by Chris Woodford. Last updated: August 8, 2016.
Turn your eyes to the sky and it's likely you'll see more than a
few vapor trails—the wispy white lines that jet planes scribble on
the great blue canvas stretched above our heads. At the dawn of the
20th century, the very idea of powered flight seemed, to many, like a
ludicrous dream. How things have changed! At any given moment,
there are something like 7,000 flights zipping through the sky over the United States alone;
we're so used to the idea of flight that we barely even
notice all the planes screaming above us, hauling hundreds of
people at a time to their homes and holidays.
Most modern planes are powered by jet engines
(more correctly, as we'll see in a moment, gas turbines).
What exactly are these magic machines and what makes them different
from the engines used in cars or trucks?
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 that converts energy-rich, liquid fuel
into a powerful pushing force called thrust.
The thrust from one or more engines pushes a plane forward, forcing air past its scientifically shaped
wings to create an upward force called lift that powers it into the sky. That, in short, is
how planes work—but how do jet engines work?
Jet engines and car engines
One way to understand modern jet engines is to compare them with
the piston engines used in early airplanes, which are very similar
to the ones still used in cars.
A piston engine
(also called a reciprocating engine, because the pistons move back and forth or "reciprocate")
makes its power in strong steel "cooking
pots" called cylinders. Fuel is squirted into the cylinders with
air from the atmosphere. The piston in each cylinder compresses the mixture, raising its
temperature so it either ignites spontaneously (in a diesel engine)
or with help from a sparking plug (in a gas engine). The burning fuel
and air explodes and expands, pushing the piston back out and driving
the crankshaft that powers the car's wheels (or the plane's propeller), before the whole
four-step cycle (intake, compression, combustion, exhaust) repeats
itself. The trouble with this is that the piston is driven only
during one of the four steps—so it's making power only a fraction of
the time. The amount of power a piston engine makes is directly
related to how big the cylinder is and how far the piston moves;
unless you use hefty cylinders and pistons (or many of them), you're limited
to producing relatively modest amounts of power. If your piston engine
is powering a plane, that limits how fast it can fly, how much lift it can make,
how big it can be, and how much it can carry.
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. That sounds like a lot of power, but it's less than half the thrust
produced by one of the vast jet engines (turbofans) on an airliner, as you can see from the bar chart further down this article. Picture by Albert Bosco courtesy of US Air Force.
A jet engine uses the same scientific principle as a car engine:
it burns fuel with air (in a chemical reaction called combustion) to
release energy that powers a plane, vehicle, or other machine. But
instead of using cylinders that go through four steps in turn, it
uses a long metal tube that carries out the same four steps
in a straight-line sequence—a kind of thrust-making production line!
In the simplest type of jet engine, called a turbojet, air is drawn in
at the front through an inlet (or intake), compressed by a fan, mixed
with fuel and combusted, and then fired out as a hot, fast moving
exhaust at the back.
Three things make a jet engine more powerful
than a car's piston engine:
- A basic principle of physics called the law of conversation of energy tells us that if a jet engine needs to make more power each second, it has to burn more fuel
each second. A jet engine is meticulously designed to hoover up
huge amounts of air and burn it with vast amounts of fuel (roughly in
the ratio 50 parts air to one part fuel), so the main reason why it makes more power is because
it can burn more fuel.
- Because intake, compression, combustion,
and exhaust all happen simultaneously, a jet engine produces maximum
power all the time (unlike a single cylinder in a piston engine).
- Unlike a piston engine (which uses a single stroke of the
piston to extract energy), a typical jet engine passes its exhaust
through multiple turbine "stages" to extract as much energy as
possible. That makes it much more efficient (it gets more power from
the same mass of fuel).
A more technical name for a jet engine is a gas
turbine, and although it's not immediately obvious what that
means, it's actually a much better description of how an engine like
this really works. A jet engine works by burning fuel in air to release hot
exhaust gas. But where a car engine uses the explosions of exhaust to
push its pistons, a jet engine forces the gas past the blades of a windmill-like
spinning wheel (a turbine), making it rotate. So, in a jet engine,
exhaust gas powers a turbine—hence the name gas turbine.
Action and reaction
When we talk about jet engines, we to tend think of rocket-like tubes
that fire exhaust gas backward. Another basic bit of physics,
Newton's third law of motion, tells us that as a jet engine's exhaust
gas shoots back, the plane itself must move forward.
It's exactly like a skateboarder kicking back on the pavement to go forward;
in a jet engine, it's the exhaust gas that provides the "kick".
In everyday words, the action (the force of the exhaust gas shooting backward) is equal and
opposite to the reaction (the force of the plane moving forward); the action moves the exhaust gas,
while the reaction moves the plane.
But not all jet engines work this way: some produce hardly any rocket
exhaust at all. Instead, most of their power is harnessed by the
turbine—and the shaft attached to the turbine is used to power a
propeller (in a propeller airplane), a rotor blade (in a
helicopter), a giant fan (in a large passenger jet), or an
(in a gas-turbine power plant). We'll look at these different
types of gas turbine "jet" engines in a bit more detail in a moment.
First, let's look at how a simple jet engine makes its power.
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. (It uses a small part of the top photo on this page, taken
by Ian Schoeneberg courtesy of US
- 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 its 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.
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
All jet engines and gas turbines work in broadly the same way
(pulling air through an inlet, compressing it, combusting it with fuel, and
allowing the exhaust to expand through a turbine), so they all share
five key components: an inlet, a compressor, a combustion chamber,
and a turbine (arranged in exactly that sequence) with a driveshaft
running through them.
But there the similarities end. Different types of engines have
extra components (driven by the turbine), the inlets work in
different ways, there may be more than one combustion chamber, there
might be two or more compressors and multiple turbines. And the
application (the job the engine has to do) is also very important.
Aerospace engines are designed through meticulously engineered
compromise: the need to produce maximum power from minimum fuel (with
maximum efficiency, in other words) while being as small, light, and
quiet as possible. Gas turbines used on the ground (for example, in
power plants) don't necessarily need to compromise in quite the same
way; they don't need to be either small or light, though they
certainly still need maximum power and efficiency.
Artwork: A summary of six main types of jet engine. Each one is explained further in the text below, followed by a link to an excellent NASA website where you'll find even more graphics and animations.
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.
Whittle's original design was called a turbojet and it's still widely used in
airplanes today. A turbojet is the simplest kind of jet engine based on a gas turbine: it's a basic "rocket"
jet that moves a plane forward by firing a hot jet of exhaust
backward. The exhaust leaving the engine is much faster than the
cold air entering it—and that's how a turbojet makes its thrust. In
a turbojet, all the turbine has to do is power the compressor, so it
takes relatively little energy away from the exhaust jet.
Turbojets are basic, general-purpose jet engines that
produce steady amounts of power all the time, so they're suitable for
small, low-speed jet planes that don't have to do anything
particularly remarkable (like accelerating suddenly or
carrying enormous amounts of cargo). 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: The gray tube you can see under the rotor of this US military Seahawk helicopter is one of its twin turboshaft engines. There's another one exactly the same on the other side. Photo by Trevor Kohlrus courtesy of US Navy.
You might not think helicopters are driven by jet engines—they
have those huge rotors on top doing all the work—but you'd be
wrong: the rotors are powered by one or two gas-turbine engines
called turboshafts. A turboshaft is very different from a turbojet,
because the exhaust gas produces relatively little thrust. Instead,
the turbine in a turbojet captures most of the power and the
driveshaft running through it turns a transmission and one or more
gearboxes that spin the rotors. Apart from helicopters, you'll also
find turboshaft engines in trains, tanks, and boats. Gas turbine engines mounted in things like
power plants are also turboshafts.
Photo: A turboprop engine uses a jet engine to power a propeller. Photo by Eduardo Zaragoza courtesy of US Navy.
A modern plane with a propeller typically uses a turboprop engine.
It's similar to the turboshaft in a helicopter but, instead of
powering an overhead rotor, the turbine inside it spins a propeller
mounted on the front that pushes the plane forward. Unlike a
turboshaft, a turboprop does produce some forward thrust from its
exhaust gas, but the majority of the thrust comes from the propeller.
Since propeller-driven planes fly more slowly, they waste less energy
fighting drag (air resistance), and that makes them very efficient
for use in workhorse cargo planes and other small, light aircraft.
However, propellers themselves create a lot of air resistance, which
is one reason why turbofans were developed.
Read more about turboprops 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.
Giant passenger jets have huge fans mounted on the front, which
work like super-efficient propellers. The fans work in two ways. They
slightly increase the air that flows through the center (core) of the
engine, producing more thrust with the same fuel (which makes them
more efficient). They also blow some of their air around the outside
of the main engine, "bypassing" the core completely and producing
a backdraft of air like a propeller. In other words, a turbofan
produces thrust partly like a turbojet and partly like a turboprop.
Low-bypass turbofans send virtually all their air through the core,
while high-bypass ones send more air around it. Impressive power and
efficiency make turbofans the engines of choice on everything from
passenger jets (typically using high-bypass) to jet fighters
(low-bypass). The bypass design also cools a jet engine and makes it
quieter. Read more about turbofans from NASA.
Ramjets and scramjets
Photo: A Pegasus ramjet/scramjet engine developed for space planes in 1999.
Photo by courtesy of NASA Armstrong Flight Research Center.
Jet engines scoop air in at speed so, in theory, if you designed
the inlet as a rapidly tapering nozzle, you could make it compress
the incoming air automatically, without either a
compressor or a turbine to power it. Engines that work this way are called ramjets, and since
they need the air to be traveling fast, are really suitable only for
supersonic and hypersonic (faster-than-sound) planes. Air moving
faster than sound as it enters the engine is compressed and slowed
down dramatically, to subsonic speeds, mixed with fuel, and ignited
by a device called a flame holder, producing a rocket-like exhaust
similar to that made by a classic turbojet. Ramjets tend to be used
on rocket and missile engines but since they "breathe" air, they cannot
be used in space. Scramjets are similar, except that the
supersonic air doesn't slow down anything like as much as it speeds
through the engine. By remaining supersonic, the air exits at much
higher speed, allowing the plane to go considerably faster than one
powered by a ramjet (theoretically, up to Mach 15, or 15 times the speed of
sound—in the "high hypersonic" region).
Read more about ramjets
and scramjets from NASA.
Chart: Modern jet engines are about 100 times more powerful than the ones invented by Frank Whittle and his German rival Hans von Ohain. The red block shows the GE90, currently the world's most powerful engine. In the timeline below, you can discover how engines developed—and the engineering brains behind them.
A brief history of jet engines
- ~1800s: Using simple models, English inventor Sir George Cayley
(1773–1857) figures out the basic design and operation of the modern,
wing-lifted airplane. Unfortunately, the only practical power source available during his lifetime
is the coal-powered steam engine, which is too big, heavy, and inefficient to power a plane.
- 1860s–1870s: Working independently, French engineers Joseph
Étienne Lenoir (1822–1900), German engineer Nikolaus Otto
(1832–1891), and Karl Benz develop the modern car engine, which runs on
relatively light, clean, energy-rich gasoline—a much more practical
fuel than coal.
- 1884: Englishman Sir Charles Parsons (1854–1931) pioneers
steam turbines and compressors, key pieces of technology in
future airplane engines.
- 1903: Bicycle-making brothers Wilbur Wright (1867–1912) and
Orville Wright (1871–1948) make the first powered flight using a
gas engine to power two propellers fixed to the wings of a simple
- 1908: Frenchman René Lorin (1877–1933) invents the ramjet—the simplest possible jet engine.
- 1910: Frenchman Henri-Marie Coandă (1885–1972) builds the world's first jet-like plane, the
powered by a large air fan instead of a propeller.
- 1914: US space pioneer Robert Hutchings Goddard (1882–1945) is granted his first two patents describing liquid-fueled, multi-stage rockets—ideas that will, many years later, help fire people into space.
- 1925: Pratt & Whitney (now one of the world's biggest aero-engine makers) builds its first engine, the
- 1928: German engineer Alexander Lippisch (1894–1976) puts rocket
engines on an experimental glider to make the world's first rocket
plane, the Lippisch Ente.
- 1926: British engineer Alan Griffith (1893–1963) proposes using
gas turbine engines to power airplanes in a classic paper titled An
Aerodynamic Theory of Turbine Design. This work makes Griffith,
in effect, the theoretical father of the jet engine (his many
contributions include figuring out that a jet engine compressor needs
to use curved airfoil blades rather than ones with a simple, flat
profile). Griffith later becomes a pioneer of turbojets, turbofans, and vertical takeoff and
landing (VTOL) aircraft as the Chief Scientist to Rolls-Royce, one of
the world's leading aircraft engine makers.
- 1928: Aged only 21, English engineer Frank Whittle (1907–1996)
designs a jet engine, but the British military (and Alan Griffith,
their consultant) refuse to take his ideas seriously. Whittle is
forced to set up his own company and develop his ideas by himself. By
1937, he builds the first modern jet engine, but only as a
- 1936: Whittle invents and files a patent for the bypass turbofan
- 1933–1939: Hans von Ohain (1911–1998), Whittle's German rival, simultaneously
designs jet engines with compressors and turbines.
His HeS 3B engine, designed in 1938, powers the Heinkel He-178 on its maiden flight
as the world's first turbojet airplane on August 27, 1939.
- 1951: US aerospace engineer Charles Kaman (1919–2011) builds the first helicopter with a gas-turbine
engine, the K-225.
- 2002: General Electric's GE90-115B turbofan becomes the world's most powerful engine, with a maximum thrust of 569kN (127,900 lbf).
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. Suitable for young teens.
- Super Jumbo Jets: Inside and Out
by Holly Cefrey, PowerPlus Books/Rosen, 2002. This book goes into just enough technical detail for younger readers, covering different types of jet engines, as well as broader details of how big planes stay in the sky. Suitable for ages 9–12.
- 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: