By using the second law of thermodynamics it is possible to show that no heat
engine can be more efficient than a reversible heat engine working between two fixed temperature limits.
This heat engine is known as Carnot cycle and consists of the following processes:
Q1 = T1 s
The rejected heat from the cycle per unit mass flow is:
Q2 = T2 s
By applying the first law of thermodynamics to the cycle, we obtain:
Q1 - Q2 - W = 0
And the thermal efficiency of the cycle will be:
= W/Q1 = 1 - T2/T1
Due to mechanical friction and other irreversiblities no cycle can achieve this efficiency. The gross work output of cycle, i.e. the work done by the system is:
Wg = W41 + W12
and work ratio is defined as the ratio of the net work, W, to the gross work output, Wg, i.e.
W / Wg
The Carnot cycle has a low work ratio. Although this cycle is the most efficient system for power generation theoretically, it can not be used in practice. There are several reasons such as low work ratio, economical aspects and practical difficulties.