Turbofan Technology

 - December 18, 2006, 5:26 AM

Engine manufacturers are showing renewed interest in the 10,000-pound-thrust segment. They see the aging of the General Electric (GE) CF34-3B, the only engine in production in the class, and at least two companies–Snecma and Pratt & Whitney Canada–are eyeing future large business jets, the size of the Bombardier Challenger 600 series, as potential applications. Meanwhile, GE is modernizing the CF34-1 for the Challenger 601.

Snecma revealed plans last year to enter the business-jet engine market. Although well known in commercial aviation for the CFM56 engine, which it produces jointly with GE, Snecma only now is starting to explore smaller-aircraft segments, such as regional airlines with the SaM146 and business aviation with the newly christened Silvercrest series.

Jean-Pierre Cojan, Snecma’s executive vice president for commercial engines, told AIN that the company chose the name Silvercrest to denote exclusivity, altitude, clean mountain air and lineage. The project was known as the SM-X until it was christened at October’s NBAA Convention.

The Silvercrest series produces between 8,000 and 11,000 pounds of thrust. The typical application is said to be a large-cabin, long-range business jet with an mtow of between 45,000 and 60,000 pounds.

“The actual program is likely to be launched in two years,” Cojan said. Before that, the company will evaluate a core engine demonstrator as part of a $100 million technology program. This core should run by the end of this year. Engine certification is scheduled for 2010 or 2011.

The Silvercrest has a one-stage fan, no booster, a five-stage high-pressure compressor, a direct-flow combustor, a one-stage HP turbine and a three-stage low-pressure turbine. “We want to have business aviation reaping benefits from commercial aviation,” Cojan said. To that end, Snecma engineers employed the same 3-D aerodynamic tools they used on the CFM56 to design the engine’s titanium, solid, wide-chord, swept fan blades. Fan diameter will be 40 inches and the bypass ratio will be 4.5.

Cojan believes that makers of business jet engines can improve the performance of their powerplants. “Current turbofans are below commercial reliability standards,” he asserted. He expects the Silvercrest to provide time-on-wing performance that will be significantly greater than that of existing engines.

Nevertheless, the final stage of the compressor will be centrifugal, which is possible only because of the engine’s relatively low thrust. Centrifugal compressors are not suitable for engines with more than 12,000 pounds of thrust because of the stress on the engine parts. However, for engines that produce less than 12,000 pounds of thrust, there are significant advantages. “The pressure ratio with this single stage is very good. In our case, it translates into a pressure ratio greater than 17 on the entire five-stage compressor,” Cojan said. The engine’s overall pressure ratio is 27.

Also, blade tip clearance is an issue on the final stages of an axial compressor. Clearance between blade tips and the interior wall of the casing cannot be reduced below a given value. As a percentage of blade diameter, this value becomes greater as the blades decrease in size. Thanks to its different shape, a centrifugal compressor solves the problem.

The main challenge, Cojan explained, is the high temperature at the compressor’s exhaust. Cooling and high-quality hardware should help the engineers meet that challenge.

Because protecting the environment has become a key design driver in the industry, Snecma has opted for a direct-flow combustor rather than a reverse-flow one for lower emissions.

In terms of noise, Snecma is targeting 15 to 20 dB less than the current Chapter 4 standard (or 25 to 30 dB less than the Chapter 3 standard with which operators are more familiar). Emissions should be 50 percent cleaner than current CAEP6 standards. Specific fuel consumption is expected to be 5 to 15 percent better than that of existing engines in this class, and climb and cruise thrust should improve by 25 percent over those engines.

Snecma says it is already talking to several aircraft manufacturers.

Other OEMs Examine the Market
Snecma is not the only manufacturer interested in engines in the 10,000-pound-thrust class. Honeywell in October revealed that it is exploring the development of a turbofan in the same class, a project still in the early demonstration phase. The idea is to base the engine on the existing HTF7000, which powers the Bombardier Challenger 300.

The Arizona-based engine maker says that the new turbofan would improve fuel efficiency and durability and increase operating pressures and temperatures. This should raise the power-to-weight ratio as well.

A new forward-swept fan is aimed at cutting noise. In addition, Honeywell design engineers are planning to use novel acoustic treatments and advanced nozzles to beat Chapter 4 by 20 dB.

In terms of maintenance, Honeywell insists that “an engine in this thrust class should be a true ‘on condition’ engine, with line replaceable units that can be serviced quickly with common tools.” The design would enable replacements that could be accomplished in hours rather than days.

Pratt & Whitney Canada business aviation vice president Andrew Tanner told AIN, “We are working on the technologies required for [a 10,000-pound-thrust engine].” Discussions are already under way with airframers, he added.

Rolls-Royce is believed to be working on a preliminary design in the same class.
Meanwhile, GE has launched a modernization program that will allow Challenger 601 engines to be maintained like the engines on the Challenger 604/605. The program lets operators of GE’s CF34-1A/3A/3A2 (9,200 pounds of thrust) upgrade from a hard-time maintenance schedule to on-condition. The modernization program retrofits the hot section of the CF34-1A/3A/3A2 to the CF34-3A1 hot section, which has been flying with various airlines.