Without doubt, the greatest single catalyst for the rise of our modern industrial world was the development of the steam engine. From the earliest pumping engines to the latest steam turbines, the harnessing of steam power has fueled modernity. Yet the basic principles of steam power have not changed since the earliest steam pumps began to crank almost three hundred years ago.
Simply stated, a steam engine is an engine which converts heat energy into usable mechanical energy, usually by means of heating water in a boiler to produce steam that is expanded in a cylinder to drive a reciprocating (to and fro) piston that in turn is changed into a rotary motion which drives the shafts or wheels of machinery.
The most familiar form of steam engine to most of us will be the piston steam engine that powered railroad locomotives. Water is heated in a boiler to produce high pressure steam that, by means of valves, is introduced into a cylinder to push a piston. With a double-acting steam valve the high pressure steam acts alternately on either side of the piston driving it back and forth. The steam valves are opened and closed by a mechanical valve rod. When on one side the piston is being pushed by steam pressure, on the other side the piston is expelling the exhaust steam. The piston moves a piston rod that is attached to a horizontal linkage called a cross-head; This controls the movement of both the valve rod (allowing steam into alternate ends of the cylinder, remember) and the drive rods that move the wheels. To ensure they move in unison, the wheels are joined together by horizontal coupling rods. This is a familiar picture for most of us. But the earliest practical steam engines were a lot more simple.
Thomas Newcomen's beam engine of 1712 was the first practical application of steam power. However it produced its power not by steam expansion but by condensation. Steam was introduced into a vertical cylinder which forced up the piston thereby tilting the beam. But in order to move the piston back, water was introduced into the cylinder to condense the steam and create a partial vacuum that 'sucked' the piston downwards to tilt the beam in the opposite direction. The constant heating and cooling of the cylinder made this engine extremely inefficient; Yet it was a start.
The first significant improvements to Newcomen's design came later in the century through the work of the Scottish engineer, James Watt. Watt developed a separate condenser to allow the steam to condense outside the cylinder. This meant that the cylinder remained constantly hot, significantly improving fuel efficiency. Watt also introduced the 'double-acting' engine (mentioned above) that produced power on each stroke of the piston, a throttle valve that harmonized engine speed and the beginnings of the transfer of linear to circular motion that led to the 19th century explosion in steam applications.
Nowadays the old piston steam engines have been replaced by steam turbines, that are markedly more efficient and have a much higher power-to-weight ratio. Steam turbines are particularly suited to the generation of electricity, and these days most of the world's electricity generators are driven by steam turbines. However, despite the ultra modern application of technologies, the almost primeval love of steam power is still strong; Perhaps because we instinctively understand that the world we live in now was built by this simple yet revolutionary application of water and iron.