Possibilities and limits of contrail avoidance through aircraft engine design for kerosene- and hydrogen-fuelled engines

In this paper, we present a thermodynamic cycle-based analysis of the effect of aircraft engine design and fuel selection on contrail formation, which is evaluated using the Schmidt–Appleman criterion (SAC). Our goal is to examine the possibilities and limits of contrail avoidance to reduce the climate impact of aviation. We use the gas turbine performance software GasTurb to study geared-turbofan and open-rotor engine designs for contemporary short- to medium-range aircraft. The design parameters are overall pressure ratio, burner exit temperature, and, for the geared-turbofan, the design bypass ratio. We vary these parameters in wide ranges to cover all technically feasible engine designs. We find that contrail formation at current cruise altitudes cannot be avoided with the examined contemporary and future engine cycles. In the best case, the contrail formation altitude is increased by just over 230 m. This comes with a significant engine efficiency penalty. In contrast, future, more efficient engine cycles will decrease the contrail formation altitude, leading to the formation of more contrails. When using hydrogen as fuel instead of kerosene, contrails form at approximately 1.6 km lower altitudes according to the Schmidt–Appleman criterion. We thus find that it is practically impossible to avoid contrail formation, as predicted by the SAC, using the engine cycles and fuels examined in this paper. Operational contrail avoidance measures like cruise altitude adjustment or flying around ice-supersaturated atmospheric layers present themselves as more practicable.