How does the engine work?

        The components that provide the actual power are the cylinder, fitted with a normal piston, and the air chamber which provides the pressure changes required to drive the piston.  In this engine the air chamber consists of the 'hot cap' and the cooler.  (Unfortunately the name 'hot cap' is rather misleading in this case as only about 3/4" at the closed end is heated and the rest of the tube is simply the air chamber wall.  It is used in this article because, being all heated in some designs of engine, it is the name under which it is normally sold.)
  Working inside the chamber is the displacer, which can be moved from end to end, with about 1/32" radial clearance to allow the air to flow past it.
  When the displacer is at the cold end of the chamber, most of the air is at the hot end and, being heated, expands to increase pressure above atmospheric and push the piston out to turn the crank.  This turns the return crank which moves the displacer across to the hot end of the chamber, displacing the hot air.
  As the air flows through the narrow annular passage it gives up most of its heat to the walls of the chamber and the displacer to emerge at the cold end already partially cooled.  The cooler extracts the remaining heat and the air contracts to below atmospheric pressure to 'pull the piston back in'.
  However, this again turns the return crank to move the displacer back to the cold end, and the air, flowing back through the gap, is re-heated, by the heat it left on the walls, to arrive at the hot end already partially heated.  The heater supplies the rest of the heat needed to raise it back to its original temperature/pressure and the cycle begins again.

  This alternating interchange of heat between surfaces and air is known as 'regeneration' although Dr Stirling, the first person to exploit it in both engines and furnaces, called it 'recuperation'.

  Note that Fizgig is a non-snifting engine and therefore double acting.  Snifting engines are fitted with a non-return valve which prevents the internal pressure dropping below atmospheric and rely on their flywheels to return the piston to the top of the cylinder.

  " How large a water tank is required?"

    Any metal container or can, of over a pint in capacity, will serve, although a closed one witha vented filler cap is preferable to an open one as it won't splash if things vibrate.  Either circular or rectangular, it should be at least 3" higher then the top of the engine to promote thermosyphoning.
  Solder 1" of 5/16" copper pipe into the bottom of the can, as low as possible and about 2" of pipe into the top, about 1/2" down from the top edge.  (Water level must be kept above this)   This top pipe should enter the tank horizontally and may be bent down approx 80 degs to aim at Fizgig's top pipe.
  Connect the engine to the tank using soft plastic tubing. If this can be obtained a push fit on the stubs, no clips are needed.  If you are unlucky, and can only get larger tubing you will need to fit clips - a twist of iron wire may suffice, there is no pressure in the system.
  Two or three feet should be soldered to the bottom of the can so that it can be screwed down.   See Picture