CFM56 Double Annular Combustor (DAC) development was initiated in 1989 in response to growing airline concerns over future planned reductions in allowable emissions and the institution of taxes on emissions in some countries. The first CFM56-5B engine equipped with the DAC entered service in early 1995 with Swissair and Austrian Airlines. These two carriers provided the impetus for the development of DAC technology and worked closely with CFM. The CFM56 is the only engine in this thrust class to offer this technology. Later this year, the first CFM56-7-powered Boeing Next-Generation 737-600 equipped with DAC will enter service with Scandinavian Airlines System.
The advanced CFM56 DAC is produced by CFM International, a 50/50 joint company between Snecma (Safran Group) of France and General Electric of the United States.
Although the CFM56 single annular combustor meets current cycle regulations by a significant margin, the DAC significantly reduces NOx (oxides of nitrogen) emissions. Similarly, the improved fuel consumption of the CFM56-5B reduces CO2 (carbon dioxide) emissions by as much as 15 percent. The CFM56-5B and CFM56-7 DAC maintain emissions levels that are nearly 50 percent better than current requirements.
Aircraft engines produce four pollutants: smoke, unburned hydrocarbons, carbon monoxide, and NOx. Over the past two decades, emissions of all but NOx have been reduced to very low levels with the development of cleaner-burning engines. NOx contributes about 80 percent by weight of aircraft engine emissions during a 500 nautical mile mission, primarily during takeoff, climb, and cruise.
NOx is formed by the reaction of oxygen and nitrogen at very high temperatures. In a combustor, the highest temperatures occur when there is an optimum mixture of fuel and air. The amount of NOx formed is then determined by the "residence" time that the burning fuel/air mixture stays at the high temperature. The CFM56 DAC reduces flame temperature and residence time by increasing the airflow velocity in the burning zone and physically shortening the length of the combustor.
In addition, the DAC incorporates a second dome, or inner ring, of fuel nozzle ports. Each nozzle has a second tip that serves this inner ring. At low power levels, only the outer (pilot) stage is used. This stage is designed with low throughflow velocities and low airflow to promote stable operation and complete combustion. At high power, both stages are operational but the majority of the fuel and air is burned in the inner (main) stage. The higher throughflow velocities in this stage reduce combustor residence time. Total combustion airflow through the swirlers is more than twice as much as a conventional combustor, so the DAC also has lower temperatures.
Although the DAC program focuses on NOx emissions CFM is also concerned about CO2. Significant improvements in the fuel efficiency of CFM56 engines have brought significant reductions in CO2 emissions. For example, the recent introduction of three-dimensional aerodynamic analysis to design compressor blades and vanes for the CFM56-5B has brought about an improvement in the engine's fuel consumption and, hence, CO2 emissions.
To date, Austrian Airlines, Edelweiss, Finnair, Scandinavian Airlines System, Sabena Belgian Airlines, and Swissair have select DAC-equipped CFM56 engines to power more than 250 firm and option aircraft.