Jet engines

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!

What is a jet engine?

A jet engine is a reaction engine discharging a fast moving jet that generates thrust by jet propulsion in accordance with Newton’s laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, and pulse jets. In general, jet engines are combustion engines but non-combusting forms also exist.

In common parlance, the term jet engine loosely refers to an internal combustion
airbreathing jet engine (a duct engine). These typically consist of an engine with a rotary (rotating) air compressor powered by a turbine (“Brayton cycle“), with the leftover power providing thrust via a propelling nozzle. Jet aircraft use these types of engines for long-distance travel. Early jet aircraft used turbojet engines which were relatively inefficient for subsonic flight. Modern subsonic jet aircraft usually use high-bypass turbofan engines. These engines offer high speed and greater fuel efficiency than piston and propeller aeroengines over long distances

Also 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 have a look inside the engine…

How a jet engine works

1. 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.
   
2. A fan at the front sucks the cold air into the engine.
   
3. A second fan called a compressor squeezes the air (increases its pressure) by about eight times. This slows the air down by about 60 percent and it’s speed is now about 400 km/h (240 mph).
   
4. Kerosene (liquid fuel) is squirted into the engine from a fuel tank in the plane’s wing.
   
5. 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).
   
6. The exhaust gases rush past a set of turbine blades, spinning them like a windmill.
   
7. 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 fan.
   
8. 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 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.
   

Whittle’s engines

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.

1. Turbojet
   

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)

Turboprop

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.

Turbofan

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.

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