SE540584C2

SE540584C2 - A method for detecting insufficient working fluid level in awaste heat recovery system and a waste heat recovery system

Info

Publication number: SE540584C2
Application number: SE1750199A
Authority: SE
Inventors: Höckerdal Erik; Johansson Svante; Johansson Björn
Original assignee: Scania Cv Ab
Priority date: 2017-02-24
Filing date: 2017-02-24
Publication date: 2018-10-02
Also published as: DE102018001235A1; SE1750199A1

Abstract

The present invention relates to a method and system for detecting insufficient working fluid (WF) level in a waste heat recovery system (4), comprising a working fluid circuit (20), a working fluid reservoir (28) for the working fluid and a pump (30) arranged to pump the working fluid through the circuit (20). The working fluid is arranged to pass through an evaporator (22), an expander (24) and a condenser (26) connected to a cooling arrangement. The waste heat recovery system (4) is connected to a heat source (32), such as an exhaust gas flow which is connected to the evaporator 22. The method comprises the steps of:- pressurizing (s101) the working fluid reservoir (28);- detecting (s102) the working fluid (WF) pressure in the working fluid circuit (20); and- determining (s103) if pressure is increased in the circuit (20) during the pressurizing step (s101). Suitably it is detected whether the pressure is increased more than a pre-determined pressure increase value. A pressure increase value below the pre-determined pressure increase value indicates insufficient level of working fluid (WF) in the circuit (20).

Description

A method for detecting insufficient working fluid level in a waste heat recovery system and a waste heat recovery system TECHNICAL FIELD The present invention relates to a method for detecting insufficient working fluid level in a waste heat recovery system and a waste heat recovery system and a vehicle comprising such a waste heat recovery system, a computer program and a computer-readable medium according to the appended claims.

BACKGROUND Vehicle manufacturers are today striving to increase engine efficiency and reduce fuel consumption. This is specifically an issue for manufacturers of heavy vehicles, such as trucks and buses. One way of improving engine efficiency and fuel consumption is waste heat recovery. In vehicles with an internal combustion engine some of the energy from the fuel is dissipated as heat through the exhaust pipe and the engine cooling system. By the use of a waste heat recovery (WHR) system the heat from the exhaust gases may instead be used for example to heat various vehicle components or to produce mechanical work or electricity. Such mechanical work may for example be transferred to the powertrain and thus be used to propel the vehicle.

A waste heat recovery system typically comprises at least one heat exchanger which transfers heat between a heat source, such as exhaust gases, and a working fluid. The heat transfer between the heat source and the working fluid is an exchange of energy resulting in a change in temperature. A waste heat recovery system may also be based on a Rankine cycle and thus comprise a working fluid, a pump for circulating the working fluid in a circuit, at least one evaporator (heat exchanger), an expansion device, i.e. an expander, and a condenser. The condenser is typically connected to a cooling system, which may be part of the engine cooling system. The working fluid in such waste heat recovery system is suitably in a liquid state to start with. The pump pressurizes the working fluid which is pumped through the evaporator. The working fluid is heated by for example exhaust gases led through the evaporator and the working fluid thereby evaporates. The vapour may subsequently be expanded in the expander. By means of the expander the recovered heat may thereby be converted into mechanical work. The vapour is thereafter cooled in the condenser, such that the working fluid is brought back to its initial liquid state.

The waste heat recovery system is hermetically closed to keep the working fluid inside the system. The working fluids used in the WHR-systems often have properties that make them unsuitable for release in the environment and can be for example flammable alcohols, cyclopentane and refrigerants. Therefore, the WHR system is made liquid tight and thus, during normal operation there is no need to refill fluid in the system. However, in case of leakages of working fluid in the WHR system, liquid needs to be re-filled, and service of the system is required. Therefore, there is a need for a reliable method of detecting leakage in the system.

JP2014-134175A discloses a method for volume adjustment mechanism which is able to detect leakage of a working fluid in a waste heat recovery system. The system comprises a volume adjustment mechanism 60 which may supply a refrigerant to a radiator 30. The volume adjustment mechanism 60 includes a storage chamber 63 which is disposed between the downstream side of the radiator 30 and the upstream side of a boiler 50 when viewed in a circulation direction of the working fluid circuit 11 and stores the refrigerant. Further, the volume adjustment mechanism 60 includes: a piston 62 which moves in the storage chamber 63 and varies the volume of the storage chamber 63; and a control part 75 which controls the piston 62 and determines if refrigerant leakage from the working fluid circuit 11 occurs on the basis of a difference between a reference travel distance and an actual travel distance of the piston 62. The system requires continuous monitoring of the difference in the reference movement amount and the actual amount of the piston 62 in the volume adjustment mechanism 60 to be able to confirm the presence or absence of working fluid leakage in the circuit.

Thus, despite known solutions there is a need to simplify and improve the existing leakage detecting methods of working fluid in waste heat recovery systems.

SUMMARY OF THE INVENTION In view of the prior art, there is still a need to develop a method for detecting insufficient level of working fluid in a waste heat recovery system in a simple and reliable way. There is also a need for a detection method and system with simple data processing needs. Further, there is a need for a leakage detecting method which is robust, simple and which does not essentially increase the weight of the system and thus does not negatively affect fuel consumption of a vehicle.

An objective of the present invention is thus to provide an advantageous method for detecting insufficient level of working fluid in a waste heat recovery system associated with an internal combustion engine of a vehicle, which method is reliable and simple. By means of detecting insufficient level of working fluid, it will be possible to indicate presence of leakage of working fluid in a waste heat recovery system.

A further objective of the invention is to provide a method which reduces the risk for operational disturbances.

Another object is to provide a waste heat recovery system in which means for detecting insufficient level of working fluid are provided while the system is robust, simple and does not essentially increase the weight and cost of the system and thus does not negatively affect fuel consumption of a vehicle.

The herein mentioned objectives are achieved by a method for detecting insufficient level of a working fluid in a waste heat recovery system associated with an internal combustion engine, a waste heat recovery system comprising means for detecting insufficient level of working fluid, a vehicle, a computer program and a computerreadable medium according to the appended independent claims.

Accordingly, the present invention relates to a method for detecting insufficient level of working fluid in a waste heat recovery system arranged in connection with an internal combustion engine of a vehicle. The waste heat recovery system comprises a working fluid circuit, a working fluid reservoir for the working fluid and a pump arranged to pump the working fluid through the circuit. The working fluid is arranged to pass through an evaporator, an expander and a condenser connected to a cooling arrangement. The waste heat recovery system is connected to a heat source, which in turn is connected to the evaporator. The method of the present disclosure comprises the steps of: - pressurizing the working fluid reservoir by actuating a pressurizing means acting on the working fluid reservoir at a fluid temperature in the working fluid circuit which is below the boiling point of the working fluid; - detecting the working fluid pressure in the working fluid circuit by means of at least one pressure sensor arranged downstream of the pressurizing means; and - determining if pressure is increased in the circuit during the pressurizing step more than a pre-determined pressure increase value, wherein a pressure increase value below the pre-determined pressure increase value indicates insufficient level of working fluid in the circuit.

In the step of determining if pressure is increased in the circuit during the pressurizing step, it is possible to use only one pressure sensor arranged to detect fluid pressure of the working fluid in the working fluid circuit located downstream of the pressurizing means. The pressure value detected may be compared with a start pressure value measured before pressurizing and if increase in the pressure is detected, there is a sufficient fluid level in the system. In the absence of increased pressure, i.e. if no increase in the pressure is detected or the pressure increase value is below the pre-determined pressure increase value, there is an insufficient fluid level in the system. When the pressure increase value is determined the detected pressure value is compared with pre-defined calibrated values. If no increase is detected or if the increase is below a pre-determined calibrated value, there is an insufficient level of working fluid in the working fluid circuit.

The present method is simple, robust and reliable. The at least one pressure sensor can be chosen from existing pressure sensors in the waste heat recovery system. Thus, since only one pressure sensor is required to detect the working fluid pressure and an existing pressure sensor device can be used for the detection, the amount of components in the system can be minimized.

The method steps are suitably performed during a cold start of the engine. In this way regular check of the system is provided. By cold start is meant that when the engine is started, the temperature in the waste heat recovery circuit and its connections to the heat sources and sinks is below the boiling temperature of the working fluid at the present ambient pressure, whereby the working fluid remains in liquid phase in the whole waste heat recovery system. Thus, the check is performed at a phase in which the working fluid has not reached the boiling point temperature. To further prevent excessive heating of the working fluid, the evaporator can be bypassed by the heat source during the method steps.

The method may further comprise a step of generating an error message in case the pressure increase value determined during the pressurizing step is below the predetermined pressure increase value. In this way it can be indicated to the operator of the vehicle that service is required. Alternatively, the error message is generated in the form of an error code which is readable during the service of the vehicle.

The step of detecting the working fluid pressure may be performed by means of a first pressure sensor arranged upstream of the evaporator and downstream of the pump, a second pressure sensor arranged downstream of the condense and upstream of the working fluid reservoir, a third pressure sensor arranged upstream of the expander and downstream of the evaporator, a fourth pressure sensor arranged downstream of the second pressure sensor and upstream of the reservoir, a fifth pressure sensor arranged downstream of the reservoir and upstream of the working fluid pump, a sixth pressure sensor arranged downstream of the expander and upstream of the condenser and/or a seventh pressure sensor arranged in connection with the working fluid reservoir. Each of these sensor devices can be a part of an existing waste heat recovery system. Preferably, the at least one pressure sensor is the first pressure sensor and/or the second sensor device, since each of these sensor devices are normally used in the waste heat recovery systems. By using several sensor devices for pressure detection, the accuracy of the method can be improved.

The step of detecting the working fluid pressure may further include using a pressurizing means pressure sensor. The pressure sensor is thus connected to the pressurizing means, and enables detection of pressure used for pressurizing. In the step of determination, the detected pressure may then be compared with the detected pressure value in the circuit obtained by means of the at least pressure sensor.

The pressurizing can be performed by means of utilizing compressed air obtained from a compressed air system of the vehicle. The compressed air can be used for actuation of the pressurizing means. In this way available means in a vehicle can be used, for example compressed air from the braking system of the vehicle.

The present invention further relates to a waste heat recovery system arranged in connection with an internal combustion engine of a vehicle, the waste heat recovery system comprising a working fluid circuit, a working fluid reservoir for the working fluid and a pump arranged to pump the working fluid through the circuit. The working fluid is arranged to pass through an evaporator, an expander and a condenser connected to a cooling arrangement. The waste heat recovery system is connected to a heat source connected to the evaporator. The waste heat recovery system further comprises a pressurizing means arranged to act on the working fluid reservoir at a fluid temperature below the boiling point of the working fluid. The working fluid circuit comprises at least one pressure sensor adapted to detect the working fluid pressure in the working fluid circuit. The at least one pressure sensor is connected to a control unit arranged to determine if pressure is increased in the circuit when the pressurizing means acts on the working fluid reservoir. Thereby, in the absence of sufficient pressure increase in the system, i.e. when the pressure increase value is below a pre-determined pressure increase value, insufficient level of working fluid in the circuit is indicated. Thereby, the control unit can be arranged to determine that there is a leakage in the working fluid circuit.

In the system the pressure sensor is suitably a pressure sensor selected from a first pressure sensor arranged upstream of the evaporator and downstream of the pump, a second pressure sensor arranged downstream of the condenser and upstream of the working fluid reservoir, a third pressure sensor arranged upstream of the expander and downstream of the evaporator, a fourth pressure sensor arranged downstream of the second pressure sensor and upstream of the reservoir, a fifth pressure sensor arranged downstream of the reservoir and upstream of the working fluid pump, a sixth pressure sensor arranged downstream of the expander and upstream of the condenser and/or a seventh pressure sensor arranged in connection with the working fluid reservoir.

The control unit may be arranged to determine if pressure is increased in the circuit when the pressurizing means acts on the working fluid reservoir by means of the at least one pressure sensor, which may be only one pressure sensor, arranged downstream of the pressurizing means. The control system comprises means to compare the pressure value detected with a start pressure value measured before the pressurizing, and if sufficient increase in the pressure is determined, there is a sufficient fluid level in the system. In the absence of sufficiently increased pressure, i.e. when the pressure increase value is below a pre-determined pressure increase value and if the control system after comparison of the pressure values does not determine increase in the pressure above the pre-determined level, there is an insufficient fluid level in the system. The accuracy of the system can be increased if additional pressure sensors are used.

The step of detecting the working fluid pressure may further include using a pressurizing means pressure sensor. The pressure sensor is thus connected to the pressurizing means, and enables detection of pressure used for pressurizing. In the step of determination, the detected pressure may then be compared with the detected pressure value in the circuit obtained by means of the at least pressure sensor. In the corresponding way as above, if the control system determines absence of a pressure increase or a pressure increase value below the predetermined pressure increase value, there is an insufficient working fluid level in the waste heat recovery system.

The control unit can be adapted to generate an error message in case a pressure increase value of the fluid pressure below a pre-determined pressure increase value is determined, i.e. in the absence of pressure increase, when the pressurizing means acts on the working fluid reservoir. In this way the operator of the vehicle can be notified that service is required. The message may be in the form of an error code which can be notified during the service of the vehicle.

The system may further comprise a bypass pipe arranged to let the heat source bypass the evaporator when the pressurizing means acts on the working fluid reservoir. In this way it can be assured that the working fluid is not heated to above boiling point when a possible leakage is being detected.

The pressurizing means is suitably arranged to be actuated by means of compressed air obtained from a compressed air system of the vehicle. The compressed air system may be for example braking system of the vehicle, whereby existing components in the vehicle can be used for the detection.

The pressurizing means can comprise an elastic bladder, such as a rubber bladder, whereby a simple device for the pressurizing is provided. Alternatively the pressurizing means may comprise bellows or a piston arrangement.

The present invention also relates to a vehicle, which comprises the waste heat recovery system as described above.

The invention also relates to a computer program, wherein said computer program comprises program code for causing an electronic control unit or a computer connected to the electronic control unit to perform the method steps defined above.

Furthermore, the present invention relates to a computer-readable medium comprising a program code stored on the computer-readable medium for performing the method steps as defined above, when said computer program is run on an electronic control unit or a computer connected to the electronic control unit.

Further objects, features and advantages of the present invention are described below in the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS 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 diagrams, and in which: Fig. 1 schematically illustrates a vehicle comprising a waste heat recovery system of the present disclosure; Fig. 2a schematically illustrates an example embodiment of a waste heat recovery system; Fig. 2b schematically illustrates an example embodiment of a waste heat recovery system; Fig. 3a-3c schematically illustrate the principle of pressurizing according to the present disclosure; Fig. 4 schematically illustrates a flow chart for a method for detecting leakage of working fluid according to an embodiment of the invention; and Fig. 5 schematically illustrates a control unit or computer according to an embodiment of the invention.

DETAILED DESCRIPTION Fig. 1 schematically shows a side view of a vehicle 1 comprising an internal combustion engine 2, a waste heat recovery system 4 associated with the internal combustion engine 2. The vehicle 1 further comprises a gearbox 6 connected to the driving wheels 8 of the vehicle 1. 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) to produce moving force to the vehicle in addition to the internal combustion engine 2.

Reference is now made to Fig. 2a and 2b which each schematically illustrates an example of a waste heat recovery system 4 of the present invention which is arranged in connection with an internal combustion engine 2 of a vehicle 1.

The waste heat recovery system 4 comprises a working fluid circuit 20, wherein the working fluid is arranged to pass through an evaporator 22, an expander 24 and a condenser 26. The working fluid circuit comprises a reservoir 28 for the working fluid WF and a pump 30 arranged to pump the working fluid WF through the circuit 20. The evaporator 22 is arranged for heat exchange between the working fluid WF and a heat source 32 in connection with the internal combustion engine 2. The condenser 26 of the waste heat recovery system 4 is suitably connected to a cooling system of the vehicle or a separate cooler, which cooling system and/or cooler are not a part of the present invention and thus not shown in detail in the drawings.

The waste heat recovery system 4 is connected, or fluidly connected, to the heat source 32, which in turn is connected to the evaporator 22. The heat source is herein exemplified as exhaust gases from the internal combustion engine 2. The exhaust gases 32 are illustrated as an arrow in an exhaust pipe 33. The exhaust gases 32 may be controlled to pass through the evaporator 22 via a first pipe 34 wherein a fluid director 36 is arranged to close a bypass pipe 35. Alternatively, the exhaust gases 32 may be controlled to pass through the bypass pipe 35 and thus bypass the evaporator 22. In this case the fluid director 36 is arranged to close first pipe 34. The waste heat recovery system 4 may comprise a plurality of evaporators 22, each connected to a different heat source 32. The evaporator 22 is suitably a heat exchanger connected to the heat source 32 and the working fluid circuit 20. The heat transfer between the working fluid WF and the heat source 32 is an exchange of energy resulting in a change in temperature.

The waste heat recovery system 4 is suitably based on an organic Rankine cycle. The waste heat recovery system is also hermetically closed to keep the working fluid inside the system and thus, during normal operation there is no need to re-fill fluid in the system. The working fluids can be for example flammable alcohols, cyclopentane or refrigerants. According to one variant the working fluid WF is organic, such as ethanol or R245fa.

The waste heat recovery system 4 is configured such that the liquid working fluid WF is pumped from low pressure to high pressure and enters the evaporator 22. The working fluid WF is heated by the heat source 32 connected to the evaporator 22 and the working fluid WF is thus evaporated. The vapour is then expanded in the expander 24 whereby mechanical work can be produced and the temperature and the pressure of the vapour is decreased. The mechanical work may for example be transferred to the crankshaft of the internal combustion engine 2 and thus be used to propel the vehicle 1 or the mechanical work may be used to drive for example a generator.

After the expander 24 the vapour enters the condenser 26 where condensation through heat exchange between the vapour and cooling fluid of a cooling system brings the working fluid WF back to its initial liquid state. Thus, the heat source 32 provides energy entering the waste heat recovery system 4 and the energy leaves the waste heat recovery system 4 as mechanical work via the expander 24 and as heat via the cooling system 6 cooling the condenser 26.

The temperature in the waste heat recovery system 4 depends on the amount of energy entering the system 4 and the amount of energy leaving the system 4. Only vapour should enter the expander 24 and the waste heat recovery system 4 therefore comprises a bypass arrangement 25, such that in the case where the working fluid WF is still in a liquid state, or steam with insufficient superheat downstream of the evaporator 22, the working fluid WF can bypass the expander 24 through the bypass arrangement 25 to avoid condensation within, or risk of damaging, the expander. The pump 30 pressurizing and circulating the working fluid WF can be electrically driven.

The waste heat recovery system 4 may comprise one or more evaporators/heat exchangers 22. The waste heat recovery system 4 may for example comprise a recuperator arranged to pre-heat the working fluid WF before entering the evaporator 22. The waste heat recovery system 4 may also comprise one or more condensers 26, such that cooling down of the working fluid WF may be performed in multiple steps. Furthermore, the waste heat recovery system 4 may comprise one or more expanders 24. The expander 24 may be a turbine or a piston expander.

In the waste heat recovery system 4, during the normal operating conditions there should always be a flow of working fluid WF through the circuit. To assure the flow of working fluid, the circuit should contain a sufficient amount of working fluid, which can be re-filled to the system during manufacture. Since the circuit is hermetically sealed, the amount of fluid should be constant unless there is a leakage of the fluid in the circuit. Therefore, there is normally no need to re-fill working fluid into the circuit. During the normal operation of the waste heat recovery system, the working fluid circuit normally comprises a first pressure sensor 41 arranged upstream of the evaporator 22 and downstream of the pump 30, and a second pressure sensor 42 arranged downstream of the condenser 26 and upstream of the working fluid reservoir 28.

To determine an insufficient fluid level and thus possible leakage in the working fluid circuit 20 the working fluid system 4 according to the present invention comprises a pressurizing means 40 arranged to act on the working fluid reservoir 28 and to increase the pressure of the working fluid WF in the reservoir 28 and consequently in the working fluid circuit 20. During the pressurizing, the pump 30 is suitably shut off or is operated at low flow for increased accuracy. In some cases the expander bypass line 25 of the expander 24, e.g. in case of piston expanders, is opened to allow the working fluid bypass the expander. The pressurizing means 40 can be arranged to be actuated by means of compressed air obtained from a compressed air system of the vehicle. The compressed air system may be for example the braking system of the vehicle. The pressurizing means 40 may be of any suitable type, and may comprise e.g. a rubber bladder filled with compressed air and emptied by means of a pressing device. Rubber may be replaced with any suitable elastic material such as synthetic plastic material.- Alternatively the pressurizing means may be a piston arrangement, or for example a bellows. According to one variant, the pressurizing means 40 is a rubber bladder, whereby a simple, light-weight and robust device is provided.

Fig. 3a, 3b and 3c show the principle of pressurizing the fluid or the fluid reservoir in a working fluid circuit 20. Flerein the figures are used only to illustrate the principle and the components shown in the figures do not correspond to the components used in the waste heat recovery system of the present disclosure. Fig. 3a shows a working fluid circuit 20 with a sufficient working fluid level 23. It can be seen that the fluid level 23 is higher than the level defined by stop means 21. Thus, the fluid is pressurized when the pressurizing means 40, which is a piston in the shown example, is pressed towards the fluid surface level 23. In Fig. 3b it can be seen that the fluid level 23 is below the level defined by the stop means 21. Therefore, when the piston 40 is pressed towards the fluid level 23, it will not reach the fluid level before it reaches the stop means 21, and thus the fluid will not be pressurized since the liquid level is too low. Since the fluid is filled to a required level during the assembly of the system and the fluid circuit is hermetically sealed during the manufacture, there should be sufficiently fluid in the system to be pressurized by the piston. In the scheme of Fig 3b, the piston does not reach the fluid level surface 23, and thus the fluid will not be pressurized. Therefore, it can be concluded that there is a leakage in the circuit 20. In the example shown by Fig. 3c, the piston reaches the fluid surface level 23, but only a minor pressure increase is obtained, since the fluid level is the lowest possible.

According to the present disclosure a warm fluid originating from the engine, e.g. exhaust gas, is led through the evaporator and used to heat the working fluid in the waste heat recovery system. Energy in the form of heat is thereby transferred from e.g. the exhaust gases to the working fluid in the waste heat recovery system and thereby the working fluid is heated so that it is evaporated. The pressurizing is performed when the fluid temperature in the circuit is below the boiling point of the working fluid WF. By boiling point is meant a temperature in which the vapour pressure of the fluid equals to the external pressure, and whereby the liquid boils. The boiling point is affected by the external pressure which means that the higher the pressure, the higher the boiling temperature. Therefore, the pressurizing means is suitably actuated during a cold start of the engine. By cold start is meant that when the engine is started, the temperature in the waste heat recovery circuit and its connections to the heat sources and sinks is below the boiling temperature of the working fluid at the present ambient pressure, whereby the working fluid remains in liquid phase in the whole waste heat recovery system. For example, if the working fluid is ethanol and the waste heat recovery system is at atmospheric pressure, the boiling point is 78.1°C, and thus, the cold start corresponds to a phase where the working fluid cannot be heated above said temperature in any way. Additionally or alternatively, the heat source, such as exhaust gases, is disconnected from the working fluid system and is arranged to bypass the evaporator. Thus, the pressurizing means 40 is actuated during a cold start of the engine the working fluid system further comprises a bypass pipe 35 arranged to let the heat source 32 bypass the evaporator 22 when the pressurizing means 40 acts on the working fluid reservoir 28 and thus when the steps of the detecting method for an insufficient fluid level are performed. In this way the working fluid will not boil and will remain in liquid state. Therefore, it is possible to detect fluid pressure in the system in a reliable way.

To detect the working fluid pressure in the working fluid circuit at least one pressure sensor arranged downstream of the pressurizing means has to be used. Since the waste heat recovery system normally comprises the first pressure sensor 41 and the second pressure sensor 42, the at least one pressure sensor may be chosen to be one or both of these sensors. The at least one pressure sensor is connected to a control unit 50 arranged to determine if the pressure is increased in the circuit 20 when the pressurizing means 40 acts on the working fluid reservoir 28. The control unit may for example be adapted to receive pressure measurement value in the form of signals from the sensors and if increase is detected, no action is necessary.

However, the absence of increased pressure or a pressure increase value below the pre-determined pressure increase value indicates insufficient working fluid (WF) level and thus potential leakage in the circuit 20.

Thus, the control unit 50 may be adapted to receive signals from the pressure sensor indicating the measured pressure value and then compare the measured pressure value with a pre-determined value. The pre-determined value may be the pressure in the system before increasing the pressure or the pre-determined value may be a pressure value corresponding to an increased value in a waste heat recovery system without leakage. In any case, if the pressure applied to the working fluid reservoir propagates to the working fluid circuit and an increase in the fluid pressure is detected by the at least one sensor connected to the control unit 50, no action needs to be taken. However, if no increase or a pressure increase value of the fluid pressure below a pre-determined pressure increase value is determined during the pressure increasing step, the control system 50 is adapted to generate an error message to indicate to the operator of the vehicle that service is required. The message may be in the form of an error code that is readable e.g. by means of a computer, when the vehicle is at service.

The waste heat recovery system 4 may have only one pressure sensor selected from several pressure sensors or may comprise several pressure sensors. Preferably, the system 4 comprises at least one of a first pressure sensor 41 arranged upstream of the evaporator 22 and downstream of the pump 30 and a second pressure sensor 42 arranged downstream of the condenser 26 and upstream of the working fluid reservoir 28, since these sensors are normally present in the system 4. Alternatively or additionally, the pressure sensor may be any or several of a third pressure sensor 43 arranged upstream of the expander 24 and downstream of the evaporator 22, and is also referred to expander-in pressure sensor detecting the pressure of the fluid fed into the expander. A fourth pressure sensor 44 may be a sensor arranged downstream of the second pressure sensor 42 and upstream of the reservoir 28 and is referred to a reservoir-in pressure sensor detecting the pressure of the fluid fed into the reservoir. Additionally or alternatively the sensor may be a fifth pressure sensor 45 arranged downstream of the reservoir 28 and upstream of the working fluid pump 30 and is referred to a pump-in pressure sensor detecting the pressure of the fluid entering the pump 30. According to one embodiment, the sensor may be a sixth pressure sensor 46 arranged downstream of the expander 24 and upstream of the condenser 26 and is referred to an expander-out pressure sensor detecting the pressure of the working fluid exiting the expander. Furthermore, the sensor may be a seventh pressure sensor 47 connected to the reservoir 28. The at least one additional pressure sensor improves the accuracy of leakage detection. The at least one additional pressure sensor is also connected to the control unit 50.

The step of detecting the working fluid pressure may further include using a pressurizing means pressure sensor 48. The pressure sensor is thus connected to the pressurizing means 40, and enables detection of pressure used for pressurizing. In the step of determination, the detected pressure may then be compared with the detected pressure value in the circuit 20 obtained by means of the at least pressure sensor. In the corresponding way as above, if the control unit 50 determines absence of a pressure increase or a pressure increase value below the pre-determined pressure increase value, there is an insufficient working fluid level in the waste heat recovery system.

Fig. 4 shows a flowchart for a method for detecting leakage of working fluid in a waste heat recovery system arranged in connection with an internal combustion engine 2 of a vehicle 1. The waste heat recovery system is suitably configured as described in Fig. 2 or 3. The method comprises the steps of: - pressurizing s101 the working fluid reservoir 28 by actuating a pressurizing means 40 acting on the working fluid reservoir 28, at a fluid temperature in the working fluid circuit 20 which is below the boiling point of the working fluid WF; - detecting s102 the working fluid WF pressure in the working fluid circuit 20 by means of at least one pressure sensor arranged downstream of the pressurizing means 40; and - determining s103 if pressure is increased in the circuit 20 during the pressurizing step s101 more than a pre-determined pressure increase value, wherein a pressure increase value below the pre-determined pressure increase value indicates insufficient level of working fluid WF in the circuit (20).

The method may further comprise a step of generating s104 an error message in case a pressure increase value of the fluid pressure below a pre-determined pressure increase value in the working fluid circuit 20 is determined in the step s103 performed during the pressurizing step s101. In this way the operator of the vehicle is informed that service is required.

The step of pressurizing is suitably performed during a cold start of the engine. In this way the heat source, e.g. exhaust gases, have not raised the temperature of the liquid above boiling point, whereby the working fluid is still in liquid phase and the pressure measured in the system will indicate liquid pressure. Alternatively or additionally the evaporator can be bypassed during the step of pressurizing the working fluid reservoir 28. In this way it can be assured in an easy way that the working fluid is not heated to the boiling point. The actuating of the pressurizing means can be performed by means of compressed air obtained from a compressed air system of the vehicle. Thus, the existing components in the vehicle may be utilized for the detection method, whereby the amount of additional components is minimized.

The step of detecting the pressure s102 comprises detecting the working fluid (WF) pressure in the working fluid circuit 20 by means of at least one of a first pressure sensor 41 arranged upstream of the evaporator 22 and downstream of the pump 30, a second pressure sensor 42 arranged downstream of the condenser 26 and upstream of the working fluid reservoir 28, a third pressure sensor 43 arranged upstream of the expander 24 and downstream of the evaporator 22, a fourth pressure sensor 44 arranged downstream of the second pressure sensor 42 and upstream of the reservoir 28, a fifth pressure sensor 45 arranged downstream of the reservoir 28 and upstream of the working fluid pump 30, a sixth pressure sensor 46 arranged downstream of the expander 24 and upstream of the condenser 26 and/or a seventh pressure sensor 47 arranged in connection with the working fluid reservoir The method steps are suitably performed by means of the control unit 50, which can be an independent control unit or connected to the internal combustion engine 2 and thus being a control unit of the engine or the vehicle. The pressure sensors are connected to the control unit 50 by means of cables or wireless connection as shown by dotted lines in Fig. 2 and 3.

Fig. 5 schematically illustrates a device 500. The control unit 50 and/or computer 52 described with reference to Fig. 2 and 3 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 nonvolatile memory 520 has also a second memory element 540.

There is provided a computer program P which comprises routines for a method for detecting insufficient level of working fluid in a waste heat recovery system 4 according to the invention. The computer program P comprises routines for determining if pressure is increased in the circuit 20 during the pressurizing step s101, wherein the absence of increased pressure, i.e. a pressure increase value below the pre-determined pressure increase value, indicates insufficient level of working fluid (WF) in the circuit (20) and thus leakage in the circuit 20. The determination is based on pressure values measured by means of the at least one pressure sensor. The computer program P comprises routines for commanding actuation of the pressurizing means when the temperature of the working fluid is below the boiling point at pre-determined intervals or when desired. For example, the method can be performed always at cold start of the vehicle. 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.

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.

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.

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.

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.

The foregoing description 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 a skilled person to understand the invention for various embodiments and with the various modifications appropriate to the intended use.

Claims

1. A method for detecting an insufficient level of working fluid in a waste heat recovery system (4) arranged in connection with an internal combustion engine (2) of a vehicle (1), the waste heat recovery system comprising a working fluid circuit (20), a working fluid reservoir (28) for the working fluid (WF) and a pump (30) arranged to pump the working fluid (WF) through the circuit (20), wherein the working fluid (WF) is arranged to pass through an evaporator (22), an expander (24) and a condenser (26) connected to a cooling arrangement, the waste heat recovery system (4) being connected to a heat source (32) connected to the evaporator (22), characterized by the steps of: - pressurizing (s101) the working fluid reservoir (28) by actuating a pressurizing means (40) acting on the working fluid reservoir (28) at a fluid temperature in the working fluid circuit (20) which is below the boiling point of the working fluid (WF) ; - detecting (s102) the working fluid (WF) pressure in the working fluid circuit (20) by means of at least one pressure sensor arranged downstream of the pressurizing means (40); and - determining (s103) if pressure is increased in the circuit (20) during the pressurizing step (s101) more than a pre-determined pressure increase value, wherein a pressure increase value below the pre-determined pressure increase value indicates insufficient level of working fluid (WF) in the circuit (20).

2. The method according to claim 1, wherein the method steps pressurizing (s101), detecting (s102) and determining (s103) are performed during a cold start of the engine.

3. The method according to any of claims 1 or 2, wherein the evaporator (22) is bypassed by the heat source (32) when the method steps pressurizing (s101), detecting (s102) and determining (s103) are performed.

4. The method according to any of claims 1 to 3, further comprising a step of generating (s104) an error message in case the pressure increase value determined during the pressurizing step (s101) is below the pre-determined pressure increase value.

5. The method according to any of claims 1 to 4, wherein the working fluid (WF) pressure is detected by means of at least one sensor, the sensor being a first pressure sensor (41) arranged upstream of the evaporator (22) and downstream of the pump (30), a second pressure sensor (42) arranged downstream of the condenser (26) and upstream of the working fluid reservoir (28), a third pressure sensor (43) arranged upstream of the expander (24) and downstream of the evaporator (22), a fourth pressure (44) sensor arranged downstream of the second pressure sensor (42) and upstream of the reservoir (28), a fifth pressure sensor (45) arranged downstream of the reservoir (28) and upstream of the working fluid pump (30), a sixth pressure sensor (46) arranged downstream of the expander (24) and upstream of the condenser (26) and/or a seventh pressure sensor (47) arranged in connection with the working fluid reservoir (28).

6. The method according to any of claims 1 to 5, wherein in the pressurizing is performed by means of utilizing compressed air obtained from a compressed air system of the vehicle.

7. A waste heat recovery system (4) arranged in connection with an internal combustion engine (2) of a vehicle (1), the waste heat recovery system comprising a working fluid circuit (20), a working fluid reservoir (28) for the working fluid (WF) and a pump (30) arranged to pump the working fluid (WF) through the circuit (20), wherein the working fluid is arranged to pass through an evaporator (22), an expander (24) and a condenser (26) connected to a cooling arrangement, the waste heat recovery system (4) being connected to a heat source (32), which is connected to the evaporator (22), characterized in that, the working fluid system (4) further comprises a pressurizing means (40) arranged to act on the working fluid reservoir (28) at a fluid temperature in the working fluid circuit (20) which is below the boiling point of the working fluid (WF), and the working fluid circuit (20) comprises at least one pressure sensor adapted to detect the working fluid (WF) pressure in the working fluid circuit (20) downstream of the pressurizing means (40), wherein the at least one pressure sensor is connected to a control unit (50) arranged to determine if pressure is increased in the circuit (20) when the pressurizing means (40) acts on the working fluid reservoir (28).

8. The waste heat recovery system (4) of claim 7, wherein the pressure sensor is at least one of a first pressure sensor (41) arranged upstream of the evaporator (22) and downstream of the pump (30), a second pressure sensor (42) arranged downstream of the condenser (26) and upstream of the working fluid reservoir (28), a third pressure sensor (43) arranged upstream of the expander (24) and downstream of the evaporator (22), a fourth pressure sensor (44) arranged downstream of the second pressure sensor (42) and upstream of the reservoir (28), a fifth pressure sensor (45) arranged downstream of the reservoir (28) and upstream of the working fluid pump (30), a sixth pressure sensor (46) arranged downstream of the expander (24) and upstream of the condenser (26) and/or a seventh pressure sensor (47) arranged in connection with the working fluid reservoir (28).

9. The waste heat recovery system (4) of claim 7 or 8, wherein the control unit (50) is adapted to generate an error message in case a pressure increase value of the fluid pressure is below a pre-determined pressure increase value when the pressurizing means (40) acts on the working fluid reservoir (28).

10. The waste heat recovery system (4) of any of claims 7 to 9, wherein the system (4) further comprises a bypass pipe arranged to let the heat source (32) bypass the evaporator (22) when the pressurizing means (40) acts on the working fluid reservoir (28).

11. The waste heat recovery system (4) of any of claims 7 to 10, wherein the pressurizing means (40) is arranged to be actuated by means of compressed air obtained from a compressed air system of the vehicle.

12. The waste heat recovery system (4) of claim 11, wherein the pressurizing means (40) comprises an elastic bladder, a bellows or a piston arrangement.

13. A vehicle, characterized in that it comprises the waste heat recovery system (4) of any of claims 7-12.

14. A computer program (P), wherein said computer program comprises program code for causing an electronic control unit (50; 500) or a computer (52; 500) connected to the electronic control unit (50; 500) to perform the steps according to any of the claims 1-6.

15. A computer-readable medium comprising a program code stored on the computer-readable medium for performing the method steps according to any of claims 1-6, when said computer program is run on an electronic control unit (50; 500) or a computer (52; 500) connected to the electronic control unit (50; 500).