Water Injection: Disruptive Technology1 to Reduce Airplane Emissions and Maintenance Costs 2004-01-3108
Water injection is an old aviation technology that was previously used to generate increased engine power during takeoff. If water injection were now to be used without increasing thrust, it could result in large reductions in takeoff NOx emissions and would most likely enable longer engine life and reduced operator costs.
Due to the cooling action of evaporating water, a large temperature reduction will be experienced at the point where the water is injected into the engine. This could improve combustion emissions, such as temperature-sensitive NOx, and help reduce temperatures throughout the turbine section of the engine.
The two current preferred methods of water injection are: (1) direct injection into the combustor, and (2) misting of the conditioned water before the engine's compressor. Combustor injection could achieve up to 90% NOx reduction and offer few implementation challenges as it has been used in aero-derivative industrial engines for over 30 years. For compressor water misting, the rate of water flow is limited to about 3% of the core air flowiii, and so could only achieve slightly more than 50% NOx reduction level but would offer larger reductions in turbine inlet temperature.
Newer high bypass ratio (i.e. low fan pressure ratio) engines experience a higher thrust lapse rate as airplane altitude increases. This results in a longer period of time that the engine is operating at high core temperatures. In order to reduce these temperatures and improve engine life, both water injection systems were evaluated for use all the way to top of climb.
Although the water misting system showed better engine performance results, the best solution may be the combustor injection method when used for takeoff only. This configuration results in minimal airplane payload weight penalties because it uses less than half the water of the compressor water misting method. The combustor water injection method reduces the turbine inlet temperature just enough during takeoff that it would greatly improve engine hot section life which would reduce engine maintenance costs.
Any airplane costs and inconvenience associated with water injection are anticipated to be more than offset by engine maintenance savings. These savings may generate enough market demand that water injection could be developed as an optional airplane emissions reduction technology.