A more realistic simplified model for an irreversible Otto cycle is established by considering the heat leak losses through the cylinder and internally dissipative friction, in which it is assumed that the piston velocity in the adiabatic processes obeys a pure sinusoidal law. The optimally operating region of the irreversible Otto heat engine is determined. The model avoids the usual hypotheses of endoreversible heat engines and the simple description of irreversible heat engines so that the results obtained here may include the performance characteristics of some simplified Otto cycle models.