Aircraft engine makers are seeking ways to reduce emissions in future propulsion systems, but "mixed messages" from the international policy community are frustrating progress, say industry officials.
The nature of combustion technology demands that reductions for any one type of emission, such as carbon dioxide, usually means an increase for another type, such as nitrogen oxides. With no clear policy guidance, engine companies risk committing to the wrong design concepts.
The engine makers seek clarity amid a raging policy debate over the regulation of aviation emissions between the European Commission and other national governments, including the USA. The focus of the EC is on reducing carbon dioxide emissions, while US officials advocate further study to adopt a more holistic approach.
The challenge for engine designers is factoring environmental performance into engine and combustor designs that may not enter service for another decade or more.
Carbon dioxide emissions are driven by the rate of fuel burn. A single kg of jet fuel creates 3.16kg of carbon dioxide as two oxygen atoms are attached to a single carbon atom. Engine designers know they can lower carbon dioxide production by increasing pressure ratios, thus allowing engines to burn less fuel.
However, the trade-off is an increase in nitrogen oxides produced in the more volatile combustion chamber. Engine designers have prototype combustors that can work to reduce nitrogen oxides, but that creates a trade-off with turbine durability and, possibly, safety, unless the problem of high-altitude restarts can be resolved.
Additional reporting by Stephen Trimble
Source: Flight International
