The Model Stirling Engine

This air engine is exhibited at the Hunterian Museum in Glasgow, Scotland. It was presented in 1827 to Glasgow University by its inventor the Reverend Dr Robert Stirling. The exhibit notes indicate that it is one of only two specimens with the other (a slightly different, earlier model) having been presented to Edinburgh University. Further, the exhibit notes read that ‘it is probable that at least parts of these models were made by Stirling’s brother James, a Dundee-based Iron Master or Founder’. Lord Kelvin claimed to have found this model in an ‘Augean stable’ in 1847. This term is generally used to refer to a messy, dirty and usually crowded or neglected place. The date is contemporary with Kelvin’s adaptation of a cellar situated in the basement of the Old College, High Street, Glasgow, into the world’s first student physical research laboratory. Kelvin subsequently repaired the model and used it in lecture demonstra-tions. The engine runs by using a recurrent cycle of the same trapped air, flowing repeatedly through a connected displacer cylinder (containing a ‘displacer’ plunger) and power piston, with momentum provided by the turn of a flywheel. Air in the large displacer cylinder is heated, rises up past the plunger and flows through a connecting pipe to the piston, causing it to rise and the flywheel to turn. The displacer (plunger) is moved downwards, forcing air into the top of its cylinder. This air cools, due to the presence of a water-filled cooling jacket, situated at the top of the cylinder. The continued momentum of the flywheel causes all the air to move to the top of the cylinder, where it quickly cools. The air is then forced downwards again into the heated part of the cylinder when the piston moves down, pulling the plunger upwards. The air, now in the heated bottom part of the cylinder, expands and increases pressure in the confined displacer cylinder space until it moves past the plunger, through the connector, and into the piston compartment causing the cycle to repeat.