A METHOD AND SYSTEM FOR CONTROLLING THE ROTATIONAL SPEED OF AN EXPANDER IN A WASTE HEAT RECOVERY SYSTEM

Abstract

The invention relates to a method, system, and computer program product for controlling a waste heat recovery system associated with a vehicle powertrain, the powertrain comprising a combustion engine and a gearbox connected to the combustion engine, the waste heat recovery system comprising a working fluid circuit; an evaporator; an expander; a condenser; a reservoir for a working fluid and a pump arranged to pump the working fluid through the circuit, wherein the evaporator is arranged for heat exchange between the working fluid and at least one heat source, wherein the waste heat recovery system further comprises a cooling circuit arranged in connection to the condenser, and wherein the expander is mechanically coupled to the powertrain. The method comprises the steps of determining the pressure and temperature of the working fluid upstream of the expander; and controlling the rotational speed of the expander based on the determined pressure and temperature.

Claims

1. A method for controlling a waste heat recovery system associated with a powertrain of a vehicle, the powertrain comprising a combustion engine and a gearbox connected to the combustion engine, the waste heat recovery system comprising a working fluid circuit; an evaporator; an expander; a condenser; a reservoir for a working fluid and a pump arranged to pump the working fluid through the circuit, wherein the evaporator is arranged for heat exchange between the working fluid and at least one heat source, wherein the waste heat recovery system further comprises a cooling circuit arranged in connection to the condenser, and wherein the expander is mechanically coupled to the powertrain, wherein said method comprises: determining a pressure and a temperature of the working fluid upstream of the expander; and controlling a rotational speed of the expander based on the determined pressure and temperature.

2. The method according to claim 1, wherein the rotational speed of the expander is controlled based on a comparison between the pressure and a predetermined maximum pressure and a comparison between a difference between the temperature and a boiling point for the working fluid and a predetermined minimum temperature difference.

3. The method according to claim 2, wherein the rotational speed of the expander is increased if there is a risk that the pressure will exceed the predetermined maximum pressure and/or that the difference between the temperature and the boiling point of the working fluid will become smaller than the predetermined minimum temperature difference.

4. The method according to claim 3, wherein the risk is determined based on a prediction of high load on the combustion engine.

5. The method according to claim 1, wherein the rotational speed of the expander is controlled by controlling the gearbox and thereby a speed of the powertrain.

6. The method according to claim 1, wherein the rotational speed of the expander is controlled based on a combustion engine efficiency, an expander efficiency and/or a gearbox efficiency.

7. A waste heat recovery system associated with a powertrain of a vehicle, the powertrain comprising a combustion engine and a gearbox connected to the combustion engine, the waste heat recovery system comprising: a working fluid circuit; an evaporator; an expander; a condenser; a reservoir for a working fluid; a pump arranged to pump the working fluid through the circuit, wherein the evaporator is arranged for heat exchange between the working fluid and at least one heat source; a cooling circuit arranged in connection to the condenser, and wherein the expander is mechanically coupled to the powertrain; and a control unit adapted to determine a pressure and a temperature of the working fluid upstream of the expander, and to control a rotational speed of the expander based on the determined pressure and the temperature.

8. The system according to claim 7, wherein the control unit is adapted to control the rotational speed of the expander based on a comparison between the pressure and a predetermined maximum pressure and a comparison between a difference between the temperature and a boiling point for the working fluid and a predetermined minimum temperature difference.

9. The system according to claim 8, wherein the control unit is adapted to increase the rotational speed of the expander if there is a risk that the pressure will exceed the predetermined maximum pressure and/or that the difference between the temperature and the boiling point for the working fluid will become smaller than the predetermined minimum temperature difference.

10. The system according to claim 9, wherein the control unit is adapted to determine the risk based on a prediction of high load on the combustion engine.

11. The system according to claim 7, wherein the control unit is adapted to control the rotational speed of the expander by controlling the gearbox and thereby a speed of the powertrain.

12. The system according to claim 8, wherein the predetermined maximum pressure depends on constraints of the components of the waste heat recovery system.

13. The system according to claim 8, wherein the predetermined minimum temperature difference is between 10-60 degrees, preferably 20-30 degrees.

14. The system according to claim 7, wherein the control unit is adapted to control the rotational speed of the expander based on a combustion engine efficiency, an expander efficiency and/or a gearbox efficiency.

15. A vehicle comprising a waste heat recovery system associated with a powertrain of a vehicle, the powertrain comprising a combustion engine and a gearbox connected to the combustion engine, the waste heat recovery system comprising: a working fluid circuit; an evaporator; an expander; a condenser; a reservoir for a working fluid; a pump arranged to pump the working fluid through the circuit, wherein the evaporator is arranged for heat exchange between the working fluid and at least one heat source; a cooling circuit arranged in connection to the condenser, and wherein the expander is mechanically coupled to the powertrain; and a control unit adapted to determine a pressure and a temperature of the working fluid upstream of the expander, and to control a rotational speed of the expander based on the determined pressure and the temperature.

16. (canceled)

17. (canceled)

18. A computer program product stored on a non-transitory computer-readable medium, said computer program product for controlling a waste heat recovery system associated with a powertrain of a vehicle, the powertrain comprising a combustion engine and a gearbox connected to the combustion engine, the waste heat recovery system comprising a working fluid circuit; an evaporator; an expander; a condenser; a reservoir for a working fluid; and a pump arranged to pump the working fluid through the circuit, wherein the evaporator is arranged for heat exchange between the working fluid and at least one heat source, wherein the waste heat recovery system further comprises a cooling circuit arranged in connection to the condenser, and wherein the expander is mechanically connected to the powertrain, said computer program product comprising computer instructions to cause one or more electronic control units or computers to perform the following operations: determining a pressure and a temperature of the working fluid upstream of the expander; and controlling a rotational speed of the expander based on the determined pressure and temperature.

19. The computer program product according to claim 18, wherein the rotational speed of the expander is controlled based on a comparison between the pressure and a predetermined maximum pressure and a comparison between a difference between the temperature and a boiling point for the working fluid and a predetermined minimum temperature difference.

20. The computer program product according to claim 19, wherein the rotational speed of the expander is increased if there is a risk that the pressure will exceed the predetermined maximum pressure and/or that the difference between the temperature and the boiling point of the working fluid will become smaller than the predetermined minimum temperature difference.

21. The computer program product according to claim 20, wherein the risk is determined based on a prediction of high load on the combustion engine.

22. The computer program product according to claim 18, wherein the rotational speed of the expander is controlled by controlling the gearbox and thereby a speed of the powertrain.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] For fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various drawings, and in which:

[0036] FIG. 1 schematically illustrates a vehicle according to an embodiment of the invention;

[0037] FIG. 2 schematically illustrates a waste heat recovery system according to an embodiment of the invention;

[0038] FIG. 3 illustrates a diagram over the temperature-pressure relationship for working fluids according to an embodiment of the invention;

[0039] FIG. 4 schematically illustrates a flow chart for a method for controlling a waste heat recovery system according to an embodiment of the invention; and

[0040] FIG. 5 schematically illustrates a control unit or computer according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0041] FIG. 1 schematically shows a side view of a vehicle 1 according to an embodiment of the invention. The vehicle 1 has a powertrain 3 comprising a combustion engine 2 and a gearbox 4 connected to the combustion engine 2 and the driving wheels 6 of the vehicle 1. The vehicle 1 further comprises a waste heat recovery system 10 associated with the powertrain 3. The vehicle 1 may be a heavy vehicle, e.g. a truck or a bus. The vehicle 1 may alternatively be a passenger car. The vehicle may be a hybrid vehicle comprising an electric machine (not shown) in addition to the combustion engine 2.

[0042] FIG. 2 schematically shows a waste heat recovery system 10 associated with a powertrain 3 of a vehicle 1 according to an embodiment of the invention. The vehicle 1 is suitably configured as described in FIG. 1. The waste heat recovery system 10 comprises a working fluid circuit 12; an evaporator 14; an expander 16; a condenser 18; a reservoir 20 for a working fluid WF and a pump 22 arranged to pump the working fluid WF through the circuit 12, wherein the evaporator 14 is arranged for heat exchange between the working fluid WF and at least one heat source 24, wherein the waste heat recovery system 10 further comprises a cooling circuit 26 arranged in connection to the condenser 18 and wherein the expander 16 is mechanically connected to the powertrain 3.

[0043] The waste heat recovery system 10 comprises a control unit 30 adapted to determine the pressure P and temperature T of the working fluid WF upstream of the expander 16 and to control the rotational speed of the expander 16 based on the determined pressure P and temperature T. A computer 32 may be connected to the control unit 30. The waste heat recovery system 10 further comprises at least one pressure sensor 36 and at least one temperature sensor 38 for determining the current pressure P and the current temperature T of the working fluid WF. The at least one pressure sensor 36 and the at least one temperature sensor 38 are suitably arranged in communication with the working fluid circuit 12 upstream of the expander 18 and downstream of the pump 22. The control unit 30 is arranged in connection to the evaporator 14, the expander 16, the cooling circuit 26, the pump 22, the at least one pressure sensor 36 and the at least one temperature sensor 38.

[0044] The at least one heat source 24 connected to the evaporator 14 may be exhaust gases from the combustion engine 2, an exhaust gas recirculation system (EGR), the cooling fluid of the combustion engine 2, the combustion engine 2 itself or any other hot component associated with the combustion engine 2. The at least one heat source 24 is herein illustrated as a medium passing through the evaporator 14. The at least one heat source 24 is herein illustrated as arrows and may be exhaust gases from the combustion engine 2. The waste heat recovery system 10 may comprise a plurality of heat sources 24. The evaporator 14 is suitably a heat exchanger connected to the at least one heat source 24 and the working fluid circuit 12. The heat transfer between the working fluid WF and the heat source 24 is an exchange of energy resulting in a change in temperature. The waste heat recovery system 10 is suitably based on an organic Rankine cycle. The working fluid WF is thus suitably organic, such as ethanol or acetone. The waste heat recovery system 10 is thus configured such that the liquid working fluid WF is pumped from low pressure to high pressure and enters the evaporator 14. The working fluid WF is thereby heated by the at least one heat source 24 connected to the evaporator 14 and the working fluid WF is thus evaporated. The vapour is then expanded in the expander 16 whereby mechanical work is produced and transferred to the powertrain 3, whereby the temperature and the pressure of the vapour is decreased. The vapour thereafter enters the condenser 18 where condensation through heat exchange between the vapour and the cooling fluid of the cooling circuit 26 brings the working fluid WF back to its initial liquid state. Thus, the heat source 24 is providing the energy entering the waste heat recovery system 10 and the energy is leaving the waste heat recovery system 10 as mechanical work via the expander 16 and as heat via the cooling circuit 26 cooling the condenser 18. The temperature of the working fluid WF in the waste heat recovery system 10 thus depends on the amount of energy entering the system 10 and the amount of energy leaving the system 10.

[0045] Only vapour should enter the expander 16 and the waste heat recovery system 10 therefore comprises a bypass arrangement 34, such that in the case where the working fluid WF is still in a liquid state downstream of the evaporator 14, the working fluid WF is bypassing the expander 16 through the bypass arrangement 34. The expander 16 is suitably a fixed displacement expander, such a piston expander. The expander 16 may be mechanically connected directly to the combustion engine 2 or to the gearbox 4.

[0046] The pump 22 pressurizing and circulating the working fluid WF through the circuit 12 may be damaged if the working fluid WF entering the pump 22 is not in a liquid state. Thus in the case where the temperature downstream of the condenser 18 is too high, such that the working fluid WF is not in a liquid state, the pressure in the reservoir 20 may be increased. This way, the working fluid WF is brought to a liquid state and may be pumped by the pump 22. The pump 22 is suitably electrically driven.

[0047] The cooling circuit 26 connected to the condenser 18 may be part of the combustion engine cooling system or a separate cooling system. The cooling fluid in the cooling circuit 26 may thereby be pumped by a cooling pump (not shown) driven by the combustion engine 2 or by an electric machine (not shown).

[0048] The waste heat recovery system 10 may comprise one or more heat exchangers 14. The waste heat recovery system 10 may for example comprise a recuperator arranged to pre-heat the working fluid WF before entering the evaporator 14. The waste heat recovery system 10 may also comprise one or more condensers 18, such that cooling down of the working fluid WF may be performed in multiple steps. Furthermore, the system 10 may comprise one or more expanders 16.

[0049] FIG. 3 shows a diagram over the relationship between temperature T and pressure P for working fluids WF according to an embodiment of the invention. Two different working fluids WF are illustrated in this diagram and just as an example the solid line may represent ethanol and the dashed line may represent acetone. Any of the working fluids WF may constitute the working fluid in the waste heat recovery system 10 as disclosed in FIG. 2. The diagram shows the normal boiling point BP1n, BP2n for the respective working fluid. The normal boiling point is the temperature at which the working fluid WF evaporates in atmospheric pressure. The relationship between temperature T and pressure P is thus different for different types of working fluid WF. The boiling point BP for a working fluid WF vanes with the pressure P. If the pressure increases, the boiling point BP increases.

[0050] The diagram further illustrates the temperature difference T between a determined temperature T1, T2 and the boiling point BP1, BP2 at the determined pressure P1, P2 for the respective working fluid WF. This temperature difference T is also called the level of superheat. A certain level of superheat is desired in the waste heat recovery system 10 in order to obtain optimal efficiency of the expander 16. The diagram shows the desired level of superheat defined as a predetermined minimum temperature difference T.sub.min, for the working fluid WF illustrated as a dashed line. The predetermined minimum temperature difference T.sub.min is suitably between 10-60 degrees, preferably between 20-30 degrees.

[0051] The components of the waste heat recovery system 10 set a constraint on the maximum pressure P.sub.max of the working fluid WF that the system 10 can handle without problems. If the pressure P of the working fluid WF is higher than the maximum pressure P.sub.max on the high pressure side of the waste heat recovery system 10, the various components may be damaged. Such maximum pressure P.sub.max is predetermined for the relevant waste heat recovery system 10.

[0052] FIG. 4 shows a flowchart for a method for controlling a waste heat recovery system 10 associated with a powertrain 3 of a vehicle 1. The powertrain 3 comprises a combustion engine 2 and a gearbox 4 connected to the combustion engine 2. The waste heat recovery system 10 comprises a working fluid circuit 12; an evaporator 14; an expander 16; a condenser 18; a reservoir 20 for a working fluid WF and a pump 22 arranged to pump the working fluid WF through the circuit 12, wherein the evaporator 14 is arranged for heat exchange between the working fluid WF and at least one heat source 24, wherein the waste heat recovery system 10 further comprises a cooling circuit 26 arranged in connection to the condenser 18, and wherein the expander 16 is mechanically coupled to the powertrain 3. The method comprises the steps of: [0053] determining s101 the pressure P and temperature T of the working fluid WF upstream of the expander 16; and [0054] controlling s102 the rotational speed of the expander 16 based on the determined pressure P and temperature T.

[0055] The waste heat recovery system 10 is suitably configured as disclosed in FIG. 2, wherein the control unit 30 is adapted to perform the method steps described herein.

[0056] The rotational speed of the expander 16 may be controlled based on a comparison between the determined pressure P and a predetermined maximum pressure P.sub.max and a comparison between a difference between the determined temperature and the boiling point for the working fluid T and a predetermined minimum temperature difference T.sub.min. The temperature T of the working fluid WF upstream of the expander 16 is higher than the boiling point BP when it is a vapour. The boiling point BP for the working fluid WF depends on the pressure P. The difference between the actual temperature of the working fluid and the boiling point T at the current pressure P thus indicates the level of superheat of the working fluid WF. An example of the relationship between the pressure P and temperature T of the working fluid is illustrated in FIG. 3.

[0057] The rotational speed of the expander 16 is suitably increased when the determined pressure P exceeds the predetermined maximum pressure P.sub.max and/or the difference between the determined temperature and the boiling point for the working fluid T is smaller than the predetermined minimum temperature difference T.sub.min. When the rotational speed of the expander 16 is increased a greater mass flow of working fluid WF can be handled by the expander 16 and the pressure of the working fluid WF in the circuit 12 upstream of the expander 16 will decrease. When the temperature difference T is smaller than the predetermined minimum temperature difference T.sub.min, there is a great risk that the working fluid WF will shift to liquid phase which may damage the expander 16. The higher the pressure P of the working fluid WF the higher is the boiling point BP of the working fluid WF. Thus, by decreasing the pressure P the boiling point BP of the vapour is decreased and the difference between the determined temperature and the boiling point T will thereby increase.

[0058] The rotational speed of the expander 16 may be increased if there is a risk that the pressure P of the working fluid WF will exceed the predetermined maximum pressure P.sub.max and/or that the difference between the temperature and the boiling point of the working fluid T will become smaller than the predetermined minimum temperature difference T.sub.min. The rotational speed of the expander 16 is thus suitably increased if the determined level of superheat is lower than the predetermined minimum level of superheat. This way, the pressure P of the working fluid WF is pre-emptively decreased and the damage of the waste heat recovery system 10 is avoided while optimizing the energy recovery.

[0059] The risk that the pressure P will exceed the predetermined maximum pressure P.sub.max and/or that the difference between the temperature and the boiling point of the working fluid T will become smaller than the predetermined minimum temperature difference T.sub.min may be determined based on a prediction of high load on the combustion engine 2. When the load on the combustion engine 2 is high, the temperature of the at least one heat source 24 will increase and the temperature T and pressure P of the working fluid WF will thereby also increase. Thus, by predicting that the load on the combustion engine 2 will be high, it is predicted that the temperature T and pressure P of the working fluid WF will increase. The high load on the combustion engine 2 may be predicted based on the topography of the route of the vehicle 1.

[0060] The rotational speed of the expander 16 may be controlled by controlling the gearbox 4 and thereby the rotational speed of the powertrain 3. Since the expander 16 is mechanically connected to the powertrain 3, the rotational speed of the expander 16 is directly connected to the speed of the powertrain 3 and thus the speed of the combustion engine 2. The rotational speed of the expander 16 is thus suitably increased by controlling the gearbox 4 to a lower gear. By controlling the gearbox 4, such that a lower gear is engaged, the speed of the combustion engine 2/the powertrain 3 will increase and the rotational speed of the expander 16 will thereby also increase. If the gearbox 4 is controlled to shift to a higher gear, the speed of the combustion engine 2 and the powertrain 3 will decrease, and the rotational speed of the expander 16 will thereby also decrease.

[0061] The rotational speed of the expander 16 may be controlled based on the combustion engine efficiency, the expander efficiency and/or the gearbox efficiency. The rotational speed of the expander 16 may thus be controlled based on the resulting impact on the overall efficiency of the powertrain 3. By considering the overall efficiency of the powertrain 3 the rotational speed of the expander 16 can be controlled while obtaining the currently most energy optimal engine speed. The gearbox 4 is thus preferably controlled based on the resulting impact on the combustion engine efficiency, the expander efficiency and/or the gearbox efficiency when controlling the rotational speed of the expander.

[0062] The method may comprise to increase the rotational speed of the expander 16 by controlling the gearbox 4 to a lower gear, only if the resulting negative impact on the overall efficiency of the powertrain 3 is smaller than the resulting increase of energy recovery. That is, if the decrease in overall efficiency of the powertrain 3 will be greater than the increase of recovered energy by shifting to a lower gear, the gearbox 4 will not be controlled to a lower gear. Instead, the at least one heat source 24 will be controlled to bypass the evaporator 14.

[0063] FIG. 5 schematically illustrates a device 500. The control unit 30 and/or computer 32 described with reference to FIG. 2 may in a version comprise the device 500. The term link refers herein to a communication link which may be a physical connection such as an optoelectronic communication line, or a non-physical connection such as a wireless connection, e.g. a radio link or microwave link. The device 500 comprises a non-volatile memory 520, a data processing unit 510 and a read/write memory 550. The non-volatile memory 520 has a first memory element 530 in which a computer program, e.g. an operating system, is stored for controlling the function of the device 500. The device 500 further comprises a bus controller, a serial communication port, I/O means, an A/D converter, a time and date input and transfer unit, an event counter and an interruption controller (not depicted). The non-volatile memory 520 has also a second memory element 540.

[0064] There is provided a computer program P which comprises routines for a method for controlling a waste heat recovery system 10 associated with a powertrain 3 of a vehicle 1 according to the invention. The computer program P comprises routines for determining a pressure P and temperature T of the working fluid WF upstream of the expander 16. The computer program P comprises routines for controlling the rotational speed of the expander 16 based on the determined pressure P and temperature T. The computer program P comprises routines for controlling the rotational speed of the expander 16 based on a comparison between the determined pressure P and a predetermined maximum pressure P.sub.max and a comparison between a difference between the determined temperature and the boiling point for the working fluid T and a predetermined minimum temperature difference T.sub.min. The computer program P comprises routines for increasing the rotational speed of the expander 16 when the determined pressure P exceeds the predetermined maximum pressure P.sub.max and/or the difference between the determined temperature and the boiling point for the working fluid T is smaller than the predetermined minimum temperature difference T.sub.min. The computer program P comprises routines for increasing the rotational speed of the expander 16 if there is a risk that the pressure P of the working fluid WF will exceed the predetermined maximum pressure P.sub.max and/or that the difference between the temperature and the boiling point of the working fluid T will become smaller than the predetermined minimum temperature difference T.sub.min. The computer program P comprises routines for controlling the rotational speed of the expander by controlling the gearbox 4 and thereby the rotational speed of the powertrain 3. The computer program P comprises routines for controlling the rotational speed of the expander 16 based on the combustion engine efficiency, the expander efficiency and/or the gearbox efficiency. The program P may be stored in an executable form or in a compressed form in a memory 560 and/or in a read/write memory 550.

[0065] Where the data processing unit 510 is described as performing a certain function, it means that the data processing unit 510 effects a certain part of the program stored in the memory 560 or a certain part of the program stored in the read/write memory 550.

[0066] The data processing device 510 can communicate with a data port 599 via a data bus 515. The non-volatile memory 520 is intended for communication with the data processing unit 510 via a data bus 512. The separate memory 560 is intended to communicate with the data processing unit 510 via a data bus 511. The read/write memory 550 is adapted to communicating with the data processing unit 510 via a data bus 514.

[0067] When data are received on the data port 599, they are stored temporarily in the second memory element 540. When input data received have been temporarily stored, the data processing unit 510 is prepared to effect code execution as described above.

[0068] Parts of the methods herein described may be effected by the device 500 by means of the data processing unit 510 which runs the program stored in the memory 560 or the read/write memory 550. When the device 500 runs the program, methods herein described are executed.

[0069] The foregoing description of the preferred embodiments of the present invention is provided for illustrative and descriptive purposes. It is not intended to be exhaustive or to restrict the invention to the variants described. Many modifications and variations will obviously be apparent to one skilled in the art. The embodiments have been chosen and described in order best to explain the principles of the invention and its practical applications and hence make it possible for specialists to understand the invention for various embodiments and with the various modifications appropriate to the intended use.