CFM International, the engine manufacturing joint venture between General Electric and Snecma of France, is forging ahead with a range of advanced engine studies as part of its leading edge aviation propulsion (LEAP56) program.

“LEAP56 lays the foundations for game-changing architectures, including what could be a breakthrough counter-rotating fan and an open-rotor design,” said CFM executive vice president Francois Planaud. “Whether LEAP56 leads to an all-new engine or improvement of existing engines is still an open question,” he added.

The company is starting a “voice of customer” effort to evaluate views on the proposed new propulsion technology. At the same time, it is also paying close attention to pressures to improve environmental performance.

CFM has been trawling through design and test data of the unducted fan studies carried out by GE and Snecma in the mid-1980s. “The work done then is still very useful. We had clearly identified the fuel burn benefits, which had been measured on flight tests…We still think that if we combine it with current technology and that configuration, we should get a 10- to 15-percent improved fuel burn,” said Planaud. However, for the more radical options, he admitted that to accommodate rear-mounted engines, the airframers would also have to produce the next generation of narrowbodies, which are expected to enter service in around 2015.

Another key aspect of CFM’s LEAP56 effort is the development of a second-generation twin-annular pre-swirl (TAPS II) combustor. It is also aiming to raise by-pass ratio from five (in its current CFM56 engines) to around nine, while also focusing heavily on weight savings of up to 400 pounds per engine. A composite fan produced using resin-transfer molding would be central to this design to lower weight while also improving durability and ease of maintenance. “Low engine weight has a snowball effect in terms of lower fuel burn of the aircraft,” explained Planaud.

Environmental Pressure
Francis Couillard, CFM general manager of environmental affairs, told a pre-Paris Air Show press briefing that CFM is also looking into areas such as alternative fuels, although temperature and viscosity requirements rule out alternatives such as biodiesel and fatty-acid methyl ester. This has prompted the company to look at synthetic kerosene, which brings the added advantages of reducing sulphur and hazardous air pollutant emissions to meet the stringent jet-A1 fuel specifications.

Another less mature fuel technology is biomass, but this is a longer term possibility subject to a protracted period of verification testing, and like all alternative fuels, dependent on a wholesale reconfiguration of the worldwide support infrastructure.
The biggest challenge for engine designers, argued Couillard, is how to strike a balance between carbon dioxide (CO2) and oxides from nitrogen (NOx) emissions, based on the fact that increasing engine temperatures leads to reduced carbon emissions but increases nitrous oxides, which is where focusing on more efficient mixing and burning of fuel in the combustor comes in.

The pressure for such changes is coming regionally–for example, from the European Union–and internationally, or both (as regions look to implement international obligations). NOx from aircraft engines is regulated by ICAO’s Committee on Aviation Environmental Protection (CAEP), whose latest requirements (CAEP6) call for a 12-percent reduction in NOx levels by 2008 (compared to a 2004 baseline).

Meanwhile in Europe, CFM is working to meet the goals of the Advisory Council for Aeronautics Research in Europe. This body has set ambitious targets for its “Vision 2020” of a 50-percent reduction in fuel burn, 50-percent reduction of CO2 (as a consequence), an 80-percent reduction in NOx and a 50-percent cut in noise levels, which Couillard admits could be difficult with open-rotor designs.

CFM also participates in various other research efforts in Europe, many under the auspices of the European Commission with various acronyms such as MAESTRO, SILENCER, VITAL and NACRE supplementing its efforts through LEAP56. It has even looked at a slanted engine inlet to deflect noise upward.

Hot on CFM’s heels however is Pratt & Whitney, which lost engine market dominance to the CFM56 when it failed to follow up its ubiquitous JT8 engine. It extols the virtues of its geared turbofan architecture, which it believes could enter service in around five to six years and beat any CFM56 replacement to market.

Current Engines
According to CFM president Eric Bachelet, his customers have seen an average 33-percent improvement in labor productivity, 10-percent drop in sales and distribution costs and a 13-percent reduction in non-fuel distribution costs. However, this has happened in the face of the “headwind” of fuel-price increases, which have increased from 13 percent of operating costs in 2001 to an estimated 26 percent in 2007.

With 17,000 engines in service and a 4,000 engine backlog, CFM has 55 percent of the market for aircraft of 100 seats or more and anticipates all-time high production numbers approaching 1,400 engines a year in the period 2008 to 2010. It has, over the years, invested some $1.5 billion in improving its line of CFM56 engines, with the TECH56 program–launched in 1998–now coming to fruition on both the Airbus A320 and Boeing 737 families.

Engine certification on the Boeing platform came in September 2006 and the first upgraded engine (a CFM56-7B) entered service in May 2007, said Planaud, while the Airbus application (the CFM56-5B) is scheduled to enter service by the end of this year. The upgrades include various compressor, combustor and turbine improvements such as better cooling, a five-degree increase in engine gas temperature margin, 20-percent lower NOx and reduced fuel burn, which should help CFM customers cope with sustained high fuel costs.

CFM’s arch rival, International Aero Engines (IAE)–a P&W joint venture with Rolls-Royce, Japan Aero Engines Corp. and MTU Aero Engines–is not standing still. However, the company claims its IAE-2500-A5 engine produces “lower total emissions than its competitors” and will continue to improve through IAE’s new-build standard, SelectOne, which is currently undergoing tests.

According to IAE senior vice president of customers Phil Harris, “SelectOne will provide a significant four-percent fuel burn advantage over our competition with a corresponding difference in regard to CO2 emissions.”

For the older CFM56-3 engine upgrade program, CFM has sold 154 kits, most recently with a large order from Southwest Airlines, the main improvement being an extra 3,000 hours on-wing life, representing a further year in operation for a typical airline.