Jet Propulsion/Thermal efficiency
For heat engines the thermal efficiency is the mechanical work output for each unit of heat input. For the Carnot cycle it is
ambient absolute temperature
maximum cycle absolute temperature
Actual engines using other cycles will have lower efficiencies than the Carnot cycle since they have irreversible processes such as heat flow across finite temperature differences and non-adiabatic compression and expansions, as well as friction.
Heat engines use as high a temperature as possible to maximize the efficiency and are limited by temperature capabilities of real materials used to make the engines.