How do thermodynamic processes apply to heat engines?

What is a Heat Engine?

A heat engine is a device that converts heat energy into mechanical work by operating between two thermal reservoirs — a hot source and a cold sink.

Role of Thermodynamic Processes

1. Heat Absorption (Isothermal or Constant Temperature Process)
   In many engines, heat is absorbed from the hot reservoir during an isothermal expansion, where the system expands and does work on the surroundings while maintaining a constant temperature.
2. Work Output (Expansion Process, Usually Adiabatic or Isentropic)
   The working fluid expands and does work on the piston or other mechanical parts. If the expansion is adiabatic (no heat exchange), the temperature and pressure drop as work is done.
3. Heat Rejection (Isothermal or Constant Temperature Compression)
   The working fluid releases heat to the cold reservoir, often during isothermal compression, decreasing its volume while staying at a constant temperature.
4. Work Input (Compression Process, Often Adiabatic)
   The working fluid is compressed adiabatically, increasing its temperature and pressure without exchanging heat, preparing it for the next cycle.

Common Thermodynamic Cycles in Heat Engines

• Carnot Cycle:
  Composed of two isothermal and two adiabatic processes, it represents the most efficient cycle possible between two temperatures.
• Otto Cycle:
  Characterized by adiabatic compression and expansion, with constant volume heat addition and rejection—typical of gasoline engines.
• Diesel Cycle:
  Similar to Otto but with constant pressure heat addition.
• Rankine Cycle:
  Used in steam engines; involves phase changes and includes processes like isentropic expansion and condensation.