Towards Environmentally Friendly Aero Engines

Abstract: Since the introduction of subsonic jet driven commercial aircraft, incremental improvements in overall efficiency has been a driving requirement as well as a selling argument for the engine industry. The historical overall efficiency improvement is the result of continuous advances in aerodynamic design, increase of overall pressure ratios, turbine inlet temperatures and propulsive efficiencies in parallel with the development of stronger and lighter materials. The bypass jet engine technology provided a major improvement in specific fuel consumption and noise levels. Since its introduction, the turbofan engine has manifested the aero engine industry standard.The objectives of this work have been to evaluate a number of potential aero engine technologies, with respect to the future environmental goals and the reference technology level that is used today. In order to achieve this, methods for assessing subsonic and transonic airplane aerodynamics, operating cost, aero engine emissions and noise have been developed. In particular the opportunities provided by the geared turbofan engine (GTF) are carefully analysed and compared to the advanced direct drive turbofan (DDTF) engine. Although estimated improvements are considerably lower than provided by leading engine manufacturers the engine cycle still provides a valuable reduction in fuel burn. More importantly, the GTF engine allows a substantial reduction in engine noise at a considerably lower fuel penalty than achievable by the DDTF engine configuration. Several innovative concepts, such as the intercooled recuperated aero engine, distributed propulsion, open rotor engines, catalytic combustion as well as the use of alternative fuels are discussed.It is observed that today’s jet driven commercial transport airplanes, despite six decades of airplane and engine evolution, are operating at about the same energy efficiency as the best piston driven engines of the 1940s. This is a consequence of the high productivity requirement posed on today’s commercial aviation. It seems reasonable to assume that a considerable potential for improving energy efficiency exists if society will allow a reduction in aviation productivity by accepting increased travel times.