An afterburner, also known as reheat, is a device some jet engines use for extra speed. In fact, most fighter jets rely on them to achieve supersonic speeds.
Let’s have a look at how these engineering wonders work.
Jet engine basics
Jet engines create thrust through combustion, which is the reaction between fuel and oxygen. The result of which is a rapid expansion of gases. By directing these gases backward, the engine generates thrust that propels the aircraft forward.
To obtain the oxygen, jet engines take in air from the atmosphere. The compressor compresses this air to high pressure, enhancing combustion efficiency. In the burners, fuel is mixed with the compressed air and ignited, producing the hot exhaust gases.
These exhaust gases also pass through a turbine, which spins to power the compressor and creates a continuous cycle of thrust.
Afterburner function
Afterburners boost engine thrust by spraying fuel into the exhaust for a second combustion phase. But for combustion to occur, we also need oxygen.
In turbojet engines, some air passes through burners without combusting to prevent the turbine from overheating. Afterburners then use the leftover oxygen from this air.
Both turbojets and low-bypass turbofans can use afterburners. In turbojets, all the air goes through the engine core, while turbofans allow some air to bypass it, which makes them more efficient.
For turbofans, the air that bypasses the core also provides oxygen for the afterburners, leading to better performance compared to turbojet engines.
Afterburning engines use thrust nozzles with variable geometry. By changing the nozzle diameter, these ensure thrust generation is efficient regardless of the engine output. They also keep jet pipe pressure from getting too high, which can stall the engine.
How afterburners increase jet engine thrust
Thrust is equal to the exhaust velocity times by the exhaust mass flow rate. Going by this, there are two ways to increase jet engine thrust: make the exhaust gases exit the engine faster, or have a greater volume of exhaust gases.
Afterburners boost thrust by increasing the exhaust velocity. By doing so, the mass flow rate through the engine also increases.
Another way to enhance thrust is to make the engine wider. This allows for a higher mass flow rate without increasing the exhaust velocity. High-bypass turbofans use this principle for gaining extra efficiency. However, with fighter jets, the extra engine size is not workable and leaves the afterburning engine to be the best option for thrust gains.
The afterburner process increases the thrust of turbojets by up to 50%. For turbofans, the gains can be even greater at up to 70%.
With the exhaust speeds of afterburners produce, shock diamonds often appear in the exhaust plume. These pattern distortions form when the exhaust gases travel faster than the speed of sound and have a lower pressure than the atmosphere.
Afterburner limitation - efficiency
The downside of afterburners is that they are thirsty. Aircraft typically only use them for short high-speed intervals or at takeoff to conserve fuel.
The low efficiency of afterburning engines results from the use of uncompressed air. To help explain, the following equation expresses jet engine efficiency:
η = (P1 / P2)(γ-1)/ γ
Where,
- η is the efficiency of the engine
- P1 is the pressure of the intake air before compression
- P2 is the pressure of the intake air after compression
- γ is the heat capacity ratio
As you can see, increasing P2 by compressing the intake air gives a higher efficiency value.
Primary burners use pressurised air from a compressor for this reason. However, with afterburners, the air cannot be compressed before combustion. This results in the low efficiency and high fuel consumption.
How much faster can afterburners make an aircraft go?
How much faster afterburners can make an aircraft go varies. It depends on whether the engine is optimised for cruising, or high-speed afterburner use.
One aircraft example that sees a significant afterburner boost is the F-16 Fighting Falcon. This has a cruising speed of under the speed of sound. But when it has its afterburners firing, it can exceed Mach 2, which is two times the speed of sound! So in this case, the afterburner more than doubles the speed of the aircraft.
A contrasting example is the F-22 Raptor, the world’s first 5th-gen fighter jet. It features supercruise capability that enables it to fly at supersonic speeds without using afterburners. However, when it needs an extra kick, its afterburners boost speed up to Mach 2.2. While impressive, this speed increase is not as dramatic as what the F-16 can achieve.