Robert Stirling's hot-air engine

In the C18th and early C18th men working with steam engines were always at risk from exploding boilers. In the first steam-driven locomotives only the driver, stoker, and anyone having the misfortune to be passing by at the time were killed.   

But when high-pressure steam boilers started being used in ships - e.g. in American river steamers driving side-mounted paddlewheels – an exploding boiler meant a death toll of hundreds, and tons of valuable cargo lost.

While work continued on trying to improve the quality of metal and strength of the seals used in constructing high-pressure steam boilers, one man had been exploring a radical alternative.  

Born in Scotland in October 1790, Robert Stirling was one of eight children. His grandfather, Michael Stirling had invented the first rotary threshing machine in 1756, and it was probably from him that Robert inherited his interest in engineering. After a classical education at Edinburgh University, Robert was ordained as a minister of the Church of Scotland in 1816.  This same year he patented his first ‘air’ engine. 

There had been earlier experiments with ‘hot-air’ engines both in France and England so the idea wasn’t new.   What made Stirling’s engine different was his invention of a regenerator, which stored the heat from one cycle for use in the next, making the engine far more efficient. 

His was a closed-cycle engine which meant the same air – first heated then cooled – was used over and over again. No new air was drawn in and none was expelled as exhaust.    

How it worked:  air inside the cylinder was heated by an outside source, a small wood or coal fire.  (In the model above the silver-coloured chamber is the firebox, and in front of the water tower - part of the cooling system - you can see the handle of the shovel on which the fire would be laid.)  This heated air expanded and pushed up the piston. The air then passed through the regenerator to the cold side of the engine where it cooled and contracted, pulling the piston down.  This continual heating and cooling of the air produced the pressure change that pushed and pulled the piston, making the engine run. 

It was beautifully simple, efficient and clean.  But most important of all, because there was no boiler it was totally safe.  In 1818 one of these engines was used as a quarry pump and ran for two years.  

Heavy investment in high-pressure steam meant that the air engine – an invention ahead of its time - was never fully exploited. 

But it gave me a great idea for a book.   

Jane Jackson.