Control circuit for waste heat recovery systems

1. A control circuit ( 27 ) for a waste heat recovery system ( 2 ) for a heat engine ( 36 ), wherein the waste heat recovery system ( 2 ) comprises at least one evaporator ( 21 ) for converting waste heat from exhaust gas ( 31 , 31 a ) generated by the heat engine ( 36 ) into a working medium ( 23 ), at least one expansion machine ( 24 ), which can be driven by the working medium ( 23 ), at least one condenser ( 25 ) for condensing the working medium ( 23 a ) expanded in the expansion machine ( 24 ) into a liquid state ( 23 b ) and at least one delivery device ( 26 ) for increasing a pressure of the condensed working medium ( 23 b ) and delivering it into the evaporator ( 21 ), wherein the control circuit ( 27 ) controls at least one control variable which controls (a) an energy transfer from the exhaust gas ( 31 , 31 a ) to the working medium ( 23 b ), (b) an energy transfer from the working medium ( 23 c ) to the expansion machine ( 24 ), or both (a) and (b), wherein the control circuit ( 27 ) is configured to regulate a specific enthalpy h W , a temperature T W , or both the specific enthalpy h W and the temperature T W of the working medium ( 23 c ) entering the expansion machine ( 24 ) to a set value h W,S , T W,S , or to set values h W,S and T W,S , wherein the set value h W,S , T W,S , or both, as applicable, depends on the pressure p W of the working medium ( 23 c ) entering the expansion machine ( 24 ), and a prediction module ( 28 a ) is provided, which is designed to precalculate a future development of the specific enthalpy h W , of the temperature T W or the pressure p W , on the basis of a model ( 2 a ) and at least one set of state variables of the waste heat recovery system ( 2 ).

2. The control circuit ( 27 ) as claimed in claim 1 , wherein the control circuit ( 27 ) is coupled to a performance optimizer ( 1 ), which is configured to determine a dependence of the set value h W,S , T W,S on the pressure p W from optimal operating points of the waste heat recovery system ( 2 ).

3. The control circuit ( 27 ) as claimed in claim 2 , wherein the performance optimizer ( 1 ) is designed to associate a stationary working point of the waste heat recovery system ( 2 ) which has an optimal efficiency with a set of state variables of the heat engine ( 36 ) from which at least a temperature T A and a mass flow rate m A of the exhaust gas ( 31 , 31 a ) at the site of the evaporator ( 21 ) arise.

4. The control circuit ( 27 ) as claimed in claim 1 , wherein, in the control circuit ( 27 ), in addition to a control deviation ( 27 a ) of the specific enthalpy h W , or the temperature T W , from the set value h W,S , or T W,S ,

a proportion of the exhaust gas ( 31 , 31 a ) which is not conducted through the evaporator ( 21 ) also acts as a further control deviation ( 27 b ) and/or

a frequency and/or an intensity of control interventions of the control circuit ( 27 ) also acts as a further control deviation ( 27 c ) and/or

a control deviation ( 27 a ) of the specific enthalpy h W , or of the temperature T W , from the set value h W,S , or T W,S , which exceeds a predetermined threshold value, also acts as a further control deviation ( 27 d ).

5. The control circuit ( 27 ) as claimed in claim 1 , wherein the control circuit ( 27 ) is designed to plan future control interventions ( 27 e ) within a time control horizon T ch in such a way that control deviations ( 27 a - 27 d ) to be expected at a time which is a prediction horizon T ph >T ch in the future are minimized.

6. The control circuit ( 27 ) as claimed in claim 1 , wherein both the control horizon T ch and the prediction horizon T ph are defined as a multiple of a sampling time T S , wherein the sampling time T S decreases with the increasing pressure p W .

7. The control circuit ( 27 ) as claimed in claim 1 , wherein the model ( 2 a ) is linearized around at least one stationary working point of the waste heat recovery system ( 2 ).

8. The control circuit ( 27 ) as claimed in claim 7 , wherein the model ( 2 a ) is linearized in sections for different ranges of the pressure p W .

9. The control circuit ( 27 ) as claimed in claim 1 , wherein the control circuit is coupled to a Kalman filter ( 29 ) for estimating at least one state variable ( 29 a ) of the model ( 2 a ) from a set of measured state variables of the waste heat recovery system ( 2 ).

10. The control circuit ( 27 ) as claimed in claim 1 , wherein an optimization module ( 28 b ) is provided, which is designed to determine the future control interventions ( 27 e ) as a solution of a mathematical optimization problem with side conditions, in particular in the form of a quadratic program with quadratic restrictions.

11. The control circuit ( 27 ) as claimed in claim 10 , wherein a conversion module ( 28 c ) is provided, which is designed to convert boundary conditions, present in the form of inequalities, for the pressure p W , for the temperature T W , and/or for at least one control variable into quadratic restrictions.

12. The control circuit ( 27 ) as claimed in claim 1 , wherein the control circuit ( 27 ) influences the position of at least one valve ( 21 a ) which guides all or some of the exhaust gas ( 31 , 31 a ) past the evaporator ( 21 ), and/or influences the position of at least one valve ( 24 a ) which guides all or some of the working medium ( 23 c ) past the expansion machine ( 24 ).

13. A waste heat recovery system ( 2 ) for an internal combustion engine of a vehicle ( 3 ) as a heat engine ( 36 ), wherein the waste heat recovery system ( 2 ) comprises at least one evaporator ( 21 ) for converting waste heat from exhaust gas ( 31 , 31 a ) generated by the internal combustion engine ( 36 ) into a working medium ( 23 ), at least one expansion machine ( 24 ) which can be driven by the working medium ( 23 ), at least one condenser ( 25 ) for condensing the working medium ( 23 ) expanded in the expansion machine ( 24 ) into the liquid state and at least one delivery device ( 26 ) for increasing pressure of the condensed working medium ( 23 b ) and delivering it to the evaporator ( 21 ), wherein the waste heat recovery system ( 2 ) has a control circuit ( 27 ) as claimed in claim 1 .

14. A non-transitory, computer-readable medium, containing instructions which, when run on a computer, cause the computer to control a waste heat recovery system ( 2 ) for a heat engine ( 36 ), wherein the waste heat recovery system ( 2 ) comprises at least one evaporator ( 21 ) for converting waste heat from exhaust gas ( 31 , 31 a ) generated by the heat engine ( 36 ) into a working medium ( 23 ), at least one expansion machine ( 24 ), which can be driven by the working medium ( 23 ), at least one condenser ( 25 ) for condensing the working medium ( 23 a ) expanded in the expansion machine ( 24 ) into a liquid state ( 23 b ) and at least one delivery device ( 26 ) for increasing pressure of the condensed working medium ( 23 b ) and delivering it into the evaporator ( 21 ), wherein the control circuit ( 27 ) controls at least one control variable which controls (a) energy transfer from the exhaust gas ( 31 , 31 a ) to the working medium ( 23 b ), ( b ) the energy transfer from the working medium ( 23 c ) to the expansion machine ( 24 ), or both (a) and (b), by

regulating a specific enthalpy h W , a temperature T W , or both the specific enthalpy h W and the temperature T W of the working medium ( 23 c ) entering the expansion machine ( 24 ) to a set value h W,S , T W,S , or to set values h W,S and T W,S , wherein the set value h W,S , T W,S , or both, as applicable, depends on the pressure p W of the working medium ( 23 c ) entering the expansion machine ( 24 ), and

providing a prediction module ( 28 a ), which is designed to precalculate a future development of the specific enthalpy h W , of the temperature T W or the pressure p W , on the basis of a model ( 2 a ) and at least one set of state variables of the waste heat recovery system ( 2 ).