  The Carnot cycle is a prototype of ideal heat engine to draw mechanical
energy from the heat flux between two thermal baths with the maximum
efficiency, dubbed as the Carnot efficiency $\eta_{\mathrm{C}}$. Such
efficiency can only be reached by thermodynamical equilibrium processes with
infinite time, accompanied unavoidably with vanishing power - energy output per
unit time. In real-world applications, the quest to acquire high power leads to
an open question whether a fundamental maximum efficiency exists for
finite-time heat engines with given power. We experimentally implement a
finite-time Carnot cycle with sealed dry air as working substance and verify
the existence of a tradeoff relation between power and efficiency. Efficiency
up to $(0.524\pm0.034)\eta_{\mathrm{C}}$ is reached for the engine to generate
the maximum power, consistent with the theoretical prediction
$\eta_{\mathrm{C}}/2$. Our results shall provide a new platform for studying
finite-time thermodynamics consisting of nonequilibrium processes.
