By my calculation the carnot efficiency for the jet operating temperature is about 85.5%: 1-T_cold/T_hot, where T_cold ~ 230K (it's quite cold at high altitudes

By my calculation the carnot efficiency for the jet operating temperature is about 85.5%: 1-T_cold/T_hot, where T_cold ~ 230K (it's quite cold at high altitudes) and T_hot ~ 1588K (or 2400 F, which is the hottest temperature in the Leap engine produced by GE, which powers the 737-Max [1]). This is only possible though because of GE's usage of the ceramic matrix composites (CMC) which allows for 150-200K hotter temperatures than classical alloys. Now the current CMC method employed by GE uses silicon carbide, which has a melting temperature of about 3000K. If one day they could switch to hafnium carbide, with a melting temperature of more than 4100K, the carnot efficiency could reach more than 89%. [1] https://www.ornl.gov/news/ceramic-matrix-composites-take-flight-leap-jet-engine https://www.ornl.gov/news/ceramic-matrix-composites-take-fli...