How do jet engines produce thrust?

1. An airliner is as heavy as a row of houses, yet the engines under its wings push it to very high speed. Let's make one see-through and look inside.
2. At the front, a huge fan spins many times each second. It pulls in an enormous amount of air. Some goes into the middle of the engine, and the rest flows around it.
3. Inside the core, row after row of spinning blades push the air into a smaller and smaller space. This part is the compressor. Squeezed air is packed tight, and it warms up too.
4. In the combustion chamber, fuel is sprayed into the squeezed air and burns with a steady flame. The gas becomes very hot, and hot gas pushes hard to spread out.
5. On its way out, the hot gas blows through the turbine and spins it like a pinwheel. A shaft joins the turbine to the compressor and the fan, so the engine keeps itself turning.
6. Finally the gas blasts out of the narrow nozzle, far faster than the air came in. The engine pushes the gas backward, so the gas pushes the engine and the plane forward.

Tap each part, from the front to the back, and watch what it does to the air.

• Fan. The fan is the big wheel of blades you see at the front of the engine. It pulls in air and pushes it backward.
• Bypass duct. Most of the air from the fan never goes through the hot middle. It flows around it in this outer tunnel and shoots out the back, giving a strong, quiet push.
• Compressor. The compressor is many rows of small spinning blades. Each row pushes the air into less space, so the air is squeezed more and more.
• Fuel nozzles. Fuel nozzles spray jet fuel into the squeezed air as a fine mist, so it mixes well and burns evenly.
• Combustion chamber. In the combustion chamber the fuel burns in a steady flame, like a gas stove that never goes out. It heats the air until it is far hotter than an oven.
• Turbine. The turbine is a set of blade wheels in the hot gas. The rushing gas spins them, and that spinning powers the front of the engine.
• Shaft. The shaft is a strong rod down the middle of the engine. It joins the turbine at the back to the compressor and fan at the front, so the hot gas keeps the whole engine turning.
• Exhaust nozzle. The exhaust nozzle is the narrow opening at the back. Squeezing through it makes the gas go even faster, like water from a hose when you cover part of the end.

Drag the parcel of air through the engine and watch the dials. Then change the fuel. Where is the air squeezed most? Where is it hottest? Where is it fastest?

Experiment: Where inside the engine is the air squeezed the most?

1. Drag the air parcel slowly from the front to the back.
2. Watch the pressure dial. Where does it reach its highest point?
3. Now watch the speed dial in the nozzle. Then add more fuel and look again.

What we found: The air is squeezed the most at the end of the compressor. Burning the fuel then makes it very hot. After that the pressure falls again as the gas spins the turbine and speeds up through the nozzle, so it leaves much faster than it came in.

Turn on JavaScript to try it yourself.

Put the parts in order from front to back. Then set the fuel so the thrust is in the green zone without overheating the turbine, and press Run the test.

A jet engine does four jobs, one after another, all the time. People who work on engines remember them as suck, squeeze, burn, blow. A big fan sucks air in. A compressor squeezes it. Fuel burns in it. And the hot gas blows out the back.

Why squeeze the air first? Fuel burned in open air just makes a gentle flame. Fuel burned in tightly squeezed air releases its energy in a small space, so the gas pushes very hard to get out. The engine is built so the only easy way out is backward.

On the way out the gas spins a turbine, like wind spinning a pinwheel. A shaft joins the turbine to the fan and the compressor, so once the engine is running it keeps itself going as long as it has fuel.

The push itself is the balloon idea. The engine throws gas backward very fast, and the gas pushes the engine forward just as hard. A balloon runs out of air in a second. A jet engine keeps swallowing new air, so it can push for hours. Most of the push in an airliner engine comes from the big fan blowing air around the hot core.

Key words

Thrust

The forward push made by throwing air or gas backward.

Compressor

Rows of spinning blades that squeeze air into a smaller space and raise its pressure.

Combustion chamber

The part of the engine where fuel burns in the compressed air.

Turbine

A wheel of blades that hot gas spins, like wind spinning a pinwheel.

Pressure ratio

How many times the compressor multiplies the pressure of the air.

Bypass ratio

How much air flows around the core compared with how much flows through it.

Check your thinking

1. What actually pushes a jet plane forward?

   Show the answer

   Gas thrown backward very fast pushes the engine forward. It works like a balloon you let go. It does not need air behind it to push against.

2. What is the right order of jobs inside a jet engine?

   Show the answer

   Suck, squeeze, burn, blow. Air is pulled in, squeezed, heated by burning fuel and then blown out the back.

3. Why does the engine squeeze the air before burning fuel in it?

   Show the answer

   Burning in squeezed air makes a much stronger push. Squeezed air packs the energy of the burning fuel into a small space, so the gas pushes out much harder.

4. What keeps the fan and compressor spinning while the engine runs?

   Show the answer

   The turbine, which is spun by the hot gas and joined to them by a shaft. Hot gas spins the turbine, and the shaft carries that spin to the front of the engine.

5. An engine at cruising height takes in much thinner air than at sea level. With the same exhaust speed, what happens to its thrust?

   Show the answer

   It falls, because less mass of air passes through each second. Thrust is about mass flow times the change in speed. Thinner air means less mass each second, so less thrust.

6. Where is the gas in a jet engine hottest?

   Show the answer

   In the combustion chamber, where the fuel burns. Burning fuel heats the gas far above the temperature of an oven.