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Cyclic Heat Engine And Second law of thermodynamics

For engineering purposes, the second law is best expressed in terms of the conditions which govern the production of  work by a thermodynamic system operating in a cycle. 
A heat engine cycle is a thermodynamic cycle in which there is net heat transfer to a system and a net work transfer from the system. The system which executes a heat engine cycle is called a heat engine.




A heat engine may be in the form of mass of  gas confined in a cylinder and piston machine or a  mass of water moving in a steady flow through a steam power plant. 

In the cyclic heat engine, heat Q1 is transferred to the system, work We is done by the system, work Wc is done upon the system, and then heat Q2  is rejected from the system. The system is brought back to the initial state through all these four successive processes which constitute a heat engine cycle. The heat Q1 is transferred from the furnace to the water in the boiler to form steam which then works on a turbine rotor to produce work Wt, then the steam is condensed to water in the condenser in which an amount of heat Q2 is rejected from the system, and finally work Wp is done on the system (water ) to pump it to the boiler. The system repeats the cycle. 

The net heat transfer in a cycle to either of the heat engines. 

Qnet = Q1 -Q2 

and the net work in the cycle 

Wnet = Wt - Wp  or ( Wnet = We - Wc ) 

By the First Law of Thermodynamics, we have 

Qnet = Wnet 

Q1 - Q2 = Wt - Wp 

The Efficiency of  a heat Engine or a heat Engine cycle is defined as 

n = Wnet / Q1 

n = (Wt - Wp) / Q1  

n = ( Q1 - Q2 ) / Q1 

n = 1 - (Q2 / Q1)

This is also known as the thermal efficiency of an heat engine cycle. A heat Engine is very often called upon to extract as much work (net) as possible from a certain heat input i.e, to maximize the cycle efficiency. 

  

 
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