The full mechanisms of thermodynamic internal energy
transfer have not been completely understood even though the
statistical mechanisms of thermodynamic equilibrium have been
well established. We here propose an internal energy transfer
theory for thermodynamic non-equilibrium, quasi-equilibrium,
and equilibrium, for the first time. The internal energy transfer
theory is applicable to understanding the internal energy transfer
mechanisms of heat, work, and chemical energy through both
diffusion (or conduction) and internal convection. The unified
approach of heat, work, and chemical energy transfer demonstrates
internal energy transfer in wide ranges of temperature,
pressure, and chemical potential. The theory leads to a nonequilibrium
generalization beyond the quasi-equilibrium theories
of isothermal, isentropic, isobaric, isochoric, static, and incompressible
processes. It is the thermodynamic generalization of
thermal, mechanical, and chemical energy transfer in addition
to the conventional convection and diffusion energy transfer.
The theory is also applicable to exploiting both spontaneous
and nonspontaneous energy transfer. In this paper, we therefore
describe the significant merits of the internal energy transfer
theory for non-equilibrium, quasi-equilibrium, and equilibrium
thermodynamics