Control circuit for waste heat recovery systems

Abstract

The invention relates to a control circuit (27) for a waste heat recovery system (2) for a heat engine (36). The waste heat recovery system (2) comprises at least one evaporator (21) for converting waste heat from the exhaust gas (31, 31a) 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 (23a) expanded in the expansion machine (24) into the liquid state (23b), and at least one conveying device (26) for increasing the pressure of the condensed working medium (23b) and conveying same into the evaporator (21). The control circuit (27) influences at least one control variable which controls the energy transmission from the exhaust gas (31, 31a) to the working medium (23b) and/or the energy transmission from the working medium (23c) to the expansion machine (24). The control circuit (27) is designed to regulate the specific enthalpy h.sub.W and/or the temperature T.sub.W of the working medium (23c) entering the expansion machine (24) to a target value h.sub.W,S, T.sub.W,S, wherein the target value h.sub.W,S, T.sub.W,S depends on the pressure p.sub.W of the working medium (23c) entering the expansion machine (24). The invention also relates to a waste heat recovery system (2) for an internal combustion engine of a vehicle (3) in the form of a heat engine (36) comprising the control circuit (27) and to a corresponding computer program.

Claims

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, 31a) 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 (23a) expanded in the expansion machine (24) into a liquid state (23b) and at least one delivery device (26) for increasing a pressure of the condensed working medium (23b) 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, 31a) to the working medium (23b), (b) an energy transfer from the working medium (23c) to the expansion machine (24), or both (a) and (b), wherein the control circuit (27) is configured to regulate a specific enthalpy h.sub.W, a temperature T.sub.W, or both the specific enthalpy h.sub.W and the temperature T.sub.W of the working medium (23c) entering the expansion machine (24) to a set value h.sub.W,S, T.sub.W,S, or to set values h.sub.W,S and T.sub.W,S, wherein the set value h.sub.W,S, T.sub.W,S, or both, as applicable, depends on the pressure p.sub.W of the working medium (23c) entering the expansion machine (24), and a prediction module (28a) is provided, which is designed to precalculate a future development of the specific enthalpy h.sub.W, of the temperature T.sub.W or the pressure p.sub.W, on the basis of a model (2a) 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.sub.W,S, T.sub.W,S on the pressure p.sub.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.sub.A and a mass flow rate m.sub.A of the exhaust gas (31, 31a) 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 (27a) of the specific enthalpy h.sub.W, or the temperature T.sub.W, from the set value h.sub.W,S, or T.sub.W,S, a proportion of the exhaust gas (31, 31a) which is not conducted through the evaporator (21) also acts as a further control deviation (27b) and/or a frequency and/or an intensity of control interventions of the control circuit (27) also acts as a further control deviation (27c) and/or a control deviation (27a) of the specific enthalpy h.sub.W, or of the temperature T.sub.W, from the set value h.sub.W,S, or T.sub.W,S, which exceeds a predetermined threshold value, also acts as a further control deviation (27d).

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

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

7. The control circuit (27) as claimed in claim 1, wherein the model (2a) 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 (2a) is linearized in sections for different ranges of the pressure p.sub.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 (29a) of the model (2a) 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 (28b) is provided, which is designed to determine the future control interventions (27e) 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 (28c) is provided, which is designed to convert boundary conditions, present in the form of inequalities, for the pressure p.sub.W, for the temperature T.sub.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 (21a) which guides all or some of the exhaust gas (31, 31a) past the evaporator (21), and/or influences the position of at least one valve (24a) which guides all or some of the working medium (23c) 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, 31a) 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 (23b) 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, 31a) 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 (23a) expanded in the expansion machine (24) into a liquid state (23b) and at least one delivery device (26) for increasing pressure of the condensed working medium (23b) 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, 31a) to the working medium (23b), (b) the energy transfer from the working medium (23c) to the expansion machine (24), or both (a) and (b), by regulating a specific enthalpy h.sub.W, a temperature T.sub.W, or both the specific enthalpy h.sub.W and the temperature T.sub.W of the working medium (23c) entering the expansion machine (24) to a set value h.sub.W,S, T.sub.W,S, or to set values h.sub.W,S and T.sub.W,S, wherein the set value h.sub.W,S, T.sub.W,S, or both, as applicable, depends on the pressure p.sub.W of the working medium (23c) entering the expansion machine (24), and providing a prediction module (28a), which is designed to precalculate a future development of the specific enthalpy h.sub.W, of the temperature T.sub.W or the pressure p.sub.W, on the basis of a model (2a) and at least one set of state variables of the waste heat recovery system (2).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Hereinafter, further measures which improve the invention will be illustrated in more detail with the aid of figures, together with the description of the preferred exemplary embodiments of the invention.

(2) The figures show:

(3) FIG. 1 an exemplary integration of a waste heat recovery system 2 in a vehicle 3;

(4) FIG. 2 an interaction of the control circuit 27 with a performance optimizer 1 and with the waste heat recovery system 2;

(5) FIG. 3 a selection of nominal operating points for the sectional linearization of a model 2a of the waste heat recovery system 2;

(6) FIG. 4 an exemplary control horizon T.sub.ch and prediction horizon T.sub.ph of a model-predictive control in the control circuit 27.

DETAILED DESCRIPTION

(7) FIG. 1 shows how a waste heat recovery system 2 can be integrated by way of example in a vehicle 3. Combustion air 30 is taken in via a turbocharger 30b and cooled in an intercooler 30a, wherein the arrows at the intercooler 30a indicate the flow of the coolant. The combustion air is mixed with returned exhaust gas 31 from the exhaust gas return 33 and conducted into the engine 36.

(8) In a manner controlled by the exhaust gas return valve 32, some of the exhaust gas 31 generated by the engine 36 is conducted back into the exhaust gas return 33, which is cooled by a cooler 37. Analogously to the intercooler 30a, the arrows at the cooler 37 indicate the flow of the coolant. The rest of the exhaust gas 31a which remains after the return of exhaust gas firstly drives the turbocharger 30b and is rendered harmless in the exhaust gas after-treatment 34 before heat is extracted from it in the evaporator 21. The cooled exhaust gas 31b is then fed to the exhaust 35. All or some of the exhaust gas 31a can be diverted around the evaporator 21 via an exhaust gas bypass valve 21a. The waste heat in the proportion of the exhaust gas 31a which does not pass through the evaporator 21 is conducted directly into the exhaust 35 of the vehicle 3.

(9) In the evaporator 21, the working medium 23 is converted into superheated steam 23c and supplied to the expansion machine 24. The expanded working medium 23a is condensed into liquid working medium 23b in the condenser 25, wherein the flow of the coolant is in turn indicated by the arrows at the condenser 25. The condensed working medium 23b is compressed again in the delivery device 26 and returned to the evaporator 21.

(10) Some of the superheated steam 24a can be conducted past the expansion machine 24 into the bypass throttle 24b via the bypass valve 24a. It can thus be prevented that superheated steam 23c with an inadequate steam quality arrives in the expansion machine 24 and damages it. If steam 23c is conducted into the bypass throttle 24b, the energy contained therein is not used to drive the expansion machine 24 and is instead lost.

(11) The working medium 23 is supplied to the waste heat recovery system 2 from a tank 26a in a manner controlled by a tank valve 26b.

(12) FIG. 2 shows the interaction of the control circuit 27 with the waste heat recovery system 2 and with a performance optimizer 1 connected upstream. In this example, the performance optimizer 1 retains the temperature T.sub.A and the mass flow rate m.sub.A of the exhaust gas 31a at the site of the evaporator 21 as input variables 11. From these, the performance optimizer 1 determines the set value T.sub.W,S (p.sub.W) for the temperature T.sub.W of the working medium 23c at the entry into the expansion machine 24, or the set value h.sub.W,S (p.sub.W) for the specific enthalpy h.sub.W of the working medium 23c at the entry into the expansion machine 24, in each case as a function of the pressure p.sub.W of the working medium 23c. The set values T.sub.W,S (p.sub.W), or h.sub.W,S (p.sub.W), are the defaults for the control circuit 27.

(13) The control circuit 27 is designed to regulate the control deviations 27a-27d, which are combined in a common cost function, and to output control variables 27e for this purpose. In the example shown in FIG. 2, this is primarily the mass flow rate mw of the working medium 23, which is adjusted via the delivery rate of the delivery device 26. When the delivered working medium 23 passes through the evaporator 21, it is converted into superheated steam 23c with a temperature T.sub.W and a specific enthalpy h.sub.W. The temperature T.sub.W and the specific enthalpy h.sub.W are supplied to the control circuit 27 as feedback.

(14) Based on a model 2a of the waste heat recovery system 2, the control circuit 27 contains a prediction module 28a, which is designed to precalculate the future development of the specific enthalpy h.sub.W, or the temperature T.sub.W. This serves to preplan the control interventions 27e and therefore lessen the effect of the inertia in the waste heat recovery system 2 between a control intervention 27e and the arrival of the resultant feedback. This increases the likelihood of the control being able to avoid activating the bypass valves 21a and/or 24a, in particular to avoid the violation of hard system restrictions.

(15) The optimization module 28b which determines the control interventions 27e as a solution of a quadratic program with quadratic restrictions is likewise based on the model 2a. A Kalman filter 29 is provided to determine further state variables 29a appearing in the model 2a from the accessible variables. The additional state variables 29a are supplied to the control circuit 27 and optionally also to the performance optimizer 1. The performance optimizer 1 can use, for example, the same model 2a of the waste heat recovery system 2 as the control circuit 27.

(16) FIG. 3 shows an exemplary plot of the temperature T.sub.A of the exhaust gas 31a at the entry of the evaporator 21 against the associated mass flow rate M.sub.A. Measured stationary engine operation points are marked with circles. The nominal operating points selected along a straight line in the characteristic map for piecewise linearization of the model 2a are marked with crosses.

(17) FIG. 4 shows, by way of example, how to regulate a control deviation 27a, existing at the time t.sub.K, between the actual specific enthalpy h.sub.W,K and the set value h.sub.W,S within a prediction horizon T.sub.ph with a sequence of preplanned control interventions 27e which take place within a control horizon T.sub.ch. The control variable u is firstly deflected sharply upwards in two steps (u.sub.k|k and u.sub.k+1|k) to quickly reduce the original control deviation 27a. To prevent an overshoot above the set value h.sub.W,S, an intervention then (u.sub.k+2|k) takes place in the opposite direction before the control variable u is finally (u.sub.k+3|k) stabilized to an intermediate level.

(18) Without the model-predictive control, the control variable u would have to approach its final value substantially more slowly in order to avoid overshooting it.