In this paper we use a variational approach to study an endoreversible Curzon-Ahlborn-Novikov (CAN) heat engine under both maximum power and maximum ecological function conditions. By means of this procedure we analyze the performance of a CANheat engine with a nonlinear heat transfer law (the Dulong-Petit law) to describe the heat exchanges between the working substance and its thermal reservoirs. Our results are consistent with previous ones obtained by means of other procedures. In addition, we obtain expressions for the temperatures of the isothermal branches of the working fluid under maximum power conditions. Finally, we present an expression for a kind of nonendoreversible Carnot efficiency.