Consider a semi-ideal heat engine, in which heat transfer takes time, according to Fourier's law of heat conduction: , but other operations happen instantly.
Its maximal efficiency is the standard Carnot result, but it requires heat transfer to be reversible (quasistatic), thus taking infinite time. At maximum power output, its efficiency is the Chambadal–Novikov efficiency:
Due to occasional confusion about the origins of the above equation, it is sometimes named the Chambadal–Novikov–Curzon–Ahlborn efficiency.