Abstract
We study the efficiency at the maximal power ηmax of a finite-time Carnot cycle of a weakly interacting gas which we can regard as a nearly ideal gas. In several systems interacting with the hot and cold reservoirs of the temperatures Th and Tc, respectively, it is known that 
, which is often called the Curzon-Ahlborn (CA) efficiency ηCA. For the first time numerical experiments to verify the validity of ηCA are performed by means of molecular dynamics simulations and reveal that our ηmax does not always agree with ηCA, but approaches ηCA in the limit of Tc→Th. Our molecular kinetic analysis explains the above facts theoretically by using only elementary arithmetic.