The field of science, which deals with the energies possessed by gasses and vapours, is known as Thermodynamics. It also includes the conversion of theses energies in terms of heat and mechanical work and their relationship with properties of the system. A machine, which converts hat into mechanical work or vice, verse is knows as Heat Engine. The field of engineering science which deals with the applications of thermodynamics and it laws to work producing and work absorbing devices, in order to understand their performinc, is knows as thermal engineering
the heat is usually generated by the combustion of fuel which may be solid liquid or gas. it is supplied to the working substance ( a source of conveying heat to heat engine for doing work in engine cylinder) at a higher temperature. A parts of heat energy is converted into mechanical work by expanding the soaking substance, within the engine cylinder. The remaining heat energy is rejected at a lower temperature.
The working substance widely used in the heat engines.are fluids in the gaseous or liquid state. A mixture of air and fuel is used as a working substance in internal combustion engines, water vapour (stem) in the steam engine or steam turbines and refrigerants in refrigeration plants.
All the working substances are chosen for specific applications because of their some properties particularly suiting to the given process. In view of this fact the study of behaviour and properties of thermodynamic working substance become imperative.