Stirling engine

• Robert Stirling, a Scottish minister, submitted a patent for a hot air engine

  1816
• With modern materials and new knowledge of flow and heat transfer phenomena, a highly efficient engine has been developed, but the fundamental principles of the Stirling cycle are unchanged
• Stirling cycle

  1. Gas is retained in a cylinder by a piston

  2. Cylinder head is heated, pressure of gas rises

  3. Piston is released, driven by expanding gas

  4. Cylinder head is cooled, pressure of gas decreases

  5. Piston is driven back to original position
• PV diagram

  Graphical representation of the Stirling cycle, with pressure on the y-axis and volume on the x-axis
• Heat is supplied to the cylinder

  Pressure rises
• Gas expands

  Pressure drops
• Gas is cooled

  Pressure becomes lower than 1 atmosphere, piston is driven back
• Work done in one cycle

  Proportional to the area enclosed by the curves on the PV diagram
• The power of the simple Stirling engine is small and its efficiency low
• Increasing pressure on both sides of the piston

  Increases the force induced by heating and cooling, and the energy delivered per cycle
• The pressure to the right of the piston does not remain constant throughout the cycle, but is kept reasonably constant by including a buffer space
• Displacer piston

  Can drive the gas from one end of the cylinder to the other without demanding much energy, as the pressure is almost equal on either side
• Stirling cycle with displacer piston

  1. Gas is displaced to the cylinder head where it is heated

  2. Gas is expanded and work is done on the power piston

  3. Gas is driven to the right and cooled

  4. Gas is compressed and work is done
• Regenerator

  A chamber filled with a porous metal that acts like a sponge, recovering most of the heat otherwise lost
• With the modern version, more than 99% of the heat otherwise lost is recovered
• Rhombic drive

  Mechanism that regulates the movement of the displacer piston by the power piston, resulting in the four phases of the Stirling cycle partly overlapping
• The rhombic drive is inherently balanced, allowing the engine to run at high speed and high power
• Modern Stirling engine

  • Improved heat transfer system with more heat exchange units in parallel and efficient cooling
  • Improved heater with extended tubes, increased number, and added fins
• Heating system

  1. Burner placed inside the heater

  2. Fuel and air associate to become hot combustion gases that circulate around the heater tubes

  3. Pre-heater uses the combustion gases to pre-heat the air, reducing chimney losses
• The heating system is external and very flexible, allowing adaptation to use nuclear or solar energy
• The modern Stirling engine has a high efficiency, nearly 40%, and can be completely balanced, providing extremely steady torque