In this paper we show how the nonlinear kinetics of energy transfer in propelling fluids and imperfect (non-Carnot) thermal machines can be imbed into the contemporary theory of irreversible energy generators and beat pumps. We quantitatively describe effects of nonlinear heat transfer assuming that heat fluxes are proportional to the difference of temperature in certain power, T-a. We also show that the energy and particle transports can be treated either in a conventional way or as peculiar chemical reactions. In the latter case a recent approach distinguishes in each elementary transfer step two competitive (unidirectional) fluxes and the resulting flux follows as their difference. Nonlinear imperfect systems are investigated in the context of efficiency, heat flux, entropy production and mechanical power, for steady and unsteady operations. (C) 2004 Elsevier Ltd. All rights reserved.