Techy Tuesday - Pratt & Whitney's New Geared Turbofan Jet Engine

Despite getting four newsletters every day covering developments in electronics, radio and computers, I almost came up with nothing to write about tonight.  Instead, thanks (again) to the Arts Mechanical for a story on the Pratt and Whitney PW100G engine.  It's the industry's first high bypass jet engine with a turbofan (those big blades visible in the front of a jet engine) that spins slower than the "engine" that drives it.  It does this by using a gearbox connected to the shaft that gets driven by the combustion taking place closer to the rear of the engine.  The engine was first called the GTF - the Geared Turbo Fan - and probably most surprising to readers used to living on "internet time"; work on it began in 1988.   Almost 30 years and $10 billion dollars. 

To people outside the aircraft business, what may be most remarkable about the engines is that they took almost 30 years to develop. That’s about 15 times as long as the gestation period of an elephant and unimaginably longer than it takes to pop out a smartphone app. Could Pratt have gotten the hardware out faster? Probably. But industrial innovation on the scale of a commercial jet engine is inevitably and invariably a slog—one part inspiration to 99 parts perspiration.

In Pratt’s case, it required the cooperation of hundreds of engineers across the company, a $10 billion investment commitment from management, and, above all, the buy-in of aircraft makers and airlines, which had to be convinced that the engine would be both safe and durable. “It’s the antithesis of a Silicon Valley innovation,” says Alan Epstein, a retired MIT professor who is the company’s vice president for technology and the environment. “The Silicon Valley guys seem to have the attention span of 3-year-olds.”

The PurePower GTF began to take shape in 1988, when Pratt staffers in East Hartford, Conn., including a 28-year-old engineer named Michael McCune, started developing a gizmo to slow the fan—the big rotating blades at the front of the engine that provide most of a jetliner’s propulsion. For planes flying at typical speeds, a slow fan that moves large volumes of air at a moderate velocity is more efficient than a fast-spinning fan that accelerates a smaller volume of air. (The slow fan’s also quieter.)

The conditions for extracting the most efficiency out of the jet engine force vary from front to back (right to left in this picture).  The fan is more efficient moving large volumes of air at slow speeds.  The compressor and combustion are more efficient when moving faster.  The reward for making the engine this way, what they spent 30 years and $10 B chasing, was an engine with 10 to 15% better fuel efficiency but up to 75% quieter.  Quiet matters to aircraft operators (and with them, the entire "food chain" of suppliers) because jet engine noise became a big issue in the early '70s, causing municipalities to charge airlines for the amount of noise they make, and even making the airlines fly different routes to reduce their noise footprint. 

The challenge for McCune and the design team was to make opposite ends of the same shaft move at different rates.  Of course you assume the answer involves a gearbox, but the straitjacket of requirements for this gear box was exceptionally tight.  Gearing hadn’t been tried at the scale of a commercial jetliner because the conventional wisdom was that it would be too heavy and wear out too quickly.  Jet engines, lest you forget, are subject to bird strikes and are tested by throwing chickens (not frozen!) into the engines to ensure they survive. 

The biggest challenge in scaling up was how to keep the gearbox, which is about 20 inches in diameter and weighs about 250 pounds, from being torn apart if there was a shock that wrenched the fan in one direction and the shaft in another. Adding steel for stiffness would make the engine too heavy. To put some give into the system, McCune’s team attached the gearbox rigidly to the fan but somewhat loosely, with bendable metal baffles, to the compressor/turbine shaft and the engine case.

All in all, it's a pretty cool story.  Having just retired from that world, a lot of the things they devote column space to are old hat to me but would probably surprise people from other fields.  I heartily recommend you click through to Arts Mechanical for the long version of the story, including several embedded videos.

 I note that the Wiki article says:

[The engine is] currently selected as the exclusive engine for the Bombardier CSeries, Mitsubishi Regional Jet (MRJ), and Embraer's second generation E-Jets, and as an option on the Irkut MC-21 and Airbus A320neo.

I have personally worked on systems for all of those aircraft except the Irkut MC-21.