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What is the difference between a turbojet and a turbofan?

Boeing 737 Max Engine Inlet Fan
Boeing 737 Max Turbofan Engine Inlet Fan (Shutterstock – Zaheed Elahi)

The main difference between a turbofan and a turbojet is that all the air goes into the engine core (compressor, combustion chamber, turbine) in a turbojet.

A turbofan uses the main elements of a turbojet but has a large fan at the front of the engine. A certain amount of the airflow ‘bypasses’ the core (compressor, combustion chamber and turbine) flowing around the engine.

On a turbofan engine, some of the thrust is generated by the fan and the engine core, whereas on a turbojet engine, all the thrust is generated by the acceleration of flow through the engine core.

What are the differences in construction between a Turbofan vs Turbojet?

Turbojet construction

A turbojet’s main components typically consist of an air inlet to direct the flow into a compressor. From the compressor (where the flow pressure and temperature are increased), the flow is then introduced into the combustion chamber, where it is ignited.

After combustion, high temperature and high-pressure gas are accelerated and expanded in the nozzle, giving thrust. The turbine drives the compressor in a turbojet. Turbojets get all their energy from exhaust gasses.

Turbojets have zero bypass flow (i.e. no ‘cold’ flow around the core), so the airflow exits at high velocity. Turbojets are suited for high-speed supersonic flight where they are most efficient. Turbojets are often used on supersonic aircraft like Concorde.

Air France Concorde at CDG Airport in 2003
Air France Concorde at CDG Airport in 2003 – By Alexander Jonsson – http://www.airliners.net/photo/Air-France/Aerospatiale-British-Aerospace-Concorde/0432634/L/, GFDL 1.2, https://commons.wikimedia.org/w/index.php?curid=16590988

Turbojet main elements

Inlet > compressor > combustion chamber > turbine driving compressor > nozzle

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Turbofan construction

A Turbofan, on the other hand, has an air intake that leads to a fan. The air then takes two paths from the fan: ‘cold’ or bypass path and a ‘hot path’ or a core path.

Depending on the fan’s size and the engine’s bypass ratio, anywhere between 30-80% of the airflow goes around the cold path. The ‘cold’ air is diverted around the central engine core and is mixed back at the rear of the engine in the nozzle area with the ‘hot stream’.

Schematic diagram illustrating a modern 2-spool turbofan engine installation in a nacelle. The low-pressure spool is colored blue and the high-pressure one orange.
Schematic diagram illustrating a modern 2-spool turbofan engine installation in a nacelle. The low-pressure spool is colored blue and the high-pressure one orange. CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=21007866

The hot path on a turbofan engine is similar to a turbojet where the airstream goes into a compressor, then into the combustion chamber, a turbine and a nozzle. The turbine drives both the compressor and the fan in a turbofan engine. 

Turbofan components

Air intake > Fan > compressor > combustion chamber > turbine driving compressor & fan > nozzle

The other difference between turbofans and turbojets is that afterburners are not used on turbofans but are used on turbojets.

MiG-25RU trainer in September 2008
MiG-25RU trainer in September 2008 (Afterburners lit) – By Dmitriy Pichugin – http://www.airliners.net/photo/Russia—Air/Mikoyan-Gurevich-MiG-25RU/2144905/L/, GFDL 1.2, https://commons.wikimedia.org/w/index.php?curid=20639259

Bypass ratio

Bypass ratio is the ratio of air flowing through the ‘core’ compared to the amount of airflow bypassing the engine.

Bypass ratio 5 = 5 times more flow around the core vs flow going through the core > compressor > combustor > turbine.

Why is the bypass ratio important?

Having a fan and air bypassing the main engine core offers lower fuel consumption for the same amount of thrust when compared to a turbojet engine. Essentially, a low bypass ratio means more thrust is produced via the engine’s core compared to bypass. 

Jet engine internal
High bypass jet engine

On a high bypass engine, the fan produces more thrust than the core.

Turbojets have no bypass. Thrust on turbojets is produced by accelerating a small amount of air at very high speeds. Turbofans create thrust by accelerating large amounts of air at lower speeds.

According to WikipediaA turbofan emits a large amount of air more slowly, whereas a turbojet emits a smaller amount of air more quickly‘ which is less efficient in generating thrust.

Noise differences between turbojet and turbofans

Turbofan engines are much quieter than turbojet engines. The larger the bypass ratio of the engine, typically, the quieter it is.

737 Max Leap 1B Engine
Quieter 737 Max Leap 1B Engine (Shutterstock – Wirestock Creators)

Noise in jet engines is caused by changes in pressure, temperature and speed between the environment and jet flow. The main reason why high bypass turbofans are much quieter is that the airflow is much slower once accelerated through the engine.

A turbojet flow velocity at the exhaust is much higher than the flow velocity of a turbofan. The temperature and pressures changes in a turbofan at exhaust are not as significant as a turbojet which further reduces noise. 

Modern turbofans also include acoustic liners to dampen noise. To aid in mixing the bypass engine flow with the environment, chevrons / saw tooth designs have helped reduce noise, e.g. 747-8 and Dreamliner engines.

Efficiency of turbojets vs turbofans

Turbojets were created to increase efficiency and are most efficient at high subsonic speeds approx. 300 – 600mph.

The king of efficiency is the turboprop engine. The turboprop is effectively a turbojet engine with a gearbox driving a propeller. Turboprops steal the show as they effectively have an infinite amount of ‘bypass’ ratio as all the propeller and core sit in free air.

For shorter routes, where a high top speed is not crucial, slower turboprop aircraft have replaced jet aircraft because of their much greater fuel efficiency. Turbofan engines are similar to turboprops in using turbine power to drive a fan.

dash 8 turboprop
Dash 8 Turboprop Aircraft

Improved propulsive efficiency and fuel consumption gains more than makes up for mechanical losses of driving a large fan from the turbine on a turbofan engine. High bypass turbofans can get 70% or more of their thrust from the fan/ ducted component of the flow.

Turbojets suffer from low efficiency at low speeds. Turbojets are optimised and are most efficient for high-speed supersonic flight.

Efficiency gains from turbojet engines come by increasing compression or temperature of combustion. High-speed flight increases the ram pressures at the air intake and as a bi-product temperature, further improving efficiency.

SR71 Blackbird
SR71 Blackbird could cruise at Mach 3 with its turbojet engines

Turbojets are inefficient for subsonic aircraft because of high-velocity turbine exhaust gas and noise, which is wasted energy at the exhaust.

Uses of turbofan vs turbojet engines

Turbofans are used mainly on high-speed subsonic jet aircraft in civilian and military applications. Turbojets are used for supersonic applications. Concorde, for example, used the Rolls Roye 593 Olympus turbojet engines fitted with reheat or afterburners to cruise at Mach 2 at 60,000ft. 

The world’s largest twin-engined passenger jet aircraft, the Boeing 777-300ER, uses the GE90-115B turbofan jet engines to carry 386 passengers over 7,825nm (14,490 km).

Air France Boeing 777-300ER
Air France Boeing 777-300ER

Which came first, turbojets or turbofan?

The turbojet was first invented in 1928 by Frank whittle. Heinkel He 178 was the first aircraft to fly under turbojet power. The Gloster E.28/39 Gloster Whittle made the first British jet engine flight in 1941.

If you have any questions about turbojets or turbofans, please leave a comment in the section below.

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