by Chris Woodford. Last updated: April 18, 2016.
Jet engines helped to inspire the rocket engines that
put men on the Moon, and powered boats and automobiles to world speed records, but they're much more
familiar as the engines on airplanes such as Concorde and the Jumbo Jet.
Unlike internal combustion engines in cars and trucks, which convert an
up-and-down movement of pistons into rotary movement in a
crankshaft, jet engines produce power by sucking in air at the front
and blasting out hot exhaust gases at the back. Let's take a closer
look at how they work!
Photo: A jet engine taken apart during testing.
You can clearly see the giant fan at the front. This spins around to suck air into the
engine as the plane flies through the sky. Picture by Ian Schoeneberg courtesy of
What is a jet engine?
A jet engine is a machine for turning fuel into thrust (forward
motion). The thrust is produced by action and
piece of physics also known as Newton's third law of motion. The force
of the exhaust gases pushing backward produces an equal and opposite
force (reaction) called thrust that powers the vehicle forward. Exactly
the same principle pushes a skateboard forward when you kick backward
with your foot. In a jet engine, it's the exhaust gas that provides the
"kick". Let's have a look inside the engine...
How a jet engine works
This simplified diagram shows you the process through which a jet engine converts the energy in fuel into kinetic energy that makes a plane soar through the air:
- For a jet going slower than the speed of sound, the engine is moving through the air at about 1000 km/h (600 mph).
We can think of the engine as being stationary and the cold air moving
toward it at this speed.
- A fan at the front sucks the cold air into the engine and forces it through the
inlet. This slows the air down by about 60 percent and it's speed is now about 400 km/h (240 mph).
- A second fan called a compressor squeezes the air
(increases its pressure) by about eight times, and this dramatically increases its temperature.
- Kerosene (liquid fuel) is squirted into the engine from a fuel
tank in the plane's wing.
- In the combustion chamber, just behind the compressor,
the kerosene mixes with the compressed air and burns fiercely, giving
off hot exhaust gases and producing a huge increase in temperature. The burning mixture reaches a temperature of
around 900°C (1650°F).
- The exhaust gases rush past a set of turbine blades,
spinning them like a windmill. Since the turbine gains energy, the gases must lose
the same amount of energy—and they do so by cooling down slightly and losing pressure.
- The turbine blades are connected to a long axle
(represented by the middle gray line) that runs the length of
the engine. The compressor and the fan are also connected to this axle.
So, as the turbine blades spin, they also turn the compressor and the
- The hot exhaust gases exit the engine through a tapering exhaust
nozzle. Just as water squeezed through a narrow pipe accelerates dramatically
into a fast jet (think of what happens in a water pistol), the tapering design of the exhaust nozzle helps to accelerate the gases to a speed of over 2100 km/h (1300 mph). So the hot air leaving the engine at the back is traveling over
twice the speed of the cold air entering it at the front—and
that's what powers the plane. Military jets often have an after
burner that squirts fuel into the exhaust jet to produce extra
thrust. The backward-moving exhaust gases power the jet forward. Because
the plane is much bigger and heavier than the exhaust gases it
produces, the exhaust gases have to zoom backward much faster than the
plane's own speed.
In brief, you can see that each main part of the engine does a different thing to the air
or fuel mixture passing through:
- Compressor: Dramatically increases the pressure of the air (and, to a lesser extent) its temperature.
- Combustion chamber: Dramatically increases the temperature of the air-fuel mixture by releasing heat energy from the fuel.
- Exhaust nozzle: Dramatically increases the velocity of the exhaust gases, so powering the plane.
Photo: Top artwork: This composite artwork uses part of the top photo on this page, taken
by Ian Schoeneberg and courtesy of US Navy, combined with a photo of a turbine exhibit at Think Tank, the science museum in Birmingham, England, which we took ourselves.
Photo: Bottom photo: Massive thrust! A Pratt and Whitney F119
jet aircraft engine creates 156,000 newtons (35,000 pounds) of thrust
during this US Air Force test in 2002. Picture by Albert Bosco courtesy of US
British engineer Sir Frank Whittle (1907–1996) invented the jet engine in 1930, and here's one of his
designs taken from a patent he filed in 1937. As you can see, it bears a resemblance to the modern design up above, although it works a little differently (most obviously, there is no fan at the inlet). Briefly, air shoots in through the inlet (1) and is pressurized and accelerated by a compressor (2). Some is fed to the engine (3), which drives a second compressor (4), before exiting through the rear nozzle (5). The rear compressor's exhaust drives the compressor at the front (6).
Artwork: Gas turbine engine designed by Frank Whittle in 1937 and formally patented two years later. Drawing taken from US Patent: 2,168,726: Propulsion of aircraft and gas turbines, courtesy of US Patent and Trademark Office, with colors and numbers added for clarity. The patent document explains how this engine works in a lot more detail.
Types of jet engines
Jet engines have evolved quite a bit since Whittle's era. Now there are several distinctly different types, each working in
a slightly different way.
Whittle's original design was called a turbojet and it's still widely used in
airplanes today. Turbojets are basic, general-purpose jet engines.
The engine we've explained and illustrated up above is an example.
Read more about turbojets from NASA (includes an animated engine you can play about with).
Photo: Early Turbojet engines on a Boeing B-52A Stratofortress plane, pictured in 1954.
The B-52A had eight Pratt and Whitney J-57 turbojets, each of which could produce about 10,000 pounds of thrust.
Picture courtesy of US Air Force.
Turboprop engines have a propeller at the front and are popular in smaller, more economical aircraft and helicopters. The propeller is
driven by a jet engine mounted directly behind it.
Read more about turboprops from NASA.
Photo: A turboprop engine uses a jet engine to power a propeller. Photo by Eduardo Zaragoza courtesy of US Navy.
Turbofan engines are much quieter than turbojets and are typically used in large airliners. A turbofan
engine has a large fan that sucks in air at the front.
Some of the air is blown into the compressor; the rest is blown around
the outside of the combustion chamber and straight out of the back.
This "bypass" arrangement cools the engine and makes it much quieter.
It also produces much more thrust at both takeoff and landing.
Read more about turbofans from NASA.
Photo: A turbofan engine produces more thrust using an inner fan and an outer bypass (the smaller ring you can see between the inner fan and the outer case). Each one of these engines produces 43,000 pounds of thrust (almost 4.5 times more than the Stratofortress engines up above)! Photo by Lance Cheung courtesy of US Air Force.
Ramjets and scramjets
Ramjets are simple and compact jet engines—little more than gas-burning pipes, typically used to power rockets and guided missiles. Scramjets are supersonic ramjets (ones in which air travels through the engine faster than the speed of sound).
Read more about ramjets
and scramjets from NASA.
Photo: A Pegasus ramjet/scramjet engine developed for space planes in 1999.
Photo by courtesy of NASA Armstrong Flight Research Center.
Find out more
On this website
On other websites
For older readers
For younger readers
- Air and Space Travel by Chris Woodford, Facts on File, 2004. This is my own 96-page introduction to the history of air and space travel; the invention of the jet engine was a crucial bridge between the two.
- Watch GE Test Its Jet Engines by Putting Them Through Hell by Alexander George. Wired, November 27, 2014. How do you test an engine against water, ice, bird strikes, and snapping blades?
- An Airline Fleet Fueled by Natural Gas by Matthew L. Wald. The New York Times, October 25, 2012. Can we make planes cleaner and greener by running their engines on natural gas?
- Electric Arcs to Quiet Jets by Saswato R Das. IEEE Spectrum. August 1, 2004. How engineers are trying to redesign the airflow through engines to make them quieter.
- Biggest Jet Engine by Paul Eisenstein. Three-page article in Popular Science, July 2004. How the drive for faster, more economical, and quieter jet engines is making them even bigger.
- 21st-century Hot Jet Engines by Stuart F. Brown. Popular Science, June 1990. How engineers are trying to perfect engines with double the thrust.
I find it fascinating to explore inventors' ideas in their own words (and diagrams)—which is something you can do very easily by browsing patents. Here are a few I've selected that cover various types of jet engines:
If you liked this article...
You might like my new book, Atoms Under the Floorboards: The Surprising Science Hidden in Your Home, published worldwide by Bloomsbury.