Within the field of the so-called endoreversible thermodynamics (ET) i
t has been established that most of the optimum performance conditions
for finite-time thermal engines models are highly sensitive to the he
at transfer law used. For example, this is the case for maximum power
output conditions. In the present work, by means of the De Vos' method
we develop a systematic way to obtain several typical ET-results and
we find that there exists a general property which is maintained for e
ndoreversible engines under changes in the heat-transfer law.