At the recent Pacific Northwest Aerospace Alliance 2016 sub-supplier conference in Seattle, GE, Rolls-Royce and Pratt & Whitney all talked about their latest engine projects and the technology development that was critical to their success.
The engines they talked about, the GE9x, Rolls-Royce Advance and Pratt & Whitney’s Geared Turbofan, can all be characterised as the best of breed for their intended use but they could not be more different in how their level of excellence is achieved.
It made for interesting listening. Here’s the gist of what was told.
Jason Brewer, General Manager, GE Aviation Technical Marketing, described the work and technologies behind GE Aviation’s new engine for the Boeing 777X, the GE9X. The GE9X builds on the architecture from the successful GE90, employing a range of new technologies to achieve efficiencies that hasn’t been attained before.
The GE90 in its latest incarnation, the ones for the 777-300ER and -200LR, is an efficient Turbofan. The GE9X increases this efficiency level a further 10%. To get there, several industry firsts were needed. The GE9X will be the first engine to pass an Overall Pressure Ratio of 60:1.
To achieve that level, GE had to develop a high pressure compressor of 27:1 pressure ratio achieved over 11 stages. An average stage pressure ratio of 1.35 is an industry first for a turbofan. Brewer described the extensive work at their compressor testing facility in Massa, Italy, where the correct functioning of this complex compressor was verified.
To drive this compressor at take-off power, a turbine of 130,000 hp is needed. This requires a two stage turbine with very high temperatures and which employs advanced cooling techniques. To minimise the cooling needed, Ceramic Matrix Composites (CMC) is employed in the two high pressure turbines. This material can withstand higher temperatures than Nickel Alloys and therefore require less cooling air to be zapped from the compressor.
The high pressure and temperatures in the engine’s core generates high levels of NOx in the combustor if nothing is done to stop these from developing. GE has developed a two-stage burner called TAPS to control this process. The mixture of air and fuel is done before the combustion, in the fuel nozzle, to allow a flame that generates low NOx levels.
This requires a complicated build of the nozzle, with many parts. For the GE9X, the complex form is achieved in one step by means of 3D printing the nozzles. To lower the need for cooling of the combustor, CMC is used in the combustor’s liners.
GE9X will also be the first engine to have a fan diameter over 130 inches. The final design went to 133.5 inches. To control the weight of such a massive fan, Carbon Fiber Reinforced Plastic (CFRP) is used both for the fan blades and the fan case. To let the maximum of air through the fan, it’s made up of only 18 blades, down from 22 for the GE90.
Rolls-Royce’s Richard Goodhead, SVP, Customer Strategy & Marketing described the work Rolls-Royce is dong for the Advance project. Advance is a technology demonstrator engine which introduces the biggest changes ever made to the company’s three shaft engines.
The idea is to change the work division in the engine. In RB211 and Trent engines to date, a large part of the compression work has been done by the intermediate pressure (IP) spool. Part of this work is now shifted to the High Pressure (HP) spool in Advance.
The rationale for doing this is twofold:
The Advance would be the first engine to employ Rolls-Royce’s CFRP fan and fan case technology called CTI. CTI used resin infusion into a 3D woven carbon fibre structure (similar to the LEAP) instead of the hand layup method that GE is now tuning for the GE9X.
To prove the Advance’s new core, Rolls-Royce will have a demonstrator engine running on a test stand summer 2016. This engine will be made with existing components outside the core and be in the 75,000 lbf class. The front with the fan will come from the Trent XWB and the rear, including the low pressure turbine, will come from the Trent 1000. Between these parts will be the new Advance core where the principle challenges are for the new design.
The Advance core will further develop the pioneering work that Rolls-Royce has done in controlling the cooling flows for the engine’s hot sections. The company was first with throttled cooling of the high pressure turbines and is still the only one employing a fluidistor control valve for the throttling (a valve where the main airstream is control by a weaker pilot airstream). Rolls-Royce is also planning to employ CMC in the demonstrator, the first applications being new types of seals and hybrid bearings.
The subsequent demonstrator program for Rolls-Royce will be the Ultrafan program which will also include flying engines. A curiosum is that Rolls-Royce has decided to call its Ultrafan two spool engine’s low spool the “IP spool”, for intermediate pressure spool. They thereby emphasize that it’s a fast running spool with elevated pressure gains over the stages. Everyone else in the industry call this the low pressure spool.
Rolls-Royce might even classify the geared fan stub shaft as the engine’s third low speed shaft. It has tricked many to classify the Ultrafan as a three shaft engine. This would only be true if one classify the slow running fan shaft, from the gearbox forward, as a separate shaft. But then every geared turbofan in the world is a tri-shaft engine.
Pratt & Whitney
Geared Turbofans turns us to Pratt & Whitney (PW) and the presentation from Senior Marketing Manager Paul Burke.
Burke presented the advantages that a geared architecture gives an engine manufacturer. The principle has been known for a long time: the fan spins at its ideal speed (around 4,000-5,000 RPM), the low spool at its optimal speed (around 12,000-15,000 RPM) and finally the high spool at its ideal speed (more than 20,000 RPM).
By putting a 3:1 gearbox between the fan and the low speed spool, each can be optimized without needing compromises for its ideal blade speeds. The result is an engine which easily can be given a very low fan speed without compromising the booster. Together with a low fan pressure ratio, this allows the air to be accelerated to less over-speed through the engine. The low air speed has to be compensated with a large fan, giving a high By Pass Ratio (BPR). The result is higher propulsive efficiency for the engine.
At the same time, the compressors and turbines which are needed can be constructed with a fewer number of stages and blades, as each stage has a higher effectiveness (it spins at optimal speed). This has positive effects on weight, build and maintenance costs.
The drawback is that the engine needs a bulletproof 30,000 hp gearbox, one that should never fail and need no maintenance. It took Pratt & Whitney 20 years to develop and prove this piece of the engine. The gearbox adds cost and weight to the engine, but Pratt & Whitney claims that the sums still add to lower weight and costs than a direct drive design.
A bonus of the design principle that has not been talked about so much is the fact that the gearbox shifts work from the high spool to the cooler low spool. With a faster spinning booster compressor (the one behind the fan on a direct drive engine and behind the gearbox on a geared engine), it’s part of the Overall Pressure Ratio (OPR) increases. This means the high pressure compressor has less work to do which alleviates the stressed high pressure turbine. Work is therefore shifted to the low pressure turbine where it’s easier to accommodate.
The geared architecture is nothing new in the Turbofan industry. Its advantages and challenges have been known for decades (as I written before, my jet trainer had a geared turbofan engine and that was 1970!). Pratt & Whitney should be credited with taking the principle to new thrust levels, for making it known to a broader public and for perfecting the technology. PW has designed their PW1100G gearbox as a lifetime item with no scheduled maintenance (other than changing oil).