NOVELTY - The heat exchanger comprises a first flow space (19) for passing a first fluid such as a working medium, a second flow space for passing a second fluid such as an exhaust gas (18) so that it is transferable from the second fluid to the first fluid, a first inlet opening (32) for introducing the first fluid, a first outlet opening (33) for discharging of the first fluid, a second inlet opening (11) for introducing the second fluid, and a second outlet opening for discharging the second fluid.    USE - Used as a heat exchanger.    ADVANTAGE - The heat exchanger has high efficiency.    DETAILED DESCRIPTION - The heat exchanger comprises a first flow space (19) for passing a first fluid such as a working medium, a second flow space for passing a second fluid such as an exhaust gas (18) so that it is transferable from the second fluid to the first fluid, a first inlet opening (32) for introducing the first fluid, a first outlet opening (33) for discharging of the first fluid, a second inlet opening (11) for introducing the second fluid, and a second outlet opening for discharging the second fluid. The first flow space is divided into a first part, a second part and a third part and flows the first inlet opening for the first fluid into the first part and/or the second part is formed in a flow direction of the first fluid after the first part. The first part is formed in flow direction of the second fluid before the second part so that the second fluid is guidable through the first part and subsequently through the second part. The first and second fluids are conducted in a countercurrent flow through the second part and/or comprise heat transformer valves and heat exchanger pipes. The first inlet opening is formed for the first fluid in place of the first part on the second part in an interconnection with the valves. The first outlet opening is formed for the first fluid to the first part. A first inlet portion of the first fluid is formed into the second part in the flow direction of the second fluid: in an interval of 50%, 70% or 80% of a flow path of the second fluid into the second part to a second inlet portion of the second fluid in the second part; and in a region of a second outlet portion of the second fluid having the interval of less than 50%, 30% or 20% of the flow path of the second fluid in the second part. The first and the second parts are fluidly connected to each other with respect to the first fluid having the heat exchanger-pipes. The first and second fluids are conducted in a counter- or cocurrent flow by the first part. The first and second parts of the heat exchanger are formed from a separate construction unit of the heat exchanger or by a construction unit of the heat exchanger. The heat exchanger and/or the construction unit of the heat exchanger is formed in a pane design, a shell- and tube-design, and/or plate design. The first part comprises an other construction than the second part and/or the third part. The first part is designed as a tube bundle heat exchanger. A bundle is provided with tubes by which the first and second fluids flow. INDEPENDENT CLAIMS are included for:    (1) a system for utilizing waste heat of an internal combustion engine by Clausius- Rankine-cycle process; and    (2) a reciprocating internal combustion engine.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a heat exchanger.    Second inlet opening (11)    Exhaust gas (18)    First flow space (19)    First inlet opening (32)    First outlet opening. (33)