Airbus To Support Pratt & Whitney In Future Engine Technology Effort

Airbus has agreed to help CFM International with an engine demonstration program that could lead to a Leap replacement to power future single-aisle aircraft. Airbus is also embarking on a similar project with Pratt & Whitney to succeed the PW1000 geared turbofan family in the 2030s.

The airframer’s early involvement marks the beginning of new relationships being forged between airframers and engine-makers. New propulsion systems might require more integration with the aircraft, notably through a hybrid-electric architecture—the integration of gas turbines and electrical motors.

• Companies are involved in MTU-led water-enhanced turbofan project
• Airbus is partnering with Renault on a hybrid-electric layout

Airbus and Boeing are years away from defining replacements for their A320neo and 737 MAX families, respectively, but engine-makers are already preparing the technologies they may use for them. An engine has a long development cycle, and when an airframer expresses a need, manufacturers prefer to pitch relatively mature solutions. That process entails the launch of engine research and technology programs, including demonstrators, well ahead of new aircraft programs.

Airbus and Boeing are definitely expected to require drastic cuts in fuel consumption. Current technologies cannot meet such specifications, so engine-makers are considering new ways to improve efficiency. That is what CFM has been doing with its Revolutionary Innovation for Sustainable Engines (RISE) program, to be supported by Airbus in flight testing.

Pratt & Whitney’s answer to RISE is beginning to emerge, as an MTU Aero Engines partner in the EU-funded Sustainable Water-Injecting Turbofan Comprising Hybrid-Electric (Switch) program. Airbus is a partner, too. “It is not the time for competition yet; it is the time for assessment,” says Karim Mokaddem, Airbus’ head of electrification.

MTU is leading Switch as part of the EU’s Clean Aviation public-private partnership. Switch aims to combine the benefits of MTU’s water-enhanced engine concept with a hybrid-electric architecture.

MTU’s other major partners are Collins Aerospace and GKN Aerospace. Together, they are pursuing a 25% reduction in CO₂ emissions, an 80% cut in nitrogen oxide (NOx) emissions and an unspecified decrease in contrail-inducing emissions.

“This project will enable us to advance several key technologies on our road map to further extend the efficiency of the geared turbofan [GTF] engine architecture,” says Geoff Hunt, Pratt’s senior vice president of engineering and technology.

A water-enhanced turbofan (WET) engine adds a condenser to collect water from the GTF exhaust gas and a heat exchanger to vaporize that water into steam. The steam is then injected into the combustor, improving fuel efficiency. The temperature profile eliminates most of the hot spots where NOx forms, and most particles are washed out instead of forming nuclei in the contrail process.

The large heat exchanger, however, has an unfavorable effect on transient regimes, making the engine less responsive. That is what the hybrid-electric architecture can counteract, says Claus Riegler, MTU’s senior vice president for technology and engineering advanced programs.

“WET technology is relevant for the improvement it brings in the cruise phase’s climate impact, while hybrid-electric is for better local air quality at airports,” he adds.

For Pratt’s parallel hybrid propulsion system, Collins plans to supply a 500-kW motor generator on the high-pressure spool and a 1-megawatt motor generator on the low-pressure one. They would be useful for electric taxiing and for a boost at takeoff as well as in other transient phases, says Pratt Chief Sustainability Officer Graham Webb.

Phase 1 of the program, in 2023-25, is fully funded and is planned to include various assessments. A hybrid-electric GTF engine is set to be tested on the ground, and WET technology to be evaluated at the component and subsystem levels in a laboratory. Design activity would take place around the combination of the two concepts.

In addition, aircraft integration is set to be studied. “There will be an impact on the architecture,” Mokaddem says. To start with, batteries are more likely to be located in the fuselage than in the nacelle, and the electrical network’s voltage would change. In addition, global energy management is needed to ensure a benefit at the aircraft level.

The program begins at relatively low technology readiness levels (TRL). WET is hoped to stand at TRL 2.5-3 by year-end and reach TRL 4 at the end of Phase 1 in 2025, Riegler says. If the program follows on with a 2025-30 Phase 2, possibly including flight tests, WET may hit TRL 6 in 2029-30, he adds. OEMs usually prefer TRL 6 as the minimum technology basis from which to launch an engine program.

Meanwhile, the hybrid-electric propulsion system is starting at TRL 3, and Pratt is targeting TRL 5 at the 2025 end of Phase 1, according to Webb. Simultaneously, integration will be at TRL 4-5, Mokaddem says.

Among the challenges are the engine’s weight and size. Riegler expects a 50% weight penalty. While the fan diameter will be unaffected, length will increase to accommodate the heat exchanger, he says. Drag is therefore anticipated to increase, and the plan for a 25% improvement factors in those drawbacks.

Switch technologies are planned to be compatible with conventional Jet A1 fuel and sustainable aviation fuels and will be evaluated for use with hydrogen.

In a hybrid-electric architecture, batteries are seen as the weak link that drags down efficiency. To address that, Airbus has signed a partnership agreement with car manufacturer Renault to improve the technology. As with Switch, CFM’s RISE demonstrator is planned to include hybrid-electric systems, but Pratt is putting a greater emphasis on their use.

A hybrid-electric configuration alone would lead to a 5% improvement in fuel consumption, Mokaddem says, despite a 1-metric-ton weight penalty. But to meet the 5% target, specific energy—the energy-to-weight ratio—is still too low. “A current-technology 200-Wh battery weighing 1 kg [(2.2 lb.)] would only contain enough energy to carry itself on a 1,000-km [(540-nm)] flight,” he says. He sees a need for a twofold improvement.

Meanwhile, Renault has been advancing the battery technology, especially on the Zoe, a small five-seat car that has been mass-produced for 10 years. Over that period, the improvement in specific energy has been twofold, says Patrick Bastard, Renault’s vice president for research. Progress has come from chemistry and battery pack design, he explains.

Solid-state batteries might be the holy grail, potentially twice as good as the best lithium-ion batteries. A ceramic electrolyte could replace the liquid electrolyte, and a lithium metal anode would supplant the graphite anode. “Today, solid-state batteries are working in a laboratory environment, but scaling up is difficult,” Bastard says. “It will happen, but not until 2030.”

Airbus and Renault are creating joint teams. They plan to work together on cell-to-pack integration—notably on materials and connectors—and on the battery management system’s efficiency and safety. Life-cycle assessment is set to be a focus, too. Renault has transformed one of its factories to prepare batteries for reuse or recycling.

It took almost two years and several design iterations for Airbus to be convinced of the benefit of a hybrid-electric architecture. At first, adding technology bricks was not creating an efficient system, Mokaddem recalls. Nonpropulsion needs proved to be the right starting point, even though engines must eventually be part of the layout. Airbus at some point discovered that engine manufacturers were studying hybrid-electric, too, under a similar approach. The companies’ teams easily converged.

In a hybrid-electric architecture, an engine’s transient regime is helped by electric motors, says Frank Haselbach, Airbus’ senior vice president for propulsion engineering. The motors could contribute at takeoff and brief climbs while in cruise. Moreover, the high-pressure compressor can be designed with a smaller operability margin, increasing its efficiency. Electric motors can accelerate the high-pressure spool if needed, a task that requires a higher margin if performed thermodynamically, Haselbach explains.

Airbus’ target is 800 volts, a level that is an option for cars, Bastard says, which usually use 400 volts. Increasing the voltage improves the electric system’s efficiency but comes with a higher risk of arcing. “We explored up to 3,000 volts, but that was too much. We want to stick around 800 volts,” Mokaddem says.

A hybrid-electric commercial aircraft could enter into service in 2030-35, he adds. A step in that direction is set to be implementation of the Ecopulse demonstration program in cooperation with Daher and Safran. That program is planned to use a 300-kW, 800-volt system as part of a distributed propulsion arrangement on a modified TBM 900 turboprop single.