SE542148C2

SE542148C2 - A method for operating a waste heat recorvery system and a waste heat recovery system

Info

Publication number: SE542148C2
Application number: SE1750337A
Authority: SE
Inventors: Björn Johansson; Erik Höckerdal; Matthias Ussner; Svante Johansson
Original assignee: Scania Cv Ab
Priority date: 2017-03-22
Filing date: 2017-03-22
Publication date: 2020-03-03
Also published as: DE102018001828A1; SE1750337A1; DE102018001828B4

Abstract

The present disclosure relates to a waste heat recovery system (4) for an internal combustion engine (2) and a method for operating such a system with reduced risk for damage of constituent components of the system. The operation is performed in such a manner that it takes into account upcoming expected transient operating conditions of the internal combustion engine. The operation is performed such as to use a low safety margin of a parameter of the working medium of the waste heat recovery system during steady-state operating conditions and by increasing the safety margin of a parameter of the working medium before a transient operating condition of the internal combustion engine occurs.

Description

A METHOD FOR OPERATING A WASTE HEAT RECORVERY SYSTEM AND A WASTE HEAT RECOVERY SYSTEM TECHNICAL FIELD The present disclosure relates to a method for operating a waste heat recovery system for an internal combustion engine. The present disclosure also relates to a waste heat recovery system for an internal combustion engine. The internal combustion engine may be an internal combustion engine of a vehicle, a stationary engine, power pack or the like.

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 internal combustion engines, some of the energy from the fuel is dissipated as heat through the exhaust pipes and the engine cooling system. By the use of a waste heat recovery system, the heat from the exhaust gases may instead be used to heat various vehicle components or to produce mechanical work or electricity. Such mechanical work may for example be transferred to the drivetrain or crankshaft and thus be used to help to propel the vehicle. A waste heat recovery system may also recover heat from other heat sources of the internal combustion engine, such as EGR gases or cooling fluids.

A waste heat recovery system typically comprises a circuit in which a working medium is circulated. The circuit comprises a heat exchanger, an expansion device, a condenser and a working medium conveyor. Before entering the heat exchanger, the working medium is in a liquid state. The heat exchanger is adapted to evaporate the working medium such as to create a superheated steam. To achieve this, the heat exchanger transfers heat between a heat source, such as exhaust gases from the internal combustion engine, and the working medium. The superheated steam generated by the heat exchanger then passes into the expansion device wherein it is expanded. By means of the expansion device, the recovered heat may be converted into mechanical work or electricity. By way of example, the expansion device may be mechanically connected to the powertrain using a clutch or a freewheel. The working medium is thereafter cooled in the condenser such that the working medium is reverted to a liquid state. The condenser may typically be connected to a cooling system, which in turn may be a part of the engine cooling system or be a separate cooling system. The conveyor, which may typically be a pump, is adapted to control the mass flow of the working medium in the circuit, for example by pressurising the working medium. The waste heat recovery system is thus based on for example a Rankine cycle. The waste heat recovery system may further comprise a reservoir for storing the working medium and ensure that there is sufficient working medium available in the circuit at all times.

It is important to control the temperature and pressure in the waste heat recovery system so as to ensure that the heat exchanger fully evaporates the working medium to a steam. If the working medium steam is not dry when it enters the expansion device, it may risk causing damage to the expansion device. In the expansion device, some liquid of the working medium may be formed as a result of the expansion of the steam of the working medium in case the working medium is not in the form of a superheated steam when it enters the expansion device. Furthermore, there are always small fluctuations in the operating conditions of the waste heat recovery system, and possibly a loss of heat between the heat exchanger and the expansion device, which have to be accounted for. Therefore, waste heat recovery systems are generally operated such as to create a safety margin to the threshold value whereat a dry steam is obtained by the heat exchanger. In other words, waste heat recovery systems are operated such as to create a superheated steam in the heat exchanger with a certain degree of superheating. However, a too great safety margin may risk reducing the fuel saving potential of the waste heat recovery system, as well as, in some cases, risking destroying the working medium due to the high temperature exposure thereof. A too high temperature of the working medium may furthermore risk damaging other constituent components of the waste heat recovery system, such as sealings or the like. This can in worst case scenario lead to leakage of the working medium from the waste heat recovery system.

The operation of the waste heat recovery system can be controlled by controlling various factors, such as the mass flow rate of the working medium in the conveyor or by the cooling of the working medium in the condenser. During steady-state operating condition of the internal combustion engine, prior art waste heat recovery systems work very well. However, the adjustment of the waste heat recovery system is generally fairly slow, and therefore a transient operating condition of the internal combustion engine can cause a substantial disturbance in the waste heat recovery system and/or damage constituent components of the waste heat recovery system.

For example, a case can be considered where the expansion device is used to produce mechanical work utilised to propel the vehicle. During a gear shift in such a case, the engine rotational speed, and that of the coupled expansion device, changes very quickly and the volume flow through the expansion device also changes very quickly. This can cause a risk of the steam not being superheated before entering the expansion device which in turn may risk causing damage to the expansion device. These problems caused by transient operating conditions are usually solved by bypassing the expander when the pressure or temperature changes in the waste heat recovery system make the steam non-superheated. An example of such a solution is disclosed in SE 533402 C2. However, bypassing the expansion device has the drawback that the mechanical work generated by the expansion device also will change quickly. By way of example, in case the expansion device is connected to the drivetrain or crankshaft such that mechanical work produced by the expansion device is used to propel the vehicle, the quick change in torque generated by the expansion device can disturb a gear shift.

Another way of solving the problem is to increase the level of superheating of the steam produced by the heat exchanger. However, this inter alia has the drawback of reducing the fuel saving potential of the waste heat recovery system.

US 2015/0361832 A1 discloses one example of a method and device for operating a waste heat recovery device for an internal combustion engine of a vehicle. A working medium is circulated in a waste heat utilization circuit comprising a conveyor, an evaporator, an expansion machine and a condenser. A basic adjustment of the waste heat utilization circuit is provided, which as a function of heat input into the working medium adjusts the mass flow rate at the conveyor and/or the ratio between high pressure and low pressure at the expansion machine. In addition, a pilot control is provided which recognizes a change of the operating point of the internal combustion engine and, when the operating point is changed, controls the condensation output of the condenser such that a mass flow of the working medium in the circuit changes and is adapted to the new operating point.

DE 102015007104 A1 discloses one further example of a method for operating a waste heat recovery device for a vehicle. A working medium is circulated in a Rankine cycle comprising a working medium conveying means, an evaporator, an expander and a condenser. The medium conveying means is controlled in dependence of at least one parameter of a drive train, a brake device, and/or a navigation system of the vehicle. The parameter is used for predicting a future exhaust gas mass flow, a future waste heat power and/or future exhaust gas temperature, and is disclosed to thereby control the medium conveying means with a temporal perspective. DE 102015007 104 A1 is however silent as to how the medium conveying means is controlled in response to the parameter.

SUMMARY The object of the present invention is to provide a control of a waste heat recovery system for an internal combustion engine which may take into account transient operation of the internal combustion engine and which provides an efficient waste heat recovery as well as minimising the risk of damaging any constituent component of the waste heat recovery system.

In accordance with the present invention, the operation of the waste heat recovery system is such that it takes into account an expected upcoming change in the operating conditions of the internal combustion engine. Thereby, the operating conditions of the waste heat recovery system can be adapted to the expected upcoming change of the operating conditions of the internal combustion engine before the change actually occurs. This minimises the risk for damage of any of the constituent components of the waste heat recovery system resulting from a sudden change of the operating condition of the waste heat recovery system as well as minimises the risk for disturbing the operation of the waste heat recovery system in terms of the mechanical or electrical energy resulting from the expansion device of the waste heat recovery system.

Furthermore, the present invention is based on an operation of the waste heat recovery system which under "normal" operating conditions, i.e. steady-state operating conditions, of the internal combustion engine can utilise optimised conditions such that the heat exchanger produces a superheated steam but with a relatively low safety margin in terms of the degree of superheating of the steam. An example of such a steady-state operating condition of the internal combustion engine is, in the case of an internal combustion engine of a vehicle, when driving on a flat road with substantially constant speed. However, under transient operating conditions of the internal combustion engine, the waste heat recovery system is operated at a greater safety margin such that the heat exchanger produces a relatively high degree superheating of the steam. The fact that the safety margin is increased before the transient load on the internal combustion engine occurs ensures that the risk of a quick change at a transient load does not disturb the operation of the waste heat recovery system and in particular minimises the risk for damage to the expansion device of the system.

Alternatively, or in addition, the present invention is based on an operation of the waste heat recovery system which under normal operation conditions of the internal combustion engine can, if desired, utilise a fairly high maximum temperature of the working medium in the circuit, i.e. a fairly low safety margin to the temperature at which the working medium may risk being damaged or where the temperature of the working medium may risk causing damage to constituent components of the waste heat recovery system. However, during transient operating conditions of the internal combustion engine, the waste heat recovery system can be operated using a greater safety margin to the temperature at which the working medium may risk being damaged or cause damage, for example thermal damage, to constituent components of the waste heat recovery system.

The present invention can thus avoid the drawback of prior art of having to bypass the expansion device during transient operating conditions of the internal combustion engine to avoid the risk of damaging the expansion device. Consequently, the expansion device can be used for producing mechanical work and/or electricity also during a transient operating condition of the internal combustion engine and therefore optimises the efficiency of the waste heat recovery of the system. This also leads to a minimal disruption of the waste heat recovery and efficient fuel consumption of for example a vehicle comprising the waste heat recovery system.

In contrast to prior art, the operation of the waste heat recovery system according to the present invention is not performed such as to optimise the system to the operating condition of the internal combustion engine during the transient operating condition thereof, but simply to increase the safety margin of a parameter of the working medium just before and during the transient operating condition of the waste heat recovery system. This has the advantage of an easy and efficient control of the waste heat recovery system.

The present invention thus provides a method of operating a waste heat recovery system for an internal combustion engine, wherein the waste heat recovery system comprises: a circuit in which a working medium circulates; the circuit further comprising a heat exchanger adapted to evaporate the working medium to create a steam thereof, an expansion device adapted to produce mechanical or electrical energy by expanding the steam created by the heat exchanger, a condenser arranged downstream of the expansion device and adapted to condense the steam into a liquid state of the working medium, and a working medium conveyor adapted to circulate the working medium in the circuit; the waste heat recovery system further comprising a control unit adapted to receive information regarding an operating condition of the internal combustion engine and control the mass flow of the working medium in the circuit in response to said information regarding an operating condition of the internal combustion engine.

The method of operating a waste heat recovery system for an internal combustion engine comprises the steps of: a) providing a predetermined threshold value of a parameter of the working medium, b) if the control unit receives information regarding a first steady-state operating condition of the internal combustion engine, then the control unit selects a predetermined first target value of the parameter of the working medium and controls the mass flow of the working medium in the circuit to reach said first target value, wherein the first target value differs from the threshold value of the parameter by a first safety margin ?1;and c) if the control unit receives information regarding an expected upcoming transient operating condition of the internal combustion engine, then the control unit selects a second target value of the parameter of the working medium, wherein the second target value differs from the threshold value by a second safety margin ?2which is greater than the first safety margin ?1, and the control unit controls the mass flow of the working medium in the circuit to reach said second target value.

The method may further comprise a step of: d) if the control unit receives information that the transient operating condition of the internal combustion engine has terminated and the first or a second steady-state operating condition of the internal combustion engine has been reached, then the control unit selects a third target value of the parameter corresponding to a safety margin which is lower than the second safety margin ?2and controls the mass flow of the working medium in the circuit to reach said third target value.

Thereby, when the transient operating condition of the internal combustion engine has terminated, a lower safety margin of the parameter of the working medium may be used in order to optimise the waste heat recovery.

The control unit may suitably control the mass flow of the working medium in the circuit by adjusting the mass flow rate of the working medium conveyor. This is generally the fastest way of adjusting the mass flow of the working medium in the circuit and therefore provides the best possibility for enabling the waste heat recovery system to quickly adapt to expected changes of the operating conditions of the internal combustion engine. The expected changes of the operating conditions of the internal combustion engine may for example affect the operation of the waste heat recovery system by a change of heat input into the heat exchanger.

The parameter of the working medium may be the degree of superheating of the steam of the working medium exiting the heat exchanger. In such a case, the threshold value of the parameter corresponds to the vaporization point of the working medium.

Alternatively, the parameter of the working medium may be the maximum temperature of the working medium in the circuit. In such a case, the threshold value corresponds to a temperature at which the working medium may risk being damaged, or a temperature of the working medium at which it avoids the risk of causing any damage to any of the constituent components of the waste heat recovery system. An example of a temperature at which the working medium may risk being damaged can be a temperature at which the working medium decomposes or separates into different working medium components.

Naturally, the waste heat recovery system may be operated such as to take into account both the degree of superheating and the maximum temperature of the working medium, if desired.

The control unit may suitably be adapted to receive information regarding an upcoming transient operating condition of the internal combustion engine based on information from at least one of an engine management system of the internal combustion engine, a gearbox management system, a navigation system, a cruise control system and/or a radar system configured to determine traffic conditions.

The second safety margin ?2may suitably be at least 25% greater than the first safety margin ?1. Preferably, the second safety margin ?2may suitably be at least 50% greater than the first safety margin ?1.

The present invention also relates to a waste heat recovery system for an internal combustion engine, the waste heat recovery system comprising a circuit in which a working medium circulates. The circuit comprises a heat exchanger adapted to evaporate the working medium to create a steam thereof, an expansion device adapted to produce mechanical or electrical energy by expanding the steam created by the heat exchanger. The circuit further comprises a condenser, arranged downstream of the expansion device and adapted to condense the steam into a liquid state of the working medium, and a conveyor adapted to circulate the working medium in the circuit. The waste heat recovery system further comprises a control unit adapted to receive information regarding an operating condition of the internal combustion engine, and in response to said information regarding the operating condition of the internal combustion engine, control the mass flow of the working medium in the circuit. The control unit is adapted to receive information regarding a steady-state operating condition of the internal combustion engine, and in response thereto, select a first predetermined target value of a parameter of the working medium which differs from a predetermined threshold value of the parameter by a first safety margin ?1, and control the mass flow of the working medium in the circuit to reach said target value. The control unit is further adapted to receive information regarding an expected upcoming transient operating condition of the internal combustion engine, and in response thereto, select a second target value of the parameter of the working medium and control the mass flow of the working medium in the circuit to reach said second target value, wherein the second target value differs from the predetermined threshold value by a second safety margin ?2which is greater than the first safety margin ?1.

The control unit may further be adapted to receive information regarding termination of a transient operating condition of the internal combustion engine and that a steady-state operating condition has been reached, and in response thereto, select a new target value of the parameter corresponding to a safety margin which is lower than the second safety margin ?2and control the mass flow of the working medium in the circuit to reach said third target value.

In case the control unit receives information that the first steady-state operating condition of the internal combustion engine has been reached again, the third target value of the parameter may suitably be the same as the first target value of the parameter. In other words, each steady-state operating condition of the internal combustion engine may be associated with a specific predetermined target value of the parameter of the working medium.

The control unit may suitably be connected to the conveyor and adapted to control the mass flow of the working medium in the circuit by adjusting the mass flow rate of the conveyor.

The control unit may further be adapted to receive information regarding an upcoming transient operating condition of the internal combustion engine based on information from at least one of an engine management system of the internal combustion engine, a gearbox management system, a navigation system, a cruise control system and/or a radar system configured to determine traffic conditions.

The invention is particularly suitable for use in a vehicle comprising an internal combustion engine. Thus, the present invention also relates to a vehicle comprising an internal combustion engine and a waste heat recovery system as disclosed above. The vehicle may be a heavy vehicle, such as a bus or a truck. The vehicle may alternatively be a passage car or a marine vessel.

The present invention also relates to a computer program, wherein said computer program comprises program code for causing a control unit or a computer connected to the control unit to perform the method for operating a waste heat recovery system as disclosed above.

Moreover, the present invention relates to a computer-readable medium comprising instructions, which when executed by a control unit or a computer connected to the control unit causes the control unit or the computer to perform the method of operating a waste heat recovery system as disclosed above.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 schematically illustrates a vehicle according to an embodiment of the invention; Fig. 2 schematically illustrates a waste heat recovery system according to one exemplifying embodiment of the invention; Fig. 3 schematically illustrates a flow chart of a method of operating a waste heat recovery system according to an exemplifying embodiment of the invention; Fig. 4 schematically illustrates a flow chart of a method of operating a waste heat recovery system according to another exemplifying embodiment of the invention; Fig. 5 schematically illustrates the result over time of the method for operating a waste heat recovery system according to an exemplifying embodiment wherein the parameter is the degree of saturation of the steam of the working medium exiting the heat exchanger; Fig. 6 schematically illustrates the result over time of the method for operating a waste heat recovery system according to another exemplifying embodiment wherein the parameter is the maximum temperature of the working medium in the circuit at any point thereof; Fig. 7 schematically illustrates a control unit or a computer according to an exemplifying embodiment of the invention.

DETAILED DESCRIPTION The invention will be described in more detail below with reference to exemplifying embodiments and the accompanying drawings. The invention is however not limited to the exemplifying embodiments discussed and shown in the drawings, but may be varied within the scope of the appended claims. Furthermore, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate the invention or features thereof.

In the present disclosure, the term "steady-state" is used. "Steady-state" can be defined as a state wherein the variables which defines the behaviour of a system or a process are unchanging in time. It should however be noted that an operating condition of an internal combustion engine is normally not a "steady-state" in terms of the literal meaning of the term in view of normally occurring fluctuations in the operating conditions. The normally occurring fluctuations which may be due to factors which are not possible to influence or completely monitor. Thus, when the term "steady-state operating condition" is used herein, it should be understood as an essentially steady-state operating condition but with small unintentional and normal fluctuations in the operating condition. In other words, "steady-state operating condition" should be understood as it would be understood by a person skilled in the art.

Furthermore, in the present disclosure, the expression "to reach said target value" or similar expressions are used when discussing the control unit controlling the mass flow of the working medium in the circuit. This shall however not be construed as necessarily obtaining said target values, but controlling the mass flow of the working medium so as to strive towards the target value. Therefore, the term "reach" is used herein.

While the waste heat recovery system in the following is disclosed in connection with an internal combustion engine of a vehicle, the present invention is not limited to the waste heat recovery system being one in a vehicle. The waste heat recovery system may be a waste heat recovery system of any internal combustion engine, including but not limited to an internal combustion engine of a vehicle, a stationary engine (such as a diesel generator), power pack or the like.

Moreover, while the waste heat recovery system in the following is disclosed as using exhaust gases from the internal combustion engine as a heat source in the heat exchanger, the present invention is not limited to the use of exhaust gases as a heat source. For example, the heat source may be EGR (Exhaust Gas Recirculation gases) or coolant fluid.

Figure 1 schematically illustrates a side view of a vehicle 1 comprising an internal combustion engine 2, and a waste heat recovery system 4 associated with the internal combustion engine 2. The vehicle may furthermore comprise a cooling system 6 associated with the internal combustion engine 2 and connected to the waste heat recovery system 4. The vehicle further comprises a gearbox 8 connected to the driving wheels 5 of the vehicle 1. The vehicle 1 may be a heavy vehicle, e.g. a truck or a bus. The vehicle may alternatively be a passenger car. Furthermore, the vehicle may be a hybrid vehicle comprising an electric machine (not shown) in addition to the combustion engine 2. The vehicle may alternatively be a marine vessel, such as a ship.

Waste heat recovery can be accomplished by using heat from for example the exhaust gases to heat a working medium to create steam, i.e. the vaporized working medium arising from heating the working medium. This steam can then be expanded and the produced mechanical work can be used for example to propel the vehicle, generate electricity or drive auxiliary units of the vehicle.

While previously known waste heat recovery systems work very well under steady-state conditions, they are not fully adapted to transient conditions. The internal combustion engine which is frequently used as a heat source for operating the waste heat recovery system, namely for delivering heat to the heat exchanger, is frequently operated transiently or has many different steady-state operating conditions. While it is previously known to control the waste heat recovery system in response to such conditions, as described in the background, such methods are often slow or do not allow to optimise the waste heat recovery. Moreover, the adjustment of the waste heat recovery system is often performed only after a transient operating condition of the internal combustion engine has been detected, and there might therefore be a risk of damage of constituent components of the waste heat recovery system (in particular of the expansion device) before the operation of the system has been adapted to the new operating condition of the internal combustion engine.

Figure 2 schematically illustrates a waste heat recovery system 4 according to one exemplifying embodiment of the invention. The waste heat recovery system 4 comprises a circuit 10 in which a working medium WM is circulated. In the circuit, a heat exchanger 11, expansion device 12, condenser 13 and a working medium conveyor 14 are arranged.

Before entering the heat exchanger 11, the working medium is in a liquid state. The heat exchanger 11 is adapted to evaporate the working medium such as to create a superheated steam. To achieve this, the heat exchanger 11 transfers heat between a heat source, such as exhaust gas from the internal combustion engine, and the working medium. The exhaust gas from the internal combustion engine is led to the heat exchanger via a first exhaust gas conduit 18 and exits the heat exchanger via a second exhaust gas conduit 19. Optionally, the exhaust gases from the internal combustion engine may alternatively or partly be led past the heat exchanger 11 via a third exhaust gas conduit 20. To control the amount of exhaust gases passing through the first exhaust gas conduit 18 and the third exhaust gas conduit 20, respectively, the different exhaust gas conduits may suitably comprise one or more valves 21, 22 as previously known in the art. In Figure 2, the valve 21, arranged in the second exhaust gas conduit, is shown in an open position whereas valve 22, arranged in the third exhaust conduit 21, is in a closed position. Thus, the exhaust gases would only pass through the heat exchanger 11. It should be noted that the present invention is not limited to the presence of any valves in the exhaust gas conduit or if present, their location within the exhaust gas conduits.

The superheated steam generated by the heat exchanger 11 passes into the expansion device 12 wherein it is expanded. By means of the expansion device 12, the recovered heat may be converted into mechanical work or electricity. By way of example, the expansion device 12 may be mechanically connected to the powertrain of the vehicle using a clutch or a freewheel (not shown). The circuit may further comprise a bypass conduit 16 as previously known in the art, to enable bypassing the expansion device 12, if necessary. The bypass conduit 16 may suitably comprise a bypass valve 17. During normal operation, the bypass valve 17 is in a closed position and the working medium passes the expansion device 12. The bypass valve may be controlled by the same control unit 24, i.e. the control unit adapted to receive information regarding an operating condition of the internal combustion engine, as will be further described below. Alternatively, the bypass valve may be controlled by another control unit, if desired.

After the working medium has been expanded in the expansion device 12 (or bypassed the expansion device 12), the working medium is cooled in the condenser 13 such that the working medium is reverted to a liquid state. The condenser 13 may typically be connected to a cooling system 6', which in turn may be a part of the engine cooling system 6 (as shown in Figure 1) or be a separate cooling system.

The working medium conveyor 14, which may typically be a pump, is adapted to control the mass flow of the working medium in the circuit, for example by pressurising the working medium. In accordance with the present invention, the waste heat recovery system comprises a control unit 24 adapted to receive information regarding the operating condition of the internal combustion engine and control the mass flow of the working medium in the circuit in response to said information regarding the operating condition of the internal combustion engine. The control unit 24 may suitably be adapted to control the mass flow of the working medium in the circuit by controlling the mass flow rate of the working medium conveyor 14 as shown in Figure 2, and is therefore suitably connected to the working medium conveyor. However, the control unit may alternatively, or in combination with the connection to the working medium conveyor, be adapted to control the mass flow of the working medium by means of controlling the mass flow rate through for example the condenser.

In accordance with the waste heat recovery system according to the present disclosure, the control unit is adapted to receive information regarding a steady-state operating condition of the internal combustion engine and in response thereto, select a first predetermined target value of a parameter of the working medium which differs from a predetermined threshold value of the parameter by a first safety margin ?1and control the mas flow of the working medium in the circuit to reach said first target value. The control unit is further adapted to receive information regarding an expected upcoming transient operating condition of the internal combustion engine, and in response thereto, select a second target value of the parameter of the working medium and control the mass flow of the working medium in the circuit to reach said second target value. The second target value differs from the predetermined threshold value by a second safety margin ?2which is greater than the first safety margin ?1.

The control unit may for example be adapted to receive information regarding an upcoming transient operating condition of the internal combustion engine based on information from at least one of an engine management system of the internal combustion engine, a gearbox management system. In the case of the waste heat recovery system being one of a vehicle, the control unit may alternatively or in combination be adapted to receive information regarding an expected upcoming transient operating condition of the internal combustion engine based on information from a navigation system, a cruise control system and/or a radar system of the vehicle configured to determine traffic conditions. All of the systems disclosed above as possible to provide information to the control unit regarding an upcoming transient operating condition of the internal combustion engine are previously known per se and will therefore not be discussed in further detail in the present disclosure. It is however evident that each of these systems needs to be supplemented with means for transmitting information to the control unit of the present waste heat recovery system.

The control unit may be adapted to receive information regarding an operating condition of the internal combustion engine continuously, on demand, at predetermined time intervals and/or upon occasion by any system with which the control unit communicates. However, the control unit is preferably adapted to at least continuously receive information regarding the operating condition of the internal combustion engine from an engine management system of the internal combustion engine since the engine management system can provide information regarding the current operating condition of the internal combustion engine. In contrast, the control unit may for example be adapted to only receive information from a radar system configured to determine traffic conditions when the latter system records that the traffic conditions are such that a transient operating condition of the internal combustion engine can be expected.

The waste heat recovery system 4 may further comprise a reservoir 15 for storing the working medium and ensure that there is sufficient working medium available in the circuit 10 at all times.

The working medium of the waste heat recovery system may be any previously known working medium used for this particular purpose. Examples of previously known working mediums include, but are not limited to, water, ethanol and ethanol based mixtures.

A transient operating condition of the internal combustion engine can for example lead to a different mass flow rate of exhaust gases and/or temperature of the exhaust gases used as a heat source in the heat exchanger of the waste heat recovery system to evaporate the working medium. A transient operating condition of an internal combustion engine can furthermore be a result of a gear shift.

Figure 3 schematically illustrates a flow chart of a method of operating a waste heat recovery system according to an exemplifying embodiment of the present invention. Firstly, a threshold value of a parameter of the working medium is provided (a) to the control unit, SI. Said threshold value may be a result of theoretical calculations, based on experiments or based on experience without departing from the present invention.

The control unit of the waste heat recovery system is adapted to receive information regarding the operating condition of the internal combustion engine. The control unit may further be adapted to determine if said operating condition is a steady state, or alternatively the control unit is adapted to receive information whether or not the operating condition is a steady-state operating condition, both options illustrated in figure 3 as S2. If there is a steady-state operating condition of the internal combustion engine, then the control unit selects a predetermined first target value of the parameter of the working medium and controls the mass flow of the working medium in the circuit to reach said first target value, b. This is illustrated by S3 in figure 3. The first target value differs from the threshold value of the parameter by a first safety margin ?1.

If there is not a steady-state operating condition of the internal combustion engine, or the control unit receives information that there is an expected upcoming transient operating condition of the internal combustion engine, then the control unit selects a second target value of the parameter of the working medium. The second target value differs from the threshold value by a second safety margin ?2which is greater than the first safety margin ?1. The control unit then controls the mass flow of the working medium in the circuit to reach said second target value. This constitutes step c in the method and is illustrated by S4 in figure 3.

Thereafter, the control unit is adapted to repeatedly investigate if there is a steady-state operating condition of the internal combustion engine as illustrated by arrows 31 and 32, respectively.

Thus, the method of operating a waste heat recovery system for an internal combustion engine according to the present disclosure comprises: a) providing a predetermined threshold value of a parameter of the working medium, b) if the control unit receives information regarding a first steady-state operating condition of the internal combustion engine, then the control unit selects a predetermined first target value of the parameter of the working medium and controls the mass flow of the working medium in the circuit to reach said first target value, wherein the first target value differs from the threshold value of the parameter by a first safety margin ?1;and c) if the control unit receives information regarding an expected upcoming transient operating condition of the internal combustion engine, then the control unit selects a second target value of the parameter of the working medium, wherein the second target value differs from the threshold value by a second safety margin ?2which is greater than the first safety margin ?1, and the control unit controls the mass flow of the working medium in the circuit to reach said second target value.

In accordance with the present method, the operation of the waste heat recovery system is performed such that a safety margin of a parameter of the working medium can be increased before a transient operating condition of the internal combustion engine actually occurs. Naturally, the control unit may also be adapted to determine that the transient operating condition of the internal combustion engine has actually occurred and/or is ongoing.

Figure 4 schematically illustrates a flow chart of a method of operating a waste heat recovery system according to another exemplifying embodiment of the present invention. The method shown in figure 4 differs from the method shown in figure 3 in that the control unit can further receive information regarding two different steady-state operating conditions or to determine different steady-state operating conditions of the internal combustion engine.

The method as shown in figure 4 therefore provides the possibility of the control unit to receive information regarding the operating condition of the internal combustion engine and determine or receive information whether or not a second steady-state operating condition has been reached. This is illustrated in figure 4 as S5. In case a second steady-state operating condition of the internal combustion engine has been reached, the second steady-state being different from the first steadystate operating condition of the internal combustion engine, the control unit selects a third target value of the parameter corresponding to a safety margin which is lower than the second safety margin ?2. The control unit then controls the mass flow of the working medium in the circuit to reach said third target value, d. This is illustrated as S6 in figure 4.

Thus, the method may further comprise the step of: e) when the control unit receives information that the transient operating condition of the internal combustion engine has terminated and the first steady state operating condition of the internal combustion engine, or a second steady-state operating condition of the internal combustion engine, has been reached, the control unit selects a third target value of the parameter corresponding to a safety margin which is lower than the second safety margin ?2and controls the mass flow of the working medium in the circuit to reach said third target value.

In case the control unit receives information that the transient operating condition of the internal combustion engine has terminated and the first steady-state operating condition of the internal combustion engine has been reached again, the third target value may suitably be the same as the first target value.

Naturally, the method according to the present invention may be adapted to any number of steadystate operating conditions of the internal combustion engine and/or to more than one kind of transient operating condition of the internal combustion engine.

Thereby, the operation of the waste heat recovery system is performed such that the waste heat recovery system is operated with an optimal safety margin for any steady-state operating condition of the internal combustion engine.

The control unit may suitably be adapted to control the mass flow of the working medium in the circuit by adjusting the mass flow rate of the conveyor. Using the working medium conveyor for adjusting the mass flow rate in the circuit is generally the fastest way of adapting the operation of the waste heat recovery system and is therefore the most appropriate alternative in the case of transient operating conditions of the internal combustion engine. It is however also plausible to adjust the mass flow of the working medium in the circuit by other methods, such as adjusting the condensation output in the condenser.

The parameter of the working medium may, according to one exemplifying embodiment of the method described above, be the degree of superheating of the steam of the working medium exiting the heat exchanger. In such a case, the threshold value of the parameter corresponds to the vaporization point of the working medium. In other words, the threshold value at which the steam will be a dry steam but not yet superheated.

Figure 5 schematically illustrates the result of the method according to the present invention over time t in case the parameter is the degree ?WMof superheating of the steam. The threshold value of the parameter, i.e. the vaporization point, is illustrated in the figure as ?WMand the actual degree of superheating of the working medium exiting the heat exchanger over time is illustrated by the dashed line ?WM. The time interval t0to t1corresponds to the time at which the control unit receives information regarding a steady-state operating condition of the internal combustion engine. During said period of time, the control unit selects the first target value ?1providing a safety margin ?1to the threshold value. The first target value is selected to take into account the normal fluctuations during said steady-state operating condition as demonstrated by the dashed line ?WM.

At ti, the control unit receives information regarding an expected upcoming transient working condition of the internal combustion engine which may alter the degree of superheating, or as illustrated in the figure, would increase the fluctuations. The control unit then selects a second target value ?2of the degree of superheating and controls the mass flow in the circuit to seek to reach said second target value. The second target value ?2provides a second safety margin ?1which is greater than the first safety margin ?1.

At time t2, the control unit may receive information that the transient operating condition of the internal combustion engine has terminated and the first steady-state operating condition has been reached. The control unit may then reselect the first target value and control the mass flow in the circuit in response thereto.

Alternatively, the parameter of the working medium may, according to another exemplifying embodiment of the method described above, be the maximum temperature of the working medium in the circuit, i.e. the highest temperature allowed for the working medium at any time or location in the circuit. In this case, the threshold value corresponds to a temperature at which the working medium risks being damaged, for example decomposing.

Figure 6 schematically illustrates the result of the method according to the present invention over time t in case the parameter is the maximum temperature TWMof the working medium in the circuit. The threshold value of the parameter is illustrated as TWMand the actual maximum temperature of the working medium at any point in the waste heat recovery system over time is illustrated by the dashed line TWM. The time interval t0to t3corresponds to the time at which the control unit receives information regarding a steady-state operating condition of the internal combustion engine. During said period of time, the control unit selects the first target value T1providing a safety margin ?1to the threshold value.

At t3, the control unit receives information regarding an expected upcoming transient working condition of the internal combustion engine which may alter the maximum temperature of the working medium in the circuit, or as illustrated in the figure, would increase the fluctuations. The control unit then selects a second target value T2of the maximum temperature of the working medium and controls the mass flow in the circuit to seek to reach said second target value. The second target value T2provides a second safety margin ?1which is greater than the first safety margin ?1.

At time t4, the control unit may receive information that the transient operating condition of the internal combustion engine has terminated and the first steady-state operating condition has been reached. The control unit may then reselect the first target value Ti and control the mass flow in the circuit in response thereto.

It is naturally also plausible that the waste heat recovery system is adapted to be operated in such a manner as to take into account both the parameter of the degree of superheating of the steam of the working medium as well as the parameter of the maximum temperature of the working medium.

Figure 7 schematically illustrates one exemplifying embodiment of a device 100. The control unit 24 described with reference to Fig. 2 may in a version comprise or constitute the device 100. 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 100 comprises a non-volatile memory 120, a data processing unit 110 and a read/write memory 150. The non-volatile memory 120 has a first memory element 130 in which a computer program, e.g. an operating system, is stored for controlling the function of the device 100. The device 100 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 120 has also a second memory element 140.

There is provided a computer program P which comprises routines for controlling the operation of a waste heat recovery system of an internal combustion engine. The computer program P comprises routines for receiving information regarding an operating condition of the internal combustion engine and routines for controlling the mass flow of the working medium in the circuit of a waste heat recovery system in response to said information regarding the operating condition of the internal combustion engine. The computer program comprises routines for receiving information of a predetermined threshold value of a parameter of the working medium of the waste heat recovery system. The computer further comprises routines for receiving information regarding a first steadystate operating condition of the internal combustion engine and routines to select a predetermined first target value of the parameter of the working medium, the first target value differing from the threshold value of the parameter by a first safety margin. The computer program also comprises routines for controlling the mass flow of the working medium in the circuit to reach said first target value. Moreover, the computer program P further comprises routines for receiving information of an expected upcoming transient operating condition of the internal combustion engine and routines for selecting a second target value of the parameter of the working medium, the second target value different from the threshold value of the parameter by a second safety margin greater than the first safety margin. The computer program further comprises routines for controlling the mass flow of the working medium in the circuit to reach said second target value. The computer program preferably also comprises routines for receiving information that a transient operating condition of the internal combustion engine has terminated and to select a new target value of the parameter of the working medium in response thereto, wherein the new target value provides a lower safety margin than the second safety margin.

The program P may be stored in an executable form or in a compressed form in a memory 160 and/or in a read/write memory 150.

Where the data processing unit 110 is described as performing a certain function, it means that the data processing unit 110 effects a certain part of the program stored in the memory 160 or a certain part of the program stored in the read/write memory 150.

The data processing device 110 can communicate with a data port 199 via a data bus 115. The nonvolatile memory 120 is intended for communication with the data processing unit 110 via a data bus 112. The separate memory 160 is intended to communicate with the data processing unit 110 via a data bus 111. The read/write memory 150 is adapted to communicating with the data processing unit 110 via a data bus 114.

When data are received on the data port 199, they are stored temporarily in the second memory element 140. When input data received have been temporarily stored, the data processing unit 110 is prepared to effect code execution as described above.

Parts of the methods herein described may be effected by the device 100 by means of the data processing unit 110 which runs the program stored in the memory 160 or the read/write memory 150. When the device 100 runs the program, methods herein described are executed.

Claims

1. Method of operating a waste heat recovery system (4) for an internal combustion engine (2), wherein the waste heat recovery system (4) comprises: a circuit (10) in which a working medium (WM) circulates; the circuit (10) further comprising a heat exchanger (11) adapted to evaporate the working medium (WM) to create a steam thereof, an expansion device (12) adapted to produce mechanical or electrical energy by expanding the steam created by the heat exchanger (11), a condenser (13) arranged downstream of the expansion device (12) and adapted to condense the steam into a liquid state of the working medium (WM), and a working medium conveyor (14) adapted to circulate the working medium (WM) in the circuit (10); the waste heat recovery system (4) further comprising a control unit (24) adapted to receive information regarding an operating condition of the internal combustion engine (2) and control the mass flow of the working medium (WM) in the circuit (10) in response to said information regarding an operating condition of the internal combustion engine (2); the method comprising the steps of: a) providing a predetermined threshold value (?WM, TWM) of a parameter (?WM,TWM) of the working medium (S1); b) if the control unit (24) receives information regarding a first steady-state operating condition of the internal combustion engine (2), then the control unit (24) selects a predetermined first target value (?1, T1) of the parameter of the working medium and controls the mass flow of the working medium in the circuit (10) to reach said first target value (?1, T1) of the parameter of the working medium (S3), wherein the first target value (?1, T1) differs from the threshold value (?WM, TWM) of the parameter by a first safety margin (?1);and c) if the control unit (24) receives information regarding an expected upcoming transient operating condition of the internal combustion engine (2), then the control unit (24) selects a second target value (?2, T2) of the parameter of the working medium, wherein the second target value (?2, T2) differs from the threshold value (?WM, TWM) by a second safety margin (?2) which is greater than the first safety margin (?1), and the control unit (24) controls the mass flow of the working medium in the circuit (10) to reach said second target value (?2, T2) of the parameter of the working medium (S4).

2. Method according to claim 1, further comprising: d) if the control unit (24) receives information that the transient operating condition of the internal combustion engine (2) has terminated and the first or a second steady-state operating condition of the internal combustion engine (2) has been reached, then the control unit (24) selects a third target value of the parameter corresponding to a safety margin which is lower than the second safety margin ?2and controls the mass flow of the working medium (WM) in the circuit (10) to reach said third target value (S6).

3. Method according to any one of the preceding claims, wherein the control unit (24) controls the mass flow of the working medium (WM) in the circuit (10) by adjusting the mass flow rate of the conveyor (14).

4. Method according to any one of the preceding claims, wherein the parameter is the degree of superheating ( ?WM) of the steam of working medium exiting the heat exchanger (11), and the threshold value (?WM) of the parameter corresponds to the vaporization point of the working medium.

5. Method according to any one of claims 1-3, wherein the parameter is the maximum temperature (TWM) of the working medium in the circuit (10) and the threshold value (TWM) corresponds to a temperature at which the working medium (WM) may risk being damaged.

6. Method according to any one of claims 1-3, wherein the parameter is the maximum temperature (TWM) of the working medium in the circuit (10), and the threshold value (TWM) corresponds to a maximum temperature at which the working medium (WM) does not risk causing any damage to the constituent components of the waste heat recovery system (4).

7. Method according to any of the preceding claims, wherein the control unit (24) is adapted to receive information regarding an upcoming transient operating condition of the internal combustion engine (2) based on information from at least one of an engine management system of the internal combustion engine, a gearbox management system, a navigation system, a cruise control system and/or a radar system configured to determine traffic conditions.

8. Method according to any one of the preceding claims, wherein the second safety margin (?2) is at least 25 % greater, preferably at least 50% greater, than the first safety margin (?1).

9. A waste heat recovery system (4) for an internal combustion engine (2), the waste heat recovery system comprising a circuit (10) in which a working medium (WM) circulates; the circuit (10) further comprising a heat exchanger (11) adapted to evaporate the working medium (WM) to create a steam thereof, an expansion device (12) adapted to produce mechanical or electrical energy by expanding the steam created by the heat exchanger (11), a condenser (13), arranged downstream of the expansion device (12) and adapted to condense the steam into a liquid state of the working medium (WM), and a conveyor (14) adapted to circulate the working medium (WM) in the circuit (10); the waste heat recovery system (4) further comprising a control unit (24) adapted to receive information regarding an operating condition of the internal combustion engine (2) and control the mass flow of the working medium (WM) in the circuit (10) in response to said information regarding an operating condition of the internal combustion engine (2); characterised in that the control unit (24) is adapted to receive information regarding a steady-state operating condition of the internal combustion engine (2) and, in response thereto, select a first predetermined target value (?1, T1) of a parameter (?WM,TWM) of the working medium which differs from a predetermined threshold value (?WM, TWM) of the parameter by a first safety margin (?1), and control the mass flow of the working medium in the circuit to reach said first target value (?1, T1) of the parameter of the working medium, and wherein the control unit (24) furthermore is adapted to receive information regarding an expected upcoming transient operating condition of the internal combustion engine (2), and in response thereto, select a second target value (?2, T2) of the parameter of the working medium and control the mass flow of the working medium in the circuit to reach said second target value (?2, T2) of the parameter of the working medium, wherein the second target value (?2, T2) differs from the predetermined threshold value (?WM, TWM) by a second safety margin (?2) which is greater than the first safety margin (?1).

10. The waste heat recovery system (4) according to claim 9, wherein the control unit (24) is further adapted to receive information that the transient operating condition has terminated and that a steady-state operating condition of the internal combustion engine (2) has been reached, and in response thereto, select a third target value of the parameter corresponding to a safety margin which is lower than the second safety margin ?2and control the mass flow of the working medium (WM) in the circuit (10) to reach said third target value.

11. The waste heat recovery system (4) according to any one of claims 9 and 10, wherein the control unit (24) is connected to the conveyor (14) and adapted to control the mass flow of the working medium (WM) in the circuit (10) by adjusting the mass flow rate of the conveyor (14).

12. The waste heat recovery system (4) according to any one of claims 9 to 11, wherein the control unit (24) is adapted to receive information regarding an upcoming transient operating condition of the internal combustion engine (2) based on information from at least one of an engine management system of the internal combustion engine, a gearbox management system, a navigation system, a cruise control system and/or a radar system of the vehicle configured to determine traffic conditions.

13. A vehicle (1) comprising an internal combustion engine (2), characterised in that the vehicle (1) further comprises a waste heat recovery system (4) according to any one of claims 9 to 11.

14. A computer program (P), wherein said computer program comprises program code for causing a control unit or a computer connected to the control unit to perform the method according to any one of the claims 1 to 8.

15. A computer-readable medium comprising instructions, which when executed by a control unit or a computer connected to the control unit cause the control unit or the computer to perform the method according to any one of claims 1 to 8.