How Gears Affect Turbofan Engines?
When breaking into a market dominated by a limited number of companies that make proven products, innovation is necessary. However, in a rigid industry like commercial aviation, where safety is critical, being innovative can also be risky. This was the challenge faced by Pratt & Whitney (P&W) as they attempted to gain a foothold in the 120-180 passenger aircraft engine market. To do this, P&W engineered the PW8000 geared turbofan, the first turbofan engine to use gears for enhanced performance. Adding a gearbox to the conventional turbofan allows engine components to spin at more efficient speeds. In this blog, we will discuss how the addition of gears affects turbofan engines.
It is not difficult to see the ways in which gears benefit turbofan engines. Ideally, the fan of an engine should turn more slowly than the compressors and turbines do. Until the PW8000, this couldn’t happen because the turbines and compressors shared the same shaft. As a result, the turbine and compressors would turn below their ideal speed, while the fan would turn faster than its ideal speed. Gear reducers would offer a solution to this, but these have largely been avoided due to their potential to increase the risk of costly maintenance and failures. However, years of testing have allowed engineers to design an engine that offers improvements to efficiency while still remaining reliable.
The 17-inch gearbox of the PW8000 is mounted just behind the fan, allowing engineers to optimize each component’s speed. The low-pressure turbine, which runs at approximately 9,000 rpm, provides input to the gearbox. The fan runs at around 3,000 rpm, allowing the low-pressure turbine and compressor to do more work while still running more quietly than if all components were running at the same speed. Though this configuration greatly improves efficiency, it is still the subject of some skepticism.
The primary concern with gears is their reliability. This is due to the constant friction and stress exerted on the gear teeth. To solve this, P&W engineers aimed to maximize smoothness while minimizing stress. The PW8000 gearbox also uses self-centering technology to eliminate misalignment. Further changes include tweaks to the gearbox bearings and bearings themselves, which P&W claim result in gearbox efficiency of greater than 99%. This allows the gearbox to produce less than half of the heat engineers expected, and means that the 32,000 horsepower gearbox is only slightly larger than a car’s transmission. To put that in perspective, if an automobile transmission had a similar power to weight ratio, it would weigh just around two pounds.
That said, the addition of gearboxes to jet engines is not without its drawbacks. For example, they bring added weight. Fortunately, because the gearbox allows the engine to run more efficiently, engineers can save weight in the turbines, compressors, and other areas. For example, the PW8000 has 13 stages instead of the 20 that conventional turbofans have, as well as 52% fewer compressor and turbine airfoils. This means there are fewer parts to repair or replace, which leads to a 10% reduction of operating costs and a 30% reduction of maintenance costs. P&W estimates suggest that these factors combine to result in an average annual savings of $600,000 for 120-180 passenger aircraft.
Prior to rolling out, P&W’s gear system underwent more than 1,000 hours of rigorous component testing in addition to over 1,000 hours of full-scale testing. The PW8000 was expected for delivery in 2002, but was ultimately shelved. It was rebranded as the PW1000G and eventually made its first run in 2007. The aircraft that use the PW1000G include the Airbus A220, the Airbus A320neo family, the Embraer E-Jets E2, the Irkut MC-21, and the Mitsubishi SpaceJet.
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