Jet engines
Last updated: May 14, 2007.
Jet engines have put rockets into space
and
powered boats and automobiles to world speed records, but they are more
familiar as the
engines on airplanes such as Concorde and the Jumbo Jet.
Unlike
internal combustion engines in automobiles 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.
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 courtesy of US
Navy.
A jet engine is a machine for turning fuel into thrust (forward
motion). The thrust is produced by action and
reaction—a
piece of physics also known as Newton's third law of motion. The force
(action)
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 take a closer look inside the engine...
How a jet engine works
- 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.
- A second fan called a compressor
squeezes the air
(increases its pressure) by about eight times. This slows the air down
to about 400 km/h (240 mph).
- 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. 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.
The turbine blades are connected to a long axle 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
fan.
- The hot exhaust gases exit the engine through a tapering exhaust
nozzle. The tapering design 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 travelling 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.
Photo: Massive thrust! A Pratt and Whitney F-119
jet aircraft engine creates 156,000 newtons (35,000 pounds) of thrust
during this US
Airforce test in 2002.
Picture courtesy of US
Airforce.
Types of jet engines
British engineer Sir Frank Whittle (1907-1996) invented the jet
engine in 1930 and his design, called a turbojet,
is still widely used
in airplanes today. It is not the only type of jet engine, however.
Turboprop engines have a propeller at the
front and are popular in
small, economical passenger aircraft.
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.
Ramjet engines, which are simple and compact
and little more than gas-burning
pipes, are used to power guided missiles.
Vertical takeoff
An Osprey 7 plane during early testing
Picture courtesy of the US Airforce.
Airplanes have to travel at high speeds to produce enough lift for
takeoff, but because they are immensely heavy and often carry
substantial cargoes, they can accelerate only very slowly. A typical
runway for a large airliner such as a Boeing 747 is around 2 miles (3
km) long, simply because the plane has to travel this far before it has
picked up enough speed to get off the ground.
Long runways may be fine for passenger aircraft, but military
fighters need to take off in much more confined spaces (for example,
from the deck of an aircraft carrier). Vertical Takeoff and Landing
(VTOL) aircraft solve this problem by having jet engines whose nozzles
can be swiveled in different directions. During takeoff and landing,
the jets point straight downward so the plane can rise or fall on the
spot or even hover like a helicopter.
Once the plane is airborne, the
jets swivel backward and shoot the plane forward like a conventional
airplane.
The best known plane of this sort is the Harrier "jump-jet"
extensively used by the UK Royal Navy and the US Marine Corps. The
Joint Strike Fighter (JSF) currently being developed by Boeing and
Lockheed for the US military will also be a VTOL aircraft.
The US Airforce Osprey plane works in a similar way, but has tilting
propellers
instead of jet engines. To land vertically, like a helicopter, it tilts
the propellers upward.
To fly horizontally, it points them forward.
Text copyright © Chris Woodford 2000. All rights reserved. 2007.
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