  We propose a generalized model of a heat engine and calculate the minimum and
maximum bounds on the efficiency at maximum power. We obtain a universal form
of generalized extreme bounds on the efficiency at maximum power. Our model
unifies the bounds on the efficiency and the universality features are observed
for various heat engine models. Even though our model is a direct
generalization of low-dissipation heat engines, the bounds on the efficiency
obtained at a single target function capture those observed in the actual power
plants working at different dissipation levels.
