Never before has commercial air transport come under such scrutiny for its environmental impact. While aircraft have made far more progress in terms of reducing fuel consumption and emissions per passenger carried in recent years, the relentless overall growth of air traffic has led to increasing pressure from the environmental lobby to reduce the noise and emissions produced by modern powerplants.
The noise footprint of a typical new generation commercial airliner is around 75 percent smaller than it was 30 years ago. The drive to reduce emissions presents a bigger challenge, however. While scientists still don’t completely understand the mechanism by which engine exhaust particles effect the upper troposphere and lower stratosphere (30,000 feet to 40,000 feet), they believe the pollution rouses a domino effect on the molecules resident at those levels, destroying the thin layer of ozone that protects the earth from excess radiation from the sun.
Oxides of nitrogen (NOx), produced as a result of the combustion of fuel at the high temperatures necessary to achieve efficient combustor operation and hence lower fuel burn, appear the main culprit. This apparent conflict has challenged combustion engineers for years as they struggle to reconcile the need to burn cooler while maintaining combustion efficiency.
Over the past two decades, emissions of all pollutants except NOx lowered sig-nificantly with the development of cleaner-burning engines. However, NOx contributes more than 80 percent by weight of aircraft engine emissions during a 500-nm mission. The most severe emissions occur at takeoff, climb, and cruise, which represent nearly 90 percent of the time of an average flight.
General Electric’s double annular combustor (DAC), developed and installed on CFM International CFM56 engines powering some European airlines, represented one attempt to solve the dilemma. The idea behind the DAC centered on reducing flame temperature and residence time, and hence NOx production, by increasing airflow velocity in the burning zone and physically shortening the length of the combustor by creating essentially two separate burning zones. At low power levels only the outer zone gets used, while at high power, both stages become operational, with a higher percentage of the fuel and air burned in the inner (main) stage. The higher velocities in this stage reduce combustor residence time.
However, the DAC was not only heavier than traditional combustors, but more complex, leading to higher maintenance costs. GE’s solution for the GEnx engine under development for the Boeing 787 centers on the simpler twin annular pre-swirl (TAPS) combustor.
The key to the TAPS design lies with premixing the air before it burns in the combustor. Air exiting the high-pressure compressor (which is already at a high temperature) goes into the combustor through two high-energy swirlers adjacent to the fuel nozzles. This swirl creates a more homogeneous and leaner mix of fuel and air, which burns at lower temperatures than before.
GE claims that the GEnx design will produce around 30 percent less NOx than one of its CF6-80 engines powering commercial aircraft today and 50 percent less than the level set by ICAO for 2008.
GE performed the first full engine test with a TAPS combustor on a CFM56-7B engine in 2002, which completed a 4,000-cycle endurance test and demonstrated high-altitude relight, a vital component of the certification process.
This February, the company completed a full-annular trial of the TAPS combustor built to GEnx standards, which it says “met all expectations in terms of emissions, efficiency, ignition and durability.” It expects to validate further improvements to the design later this year, paving the way for a full GEnx engine test in 2006.
Meanwhile, Rolls-Royce is pushing ahead with its own design for an ultra-low NOx combustor on its Trent 1000 engine for the Boeing 787. Also based on a single-annular design, the combustor uses specially coated tiles on the inner walls and needs up to 40 percent less air than the machined ring combustors previously used–air that can be used to quench temperatures to achieve the lower temperatures required for reduced NOx.