In February 1995, CFM International made history when the first engine equipped with a double annular combustor (DAC), the CFM56-5B, entered commercial service with Swissair.
CFM International (CFM) is a 50/50 joint company between Snecma (Safran Group) of France and General Electric of the United States.
CFM56 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.
Although the CFM56 single annular combustor meets current regulations established by the International Civil Aviation Organization (ICAO) by a significant margin, the DAC reduces NOx (oxides of nitrogen) emissions by an additional 35 percent. On a typical 500 nautical mile mission, these emissions are about 45 to 50 percent less than other engines in this thrust class.
Aircraft engines produce four pollutants which are regulated by ICAO: 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. However, NOx contributes more than 80 percent by weight of aircraft engine emissions during a 500 nautical mile mission. It is most severe at takeoff, climb, and cruise, which represent nearly 90 percent of the time of an average flight.
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 amount of "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 a higher percentage of the fuel and air is burned in the inner (main) stage. The higher throughflow velocities in this stage reduce combustor residence time. Swissair, along with Austrian Airlines, provided the initial impetus to incorporate DAC technology into the CFM56 family and played an integral role in the development process. Since then, Edelweiss, Finnair, and Sabena Belgian Airlines have chosen to equip their CFM56-5 engines with the DAC. In addition, CFM is also offering the DAC as an option on the Boeing Next-Generation 737 series. Scandinavian Airlines System (SAS) took delivery of its first DAC-equipped 737-600 last October; the airline also ordered its 737-700 and 737-800 aircraft with DAC. There are currently a total of more than 450 CFM56 DAC-equipped engines on order, including options.