This is Part 4 of an in-depth investigative report by Aviation Reporter and Author of aviation books Thomas E. Gardner. In Part 4 Mr. Gardner discusses how a turbofan operates, the evolving turbofan technology, and its upgrade culture. Understanding how a turbofan operates is important, to be able to understand the philosophy as to which turbofan engine was chosen for the 737 MAX. You can read Part 1 of the story HERE, Part 2 HERE, and Part 3 HERE.
Turbofan Design Overview
In order to understand why turbofan engines are so special, one needs to gain a better understanding of how a turbofan operates.
A turbofan or “fanjet” is simply a fan placed in front of the engine nacelle (a nacelle is the engine cover/housing).
The fan is powered by an axial-flow turbojet (in the core of the engine).
The most distinctive physical characteristic of a turbofan is its diameter.
The larger the engine diameter, the bigger the fan is. This is due to its unique design operating characteristic.
Jet engine bypass ratios
This relates to the thrust achieved through the bypass ratio the engine is designed for. There are principally two types:
They are high bypass ratio and low bypass ratio turbofans.
The bypass ratio is important for the type of work and performance the engine is designed for, for example, airliner performance versus military fighter performance.
Bypass ratio is defined by the mass flow of a) air bypassing the engine core, divided by the mass flow of b) air passing through the core engine itself. (Figure 1).
These two combined flow ratios not only establish the ratio but provide the total thrust (power) output from the engine itself. This ratio is reasonable for determining the overall operational efficiency of the engine regarding its power and fuel burn.
Low bypass turbofan engines (where most of the intake air goes into the combustible engine core) are used primarily for military applications such as fighters. This is due to the bulk of the thrust being produced in the core engine and facilitated by the higher thrust required for fighter operation.
High Bypass Ratio engine design is important for cost-conscientious airlines
However, the higher the bypass ratio, the more efficient these engines become, providing lower fuel burn (fuel cost savings) in the subsonic speed range of flight.
This high bypass ratio evolving turbofan technology upgrade became an important part of the upgrade of the Boeing 737 to the MAX model.
Animation of how a jet engine works
A) Low-pressure spool. B) High-ressure spool. C) Stationary components.
- Low-pressure compressor
- High-pressure compressor
- Combustion chamber
- High-pressure turbine
- Low-pressure turbine
- Core nozzle
- Fan nozzle
The kinetic energy (energy which a body possesses by virtue of being in motion) expended through the exhaust nozzle from the core engine core is carefully balanced from the kinetic energy from the fan’s mass flow through the bypass ducts surrounding that engine core.
The more energy extracted from the engines’ cores’ turbine in rotating the fan produces more thrust, especially at subsonic flight speed, increasing the overall mechanical/aerodynamic efficiency from the rotating fan blades further reducing the fuel used.
This is further facilitated by the available thrust growth found by increasing the engine core’s power. There are two accepted routes available. They are:
- Hot route. This is achieved by an increase in the high-pressure turbine inlet temperature.
- Cold route. This is accomplished by increasing the engine core’s mass flow.
Pratt & Whitney and GE Safron
Pratt & Whitney and GE Safron (CFM) are developing larger and wider engines to increase thrust capability.
Pratt & Whitney and G.E. Aviation in conjunction with Safron (formerly Snecma) have developed over this last decade the PW 1000 G geared and the CFM Leap-1 (Leading Edge Aviation Propulsion) turbofans.
The PW 1000 engine
The PW 1000 G had significant teething problems requiring a bit of tweaking on Pratt & Whitney’s behalf.
The planetary reduction gear system behind the PW 1000 G design was chosen in order for the low-pressure turbine located at the rear of the engine core in order to be in sync with the fan.
The gear reduction 3:1 employed behind the fan turbine 1/3 as fast as the low-pressure turbine. This ensures maximum undisturbed airflow around the core engine and its total thrust output as well.
The GE CFM engine
However, G.E./Safron (CFM) did not use a geared fan approach to achieve these same results. CFM used new advances in aerodynamic modeling through computational fluid dynamics (CFD), as well as composite fan blade materials.
Due to the special materials used in their fabrication, this allows for the fan blades to flex or untwist at higher rotational speeds. This flexing, untwisting action enables the blades angle of attack to change, admitting a significant increase in airflow around the engine core. This directly reduces the overall engine’s complexity.
Both engine types
In any event, by gearing or enhancing the fan blade design and its fabrication both control the mix of subsonic, transonic and supersonic flows acting on a single fan/gas compressor blade found in these newest turbofan designs.
Photo of the GFM Leap-X, or -1 engine currently used on the Airbus A320 N.E.O.
Engines placed on different B737 models
Here we can see the progression of engine sizes through continued upgrading starting from the original 737 to 737 Max. It is clear that the Boeing 737 could not utilize its current turbofans:
How Boeing lost its way
Here is Bloomberg’s take on Boeing:
This was an explanation of how a turbofan operates and its evolving turbofan technology upgrades. But, was the engine chosen for the 737 MAX correct for the wings and the frame of the aging 737 design? The reporter will continue revealing more explosive facts about the 737 MAX in the next article.
Featured Image: Boeing 737 MAX, Farnborough Airshow. Wikimedia.
- Boeing’s Re-Engining Worry by Thomas C. Hayes, Nov. 27, 1981, New York Times.
- FAA Form 41.
- AVAC Database.
Story: ©2020 Thomas E. Gardner. Renowned Aviation Journalist & Author Thomas E. Gardner has spent months on his investigative report. The author has based his story on true facts that have been learned in regard to the 737 Max-8 engineering disaster during his in-depth investigation process of the article.
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