  We construct a microscopic model of low-dissipation engines by driving a
Brownian particle in a time-dependent harmonic potential. Shortcuts to
adiabaticity and shortcuts to isothermality are introduced to realize the
adiabatic and isothermal branches in a thermodynamic cycle, respectively. We
derive an analytical expression of the efficiency at maximum power for this
kind of engines. This expression satisfies the universal law of efficiency at
maximum power up to the second order of the Carnot efficiency. We also analyze
the issue of power at any given efficiency for general low-dissipation engines,
and then obtain the supremum of the power in three limiting cases respectively.
