Potential of lowering the contrail formation of aircraft exhausts by engine re-design

The indirect effects of water vapour (contrails and contrail-cirrus) and particles from aircraft may play a major role in the climate impact of air traffic. The contrail formation is affected by the engine propulsion efficiency, which grows with decreasing exhaust gas temperature. An engine performing with a higher overall propulsion efficiency causes plume conditions which during mixing reach a higher relative humidity for the same ambient temperature, thus causing contrails also at a higher ambient temperature. This paper is aimed at finding technical solutions for the aircraft engines that temporarily decrease the propulsion efficiency, and thereby may alleviate the formation of contrails. Such a measure is to be applied when the aircraft flies through regions where persistent contrails are likely to form. The results suggest that by employing variable guide vanes for the fan of the engine, it would be possible to reduce the threshold formation temperature of contrails by about 1.5 K in the troposphere, corresponding to an altitude of about 300 m. In the stratosphere the beneficial effects are greater. This measure is accompanied by a decrease in overall efficiency by 10–11% points, giving an increase in specific fuel consumption by 42–48%, depending on altitude.