The PW1000 PurePower geared turbofan engine (GTF) has gained market attention and acceptance for its innovative gear-drive system, which enables the fan and low spool of the engine to turn at different speeds to optimize the performance in each section. But the technology of this engine, particularly in the all-new core, hasn’t received the same publicity as the gearbox, which is the unique design element. However, as an integral element of the “game-changing” performance Pratt & Whitney touts for this engine, the new engine core behind that gearbox should not be overlooked.
I recently had the opportunity to meet with Chief Engineers Chris Kmetz and Graham Webb at Pratt & Whitney to gain a better understand of the new technologies utilized in the engine beyond simply the gear, which has already been well discussed in the industry.
THE FIRST TRULY SCALABLE CORE
Perhaps the most interesting innovation for this new core is scalability. Scalability of the new core enables Pratt & Whitney to offer a variety of thrust ranges for various applications, which currently include the 70-90 seat Mitsubishi Regional Jet, the 110-130 seat Bombardier CSeries, the 130-180 seat Airbus A320 neo family and the 150-212 seat Irkut MS-21 family, without a redesign, by simply rescaling of specifications using today’s computer generated design tools. Pratt & Whitney could easily scale the PW1000G series to more than 100,000 lbs of thrust for narrow and wide body applications with minor modifications to the existing designs. An aircraft manufacturer can specify a thrust level, and PW can quickly develop an engine optimized to that performance for each application, scaled from the basic design, and mated to an appropriate fan size.
OPTIMIZING OPERATING SPEEDS
In recent years, with larger and larger fans providing high bypass ratios and fuel economy for today’s engines, their internal rotational speeds have slowed. Large fans move higher volumes of air at slower speeds, and to accommodate these larger fans, a compromise between propulsive efficiency and thermal efficiency had to be made, slowing the speed of conventional engines as fans became larger. With the gearbox, that no longer needs to happen as the fan can turn slowly while the core rotates at higher speeds.
Of course, this raises the question of higher rotational speeds on engine wear and maintenance costs — will the increase of the speed of the core have a negative impact? The answer to that question is no, as the increase in speed offered by the GTF brings this engine back to essentially the same rotational speeds that the well proven JT8D low bypass engine operated at in its applications for the Boeing 727, Douglas DC-9 and many other models.
Pratt & Whitney’s experience with engines turning at that speed, combined with advances in materials and design, makes the company confident that its new core can handle higher rotational speeds with excellent reliability and with negligible additional wear and tear to components. But accomplishing that required designing components tailored to higher rotational speeds and optimizing airfoil designs.
Designs for higher speed airfoils must differ from those in conventional engines, as higher speeds generate higher centrifugal loads. This necessitates new designs for outer shrouds, airfoils and blade roots, with particular focus flanges and hubs, to ensure that potential stresses from higher centrifugal force are eliminated. By optimizing speed, PW has been able to increase the specific work output per rotation, without the need to increase the number of stages in each engine section. This results in lower weight, and a shorter engine length, as well as fewer parts for lower maintenance costs.
With the gearbox, the combination of a low speed fan and high speed core results in a higher bypass ratio, which correlates with fuel economy. At 12:1, the PW1000G will have the highest bypass ratio in the industry and offers dramatic improvements in fuel economy over today’s engines.
INNOVATIVE TECHNOLOGIES BEYOND THE GEAR
The PW1000G has a number of elements of innovative technology that are overshadowed by the gear, but equally important in the efficiency of this new engine. Of course, one of the reasons we don’t hear much about them is that PW wants to keep the details proprietary.
Advanced Manufacturing Technology for Sophisticated Shapes
Pratt & Whitney is among the industry leaders in the application of advanced additive manufacturing technology, and has actually manufactured, engine tested and is in process of certifying some of the unique components for the engine by building up parts from powders rather than cutting metal. Using powdered metal, which is sintered using lasers to build up the part layer by layer, PW now forms parts with complex shapes with extreme precision.
Several benefits of advanced additive manufacturing technology include the ability to generate shapes and passages that would not be possible with conventional manufacturing, the ability to manufacture to extremely high tolerances, as well as energy efficiency and lower costs.
State of the Art Cooling Technologies
The one area that remains a trade secret for engine manufacturers are the cooling technologies used to reduce metal temperatures while withstanding higher gas temperatures in the turbines. PW has dual-use technology that was proven in their 5th generation fighter engine design for the military, and has been adapted for the GTF for civil use. This enables the engine to maintain higher pressure ratios and higher inlet temperatures without resulting in higher metal temperatures and wear to metal components. Their proprietary secret is apparently the ability to generate additional cooling with a minimum of cooling air, enabling both weight savings and efficiency.
EXAMINING THE DETAILS
If one starts at the front of the engine and moves backward, many of the features of the PW1000G that have been overshadowed by the gear are worth noting:
The Fan and Fan Case
While the fan isn’t the core, it is worth mentioning. The high technology fan is made of a proprietary hybrid-metallic structure that is light-weight and according to Pratt & Whitney enables thinner leading edge airfoil designs that generate improved aerodynamic performance over carbon fiber blades, which would be more difficult and costly to manufacture to those more complex aerodynamic shapes with four different curves in the blade. As is the case with most new technology engines, the fan is enclosed in a lightweight carbon fiber fan case to reduce engine weight.
FOD Ejection System
PW has a FOD-free core design, with an ejection point for foreign objects in the low pressure compressor that centrifuges particles outside the flow path, and is >99.95% effective. Combined with the larger size of the fan and smaller core opening, the PW1000G series is extremely effective at FOD rejection and is virtually FOD-free.
Everybody is talking about the gear, so we won’t – but will mention its effects in slowing the fan to about 35% of current levels to improve propulsive efficiency while bringing rotational speeds for the core back to the levels of first generation turbofans, which improves thermal efficiency.
Low Pressure Compressor
Three dimensional airfoil shapes are at the heart of the efficiency of the low pressure compressor, which turns at higher rotational velocities to generate increased efficiency. The combination of the low speed fan and high speed LPC provide a neatly 1:1 gain in fuel efficiency for each point of efficiency of this combination, which gains transfer efficiency from the gear in addition to the aerodynamic improvements made possible by new technology airfoils designed using computational fluid dynamics. With higher rotational speeds, the design of airfoils need to be adapted, as while higher tip speeds generate efficiency through less turning required for specific work, those higher speeds generate higher structural loading of the resulting airfoils and discs. PW has carefully designed its blades and rotors to avoid stress peaks and eliminate the need to “beef-up” components with additional weight.
High Pressure Compressor
PW has been able to gain the same level of efficiency from its engine with fewer stages in the all new high pressure compressor due to increased rotational speeds and improved airfoil aerodynamics achieved through three dimensional computational fluid dynamics techniques. Using primarily bladed disks (“blisks”), the advantage of higher rotational speeds eliminates the need for two fewer stages than for conventional direct-drive designs of equal thrust, which dramatically reduces the number of parts. This, all things being equal, should translate to lower maintenance costs for this section of the engine.
Talon-X Lean-Burn Combustor
The Talon-X lean-burn combustor used in the PW1000G is a refined version of the existing PW Talon combustor. This new design burner features a shorter axial length and simpler fuel nozzles coupled with float wall construction. The aerodynamics of the burner was developed using computational fluid dynamics to improve mixing of the fuel and air in a rich burn- quench lean (RQL) approach, thereby dramatically reducing NOx emissions. This design results in a significant margin to CAEP/6 standards of 50%.
High Pressure Turbine
The high pressure turbine takes advantage of the latest dual use technologies Pratt & Whitney has developed for their fifth generation fighter engine, including advanced cooling technologies. These technologies, which remain the equivalent of a “state secret” at engine companies, enable them to reduce the metal temperature of the airfoils while withstanding higher gas temperatures within the engine, which does have a higher pressure ratio and higher inlet temperatures than today’s engines. The key to PW’s advanced technology is that it provides higher cooling with the use of less cooling air, enabling the engine to operate without the penalty in cycle efficiency that typically results when additional cooling air is introduced. The technology includes a combination of airflow designs and specialized coatings, with early tests resulting in lower wear than initially expected.
Low Pressure Turbine
The efficiency of the engine enables PW to use only three stages for its low pressure turbine, half as many stages as a conventional drive engine would need to yield the same efficiency. This high level of work extraction per stage generates a significant savings in parts, and thereby maintenance costs. PW estimates that it will use 46% fewer airfoils in the GTF than today’s competing conventional direct drive engines.
The MTU design of the new high speed LPT for the PW1000G balances a number of design tradeoffs to accommodate higher rotational speeds, low maintenance costs, and improved efficiency. Designs of the blade outer shrouds, airfoils and roots have been optimized to accommodate higher speeds, higher pressures, and higher airflow temperatures, all while reducing the number of parts and thereby expected maintenance costs.
MANAGING TECHNOLOGY RISKS
The numerous innovative technologies used in the PW1000G engine introduce an element of risk, as with any new development. The key is how well those risks are managed and mitigated during the development process. Let’s examine the risks associated with the PW1000G engine:
The Gear: Geared turbofans are not new, and the Honeywell (nee Garrett) TFE-731 engine in business jets is a geared turbofan with high reliability. What is new is the design by PW that enables the use of a gearbox in a larger engine. With only 7 moving parts, and innovative journal bearings, and essentially no maintenance requirements, those risks have largely been mitigated.
Advanced Manufactured Parts: Building up parts using additive manufacturing isn’t new, but large-scale production is. PW is among the industry leaders in this technology, and is pioneering its use in the new engines Do parts built in this way have the same reliability as those developed by cutting metal rather than building it up? If the laws of physics apply, and the processes are as reliable as their computer controls should make them, they should be as reliable, and with more complex shaped airfoils, deliver better economic benefits.
Cooling Technology: The PW1000G will run at higher temperatures than today’s engines and will require advanced cooling technologies. There will be an increase in the use of specialized coatings in this engine, much of which has already been proven in the V2500 Select engine. In addition, PWs proprietary cooling technology is such that it requires less cooling air, and therefore no penalty in terms of cycle efficiency, without the need for exotic materials such as ceramics. While the PW1000G is expected to run hotter than today’s engines, it should be lower in temperature than a comparable performing direct-drive engine, enabling it to utilize existing coated metal rather than more exotic, and more expensive, ceramic materials.
Computational Fluid Dynamics: Engineering tools and technology have advanced in recent years, and have been successfully utilized in many aerospace applications. Pratt & Whitney has integrated these tools into their engineering processes, and these tools have enabled new airfoil designs to optimize airflow and cooling in its fifth generation military fighter engines. That experience helps mitigates risks with the PW1000G.
Design for Maintenance: The concept of designing for ease in maintenance has been espoused for many years, but not well implemented in the industry. Facing a competitive disadvantage in maintenance costs with existing products, PW stressed simplicity in the architecture of the new engine, focusing on minimizing the number of parts and engine stages to reduce, rather than increase, maintenance costs. With 1,500 fewer airfoils in an engine, the cost of those airfoils should be reflected in lower maintenance costs.
THE BOTTOM LINE
PW has designed a new engine that isn’t simply a high technology gearbox added to an existing core, but an all new product across the board. By incorporating new technology throughout the core, and leveraging the gear to provide game changing fuel economy while minimizing maintenance costs, PW feels that it has a “game changing” engine in terms of economics, and future growth potential. While technology risk is always a factor in new engine programs, those risks appear to be well managed in this case. We look forward to first flight on the Bombardier CSeries and seeing whether the engine will match its design promises. From what we’re hearing – so far, so good.
© 2012, Ernest S. Arvai. All rights reserved.