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Anatomy · Jet

Anatomy of a high-bypass turbofan

The engine under every airliner wing is really two machines sharing a shaft: a huge ducted propeller that makes most of the thrust, and a small, ferocious jet engine in the middle whose main job is to spin it.

Follow the air from left to right. Everything the engine does happens in one straight line: squeeze the air, burn fuel in it, then let the hot gas spin turbines on its way out — turbines that drive the very compressors and fan that started the process. The drawing is a section straight through the shaft, so each ring of blades appears twice, above and below the centreline.

The proportions are the giveaway: the fan dwarfs everything else. Nine-tenths of the air it moves never burns at all — it just gets thrown backwards, gently, in enormous quantity.

fan ⌀ ≈ 3 m≈ 90% of the air takes the bypass≈ 10% goes through the core — its real job is powering the fanhot core jet12345678910
A high-bypass turbofan in longitudinal section, air flowing left to right. Blade rows appear as mirrored pairs above and below the shaft centreline; bright rows rotate, dark rows are fixed stators.