  Following the result by Skrzypczyk et al., arXiv:1009.0865, that certain
self-contained quantum thermal machines can reach Carnot efficiency, we discuss
the functioning of self-contained quantum thermal machines and show, in a very
general case, that they can reach the Carnot efficiency limit. Most
importantly, the full analytical solution for the functioning of the machines
is not required; the efficiency can be deduced from a very small number of
fundamental and highly intuitive equations which capture the core of the
problem.
