There is little difference in appearance.

What both of them have in common is that the compressed air of the compressor does the work of combustion and expansion in the combustion chamber to push the turbine while the turbine drives the compressor. This is also the basic working principle of a gas turbine engine.

The difference is that the turbojet has only a single channel. Turbofan engines, on the other hand, are usually based on conventional two-rotor turbojet engines, with a turbo-driven fan and an enclosure (which forms an enclosure). That's the difference in structure. What you see in front of the turbojet like fan is the blade of the compressor, and in front of the turbofan engine is the fan.

The turbofan engine is structurally provided with an external channel and an internal channel. The ejected air from the external channel directly eliminates the engine from generating thrust, and the internal channel enters the combustion chamber.

The ratio of the air mass flow rate between the outer channel and the inner channel of a turbofan engine is called the bypass channel ratio. If the bypass ratio is less than 1, it is defined as small bypass ratio; if it is greater than 4, it is defined as large bypass ratio; if it is greater than 1 and less than 4, it is medium bypass ratio. Bypass ratio is an important design parameter of turbofan engine.

The advantages of the turbofan engine are that under the same condition of the gas generator, the air flow of the turbofan engine is large and the exhaust velocity is low, so the propulsion efficiency is high, the fuel consumption is low and the noise is low. The turbofan engine is an ideal power for high subsonic passenger and transport aircraft. Turbofan engines with afterforces are suitable for supersonic military aircraft.