A turbofan engine is a type of jet engine that produces thrust by moving a large volume of air with a front-mounted fan, while a smaller portion of that air passes through a hot combustion core. It powers nearly every commercial airliner flying today, and its defining feature is splitting incoming air into two streams: one that burns fuel and one that simply gets pushed out the back. That bypass air, which never touches the combustion process, is responsible for most of the engine’s thrust.
How the Two Airstreams Work
When air enters a turbofan, the large spinning fan at the front accelerates all of it rearward. From there, the air divides. A relatively small fraction enters the engine’s inner core, where it gets compressed, mixed with fuel, ignited, and expelled at high speed. The rest of the air, the majority in modern designs, flows around the outside of the core through a bypass duct and exits without ever being heated. Both streams contribute thrust, but the bypass air does so more efficiently because accelerating a large mass of air gently uses less fuel than accelerating a small mass violently.
The ratio of bypass air to core air is called the bypass ratio. An engine with a bypass ratio of 11 sends roughly 90% of its air around the core. Older designs like the CFM56, which powered earlier 737s and A320s, had a bypass ratio around 5.7. Current-generation engines have pushed much higher: the LEAP-1A, used on the Airbus A320neo family, has a bypass ratio of 11, and the Rolls-Royce Trent 7000 sits at 10. Higher bypass ratios generally mean better fuel efficiency and less noise.
Inside the Core
The core is where the actual combustion happens, and it follows the same basic cycle as every gas turbine. Air enters the compressor, which consists of multiple rows of spinning blades that squeeze the air into a denser, higher-pressure state. That compressed air flows into the combustor, where fuel is sprayed in and ignited. The resulting hot, high-pressure gas then blasts through the turbine, a set of blades that extract energy from the exhaust stream. The turbine is connected to the compressor and fan by a shaft, so as it spins, it drives those front-end components. Finally, the remaining exhaust exits through the nozzle at the rear, where its pressure is matched back to the surrounding atmosphere.
The energy balance is straightforward: the turbine extracts exactly enough work from the hot gas to keep the compressor and fan spinning. Whatever energy remains in the exhaust after the turbine produces rearward thrust. The compression stage heats the air even before fuel is added, and the combustion itself happens at roughly constant pressure, meaning the burning fuel raises the temperature dramatically without changing how tightly the air is packed.
Low Bypass vs. High Bypass
Not all turbofans look the same. The distinction between low-bypass and high-bypass designs reflects fundamentally different priorities.
High-bypass turbofans have enormous front fans, sometimes over six feet in diameter, and route most of their air around the core. They prioritize fuel efficiency and low noise, which is why they dominate commercial aviation. Every widebody and nearly every narrowbody airliner uses a high-bypass turbofan.
Low-bypass turbofans push a higher proportion of air through the core. They produce less thrust per unit of fuel but can operate efficiently at supersonic speeds, making them the standard for military fighter jets. Many military low-bypass engines also include an afterburner, which injects and ignites additional fuel in the exhaust stream for short bursts of extra thrust during takeoff or combat maneuvers.
The Geared Turbofan Advantage
One of the biggest constraints in traditional turbofan design is that the fan and the turbine sit on the same shaft. They have to spin at the same speed, but their ideal speeds are opposites: fans are more efficient spinning slowly, while turbines perform best spinning fast. For decades, engineers compromised between the two.
Pratt & Whitney’s geared turbofan architecture solved this by placing a gearbox between the fan and the low-pressure turbine, letting each component spin at its own optimal speed. The result was dramatic. According to the company’s chief engineer, the geared turbofan improved fuel efficiency by over 15% in a single leap, compared to the 1 to 1.5% annual improvement the industry had averaged over the previous 60 years. That translates to roughly $1.5 million in fuel savings per aircraft per year.
Because the fan can now spin much more slowly, the noise footprint drops as well. Pratt & Whitney reported up to a 75% reduction in noise compared to previous designs. The slower fan also allows a larger diameter, which increases the bypass ratio and further boosts efficiency.
Materials That Make It Possible
Fan blades endure extreme forces. They spin at thousands of revolutions per minute, ingest rain and hail, and occasionally strike birds, all while needing to be as light as possible. Titanium alloys, particularly Ti-6Al-4V, have long been the standard material for fan blades because they combine high strength with relatively low weight and resist corrosion well. Researchers continue exploring titanium foam structures that could reduce blade weight even further while maintaining resilience under the extreme loads of flight.
Newer engines have also introduced composite fan blades made from carbon fiber and resin. These are lighter than titanium, which means the engine structure supporting the fan can also be lighter, creating a cascade of weight savings across the aircraft. The GE9X engine powering the Boeing 777X and the LEAP family both use composite fan blade technology.
Why Turbofans Replaced Earlier Jet Engines
The first generation of jet airliners used pure turbojets, which forced all incoming air through the combustion core. These engines were loud, thirsty, and produced a narrow, high-velocity exhaust stream. Turbofans evolved from turbojets by adding the bypass fan, which immediately improved fuel economy because accelerating a large mass of cool bypass air generates thrust more efficiently than relying solely on hot core exhaust.
The efficiency gain is rooted in physics: thrust equals mass flow rate times the change in air velocity. Doubling the mass of air you move while halving the speed increase produces the same thrust with significantly less kinetic energy wasted as heat and noise. This is why turbofan engines are quieter as well as more fuel-efficient. The large volume of slow-moving bypass air also helps muffle the roar of the hot core exhaust, acting as a kind of acoustic blanket around the jet stream.
Early turbofans had bypass ratios below 2. Today’s commercial engines, with ratios above 10, move so much bypass air that the core accounts for a surprisingly small fraction of total thrust. The trend continues to push toward even higher ratios, limited mainly by the structural challenge of building ever-larger fans that can withstand bird strikes and crosswinds without adding prohibitive weight.