There is a difference between emissions such as carbon dioxide (CO2) and water vapor (H2O), which are normal byproducts of combustion inside a jet engine, and the polluting emissions, which actually represent a very small percentage of the gases released (less than 0.5% of engine throughput). Nitrogen oxides (NOx) account for the largest share of these releases, with the balance comprising carbon monoxide (CO), unburned hydrocarbons (UHC) and soot.
Carbon dioxide (CO2)
CO2 emissions from aircraft are not currently regulated. But they are still a significant environmental concern, since carbon dioxide is one of the main gases producing the greenhouse effect. Reducing these gases is one of the objectives of the Kyoto agreement, which provides for an 8% reduction in 2012, compared with the level in 1990 for the 15 countries in the European Union at the time.
Carbon dioxide is emitted by all industrial facilities burning fuel, building heating systems, cars and trucks, boats, airplanes, etc. Aviation accounts for about 3% of the world’s total CO2 production. Despite this relatively small percentage, the aviation industry is working to reduce its environmental impact.
In particular, the European Commission is studying the introduction of a system based on greenhouse gas emissions permits. This type of measure would encourage all players in the air transport industry to invest in technologies to reduce fuel consumption.
However, CO2 is a natural byproduct of combustion: the quantity emitted by an engine is proportional to its fuel consumption, and this gas must be a major concern of all engine-makers. Since the 1960s, manufacturers have reduced engine consumption by 40%, while improvements to the airplane have reduced the overall figure by 60% to 70%. The parent companies of CFM International, Snecma and General Electric, have invested in research on all engine components to improve thermal and propulsion efficiency, and thereby reduce their engines’ fuel consumption. Today, the CFM56 is one of the most economical commercial aero-engines on the market!
Spiraling oil prices have also spurred aircraft customers to demand reduced fuel consumption.
Taking a long-term view, Snecma is working actively to meet the European objectives set out in ACARE/Vision 2020 for a 50% reduction in CO2 emissions by 2020. This reduction will have to be achieved through a joint effort by all stakeholders: aircraft and engine-makers, air traffic management.
Snecma itself is working along several lines to achieve these goals, including: reducing engine weight through the development of new materials, improving the efficiency of components and the thermodynamic cycle (for example, by increasing the bypass ratio and compression ratio), and innovative engine architectures (for example, the contra-rotating turbofans being studied through the VITAL program). We are of course using the latest computation methods to achieve a better understanding of the underlying phenomena.
Another area under study is to replace the traditional hydrocarbon-based fuels with different products. These new fuels will have to meet three objectives: reduce CO2 emissions; limit our dependence on oil; and be available at competitive prices. Furthermore, any use of alternative fuels will have to ensure operational compatibility with the current fleet of engines, whose service life exceeds 40 years. Today, no replacement fuel meets all of these conditions. Eventually, only a synthetic jet fuel using the biomass to liquid (BTL) process could meet the challenge. But much remains to be done to ensure that a fuel of this type fully meets aeronautical requirements, and to boost global production capacities.
Snecma’s parent group SAFRAN is also actively working on this subject in collaboration with a number of research organizations. In fact, fellow group company Turbomeca is already carrying out turbine engine tests using synthetic fuels.
Nitrogen oxides (NOx)
NOx emissions are regulated by standards defined by the International Civil Aviation Organization (ICAO). Engines having entered service since December 31, 2003 are subject to the “CAEP 4”* standard. Local regulations may apply as well: countries such as Switzerland and Sweden, for instance, have set up systems that adjust landing fees according to NOx emissions, among other characteristics.
The new CAEP 6 standard will take effect on January 1, 2008 for newly certified engines, with regulations that are 12% stricter than CAEP 4.
CFM International partners Snecma and General Electric were among the pioneers in NOx reduction: as early as 1995 they introduced CFM56 engines featuring a low-NOx combustion chamber: the dual annual combustor (DAC), which reduces NOx emissions by up to 40% compared with a conventional combustor.
The combustors offered on latest-generation CFM56-5B and -7B Tech Insertion models will meet the stipulations of CAEP 6, with a margin of more than 20%.
(* CAEP: Committee for Aviation Environmental Protection, the ICAO committee that defines noise and pollution standards.)
Snecma and CFM International are continuing their efforts to reduce NOx. For example, today we are looking at a further upgrade to the dual annular combustor concept, based on multipoint injection.
Looking further ahead, Snecma is also working on solutions that would drastically reduce nitrogen oxide emissions by some 80% over 2000 figures, to meet the 2020 target for ACARE. This work is being carried out through joint projects with a number of different partners in Europe.
Unburned hydrocarbons, carbon monoxide and soot
ICAO regulations also apply to emissions of unburned hydrocarbons, carbon monoxide and soot. Unburned hydrocarbons result from inefficient and therefore incomplete combustion. However, modern engines produce significantly less of these pollutants than engines from the 1960s.
The majority of carbon monoxide emissions from human activities are generated by road traffic and heating systems.
Soot is produced at high temperature in the engine’s combustion chamber. Its primary impact is local, but these particles may also induce a greenhouse effect by creating condensation trails, better known as contrails, which can also exacerbate global warming. Snecma focuses on combustion technologies based on “lean” fuel mixtures, which not only reduce NOx emissions, but also the smoke ejected by the engine.