Most airliners fly on turbofans. The design wraps a large ducted fan around a jet core. The fan pushes a huge volume of air backward, and only a portion of it passes through the combustion chamber. The rest bypasses the core and produces thrust directly.

That bypass ratio is the key number. High-bypass engines, common on commercial aircraft, move much more air around the core than through it, which makes them efficient and quiet. Low-bypass engines suit fighter jets that need raw speed.

Main components

• Fan at the front
• Compressor stages
• Combustion chamber
• Turbine driving the fan and compressor
• Exhaust nozzle

Turbofans are quieter than turbojets because the bypass air shields the hot core exhaust. Modern designs keep improving the bypass ratio, which is why new engines look increasingly wide compared to their length.