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You can’t swing something as short as a static wick these days and not nick something related to the move to reduce Green-House Gas emissions (GHGs) – specifically carbon monoxide. By scientific consensus GHGs are a primary contributor to global climate change and global-warming worries. Or does it just seem that way? With international talks on a new treaty to control GHGs- a focus on alternative energy forms with little to no ...

Dave Higdon   |   1st February 2008
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Dave Higdon Dave Higdon

Dave Higdon writes about aviation from his base in Wichita Kansas. During three decades in...
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Cleaning Up:
Powerplants lead the ‘lean and green’ movement.

You can’t swing something as short as a static wick these days and not nick something related to the move to reduce Green-House Gas emissions (GHGs) – specifically carbon monoxide. By scientific consensus GHGs are a primary contributor to global climate change and global-warming worries. Or does it just seem that way?

With international talks on a new treaty to control GHGs- a focus on alternative energy forms with little to no carbon monoxide content- and Europe poised to embrace a carbon-trading system – and include aviation in the bear hug – it’s increasingly difficult to find a corner of modern industrial life untouched by the concerns spurring this so-called green movement.

Hybrid autos and trucks are here; General Motors hopes to begin delivering its totally fossil-fuel free Chevy Volt in about three years; and in aviation- research and development efforts are already working on small personal-size aircraft that use electric power to produce the motive force we call “thrust.”

Critics in the U.S. are challenging the FAA and Environmental Protection Agency to “do something” about the supposed polluting impact of aircraft. In one recurring- unsympathetic effort- critics want the EPA to invoke a wholesale ban on the tetraethyl lead still used in 100LL (Low Lead) fuel used by piston-powered aircraft. This push- once mounted in earnest by the Great Lakes Region office of the EPA- so far has come against some problems. Paramount among the issues is potentially rendering useless hundreds of thousands of piston aircraft engines- since currently no universally suitable replacement fuel exists for 100LL.

Beyond the push to eliminate lead from the dominant aviation gasoline- these same forces want steps taken to force reduction in noxious emissions from turbine aircraft- in particular carbon monoxide and nitrous oxide.

What’s unclear to some observers is from which basis point these green thinkers want reduction made. Should the reductions start from the foundation of today’s standards- which already reflect a reduction from a few years ago? Or should aviation be forced to move further by basing reductions on the greener engine technologies already in development but not yet flying?

In reality- some existing and upcoming engines already meet some of these regulatory standards; and engines that promise to go beyond even future standards already exist at some phase of development. Either way- it seems to the interested observer that some critics want to structure the debate as if aviation has made no strides in reducing both its need for fuel and the polluting products coming out of the exhaust.

The current state of affairs…
Research has long focused on moving piston aircraft to run on alcohol- a movement already swiftly gaining ground for ground vehicles capable of burning blends of gasoline and ethanol- reducing both fossil fuel needs and GHGs in the process. Ethanol production in the U.S.- today predominately based on corn- is a booming- growing concern. But even alcohol blends present some issues for aircraft. While it’s possible to make a gas-powered piston airplane run on 100 percent ethanol- changing over many aircraft is considered financially impractical due to compatibility problems with fuel-system components.

For turbine aircraft- synthetic fuels and so-called bio-diesel present other options that hold promise to reduce both America’s need for crude oil and GHG and other pollutants.

Essentially- for the uninitiated- bio-diesel is refined plant oil- with used cooking oil from fast-food restaurants the feedstock that launched the movement. Interest in bio-diesel has moved to the point that companies are exploring crops and production methods for competitively extracting oil for fuel. Already the U.S. Air Force has successfully performed initial flight tests using a heavy lift cargo aircraft fueled by a synthetic blend that includes bio-diesel.

But even within the confines of current engine fuels- the powerplant manufacturers have long pursued designs which use ever-less fuel per pound of thrust- in a natural progression- and lower emissions per mile in the process. As far back as the early 1980s- engine makers found ways to build cleaner- “greener” powerplants that also improved fuel efficiency by significant margins.

The movement toward greener turbine engines continues unabated and uninterrupted- fueled as much by customers’ demands for ever lower costs in both fuel and maintenance as by the consciences of engineers and corporations. Yes- the business community long expected a time when the eyes of the environmental regulators would focus on aviation’s impact as a huge user of fossil fuels; and yes- the aviation community also recognized its sensitivity to fuel prices.

In the end- though- it’s simply good business and smart competitive strategy to seek products that outperform the competition in every area – and lowering fuel and maintenance costs has the direct result in lowering emissions. A history of proactively lowering emissions and improving fuel efficiency puts turbine-using aviation a bit ahead of the curve in the quest to combat global climate change- just as the effort to reduce fuel consumption helped offset rising fuel prices. Nothing stands still- though- not the need to improve efficiency- not the move toward regulation - and not the pursuit of better powerplant technology to answer those challenges.

Is Geared Here?
You may have heard about Pratt & Whitney’s new geared-fan engine demonstrator running in the company’s Florida test facility. The PW8000 leverages improvements the company designed into the PW6000 engine- but with an inlet fan turned through a gearbox running off the low-pressure turbine. The engine also employs half the number of blades in the compressor and turbine section.

Further- employing a gearbox to drive the fan allows engineers to design the fan’s driving turbine to run at speeds optimal for each one- rather than employing compromises in these components when they run at the same speeds. The result: operating costs lower by about 10 percent- according to company information- a nine percent reduction in fuel consumption- and lower noise output and reduced maintenance needs.

The idea of employing a different approach to fans is an idea that’s come around before though... About a quarter century ago- the GE36 Unducted Fan UDF engine continued research toward a commercial application on a project originated by the National Aeronautics and Space Administration. The major commercial-engine manufacturer built and tested demonstrators of the concept which to many read like an effort to advance turboprop technology by blending the best of both turboprop and turbofan approaches.

The NASA and GE36 engine projects worked by directing exhaust gases through a pair of counter-rotating turbines directly connected to a pair of counter-rotating fan blades on the back of the engine. With large-span- variable-pitch blades- the engines somewhat resembled an eight-blade prop on a pusher propjet powerplant. The biggest difference in the UDF: no gearbox between the power turbine and prop blades a’la turboprop engine.

The promise of this radical-looking technology? Compared to turbofans of the day- fuel-consumption reductions of a massive 20 to 30 percent! Turboprop engines delivered about the same degree of increased efficiency- but with some significant limits on high speed.

The UDF and NASA’s original project sought to alleviate the speed issue using counter-rotating arrays with multiple- highly swept blades. Boeing and McDonnell Douglas both planned airframes around the engine and a McDonnell Douglas demonstrator confirmed the potential for 30 percent lower fuel consumption- speed potential and even Stage III noise compliance.

Pratt & Whitney and Allison partnered on another project that retained the gearbox but embraced some other aspects of the UDF idea. And then fuel prices dropped significantly and the higher cost of new the engines under-whelmed the market. This time- however- few expect a drop on fuel prices and the need to reduce emissions is expected to remain. Pratt & Whitney’s current demonstrator engine produces thrust more in line with small commercial airliners- so spin-off development applicable to larger business jets is likely – as is the prospect of the technology- if successful enough- to trickle down to smaller thrust ranges. It’s worked that way before.

More development
General Electric- mindful of the many legacy engines it has been operating- has undertaken a program to update the CF34 series engines used on Challenger 601 airframes that reduce GHG output and improve durability. A new combustor is the key to the GHG reduction.

Working with Honda Aero Engines- GE is also involved in development of the HF120 small turbofan engine tapped for the HondaJet and Spectrum’s Freedom light jet. An unusual reverse-flow combustion chamber and fuel nozzles couple with the HF120’s high-pressure design to produce significantly lower specific fuel consumption as well as greatly reduced GHG emissions.

Honeywell- meanwhile- is sticking with the linear-flow philosophy on its HTF1000 engine- which employs a single new annular combustion chamber. With some clever engineering using cooling air- Honeywell’s approach allows the fuel-air mixture to run rich at the front and go lean aft. This approach reduces GHGs- smoke and soot- as well as incrementally improving fuel efficiency.

Pratt & Whitney’s new PW307 employs a new combustion technology dubbed Talon 2 that yielded reduced specific fuel consumption- as well as significant cuts in noxious emissions. But rather than some in-the-future product- the PW307 is already flying on the Falcon 7X certificated last year.

Another new engine in development- this one in the 10-000-pound thrust range- promises even deeper cuts in noxious emissions and further incremental improvements in fuel efficiency. Pratt & Whitney expects this new engine to compete as the power for a new generation of large- long-distance corporate aircraft. And Snecma aims for its new Silvercrest project to find a place in the business aviation market thanks to its promise of a 15-percent advantage in fuel consumption in its segment- plus emission levels about 50 percent below today’s most-stringent standards. Again- innovative design and engineering play roles in how the Silvercrest engine achieves its lofty goals.

Ahead of a direct-flow combustion chamber Snecma placed a single-stage fan and a high-pressure compressor of five stages; aft of the combustor the engine sports a high-pressure turbine of a single stage and a low-pressure turbine made up of three. While the arrangement lengthens the engine compared to existing designs- the combination works to achieve the engine’s performance.

Fuel at the forefront
A tremendous amount of work is going into research seeking alternatives to the use of crude oil as the feedstock for fuels- aviation and otherwise. In November- the second annual meeting of the Commercial Aviation Alternative Fuels Initiative helped edge several new projects forward toward including recommendations for the creation of a single turbine-engine fuel for use in both commercial and military applications.

Blends made up of traditional jet fuel and some other synthetic or bio-based stock hold tremendous potential to take cost pressures off petroleum supplies – presuming the manufacturing and shipping aspects of producing the synthetic of bio-fuel work. Many of these- such as a 50/50 blend of a carbon/hydrogen synthetic creation and jet fuel also hold promise to reduce emissions of nitrous oxide and carbon monoxide. Another area of research involves using natural gas as the feedstock for conversion to liquid kerosene. Tests on this product should yield information on its suitability as a turbine fuel- as well as its impact on the engine’s maintenance requirements. And flight tests using- initially- a blend of mostly conventional jet fuel and the synthesized kerosene will progress to using a 100-percent synthesized fuel if the early results look good. In fact- CFM International has already conducted tests running its CFM56 on a mix that was 70 percent Jet A and 30 percent a vegetable oil product. And there’s no word yet on whether the jet exhaust carried the same smell as the bio-diesel used by many to fuel their diesel-powered cars and trucks: French fries. If so- don’t be surprised if the engines of tomorrow emit little more pollution than a faint aroma of fried potatoes.

At today’s fuel prices and while climate-change worries remain high- expect the drive to further improve fuel efficiency of another 20 percent in the next 20 years to be paralleled by efforts to replace petroleum as the base stock – both in the name of saving the planet as well as saving the bottom line.

More information from CFM: www.cfm56.com Honeywell: www.honeywell.com Pratt & Whitney Canada: www.pwc.ca- Snecma: www.snecma.com

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