The System Theory considers every engineering problem as a corresponding mathematical model with input and output spaces and usually with a controller. By the same token, it is presumed that an appropriate model has been constructed. It is known that methods applied in this theory are effective for linear or naturally linearizable mathematical models. This paper is designed to convince the reader that a mathematical background in System Theory for input-output models is helpful for a wider class of technical problems.Also it seems to be important for development and approbation of System Theory methods to apply them to some new admissible objects. It turns out, that the basic ideas of System Theory can be applied to complicated physical optimizational problems connected with heat and mass transfer (with working process of diesel engines in particular). Actually certain procedures, which are typical for this mathematical theory, together with some other ones allowed us to reduce the central variational problem for “Diesel Engine - Fuel Injection Equipment” (DE/FIE) system to the finite dimensional optimizational problem for the corresponding parametric mathematical model. The main attributes of the System Theory approach and their construction for a DE/FIE system are described.