2nd Law of Thermodynamics

2nd Law of Thermodynamics: Entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium - i.e. entropy increases & all processes tend towards chaos

Temperature gradient: Heat flows from a region of hot temperature to a region of cold temperature

All heat engines give up their energy to a cold reservoir

Qin:heat flow from the hot reservoir to the engine

Qout: heat flow from the engine to the cold reservoir.

Work done by heat engine = Qin – Qout


Efficiency = W/Qin = (Qin – Qout)/Qin

Limitations to Thermal Efficiency: - in an engine:

  • TH cannot be too high > components could melt
  • TC will be in the range of atmospheric temperatures
  • Analysis of the engine cycle can help to improve efficiency
  • Design of ports so that gas can get enter & exit with min. resistance
  • Lubrication reduces friction in bearings

Therefore an engine will never work at its theoretical efficiency