A modern airliner may look as though it is powered by two enormous jet pipes, but the most important part of each engine is the fan you can see from the front. That fan belongs to a turbofan: a gas-turbine engine designed to move a very large mass of air, rather than relying only on a narrow, extremely fast exhaust jet.
That distinction explains why turbofans dominate commercial aviation. They combine the compact, continuous power of a gas-turbine core with the propulsive efficiency of a ducted fan. The result is the thrust, range and reliability needed to lift anything from a regional jet to a fully laden long-haul airliner.
What is a turbofan engine?
A turbofan is a jet engine with two closely related airflow paths. One portion of the incoming air enters the engine core, where it is compressed, mixed with fuel, burned and expanded through turbines. The rest passes around the core through a bypass duct after being accelerated by the fan.
The gas-turbine core is the engine's power plant. Its turbines drive the compressors and the large front fan through concentric shafts. The fan is therefore not simply windmilling in the airflow: it is powered by energy extracted from the hot gas leaving the combustor.
On a high-bypass civil engine, the cooler bypass stream provides most of the propulsive force. That is why a modern turbofan can be thought of as a highly engineered ducted fan with a compact gas turbine at its heart.
How turbofan engines work, stage by stage
1. The inlet and fan capture the airflow
The nacelle inlet slows and straightens the approaching air before it reaches the fan. The fan's wide, carefully twisted blades then add energy to that air. A small proportion enters the core; the larger share flows into the annular bypass duct surrounding it.
Blade shape matters enormously. Each blade works across a wide range of speeds from root to tip, so its angle and aerofoil section change along its span. The spinner, inlet lip, fan case and outlet guide vanes are all designed as part of the same airflow system.
2. Compressors raise the core pressure
Air entering the core passes through alternating rows of rotating compressor blades and stationary vanes. The rotor stages add velocity; the stators diffuse and redirect the flow, converting much of that velocity into pressure. Successive stages squeeze the air into a smaller passage and deliver a dense, high-pressure flow to the combustor.
Modern engines often divide compression between low- and high-pressure sections. Each can rotate at the speed best suited to its blade diameter and aerodynamic task, rather than forcing the entire engine to turn as one unit.
3. The combustor adds heat
Fuel is finely sprayed into the compressed air and burned continuously in the combustor. Only part of the air supports combustion directly; additional air controls flame temperature, cools the liner and helps create an even temperature profile before the flow reaches the turbine.
The process is continuous, unlike the separate induction, compression, power and exhaust strokes of a piston engine. Once the engine is running, stable combustion supplies a constant stream of hot, high-energy gas.
4. Turbines power the machinery
The hot gas expands through turbine stages. These stages extract enough energy to turn the compressors and fan through nested shafts, while leaving useful energy in the flow for the exhaust. Turbine blades work in temperatures that can exceed the melting point of their base metal, so designers rely on heat-resistant alloys, internal cooling passages and protective coatings.
An engine may use two or three independent rotating assemblies, known as spools. A typical two-spool layout has a high-pressure compressor and turbine on one shaft, with the fan, low-pressure compressor and low-pressure turbine on another. Three-spool engines add an intermediate-pressure system.
5. Bypass and core flows produce thrust
Thrust comes from changing the momentum of the air. The bypass stream leaves the engine faster than it entered, while the core exhaust adds its own smaller, hotter contribution. Some engines mix the two streams before the final nozzle; others exhaust them separately.
For subsonic flight, moving a larger mass of air rearwards by a modest amount is generally more efficient than accelerating a smaller mass to a much higher speed. That principle is the reason modern airliner engines have grown such large fans.
What does bypass ratio mean?
Bypass ratio compares the mass of air flowing around the core with the mass flowing through it. A bypass ratio of 10:1 means roughly ten units of air use the bypass duct for every one unit passing through the core.
Early turbofans had relatively low bypass ratios. Rolls-Royce's pioneering Conway, introduced in 1960 for long-range passenger aircraft, had a bypass ratio of about 0.3:1. Decades of development pushed civil engines towards much higher ratios; a modern wide-body turbofan can reach around 10:1.
A higher bypass ratio usually improves propulsive efficiency and reduces jet noise because the engine produces thrust with a slower, larger airflow. It is not a free gain, however. A bigger fan needs a larger, heavier nacelle, creates more drag and must still provide sufficient ground clearance beneath the wing.
High-bypass versus low-bypass turbofans
High-bypass engines suit subsonic airliners, where fuel economy, low noise and long endurance matter. Low-bypass turbofans are slimmer and produce more thrust for a given frontal area. That makes them useful for fast military aircraft, where compact packaging, high specific thrust and supersonic performance can outweigh cruise efficiency.
Some low-bypass engines add an afterburner, injecting fuel into the exhaust downstream of the turbine. It produces a dramatic temporary increase in thrust, along with equally dramatic fuel consumption. Civil high-bypass engines do not use afterburners.
Why some turbofans use a gearbox
Without gearing, the fan and its driving low-pressure turbine share a shaft and therefore rotate at the same speed. Yet the large fan prefers a lower rotational speed while the smaller turbine works efficiently at a higher one.
A geared turbofan inserts a reduction gearbox between them. The fan can turn more slowly while the turbine rotates faster, allowing both to operate closer to their aerodynamic optimum. The trade-off is a demanding gearbox that must transmit enormous power reliably while remaining light enough for flight.
Why turbofans fascinate aviation enthusiasts
A turbofan combines several engineering disciplines in one machine: aerodynamics, combustion, metallurgy, acoustics, controls and precision manufacturing. Its apparent simplicity from the outside conceals multiple shafts turning at different speeds, rows of blades designed for distinct conditions and clearances measured with extraordinary care.
That mixture of visible scale and hidden complexity is celebrated in Camberline's Turbofan Thunder passenger-airliner T-shirt and matching Turbofan Thunder aviation hoodie. More airliner-inspired designs sit in the Commercial Aviation collection, while the wider Aviation collection covers aircraft from across the flying world.
Frequently asked questions
Does the fan really produce most of the thrust?
On a modern high-bypass airliner engine, yes. The exact share varies with engine design and flight condition, but the fan and bypass stream provide the majority of the thrust. The core's essential job is to supply the power that drives them.
What is the difference between a turbojet and a turbofan?
In a turbojet, nearly all incoming air passes through the core and thrust comes mainly from a high-velocity hot exhaust. A turbofan adds a fan and bypass duct, producing part or most of its thrust from cooler air flowing around the core.
Can a turbofan operate in heavy rain?
Yes. Certified engines are designed and tested for severe weather, including substantial water ingestion. The airflow, drainage and combustion system must continue operating safely, although pilots still avoid the most hazardous thunderstorm conditions.
How does reverse thrust work?
Reverse thrust does not make the engine rotate backwards. On most airliners, translating sleeves and blocker doors redirect part of the fan's bypass airflow forwards after landing, helping the wheel brakes slow the aircraft.
The modern jet engine, properly understood
A turbofan is not simply a turbojet with a larger intake. It is a carefully balanced system in which the core generates shaft power, the turbines harvest it, and the fan turns it into efficient thrust by moving a huge volume of air. Once that relationship is clear, the size of a modern nacelle—and the quiet authority of the aircraft it powers—makes much more sense.