A different measure of ideal heat-engine efficiency is given by considerations of endoreversible thermodynamics, where the system is broken into reversible subsystems, but with non reversible interactions between them. A classical example is the Curzon–Ahlborn engine, very similar to a Carnot engine, but where the thermal reservoirs at temperature T h and T c are allowed to be different from the temperatures of the substance going through the reversible Carnot cycle: T h ′ and T c ′ . The heat transfers between the reservoirs and the substance are considered as conductive (and irreversible) in the form d Q h , c / d t = α α ( T h , c − − T h , c ′ ) . In this case, a tradeoff has to be made between power output and efficiency. If the engine is operated very slowly, the heat flux is low, T ≈ ≈ T ′ and the classical Carnot result is found