BEGINNING OF THERMODYNAMICS ?

Thermodynamics is the the science of energy . in which we study about how energy could  be converted into  primary interest with high efficiency .

although The first commercially-successful engine that could supply continuous power to a machine, was the atmospheric engine, invented around 1712 , but there was not proper scientific explanation which can describe how system is running and at what condition's it could get more efficiency .

First scientific formulation of thermodynamic was made by a french military Engineer Sadi Carnot often described as Father of Thermodynamic . He published his work in a book  -
Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power . 

He was the first person talked about how steam engine's or system efficiency could be increased by increasing the temperature difference between source body and sink body . 

source body - Is the body from which system takes heat as input to convert it into primary interest  also known as higher thermal reservoir .

Sink - after converting input heat in to primary interest remaining heat will be ejected to a different body known as sink (generally environment) also known as lower thermal reservoir .  

More the difference between the temperature between source and sink more will be the efficiency of the system, this discovery also satisfy THE LAW OF CONSERVATION OF ENERGY .

EFFICIENCY -

This implies toward one important result that regardless of fuel used in system the efficiency of the system depends on the temperature of source and sink  .