Making a Stirling Engine
Thinking my way into this research project on Romantic Engineering I decided to make a Stirling Engine from scrap metal. I have an old Boxford Lathe that dates from the 1960s on which I’ve started to turn the parts. My progress is entirely dependent on using scrap I can find rather than buying in components. This is very much a Romantic period engine. Robert Stirling, a Church of Scotland Minister from Perthshire, first patented this hot air engine in 1816. Below is a sketch from that patent.
This is a remarkable design that relies on air moving in a closed circuit. There is no steam involved, no inlet and exhaust or condenser. It does not need water or coal. Just heat, or cold. All this engine needs is a temperature differential to work. In a normal steam engine water is heated in a boiler to produce steam. This is then directed, with valves into a cylinder where it drives a piston. A crank or Watt’s sun and planet gearing then converts linear motion into rotation.
You can see how a steam engine works here:
Here you can see in red how live steam enters the cylinder, driving down the piston. As it does so dead or exhaust steam exits at the other side of the cylinder. This then goes to a condenser for cooling. The valve gear then directs live steam to the side of the piston that the exhaust just exited from and the cycle is repeated.
You can see how the linear motion of the piston is converted to rotary motion through a crank attached to the piston rod. This drives the flywheel, which helps to carry the energy produced by the engine, smoothing out the running of the machine. The spinning balls that you can see are a centrifugal governor. Watt adapted this technology to fit his steam engines so that the speed of the engine would be self-regulating. If the engine speeds up too much the balls move outwards, working a valve that throttles in the amount of steam allowed into the engine. If it is slower than has been regulated the balls move together, opening the inlet valve so more steam is allowed in. You can see in the Gif above how steam is exhausted out of the cylinder. This is a waste of water and energy.
The Stirling Engine
The Stirling engine on the other hand is a closed cycle heat engine. The gas that drives the power piston is continuously recycled. This is a remarkable design, that did not quite catch on in the period, although it is now the subject of much investigation as it can use any heat source. The engine has a high thermal efficiency, produces low emissions, and operates with less noise or vibration than other prime movers. The cycle of the kind of Stirling engine that I will make can be seen below:
The Stirling Engine Cycle
The cylinder on the left is heated. This then moves the displacer piston, which has a slight clearance fit. The gas then pushes towards the power piston on the right, which has a close gas-tight fit in the cylinder. The cooling gas from the right hand cylinder then fills the cylinder of the displacer piston again restarting the cycle. The position of both cranks on the flywheel, which carries the energy of the power piston through to the next cycle, is crucial.
A Deceptively Simple Engine
This engine has great future potential as it can use any heat source. As Malcolm Longair writes, ‘they are of the greatest interest as an ecologically friendly source of power’ (Theoretical Concepts in Physics, CUP, 2020. p. 261). Furthermore, the running of a Stirling Engine can be carbon neutral if the heat of the sun is used to power it.
I will update the progress of my small engine once I can find and modify enough scrap parts for the rest of the it. So far I have made the power piston and cylinder. Hopefully I can provide images of the components, and it all running, in a future instalment.