Department of Physics

Stirling Engine

    Stirling engines are more efficient forms of generating power from what energy than standard combustion engines. They are not widely used due to their low power to weight ratio, high cost, and long startup times when compared to combustion engines. Their specifications mean that they are seldom used in vehicles (outside of submarines where they are somewhat common). They have common applications in Nuclear power and Solar energy generation, heat pumps, cryocoolers, and desalination plants. 

    Stirling engines come in many different forms and configurations. The most well known of which is the low temperature difference engine. They were first patented in 1816 by Robert Stirling who was attempting to create a hot air engine based on improving similar designs from french physicist Guillaume Amontons (who built the first functioning air engine in 1699). Stirling was the first to seal the air in a closed circuit allowing its use in pumping water at a rock quarry.

     Our stirling engine consists of two pistons; one in the glass bulb, the other on the top of the pressure chamber. The latter piston is connected to a lever arm which is in turn connected to a flywheel. The exterior of the flywheel is an elastic band connecting it to a small electromagnet. When spun, this electromagnet generates electricity which turns on a red LED.

PHYSICS EXPLANATION:

    As stated in the description, stirling engines are more efficient forms of generating power from heat energy than standard combustion engines. The greater the temperature difference between the interior and exterior, the more efficient the generation.  The two pistons of the engine are connected via a pressure chamber that allows air to flow between them. The ‘hot-side’ of the piston is exposed to the flame and heats up while the ‘cold-side’ is cooled by the ambient air temperature. Thermal expansion causes the heated air to push the hot-side inward, forcing the cool-side up. The air moves into the cool-sides chamber, where it cools and condenses. This condensation causes the cool-side’s piston to contract, the momentum of which draws the cooled air into the hot-side and forcibly compresses it further. The flame then again heats and expands the air, forcing the piston back inward, repeating the cycle.

DEMONSTRATION:

  1. Carefully fill glass fuel container with ethanol (or fuel of choice) and place container in raised circle directly below glass bulb
    • Ensure that wick is submerged in ethanol with only a small amount exposed.
    • For best results ensure that there is a gap between the wick and the glass tube
    • Clean any spilled fuel with a dry rag or paper towel (if there is any) before disposing of whatever instrument was used.
  2. When container is in place, carefully light the wick using a matchstick or lighter 
    • Fuel and matches can be attained from the “engine fuel” tub located in D1Y2
  3. After 30 seconds, swiftly push down the side of the lever-arm furthest from the flame. 
    • If the flywheel does not begin spinning on its own, reset the pistons then wait another 15-30 seconds and try again.
  4. To stop the engine, blow out the flame and wait for the motion to die out on its own.

  • Be patient, takes a minute or two to warm up.
  • Demo contains many glass parts and is surprisingly heavy. Be careful to avoid dropping it when transporting
  • You will be lighting matches, so exercise fire safety precautions when interacting with the demo. Do not touch any part of the demo that is above or close to the flame, as it will be very hot.
  • You will need to have a lighter or use on of the matches
  • The fuel container will need to be filled with ethanol. The top is not sealed. When filled, keep the container flat to avoid leaks. If you get fuel on your skin or clothes, wash it off as soon as possible (certainly before lighting anything on fire).
  • Alternative fuels can be used, but please only use things that don’t leave soot residue or otherwise stain the glass bulb.
  • Wait for the flywheel to spin down on its own. Do not attempt to stop the flywheel or the pistons manually, as you may injure your fingers doing so.

  • Fuel chamber (should be on the stirling engine already)
  • Ethanol (D1Y2, or near the sink)
  • Matches (D1Y2)
  • Heat resistant glove (D1Y2). 

  • Steam Engines (ID#, LOCATION)
  • Combustion engine piston cutout (ID#, LOCATION)
  • Low temperature difference stirling engine aka ‘LTDE’ (ID#, LOCATION) 

DEMO ID: D1Y01
LOCATION: D1y