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.
Jane Jackson.
