How a Jet Engine Works: Intake to Exhaust

A jet engine makes thrust by throwing air backwards very fast. It takes in air, compresses it, burns fuel in it and lets the hot gas rush out of a nozzle. The push on the aircraft is the reaction to that rush of gas. Most airliners today use a version called a turbofan, in which a big fan at the front sends extra air around the outside of the engine.

Thrust and Newton's third law

NASA's Glenn Research Center explains thrust with Isaac Newton's third law: for every action there is an equal and opposite reaction. When an engine pushes gas backwards, the gas pushes the engine, and the aircraft attached to it, forwards. A jet engine does not push against the air behind it. It would work in the same way in space, if it carried its own oxygen, which is the reason rockets do.

The four stages

NASA describes the work of a jet engine in four stages. They happen continuously and all at once, in different parts of the engine.

• Intake: a fan at the front draws in a large volume of air.
• Compression: rows of spinning blades on a shaft squeeze the air into a smaller space and raise its pressure. NASA says a turbojet raises the pressure to between 3 and 12 times the starting value.
• Combustion: fuel is sprayed into the compressed air, through as many as 20 nozzles, and ignited. NASA puts the combustor temperature at about 2,700 degrees Fahrenheit (roughly 1,480 degrees Celsius), so the walls are often lined with ceramic.
• Exhaust: the expanding gas spins a turbine, which turns the compressor and fan on a shared shaft. The gas then leaves through the nozzle and produces the thrust.

The turbine is the link that keeps the engine running by itself. Part of the energy in the hot gas is taken out to drive the compressor at the front, and the rest is left to make thrust. Starting the engine needs an outside source of power to get the shaft turning, after which the engine sustains itself.

Turbojet and turbofan

In a simple turbojet, all the air that enters goes through the core: compressor, combustor and turbine. NASA notes that an afterburner, which burns extra fuel in the exhaust, can add about 40 percent more thrust at takeoff. Afterburners use a lot of fuel, so they appear mainly on military aircraft.

A turbofan adds a large fan at the front. Some of the air goes into the core as before, and the rest bypasses it through a duct around the outside. NASA defines the bypass ratio as the fan airflow divided by the core airflow. A ratio of 6 means six times as much air goes around the core as through it.

Both streams make thrust. Because the extra fan changes the core's fuel use only a little, a turbofan makes more thrust for nearly the same fuel. NASA says high-bypass turbofans approach the fuel efficiency of propeller engines, and that even low-bypass turbofans are more efficient than plain turbojets.

Why airliners use turbofans

Sending a lot of air backwards a little faster than the aircraft's speed is more efficient than sending a little air backwards very fast. Wikipedia's turbofan article, which draws on engineering references, says turbofans are most efficient at roughly 500 to 1,000 kilometres per hour, the speed range of most commercial aircraft. It gives bypass ratios of about 12.5 to 1 for the Pratt & Whitney PW1000G and about 11 to 1 for the CFM LEAP, against 0.3 to 1 for the Rolls-Royce Conway, the first production turbofan. Modern fighter engines usually keep the ratio low.

The slower bypass air also makes the engine quieter. The same article explains that the air leaving the bypass duct moves much more slowly than the exhaust of a pure jet, so a high-bypass engine is significantly quieter at the same thrust. Sound-absorbing liners in the engine housing and serrated nozzle edges, called chevrons, reduce noise further.

Other engines in the family

The same core idea appears in other engines. A turboprop uses the turbine to drive a propeller. A turboshaft drives a helicopter rotor. A ramjet has no moving parts and depends on forward speed to compress the air, so it cannot start from a standstill.

In short

Intake, compression, combustion and exhaust turn fuel into a fast stream of gas and a push forward. The turbofan adds a bypass fan that moves more air more slowly, which saves fuel and noise at airliner speeds. This is general information from NASA and a reference article, as of 1 November 2026, and does not describe any one engine.

Sources

• Engines, NASA Glenn Research Center
• Turbofan Engine, NASA Glenn Research Center
• Turbofan, Wikipedia