Diagrams.ink / Jet engines

Same fire. More air around it.

A turbojet pushes only the air that passes through its core. A turbofan wraps a big fan around the same core and sends extra air past it. Change the throttle and the bypass ratio, then watch where the air goes and how the push changes.

Turbojet

Every bit of air goes through the core.

Air through the engine

Thrust, qualitative

Exhaust jet

Turbofan

A fan sends extra air around the same core.

Bypass ratio 6 : 1

Air through the engine

Thrust, qualitative

Exhaust jet

Air entering Core air, heated by fuel Bypass air (never burns fuel) Thrust, pushing the engine forward

Try it: drag a slider, or pull the pulsing handle on the turbofan's cowl.

Throttle

Bypass ratio

Why bypass air matters

The core makes a fixed amount of energy from burning fuel. A turbojet spends all of it speeding up a small amount of air into a very fast jet. A turbofan spends it on a lot more air, giving each bit a gentler push. Thrust comes from how much air you push and how much you speed it up, so more air at lower speed can add up to more push for the same core.

The idea behind the bars

For a still engine, thrust equals air flow times jet speed. If the jet gets a fixed share of the core's energy, then ½ × air × speed² stays fixed, and thrust grows as the square root of the air. The bars use this simplified rule, so only their ordering and rough size mean anything.

What this leaves out

Real engines depend on flight speed, altitude, fan size, weight, drag and noise, and a bigger fan has real costs. No efficiency or fuel figures are claimed here. Bypass lets the same core push more air more gently; that is the trade the picture shows, not a performance prediction.