Decoding Engine Types: Turbofan vs. Turbojet
Jet engines have revolutionised aviation, making high-speed air travel possible. Different types of turbine engines power a wide range of aircraft, from military fighters to commercial airliners. But when it comes to turbofan and turbojet engines, what sets them apart?
While both operate on the same basic principle — air intake, compression, combustion, and exhaust—their design and performance characteristics differ significantly. So, is one engine type better than the other? Let’s explore the key differences between turbofan and turbojet engines, their advantages, and disadvantages, and where each excels in aviation.
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Types of Aircraft Turbine Engines
Before diving into turbofans and turbojets, let’s briefly look at the six main types of jet propulsion systems used in aviation.
1. Turbojet – The Classic Jet Engine
The simplest type of jet engine, where all incoming air passes through the compressor, combustion chamber, and turbine, produces high-speed exhaust for thrust.
2. Turbofan – The Modern Standard
A turbojet with a large fan at the front bypasses air around the engine core. This improves fuel efficiency and reduces noise, making it ideal for commercial airliners and modern military jets.
3. Turboprop – Propeller-Powered Efficiency
A turboprop engine uses a turbine to drive a propeller, rather than producing jet thrust. This design offers high efficiency at lower speeds, making turboprops ideal for short-haul flights and regional air travel. Used in regional airliners, cargo planes, and military transport aircraft.
4. Turboshaft – Powering Helicopters
A turboshaft engine is similar to a turboprop but designed to drive a shaft instead of a propeller. This allows it to power helicopter rotors, auxiliary power units (APUs), and naval vessels.
5. Ramjet – Speed Without Moving Parts
A ramjet is a unique type of jet engine that operates at supersonic speeds but has no moving parts. Instead of using a compressor, ramjets rely on high-speed air intake to compress air before combustion. They are ineffective at low speeds and require an initial boost (from another engine or launch system) to function. Used in missiles and experimental high-speed aircraft.
6. Scramjet – Hypersonic Flight
A variation of a ramjet that works at hypersonic speeds (Mach 5+). Used in cutting-edge aerospace projects like NASA’s X-43.
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Now, let’s have a closer look at turbofan and turbojet engines, as they are the most widely used in aviation.
Turbojet Engine
A turbojet is the original jet engine design, developed in the 1930s and widely used in the early days of jet aviation. It consists of:
Compressor – Compresses incoming air for combustion.
Combustion Chamber – Mixes compressed air with fuel and ignites it.
Turbine – Extracts energy to drive the compressor.
Exhaust Nozzle – Expels high-speed gases to generate thrust.
In a turbojet engine, all incoming air is compressed, mixed with fuel, burned, and then ejected at high speed to create thrust.
Turbojets offer high speed and a compact, lightweight design, making them ideal for supersonic and high-altitude flight, particularly for fighter jets. However, they are consuming large amounts of fuel, especially at lower speeds. They also produce a sharp, high-pitched noise, and perform best above Mach 1.
This type of engine powers supersonic aircraft like the Concorde and the Lockheed SR-71 Blackbird, as well as military jets like the MiG-21 and the F-104 Starfighter.
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Turbofan Engine
A turbofan is an advanced version of a turbojet, designed for better fuel efficiency and lower noise. The key difference? It has a large fan at the front, which bypasses some air around the engine core.
The fan pulls in air — some goes through the engine core, while a large portion bypasses the core, producing additional thrust.
This bypassed air increases fuel efficiency and reduces noise, making turbofans ideal for commercial aviation.
Turbofan engines power a wide range of aircraft, from commercial airliners like the Boeing 747, Airbus A320, and Boeing 787, to modern fighter jets such as the F-22 Raptor and Eurofighter Typhoon.
Turbofans are more fuel-efficient than turbojets because they use bypass air to generate thrust, reducing the need to burn as much fuel. They are quieter, and offer better performance at subsonic speeds, making them ideal for commercial airliners. However, turbofans are larger and heavier than turbojets.
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Which Engine is Better?
There’s no one-size-fits-all answer — each engine is designed for different jobs. The key difference between a turbofan and a turbojet engine is the addition of large fan blades and a surrounding nacelle on the turbofan.
Both engine types feature turbine blades behind the combustion stage, driven by exhaust flow. These turbine blades are connected to the fan and compressor blades at the front of the engine. However, the turbofan's initial fan blades are significantly larger.
Let's quickly compare these engines based on key characteristics.
Fuel Efficiency: The turbofan engine was designed to address the fuel consumption issues of the turbojet. For engines of comparable core size, the turbofan moves more air due to its bypass airflow. This results in greater thrust for the same power output, meaning the turbofan burns less fuel than a turbojet to achieve the same thrust.
Performance: Propeller engines are most efficient at low speeds, while jet engines excel at high speeds. The turbofan's initial stage functions somewhat like a propeller, effectively combining the benefits of both propeller and jet engines. This makes the turbofan more efficient at lower airspeeds, while the turbojet remains superior at higher speeds.
Noise: Noise is another key differentiator. Jet engine noise primarily comes from the high-velocity exhaust. Turbofans, with bypass ratios as high as 12:1 (meaning 12 kg of air bypasses the core for every 1 kg that goes through it), mix the high-velocity exhaust with lower-velocity bypass air, resulting in a significantly quieter operation compared to turbojets.
Maintenance: The turbofan's increased complexity, with more components than a turbojet, translates to more maintenance requirements. More parts necessitate more frequent inspection, service, and upkeep. The larger diameter and additional components of a turbofan also contribute to increased aircraft drag and weight.
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Turbojet vs. Turbofan
Airhead's Takeaway
Both turbofan and turbojet engines have shaped aviation history, but the turbofan has become the standard for most applications due to its efficiency and versatility. However, turbojets still have their place in high-speed military aviation.
The future? With advances in hypersonic propulsion and electric jet technology, the next generation of aircraft engines might be even more efficient, faster, and quieter than today’s designs.
