  We investigate the efficiency at maximum power (EMP) of irreversible quantum
Carnot engines that perform finite-time cycles between two temperature tunable
baths. The temperature form we adopt can be experimentally realized in squeezed
baths in the high temperature limit, which makes our proposal of practical
relevance. Focusing on low dissipation engines, we first generalize the
pervious upper as well as lower bounds for the EMP to temperature tunable cases
in which they are solely determined by a generalized Carnot limit. As an
illustrative example, we then consider a minimal heat engine model with a
two-level spin as the working medium. It mimics a low dissipation engine as
confirmed by finite time thermodynamic optimization results. The so-obtained
EMP, being constrained by the generalized bounds, is well described by a
generalized Curzon- Ahlborn efficiency as consequences of a left/right symmetry
for a rate constant and low dissipations. Intriguing features of this minimal
heat engine under optimal power output are also demonstrated.
