My Science School

STEAM ENGINES

An engine is a machine that converts the energy stored in fuel into energy for operating other machines. In a steam engine, burning fuel heats water in a boiler, turning it to steam, which builds up in the boiler. The pressurized steam is used to operate the moving parts of the engine. In the first century AD, the Greek inventor Hero built a device that was turned by jets of steam, but it was a curiosity rather than a useful machine.

The first steam-powered machine was built in 1698 by English engineer Thomas Savery. It was designed to pump water from flooded mines, but was never actually used. In Savery’s engine, steam from the boiler filled a large cylinder. Then cold water was poured over the outside of the cylinder, which cooled it, making the steam condense (turn back to liquid water).This created a vacuum in the cylinder, which sucked in water from the mine through a pipe. More steam was fed to the cylinder to push the water up an outlet pipe.

Thomas Savery called his steam engine the “miner’s friend”. It pumped water from the pipe at the bottom into the pipe at the top via the two large cylinders.

In Thomas Newcomen’s atmospheric steam engine, movement of the piston was transferred to the pump by a rocking beam.

In 1712 another English engineer, Thomas Newcomen, also completed a steam engine for pumping mine water. In Newcomen’s engine, steam from the boiler went along pipes to a cylinder, where its pressure pushed a piston upwards. Then cold water was sprayed into the cylinder, which made the steam condense. This reduced the pressure in the cylinder, and the pressure of the air in the atmosphere outside pushed the piston back down. This is why Newcomen’s engine is often called an atmospheric engine. Although it used a huge amount of coal, it was very successful, especially at coal mines, where there was an endless supply of coal.

WATT’S IMPROVEMENTS

Steam engine design was greatly improved in the 1770s by Scottish engineer James Watt. He realized that Newcomen’s engine was very inefficient because the cylinder was heated and cooled on every cycle. So Watt built his own engine, which became popular for powering industrial machinery, such as spinning and weaving machines.

Steam engines are still used today in power stations in the form of the steam turbine, where high-pressure steam makes a fan-like turbine spin at high speed.

James Watt’s steam engine included many improvements over Thomas Newcomen’s. It had a separate cylinder where the steam was condensed, allowing the main cylinder to remain hot all the time. The piston was double-acting, which means it was moved both up and down by steam. This was achieved by feeding steam to one side of the piston then the other. An automatic governor controlled the flow of steam to the cylinder, and so regulated its speed. Sun-and-planet gears converted the up-and-down movement of the cylinder into a turning movement.