  We study the efficiency at the maximal power $\eta_\mathrm{max}$ of a
finite-time Carnot cycle of a weakly interacting gas which we can reagard as a
nearly ideal gas. In several systems interacting with the hot and cold
reservoirs of the temperatures $T_\mathrm{h}$ and $T_\mathrm{c}$, respectively,
it is known that $\eta_\mathrm{max}=1-\sqrt{T_\mathrm{c}/T_\mathrm{h}}$ which
is often called the Curzon-Ahlborn (CA) efficiency $\eta_\mathrm{CA}$. For the
first time numerical experiments to verify the validity of $\eta_\mathrm{CA}$
are performed by means of molecular dynamics simulations and reveal that our
$\eta_\mathrm{max}$ does not always agree with $\eta_\mathrm{CA}$, but
approaches $\eta_\mathrm{CA}$ in the limit of $T_\mathrm{c} \to T_\mathrm{h}$.
Our molecular kinetic analysis explains the above facts theoretically by using
only elementary arithmetic.
