Internal combustion engines are used in various technical applications for converting thermal energy into mechanical energy. In motor vehicles, especially in trucks, internal combustion engines are used to move the motor vehicle. The efficiency of internal combustion engines can be increased by the use of systems for utilizing the waste heat from the internal combustion engine by means of the Clausius-Rankine cycle process. In this process, the system converts the waste heat from the internal combustion engine into mechanical energy. The system comprises a circuit having lines with a working medium, e.g., water, a working medium pump for delivering the working medium, a vaporizer for vaporizing the liquid working medium, an expander, a condenser for liquefying the vaporous working medium, and a collecting and equalizing tank for the liquid working medium. By the use of such systems in the internal combustion engine, the overall efficiency of the internal combustion engine can be increased with such a system as a component of the internal combustion engine.
In this regard, a vaporizer-exhaust gas heat exchanger and a vaporizer-exhaust gas recirculation [EGR] heat exchanger are used in motor vehicles as a vaporizer for heating and vaporizing the working medium. The exhaust gas passed through the vaporizer-exhaust gas heat exchanger comes from an exhaust gas turbine as a main exhaust stream and the exhaust gas passed through the vaporizer-EGR heat exchanger comes from an exhaust gas recirculation line and after passage through the vaporizer-EGR heat exchanger is again supplied to the internal combustion engine in that the exhaust gas is introduced into a charge air line. To achieve a Clausius-Rankine cycle process efficiency as high as possible, it is necessary to heat, vaporize, and superheat the working medium to as high a temperature as possible. From the thermodynamic standpoint, it is therefore expedient to pass the exhaust gas, discharged from the condenser, first through the vaporizer-exhaust gas heat exchanger and then through the vaporizer-EGR heat exchanger. In so doing, the exhaust gas passed through the vaporizer-exhaust gas heat exchanger has lower temperatures than the exhaust gas passed through the vaporizer-EGR heat exchanger. For example, the exhaust gas passed through the vaporizer-exhaust gas heat exchanger has a temperature of 280° C. to 380° C. and the exhaust gas passed through the vaporizer-EGR heat exchanger has a temperature in the range of 500° C. to 700° C. As a result, the working medium can be first heated and at least partially vaporized in the vaporizer-exhaust gas heat exchanger and then completely vaporized and superheated in the vaporizer-EGR heat exchanger. As a departure from this, it is also known to pass the working medium after it is discharged from the condenser first partially through the vaporizer-EGR heat exchanger, then to pass the working medium, partially passed through the vaporizer-EGR heat exchanger, through the vaporizer-exhaust gas heat exchanger, and then to pass the working medium, passed through the vaporizer-exhaust gas heat exchanger, through the vaporizer-EGR heat exchanger. Moreover, it is also known to connect in parallel the vaporizer-exhaust gas heat exchanger and the vaporizer-EGR heat exchanger relative to the fluid stream with the working medium after discharge from the condenser.
Greatly increased temperatures occur at the vaporizer-EGR heat exchanger at the exhaust gas inlet side and this leads to high thermal expansions and damage of the vaporizer-EGR heat exchanger. The high temperatures in the vaporizer-EGR heat exchanger lead to a nonhomogeneous distribution of the working medium and thereby to a local dry-out in the individual regions of the vaporizer-EGR heat exchanger. The result is very high local temperatures, so-called hot spots, which may lead to decreased performance. When ethanol or organic working media of the Clausius-Rankine cycle process are used, the hot spots lead to degradations because wall temperatures above 400° C. may occur here. Lubricating oil as well, which is present in the working medium for lubricating the expander, can be degraded by the high temperatures in the vaporizer-EGR heat exchanger.
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