The primary driver of aircraft efficiency today is the engines. While advances in aerodynamics and materials do cut down on fuel burn as well, the engine is the main determinant as to how much fuel an aircraft burns, and older planes equipped with new engines tend to be more than competitive against a clean-sheet aircraft. During the start of the Jet Age, manufacturers were primarily focused on increasing power, with fuel efficiency being a second priority, but as technology has developed, this has changed.

Jet engines have essentially become as powerful as needed, and more than ever, focus is being put on cutting costs. Fuel is one of the highest costs for any airline, and the airline industry is extremely cost-sensitive. What’s more, the aviation industry is working towards becoming carbon neutral by 2050, actively decarbonizing as the industry is facing greater scrutiny, and more efficient jet engines help reduce emissions.

The Three Main Types Of Jet Engines

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Early jet engines were what’s known as a turbojet engine. These engines work by compressing air, which is then mixed with fuel and burned in the combustion chamber, before the hot, high-pressure air is pushed out through the exhaust. A turboprop engine, meanwhile, is largely the same type of motor, but the difference is that the turbine spins a propeller, rather than pushing out exhaust air. A turboprop engine burns less fuel than a turbojet, but the tradeoff is that they’re not as fast.

Turbofan engines are the most ubiquitous types of engines on commercial airliners, as they power aircraft ranging from the lowly Bombardier CRJ200 to the massive Boeing 777-9 and everything in between. Turbofans are similar to turbojets, but they feature a fan in the front of the engine, and some of the air flows through a bypass duct rather than through the engine core. The benefit of a turbofan engine is that it can be simultaneously more powerful and more efficient than a comparable turbojet, since it takes significantly more energy to push additional air through the engine core rather than the bypass duct.

There have been significant technological advances in turbofan design over the past six decades, but one of the most consequential and straightforward innovations is in bypass ratios, or how much air is bypassed compared to the amount that flows through the core. Modern aircraft engines feature significantly higher bypass ratios than 1980s-era engines or first-generation turbofans, which means that these engines are generating more power while consuming only slightly more fuel, if even that.

The Efficiency Of Modern-Day Engines

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Exact fuel burn numbers for engines vary depending on flight conditions and aircraft configurations, but trends can still be observed across time. As an example, the Boeing 737 MAX is exclusively powered by the CFM LEAP-1B, an engine that CFM advertises delivers a 15% reduction in specific fuel consumption compared to the CFM56-7B. The CFM56-7B was the exclusive engine for the 737NG, and it was advertised as delivering an 8% fuel burn reduction compared to the CFM56-3, which powered the 737 Classic. The CFM56-3, meanwhile, came with almost 20% lower fuel burn than the Pratt & Whitney JT8D, found on the 737-100/200.

The CFM LEAP is a state-of-the-art engine that debuted in the 2010s, while the CFM56-7B debuted in 1997. Not only does the engine have a much higher bypass ratio, but it also features a more advanced engine core derived from the General Electric GEnx, along with an increased use of composites. The CFM56-7B is more of an upgrade to the 737 Classic’s CFM56-3, with improved aerodynamics, an upgraded engine core, and a new fan with fewer fan blades. Even with this, the CFM56-7B still delivered meaningful improvements in fuel burn.

The difference between the CFM56-3 and the Pratt & Whitney JT8D, however, is the largest, in part due to the significant design differences. The 1960s-era JT8D was an early low-bypass turbofan, where more air was pushed through the engine core than the bypass ducts, while the CFM56 is a high-bypass engine. Combined with innovations in aerodynamics and materials, you had an engine that was significantly more efficient than the JT8D. In addition, the CFM56-3 could generate over 40% more thrust than the JT8Ds on the 737.

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Other Modern Examples Of Engine Improvements

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In the 1960s, General Electric developed the TF39 for the Lockheed C-5 Galaxy, the world’s first high-bypass turbofan engine (generally an engine with at least a 4:1 or 5:1 bypass ratio). It then developed the engine into a civilian version called the CF6 and spent decades further developing it. General Electric has since replaced the CF6 with the GEnx in its lineup, which is advertised as burning roughly 15% less fuel than the CF6. This was made possible through the GEnx’s 10:1 bypass ratio (roughly double that of the CF6), more efficient core design, and extensive use of advanced materials.

The second generation of the Boeing 777 was exclusively powered by the General Electric GE90, and for the 777X, General Electric has developed the GE9X. It’s derived from the GE90, but incorporates technologies developed for the GEnx to achieve an advertised 10% fuel savings over the GE90. This is achieved in a number of ways, such as a higher bypass ratio, fewer fan blades, and a revised core. The biggest innovation with the GE9X is the extensive use of ceramic matrix composites (CMC), which allows for higher pressure ratios that dramatically cut fuel burn.

While fuel efficiency is extremely important to airlines, there are other factors that are taken into account as well. Maintenance costs are a key consideration, and these work in tandem with durability. These factors have hampered sales of the Rolls-Royce Trent 1000 (competitor to the GEnx) and Pratt & Whitney PW1000G (competitor to the CFM LEAP). Although they’re otherwise fantastic engines, they simply aren’t lasting as long as advertised, requiring early replacements and more frequent maintenance visits.

Considerations For Aircraft Fuel Burn

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It’s a general rule that the higher the bypass ratio, the more efficient a turbofan engine is. Of course, larger fans come with higher drag, which can diminish the benefits in some cases. In addition, design elements like a larger fan can increase weight, which results in higher fuel burn. As such, weight is one of the most important considerations for engineers when developing a new engine. This can be alleviated through the use of advanced materials like composites or by simplifying certain engine components (e.g., designing fewer fan blades).

Pressure ratios and internal temperatures are extremely important, since higher pressure and temperatures lower fuel burn. However, this also increases stress on the engine components. CFM and General Electric have focused heavily on innovating with the materials of their engines to boost pressure ratios, mainly by incorporating CMCs. The GE9X, which uses more CMCs than any other commercial jet engine, has an overall pressure ratio of 60:1, whereas the GE90-115B has a pressure ratio of 42:1.

Of course, the aircraft itself matters too. A Boeing 787 powered by the General Electric GEnx burns roughly the same amount of fuel as a CF6-powered Boeing 767, despite the differences in engine technology. This is because the 787 is a larger, heavier aircraft. What makes the 787 with the GEnx so compelling is that it burns the same amount of fuel as a 767, while carrying significantly more passengers and cargo with much more range. In essence, some newer aircraft powered by the latest generation of engines aren’t burning less fuel than predecessors, as the planes themselves are larger.

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The Next Step Forward For Engine Technology

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The Pratt & Whitney PW1000G is notable for featuring a gearbox. Located between the fan and the low-pressure core, this enables each component of the engine to turn at its optimal speed, resulting in 16% lower fuel burn than the IAE V2500. While the PW1000G has experienced significant technical challenges, none are related to the gearbox, and the engine’s overall success is indicative that geared turbofans are the future. Rolls-Royce is also developing its own geared turbofan, named the UltraFan, which should be scalable for both narrowbody and widebody applications, and will boast an astounding 15:1 bypass ratio.

General Electric has been pioneering the use of CMCs in turbofan engines, as these materials are lighter, stronger, and can withstand higher temperatures than traditional metal alloys. CMCs can be expensive to obtain, and there remain limitations as to where they can be used. However, this is a field that General Electric is heavily focused on, and advancements in this area of engine design are also inevitable, just like gearboxes.

Airbus and CFM are working together on the CFM RISE, an open rotor engine designed for Airbus’s next single-aisle aircraft, to debut in the 2030s. An open rotor engine essentially removes the engine cowling of a turbofan and operates at much higher speeds than a turboprop, essentially blending the two. By removing the cowling, the CFM RISE promises 20% lower fuel burn compared to current generation engines. Of course, questions remain about the viability of an open-rotor engine, due to technical challenges such as higher noise levels and potentially lower speeds than a turbofan.