Page 138 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

The work-producing device that best fits into the definition of a heat engine are:

❑ The steam power plant

❑ The close cycle gas turbine

By the first law, in a complete cycle,

Net heat supplied = Net work done

Referring to Fig. 9.1-1, from equation  dQ =  dW, we have,

Q1 – Q2 = W

By the second law, the gross heat supplied must be greater than the net work done,

i.e. Q1 > W

The thermal efficiency of a heat engine is defined as the ratio of the net work done in the
cycle to the gross heat supplied in the cycle. It is usually expressed as a percentage.

Referring to Fig. 4.1.2-1,
W
Thermal efficiency,  = (4.2)
Q 1

Substituting equation 9.1,

(Q − Q )
 = 1 2
Q 1

 Q 
1
  = −  2  (4.3)
 Q 1 

It can be seen that the second law implies that the thermal efficiency of a heat engine

must always be less than 100% (Q1 > W ).

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