CFM International at 50: Pushing the boundaries - innovative technologies

Since its genesis 50 years ago, CFM International’s strength has been its ability to pool the talents and proprietary technologies of engineering teams on both sides of the Atlantic to create two enduring families of engines, the CFM56 and its LEAP successor, that have delivered unparalleled performances in terms of efficiency, durability and reliability.

Neither General Electric nor Snecma could have come up with the CFM56 on their own – although both had mulled their own foray into the mid-thrust market before the fateful meeting of their leaders at the 1971 Paris air show. The breakthrough came by combining their competencies – engine core know-how in the case of the Americans, and, for the French, expertise in the fan, gearbox and low-pressure (LP) elements.

Although the precise division of responsibilities between GE Aerospace and Safran Aircraft Engines has switched slightly over the years, the principle of merging the skills and resources of the two partner companies to create industry-leading products has remained in place. It continues with RISE, for which the two companies have completed some 250 tests.

The LEAP engine now makes up most production, but with almost 34,000 units delivered, its predecessor dominates the in-service fleet – some CFM56s will likely remain flying through the 2040s. More than 42 years after it entered service, it is easy to overlook how transformative the original design was, with its high bypass ratio, nine-stage high-pressure (HP) compressor, and dovetailed fan blades that could be replaced on wing.

Subsequent improvements to the CFM56 through the Tech Insertion programme in 2006 included changes to the HP compressor, HP turbine and LP turbine nozzle aimed at delivering a 1% improvement in fuel efficiency. These technologies were developed through the TECH56 initiative. The TECH56 programme also advanced a new type of combustion technology, for narrowbody engines, originally developed for the widebody GEnx. The Twin Annular Premixing Swirler, or TAPS, was designed to lower nitrogen oxide and particulate emissions.

The LEAP engine, unveiled in 2008, delivered a step-change in performance, with its optimized thermodynamic design, higher bypass and compression ratios, and greater use of lightweight materials. It was the first engine to feature one-piece woven carbonfibre fan blades and fan case, which helped reduce the engine weight by 227kg (500lb). Its 3D printed fuel nozzles are up to 25% lighter than traditional counterparts. A debris rejection feature – built into the swan-neck of the flow path into the compressor inlet – comprises variable-bypass vane doors designed to eject particles into the bypass duct. The TAPS in the LEAP engine combustor delivers best in class emissions.

The HP turbine, based on a scaled-down design of that used on the GE Aerospace GE90 and GEnx, incorporates a reconfigured cooling system that provides a more uniform flow within the blade structure. The turbine module also includes ceramic matrix composite turbine shrouds, and each stage has fewer, more efficient blades than on the CFM56.

The RISE project, announced in 2021, aims to push the boundaries of innovation further by developing technologies that could lead to a 20%+ reduction in carbon emissions compared with today’s engines. Thousands of engineers with both CFM companies, as well as supply chain and research partners, are working on a plethora of studies that are rapidly moving from component-level evaluations to trials of complete modules.

At July’s Farnborough air show, CFM said it had completed more than 250 RISE tests as “technologies continue to mature”. These include more than 200h of wind-tunnel testing by Safran Aircraft Engines and French aerospace research agency ONERA using a 1:5 scale model of an Open Fan engine, as well as high-speed, LP turbine tests using advanced turbine blades.

The Open Fan propulsive system is a central element of the RISE programme, and the feature that garners most attention, perhaps because it is the most visible. Fans suck air into a turbojet engine. In principle, the larger the fan, the greater the bypass ratio – the proportion of air pushed around the core compared with the amount moving through the core. Today’s turbofans have bypass ratios of up to 12:1.

However, fans in a conventional nacelle-enclosed engine have become about as large as is practical. Any bigger and efficiency gains from a higher bypass ratio would be cancelled out by weight and drag penalties caused by larger nacelles and support structures. An Open Fan, however, has no nacelle so can, in theory, be much larger. Bypass ratios could be between 45:1 and 60:1.

Open Fans are not new. In the 1970s, spiralling oil prices prompted engine manufacturers and NASA to research so-called “propfans”. Later, GE developed a prototype called the GE36 Unducted Fan, which the company flight tested on a Boeing 727 and McDonnell Douglas MD-80. However, efforts stalled as oil prices stabilised and over worries about noise and safety if there was an uncontained engine failure.

Despite that, lessons learned from the GE36 remained in the GE Aerospace collective memory, and engineers working on the RISE programme are convinced technological advances could today counter these concerns. While the experimental GE36 had the complex architecture of two counter-rotating fans, the structure being developed under RISE is a single fan with variable pitch blades, positioned ahead of stationary outlet guide vanes.

In other areas of the programme, GE Aerospace is working with NASA to advance next-generation compressor, combustor, and HP turbine (HPT) technologies through a compact core demonstration. After a first test run of next-generation HPT blades and nozzles on a demonstrator engine, the company has started a second HPT airfoil test campaign, focusing on another crucial element of the RISE programme: durability.

For RISE, GE Aerospace is carrying out a range of evaluations using its 20,000lb (86kN)-thrust Passport engine for the Bombardier Global 7500 business jet, convinced that the combination of an Open Fan with a high-pressure section that burns hotter and at higher pressures could result in an engine with a similar-sized core as the Passport able to generate 30,000lb of thrust. That is enough to power a narrowbody.

As part of NASA’s Hybrid Thermally Efficient Core (HyTEC) programme, GE Aerospace is also looking at the potential of hybrid electrification by integrating electric motor generators on a Passport engine. The company has already tested the motor generators separately and is preparing to carry out ground tests on the adapted powerplant.

By stretching the technological frontiers in so many areas, the CFM partners are hoping that some or all of these advances could end up as the building blocks of a truly revolutionary engine, ready in time for the introduction by Airbus and Boeing of clean-sheet narrowbodies to replace the A320 and 737 families from the mid-2030s.