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Boeing Airplane Engines
Boeing builds airplanes, but companies such as CFM, GE Aerospace and Rolls-Royce build their engines. Here is how the two complex machines are engineered to work together.
Look at a Boeing 737 MAX, 787 Dreamliner or 777X and it is easy to think of the engines as simply another Boeing component.
They are not.
Boeing designs and integrates the airplane, but the turbofan engines hanging under its wings are produced by specialist engine manufacturers. The 737 MAX uses the CFM International LEAP-1B. The 787 Dreamliner can be ordered with either GE Aerospace's GEnx-1B or Rolls-Royce's Trent 1000. The 777X uses GE Aerospace's GE9X.
That split is the starting point for understanding modern airliner design. An engine and an airframe are separate products, but neither can be designed in isolation.
A Jet Engine Is Really Two Airflows
Modern Boeing airliners use high-bypass turbofan engines.
At the front, a large fan pulls in an enormous mass of air. Some of that air enters the engine core. Compressors raise its pressure, fuel is added and burned in the combustor, and hot gas expands through turbine stages. Those turbines extract energy to drive the compressors and fan before the remaining gas leaves through the exhaust.
But much of the air never enters the combustor.
It passes around the core through the bypass duct.
NASA's explanation of turbofan engines shows why this matters: thrust comes from both the core flow and the air accelerated by the fan. High-bypass engines can move a very large mass of air without requiring the exhaust to be accelerated as violently as in older turbojet-style designs.
This is one reason modern airliner engines have enormous front fans.
GE Aerospace lists a 111.1-inch fan for the GEnx-1B used on the 787. The GE9X for the 777X has a 134-inch front fan.
The LEAP-1B used on the much smaller 737 MAX has a 69-inch fan.
Boeing Does Not Simply Choose an Engine at the End
The engine affects the aircraft around it.
Its mass creates structural loads that must pass through the engine mount and pylon into the wing. The nacelle surrounding the engine affects aerodynamic drag and noise. Fan diameter influences ground clearance.
The airplane must also connect the engine to fuel, electrical, control, monitoring and fire-protection systems.
The 737 MAX is a useful example of this integration.
Boeing says the LEAP-1B was designed specifically for the 737 MAX and describes the nacelle as being integrated with the wing to improve aerodynamic efficiency.
CFM International identifies the LEAP-1B as the sole-source engine for the MAX family.
That does not mean Boeing designed the LEAP's compressor, combustor or turbines. CFM International—a company jointly owned by GE Aerospace and Safran Aircraft Engines—developed the engine.
Boeing's task is to integrate that propulsion system into the airplane.
The 787 Shows a Different Model
The 787 Dreamliner gives airlines a choice between two engine families: the GEnx-1B from GE Aerospace and the Trent 1000 from Rolls-Royce.
The two engines do not even use identical internal architectures.
GE describes the GEnx as a counter-rotating two-spool engine. Rolls-Royce uses its characteristic three-shaft architecture for the Trent 1000.
Yet both had to meet the installation, thrust, electrical, aerodynamic and operating requirements of the same Boeing aircraft.
The Dreamliner also pushed engine-aircraft integration beyond propulsion.
Boeing describes the 787 as using more electric systems than earlier aircraft. Rolls-Royce explains that the 787 does not take conventional pneumatic bleed air from the Trent 1000 for aircraft systems; instead, substantial electrical power is extracted from the engine.
GE meanwhile uses carbon-fibre composite fan blades and a composite fan case on the GEnx, reducing weight in one of the engine's largest structures.
So an airline choosing between a GEnx-powered and Trent-powered 787 is choosing between two quite different engines that have both been engineered to work with the same Boeing airframe.
The 777X Takes the Opposite Approach
The 777X has no competing engine option.
Boeing lists the GE9X from GE Aerospace across the current 777X family.
Here the engine itself becomes part of the aircraft's extraordinary scale.
GE lists its fan at 134 inches in diameter and says the large fan helps achieve the high bypass ratio needed for efficiency. The engine also uses composite fan blades, a composite fan case and ceramic-matrix-composite materials in high-temperature sections.
Boeing combines that propulsion system with the 777X's large composite wing and folding wingtips.
The relationship illustrates why saying “Boeing built the GE9X” is inaccurate.
GE Aerospace designs and manufactures the engine. Boeing designs the airplane around a propulsion installation that uses it.
Engine Design Changes the Plane You See
Many visible features of a modern jet are consequences of propulsion engineering.
The diameter of the nacelle, its distance from the runway, the pylon connecting it to the wing and the shape of the exhaust area all reflect decisions made where engine engineering meets aircraft engineering.
Even noise-reduction features can cross that boundary.
Boeing says the 737 MAX's smaller noise footprint relative to the 737-800 is helped by both the higher-bypass LEAP-1B and the chevron design around the nacelle. Similar serrated edges can be seen on the 787's propulsion installation.
What looks from outside like simply “an engine hanging under a wing” is therefore the interface between two extremely complex machines.
A useful mental model for the rest of this series is:
engine manufacturer designs the turbofan → Boeing integrates it with the airframe → the engine and aircraft are tested as a combined propulsion installation → the complete aircraft must satisfy certification requirements
The next article will zoom in on one of the most interesting examples of that relationship: the CFM LEAP-1B and Boeing 737 MAX, where fan diameter, ground clearance, nacelle shape and engine placement all became important parts of the airplane's design.
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