The steam engine

In plain language

A steam engine uses the pressure of steam to push a piston back and forth, or to turn a wheel. Fuel is burned under a boiler, the boiler turns water into steam, and the steam is let into a cylinder where it moves a piston.

The first practical design, Thomas Newcomen's engine of about 1712, worked by condensing steam inside the cylinder with a spray of cold water. The air pressure above the piston then pushed it down. Its overall efficiency was low, and its first use was pumping water out of a coal mine. James Watt's separate condenser, patented in 1769, cooled the steam in a different vessel, so the cylinder stayed hot. The Science Museum Group says engines with the condenser burned two-thirds less coal. Later, Richard Trevithick's high-pressure engine of 1797 was in great demand in Cornwall and South Wales, where it raised ore and refuse from mines.

Technical explanation

Newcomen's atmospheric engine condensed steam inside the cylinder by injecting cold water. The resulting partial vacuum let atmospheric pressure push the piston down, and a rocking beam transferred that motion to a pump. A 2012 analysis of a historical Newcomen engine at Westfield, built from historical operating data, estimated an overall efficiency of about 1.3% and a thermal efficiency of about 2.7% (Carr and Cowart, 2012). The authors identify thermal efficiency as the main cause of the low overall figure. The cylinder was cooled by the spray, so each stroke required steam to reheat it, and the steam also had to fill the space left by the piston and drive out condensed water and air.

Watt's separate condenser, granted on 5 January 1769, moved condensation into a cooled vessel outside the cylinder, so the cylinder stayed hot. The Science Museum Group states that engines with the condenser burned two-thirds less coal (Science Museum Group, 2019). Trevithick later developed a successful high-pressure engine in 1797, which was in great demand in Cornwall and South Wales. The ASME profile says it was used for raising the ore and refuse from mines (ASME, 2012).

Steam was then applied to road vehicles. Cugnot's steam wagon, built for the army to haul cannon, was reported to move at a little over 2 miles per hour and needed wood refuelling about every 15 minutes (Linda Hall Library, 2018). Trevithick's Penydarren locomotive ran on the Merthyr tramroad on 21 February 1804 (Amgueddfa Cymru, 2008). Road use was then restricted by law. The Locomotives Act 1865 limited speed to 4 miles per hour on turnpike roads and public highways, and to 2 miles per hour through any city, town or village. It also required a person on foot, not less than 60 yards ahead, to carry a red flag (Locomotives Act 1865). The related profiles on the automobile and the internal combustion engine cover what followed.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Limitations and open problems

• Early engines wasted most of their fuel energy. A 2012 analysis of one historical Newcomen engine estimated an overall efficiency of about 1.3% (Carr and Cowart, 2012).
• Early steam road vehicles needed frequent refuelling. Cugnot's vehicle was reported to need wood refuelling about every 15 minutes (Linda Hall Library, 2018).
• Road use was restricted by law. The Locomotives Act 1865 limited speed to 4 mph on turnpike roads and public highways and 2 mph through towns, and required a person on foot, not less than 60 yards ahead, carrying a red flag (Locomotives Act 1865).

Possible future uses

Timeline

1. 1712Thomas Newcomen, with his assistant John Cawley or Calley, introduces an atmospheric engine, about 1712, according to the Science Museum Group record of a drawing made in 1897. Sources: [1]
2. 5 Jan 1769Watt's patent for a separate condenser is granted, titled 'A new method of lessening the consumption of steam and fuel in fire-engines'. Sources: [3]
3. 1770Cugnot's full-size steam wagon is unveiled, after a scaled-down working model in 1769. Sources: [4]
4. 24 Dec 1801Trevithick unveils his first high-pressure steam locomotive on Christmas Eve and takes seven friends on a short journey. Sources: [5]
5. 21 Feb 1804Trevithick's Penydarren locomotive makes its journey on the Merthyr tramroad, reportedly hauling ten tons of iron and 70 men, according to Trevithick's account. Sources: [6]
6. 5 Jul 1865The Locomotives Act 1865 is dated 5 July 1865. Section 1 says it comes into operation on 1 September 1865. Sources: [7]

Glossary

Atmospheric engine

An early steam engine in which steam in the cylinder is condensed to form a partial vacuum, so that air pressure pushes the piston down.

Separate condenser

A cooled vessel, outside the cylinder, where steam is condensed so that the cylinder stays hot.

High-pressure steam

Steam at pressures well above atmospheric pressure, which can expand in a cylinder to drive a smaller, lighter engine.

Reciprocating engine

An engine in which a piston moves back and forth inside a cylinder, which is then converted into a rotary or pumping motion.

Sources

1. other
   Drawing of Thomas Newcomen's Pumping Engine, 1712
   Science Museum Group, 1897. Accessed 22 Oct 2026.
2. conference paper
   Thermodynamic Modeling of a Newcomen Steam Engine (AC 2012-4611)
   Matthew A. Carr, Jim Cowart. American Society for Engineering Education, 2012. Accessed 22 Oct 2026.
3. other
   James Watt and the separate condenser
   Ben Russell. Science Museum Group blog, 5 Jan 2019. Accessed 22 Oct 2026.
4. other
   Nicolas-Joseph Cugnot (Scientist of the Day)
   The Linda Hall Library, 26 Feb 2018. Accessed 22 Oct 2026.
5. other
   Richard Trevithick
   Tom Ricci. ASME, 22 Jun 2012. Accessed 22 Oct 2026.
6. other
   Richard Trevithick's steam locomotive
   Amgueddfa Cymru (Museum Wales), 15 Dec 2008. Accessed 22 Oct 2026.
7. other
   An Act for further regulating the Use of Locomotives on Turnpike and other Roads for agricultural and other Purposes (Locomotives Act 1865, 28 & 29 Vict. c. 83)
   legislation.gov.uk (The National Archives), 5 Jul 1865. Accessed 22 Oct 2026.

Revision history

3 changes since this profile was created.