cylinder head is in close contact with a source at the same temperature. The gas does work on its surroundings as it is allowed to expand slowly. Unsurprisingly, the expansion causes the gas to lose heat, however at the same time heat is transferred back into the gas from the reservoir, and thus keeping the temperature of gas at TH. This is a reversible process because of the fact that the difference in temperatures between the gas and the reservoir never exceeds a differential amount, dT , and it continues until the piston reaches position 2. The amount of total heat transfered to the gas during this process is QH.
Reversible Adiabatic Expansion
At state 2, the system becomes adiabatic as the reservoir that was in contact with the cylinder head is removed and replaced with insulation. The gas continues to expand however, and since it is no longer recieving heat from the reservoir it does work on the surroundings until the temperature drops from TH to TL. This process is reversible because the piston is to be assumed frictionless and the process to be quasiequilibrium.
Reversible Isothermal Compression
At this point the insultion is removed and the cylinder head brought into contact with a sink at temperate TL. An external force pushes the piston back towards the head and so compresses the gas, causing its temperature to rise. However similar to the first process, heat is almost immediately transferred into the sink and so the gas' temperature remains constant at TL, and is reversible for the same reasons as the first process. The amount of total heat rejected by the gas during this process is QL.
Reversible Adiabatic Compression
Here, the reservoir is removed and the insulation put back into place once again. The gas is compressed, causing its temperature to rise from TL to TH, thus completing the cycle.
The cycle can be represented by the P-V diagram below, remembering that the area under the curve represents the boundary work for quasi-equilibrium processes.
