A turbofan takes the turbojet idea and splits the air in two. Follow the flow from the big front fan, through the cool bypass duct and the hot core, to the two streams that leave the back.
A turbofan is the engine that powers almost all commercial jets. It looks like a turbojet with a much larger set of blades at the front — the fan — wrapped in a big duct. The fan sends most of the air around the outside of the core as a cool stream, and only the rest goes through the burning core. That split is what makes it quieter and far more efficient.
From outside, a turbofan is just a fat tube with a hole at each end. The important idea — that one engine makes two gas streams at once — only becomes clear when you cut it open and follow the air as it divides.
At the very front, a large fan turns and draws in a great mass of air. It gives that air a first push backwards, and on its own it already accounts for a large share of the thrust.
Just behind the fan, the air divides. Most of it is turned outward into the bypass duct, where it will travel around the core. The rest is guided into the core engine at the centre.
The air that entered the core is squeezed by the core compressor to a much higher pressure, then fuel is burned in it inside the combustor. This is the same hot, high-pressure gas a turbojet makes — but here it only forms a small part of the total flow.
The hot gas spins the core turbine, and a shaft carries that rotation forward to the fan and the core compressor. This is what keeps the whole engine turning by itself.
At the back there are two exits. The hot core gas leaves through the inner core nozzle, while the cool bypass air leaves through the outer bypass nozzle. The two streams together — a big, slow, cool one and a small, fast, hot one — make the total thrust.
In a turbojet all the air passes through the core and is burned. In a turbofan, a large fan sends most of the air around the core as a cool bypass stream, which is quieter and more efficient for the same thrust.
Moving a large mass of air only slightly is more efficient than heating a small mass a great deal. The big, gently accelerated bypass stream does much of the work at lower noise and lower fuel use.
No. It passes through the bypass duct, around the outside of the core, and leaves through the bypass nozzle without ever touching the flame. Only the air that enters the core is burned.
The inner core nozzle accelerates the hot burned gas, while the outer bypass nozzle releases the cool air from the duct. Together their two streams make the total thrust.
Most of the thrust comes from the large, slow, relatively cool bypass stream rather than a small, very fast hot jet. A slower, broader exhaust mixes with the air more gently, which cuts the jet noise.