SE541172C2

SE541172C2 - A method and a vehicle for controlling a WHR-system in response to a determined recoverable energy of a downhill slope

Info

Publication number: SE541172C2
Application number: SE1651038A
Authority: SE
Inventors: Björn Johansson; Erik Höckerdal; Thomas Timren
Original assignee: Scania Cv Ab
Priority date: 2016-07-12
Filing date: 2016-07-12
Publication date: 2019-04-23
Also published as: DE102017006371A1; DE102017006371B4; SE1651038A1

Abstract

The invention relates to a method for controlling a vehicle (1) in connection with a downhill slope, the vehicle (1) comprising a powertrain (3) with a combustion engine (2); and a waste heat recovery system (10) associated with the combustion engine (2), the waste heat recovery system (10) comprising a working fluid circuit (12); an evaporator (14); an expander (16); a condenser (18); a reservoir (20) for a working fluid (WF) and a pump (22) arranged to pump the working fluid (WF) through the circuit (12), wherein the evaporator (14) is arranged for heat exchange between the working fluid (WF) and at least one heat source (24), wherein the waste heat recovery system (10) further comprises a cooling circuit (26) arranged in connection to the condenser (18), and wherein the expander (16) is mechanically connected to the powertrain (3). The method comprises the steps of: predicting (s101) a downhill slope and determining if the predicted downhill slope fulfils a predetermined requirement; determining (s102) the amount of energy that will be recovered by the waste heat recovery system (10) during the downhill slope; and controlling (s103) the vehicle (1) based on the determined amount of energy that will be recovered by the waste heat recovery system (10).The invention also relates to a vehicle (1), a computer program (Pr) and a computer program product.

Description

A method and a vehicle for controlling a WHR-system in response to a determined recoverable energy of a downhill slope TECHNICAL FIELD The present invention relates to a method for controlling a vehicle in connection with a downhill slope, a vehicle, a computer program and a computer program product 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. In vehicles with 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 some of the dissipated heat may instead be used to produce mechanical work. The mechanical work may for example be transferred to the powertrain and thus be used to propel the vehicle. This way the engine efficiency and the fuel consumption can be improved.

Waste heat recovery systems are typically based on the Rankine cycle and thus comprise a working fluid, a pump for circulating the working fluid in a circuit, at least one evaporator, an expansion device and at least one condenser. The working fluid 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 the heat source(s) lead through the evaporator and the working fluid thereby evaporates. The vapour is subsequently expanded in the expansion device. By means of the expansion device the recovered heat is 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 condenser is thus typically connected to a cooling circuit, which could be part of the engine cooling system or a separate cooling circuit.

The mechanical work generated by the expansion device may be transferred to the powertrain of the vehicle if the expansion device is mechanically connected to the powertrain. Document US2009211253 A1 discloses a waste heat recovery system where a shaft of a turbine (expansion device) is coupled to the engine crankshaft. The mechanical work generated by the expansion device is thus torque used to propel the vehicle. The extra torque provided by such waste heat recovery systems may not always be desired. When a vehicle is driving uphill high load on the combustion engine will result in higher temperature of the exhaust gases and thereby more energy transferred via the evaporator to the waste heat recovery system. This means that more torque can be provided by the expansion device. Driving uphill, this is typically an advantage. However, when the vehicle starts driving downhill the extra torque provided by the waste heat recovery system may not be desired for propulsion of the vehicle. In a long downhill slope the vehicle speed will increase due to the mass of the vehicle (potential energy) and there is a risk that the vehicle speed becomes too high. Depending on the length of the downhill slope, extra torque from the waste heat recovery system may thus not be useful. Document DE102008011213 A1 discloses a waste heat recovery system where the mechanical work from the expander is used to propel a vehicle. When the vehicle is braking or driving downhill the combustion engine continues to run but the fuel supply is stopped and the braking energy is stored thermally.

SUMMARY OF THE INVENTION Despite known solutions in the field, there is still a need to develop a method for controlling a vehicle in connection with a downhill slope, which enables energy optimal operation of the vehicle.

An object of the present invention is to achieve an advantageous method for controlling a vehicle in connection with a downhill slope, which reduces fuel consumption and enables energy optimal operation of the vehicle.

The herein mentioned objects are achieved by a method for controlling a vehicle, a vehicle, a computer program and a computer program product according to the independent claims.

According to an aspect of the present invention a method for controlling a vehicle in connection with a downhill slope is provided. The vehicle comprising a powertrain with a combustion engine, wherein the vehicle comprises a waste heat recovery system associated with the combustion engine, the waste heat recovery system comprising a working fluid circuit; an evaporator; an expander; a condenser; a reservoir for a working fluid and a pump arranged to pump the working fluid through the circuit, wherein the evaporator is arranged for heat exchange between the working fluid and at least one heat source, wherein the waste heat recovery system further comprises a cooling circuit arranged in connection to the condenser, and wherein the expander is mechanically connected to the powertrain. The method comprises the steps of: - predicting a downhill slope and determining if the predicted downhill slope fulfils a predetermined requirement; - determining the amount of energy that will be recovered by the waste heat recovery system during the predicted downhill slope; and - controlling the vehicle based on the determined amount of energy that will be recovered by the waste heat recovery system.

The predetermined requirement is suitably that the gradient of the downhill slope is such that braking of the vehicle will not be required. The predetermined requirement may be that the gradient of the downhill slope is such that braking of the vehicle will not be required in order to maintain the vehicle speed below a predetermined vehicle speed (not exceed a predetermined vehicle speed) while driving down the predicted downhill slope.

The method may comprise the steps of: - predicting a downhill slope which will not require braking of the vehicle; - determining the amount of energy that will be recovered by the waste heat recovery system during the downhill slope; and - controlling the vehicle based on the determined amount of energy.

The method may comprise the steps of: - predicting a downhill slope which will not require braking of the vehicle to maintain the vehicle speed below a predetermined vehicle speed; - determining the amount of energy that will be recovered by the waste heat recovery system during the predicted downhill slope; and - controlling the vehicle based on the determined amount of energy that will be recovered by the waste heat recovery system.

The waste heat recovery system of the vehicle is suitably based on the Rankine cycle, preferably an organic Rankine cycle. The working fluid is thus suitably organic, such as ethanol or acetone. The waste heat recovery system based on the Rankine cycle is suitably configured such that the working fluid, suitably in a liquid state, is pumped through the evaporator. The working fluid is thereby heated by the at least one heat source connected to the evaporator and the working fluid thus evaporates. The vapour is then expanded in the expander whereby mechanical work is produced. The mechanical work is transferred from the expander as torque to the powertrain. The mechanical work may for example be transferred to the crankshaft of the combustion engine or the gearbox and thus be used to propel the vehicle. The vapour is thereafter cooled in the condenser by heat exchange with the cooling fluid in the cooling circuit, such that the working fluid is brought back to its initial liquid state. The at least one heat source in the vehicle comprising the waste heat recovery system may be exhaust gases from the combustion engine, an exhaust gas recirculation system, the cooling fluid of the combustion engine, the combustion engine itself or any other hot component in the vehicle. The at least one heat source is preferably associated with the combustion engine. The evaporator is suitably a heat exchanger connected to the at least one heat source and the working fluid circuit. The heat transfer between the working fluid and the heat source is an exchange of energy resulting in a change in temperature. Thus, the heat source is providing the energy entering the waste heat recovery system and the energy is leaving the waste heat recovery system as mechanical work via the expander and as heat via the cooling circuit. The temperature in the waste heat recovery system thus depends on the amount of energy entering the system and the amount of energy leaving the system.

The torque provided by the expander helps propelling the vehicle but there might be situations where the additional torque is not needed. In a long downhill slope the vehicle speed will increase of itself due to the mass of the vehicle (potential energy). This often results in a need to brake the vehicle somewhere along the downhill slope in order not to exceed a predetermined vehicle speed. In order to save fuel vehicles are usually braked by coasting with a gear engaged when driving down a long slope. When the vehicle is coasting with a gear engaged, the engine is running and the fuel supply is cut off, such that the engine is driven by the driving wheels of the vehicle. In such cases, the additional torque provided by the expander of the waste heat recovery system is not needed or wanted. During short downhill slopes however, the additional torque provided by the expander may be useful. It is therefore important to consider the energy that can be recovered by the waste heat recovery system, and thus the mechanical work that can be provided by the expander, when planning the operation of a vehicle. By predicting a downhill slope and determining if the predicted downhill slope fulfils a predetermined requirement; determining the amount of energy that will be recovered by the waste heat recovery system during the downhill slope; and controlling the vehicle based on the determined amount of energy, the vehicle may be operated in an energy optimal way in connection with the predicted downhill slope.

The step of predicting a downhill slope which will not require braking of the vehicle suitably comprises to predict a downhill slope where the gradient is such that braking of the vehicle is not required in order to not exceed a predetermined vehicle speed. Such predetermined vehicle speed may be a desired speed requested by the operator of the vehicle, it may be a speed set by a control system (e.g. a cruise control or a downhill speed control) or it may be a speed limit. A downhill slope which will not require braking of the vehicle is typically a short downhill slope. A downhill slope which will not require braking of the vehicle is typically a downhill slope where the change in potential energy is such that the predetermined vehicle speed can be maintained without braking the vehicle. A downhill slope which will not require braking of the vehicle is suitably predicted based on road inclination, friction, length of the slope or similar. Such road data is available in the vehicle control system and may be determined according to conventional methods by means of navigation systems, sensors and/or cameras. Whether braking of the vehicle is necessary or not also depends on the vehicle characteristics, such as vehicle speed prior to the downhill slope and the weight/load of the vehicle. By predicting a downhill slope which will not require braking of the vehicle, a driving situation is predicted where mechanical work provided by the waste heat recovery system may be useful.

The step of determining the amount of energy that will be recovered by the waste heat recovery system during the downhill slope suitably involves determining the additional torque that the waste heat recovery system can provide during the downhill slope. The torque provided by the waste heat recovery system is mechanical work converted from the recovered energy in the waste heat recovery system and by determining the amount of energy recovered by the system the torque provided by the system is also determined. The method may comprise the step of determining the amount of energy that will be recovered by the waste heat recovery system during a predetermined period of time ahead. By determining the energy that can be recovered by the waste heat recovery system during the downhill slope the operation of the vehicle in connection with the downhill slope can be planned in an energy optimal way. The amount of energy that will be recovered by the waste heat recovery system during the downhill slope is suitably determined based on estimated stored heat associated with the at least one heat source and a predicted mass flow of the at least one heat source. The stored heat associated with the at least one heat source and the mass flow will determine the amount of energy entering the waste heat recovery system and thus the amount of energy that will be recovered by the waste heat recovery system. The stored heat associated with the at least one heat source will affect the temperature of the at least one heat source. The amount of energy that will be recovered by the waste heat recovery system during the downhill slope is thus an estimate. The at least one heat source is suitably exhaust gases in an exhaust system of the vehicle and the energy recovered by the waste heat recovery system is suitably determined based on the estimated stored heat in the exhaust system upstream of the evaporator and the predicted exhaust gas mass flow. The temperature of the exhaust gases entering the evaporator thus depends on the stored heat in the exhaust system upstream of the evaporator. The exhaust system may comprise an exhaust gas aftertreatment system which typically stores a lot of heat.

The step of controlling the vehicle based on the determined amount of energy that will be recovered from the waste heat recovery system suitably comprises to control the vehicle in connection to the predicted downhill slope based on the determined amount of energy. The step of controlling the vehicle may comprise to control the vehicle speed based on the amount of energy that can be recovered in the system. Vehicles today typically comprise various cruise control systems for controlling the speed of the vehicle. Different aspects, such as fuel consumption, comfort and time are considered when controlling the vehicle and different systems use different strategies for controlling the vehicle. When the vehicle comprises a waste heat recovery system which provides mechanical work to the powertrain, it is therefore important that these systems also consider the waste heat recovery system when planning the operation of the vehicle. This way, the vehicle can be operated with optimal energy utilization.

According to an aspect of the invention the step of controlling the vehicle speed comprises to decrease the vehicle speed prior to the downhill slope, wherein the decrease depends on the determined amount of energy that can be recovered by the waste heat recovery system during the downhill slope. One strategy of operating a vehicle in a short downhill slope is to keep the combustion engine connected to the powertrain with a gear engaged and the fuel supply cut off, such that the combustion engine is driven by the driving wheels of the vehicle. This strategy improves the fuel consumption and braking of the vehicle is avoided. With a waste heat recovery system according to the invention, additional torque will be provided to the powertrain. This additional torque must be considered when controlling the vehicle speed if braking of the vehicle is to be avoided. Since the expander of the waste heat recovery system is connected to the powertrain the additional torque will act on the driving wheels and thereby help propelling the vehicle. By decreasing the vehicle speed prior to the downhill slope braking of the vehicle can be avoided despite the additional torque provided by the waste heat recovery system. The vehicle speed at the crest of the downhill slope is suitably decreased. The decrease depends on the determined amount of energy that will be recovered by the waste heat recovery system during the downhill slope. The more energy that will be recovered by the waste heat recovery system the more additional torque will act on the powertrain. Thus, the more energy that will be recovered by the waste heat recovery system the more should the vehicle speed be decreased prior to the downhill slope.

According to an aspect of the invention the step of controlling the vehicle comprises to, when driving down the slope, disconnect the combustion engine from the rest of the powertrain and ensure that the combustion engine is operated with an engine speed which does not exceed a standard idling speed by means of the energy recovered by the waste heat recovery system. The step of controlling the vehicle suitably comprises to, when driving down the slope, disconnect the combustion engine from the rest of the powertrain and ensure that the combustion engine is operated with a standard idling speed at least partly by means of the energy recovered by the waste heat recovery system. One strategy of operating a vehicle in a short downhill slope is to disconnect the combustion engine from the rest of the powertrain and control the combustion engine to a standard idling speed. With a waste heat recovery system according to the invention, additional torque will be provided to the powertrain. This additional torque must be considered when controlling the engine speed. The expander of the waste heat recovery system is suitably connected to the combustion engine, such that the additional torque will act on the crankshaft of the combustion engine. This way, the additional torque will help the combustion engine maintaining the idling speed. By operating the vehicle with the combustion engine idling during the downhill slope, the waste heat recovery system will add energy to the combustion engine and less fuel is needed to achieve the idling speed of the combustion engine. The fuel injection to the combustion engine can be controlled so that the energy provided by the injected fuel, together with the energy from the waste heat recovery system, results in a standard idling speed of the combustion engine. By disconnecting the combustion engine from the rest of the powertrain and ensuring that the combustion engine is operated with a standard idling speed at least by means of the energy recovered from the waste heat recovery system, the fuel consumption is reduced and the vehicle is operated in an energy optimal way.

The step of controlling the vehicle may alternatively comprise to, when driving down the slope, disconnect the combustion engine from the rest of the powertrain, control the combustion engine to an engine speed lower than the standard idling speed and ensure that the waste heat recovery system provides additional torque to the powertrain. Since the idling speed of the combustion engine is achieved partly by means of the torque provided by the waste heat recovery system, the engine speed of the combustion engine can be decreased to a speed lower than the standard idling speed. Normally, a too low idling speed of a combustion engine will result in vibrations caused by excited resonant frequencies from the combustion engine. Such vibrations affect the comfort in the vehicle and should be avoided. However, the waste heat recovery system according to the invention is vibration-free and since the waste heat recovery system provides torque to the combustion engine, less torque is needed from combustion in the combustion engine for achieving the engine speed. The excitation amplitude from the combustion engine is thereby decreased and the engine speed can be decreased below the standard idling speed with maintained comfort. This way, the fuel consumption is decreased further.

According to an aspect of the invention the step of controlling the vehicle comprises to, when driving down the slope, turn off the combustion engine and control the working fluid in the waste heat recovery system to bypass the expander. One strategy of operating a vehicle in a short downhill slope is to turn of the combustion engine. Normally when the combustion engine is turned off, the waste heat recovery system is turned off since the at least one heat source is associated with the combustion engine. However, if the combustion engine is turned off in a short downhill slope where braking of the vehicle is not required, there is no need to turn off the waste heat recovery system. Instead the waste heat recovery system is kept active but the torque provided by the expander will not be needed since the combustion engine is turned off. The torque provided by the waste heat recovery system is not enough to alone rotate the crankshaft of the combustion engine. By controlling the working fluid in the waste heat recovery system such that it is bypassed the expander, no mechanical work is generated by the expander. The waste heat recovery system is thus kept active in a safe state with low risk of overheating or damaging the system, while storing the available heat/energy for later utilization. This way, the waste heat recovery is ready to produce useful work directly when it is needed again.

The method steps are suitably performed by means of a control unit connected to the waste heat recovery system. The predetermined vehicle speed is suitably stored in the control unit.

According to an aspect of the present invention a vehicle comprising a powertrain with a combustion engine is provided. The vehicle further comprises a waste heat recovery system associated with the combustion engine, the waste heat recovery system comprising a working fluid circuit; an evaporator; an expander; a condenser; a reservoir for a working fluid and a pump arranged to pump the working fluid through the circuit, wherein the evaporator is arranged for heat exchange between the working fluid and at least one heat source, and wherein the waste heat recovery system further comprises a cooling circuit arranged in connection to the condenser, and wherein the expander is mechanically connected to the powertrain. The vehicle further comprises a control unit adapted to predict a downhill slope and determine if the predicted downhill slope fulfils a predetermined requirement; determine the amount of energy that will be recovered by the waste heat recovery system during the downhill slope; and to control the vehicle based on the determined amount of energy. By predicting a downhill slope and determine if it fulfils a predetermined requirement; determining the amount of energy that will be recovered by the waste heat recovery system during the downhill slope; and controlling the vehicle based on the determined amount of energy, the vehicle may be operated in an energy optimal way in connection with the predicted downhill slope.

The predetermined requirement is suitably that the gradient of the downhill slope is such that braking of the vehicle will not be required. The predetermined requirement may be that the gradient of the downhill slope is such that braking of the vehicle will not be required in order to maintain the vehicle speed below a predetermined vehicle speed (not exceed a predetermined vehicle speed) while driving down the predicted downhill slope. The control unit may be adapted to predict a downhill slope which will not require braking of the vehicle; determine the amount of energy that will be recovered by the waste heat recovery system during the downhill slope; and control the vehicle based on the determined amount of energy. The control unit may be adapted to predict a downhill slope which will not require braking of the vehicle to maintain the vehicle speed below a predetermined vehicle speed; determine the amount of energy that will be recovered by the waste heat recovery system during the predicted downhill slope; and control the vehicle based on the determined amount of energy that will be recovered by the waste heat recovery system. The predetermined vehicle speed may be a desired speed requested by the operator of the vehicle, it may be a speed set by a control system or it may be a speed limit. The control unit is suitably adapted to predict a downhill slope which will not require braking of the vehicle based on road inclination, friction, length of the slope or similar. Such road data is available for the control unit and may be determined according to conventional methods by means of navigation systems, sensors and/or cameras arranged on the vehicle. The control unit may be adapted to predict a downhill slope which will not require braking of the vehicle based on vehicle characteristics, such as vehicle speed and the weight/load of the vehicle.

The control unit is suitably connected to the components of the waste heat recovery system such as the evaporator, the expander, the condenser and the pump.

The control unit may be adapted to determine the additional torque that the waste heat recovery system will provide during the downhill slope. The torque provided by the waste heat recovery system is mechanical work converted from the recovered energy in the waste heat recovery system and by determining the amount of energy recovered by the system the torque provided by the system is also determined.

According to an aspect of the invention the control unit is adapted to determine the amount of energy that will be recovered by the waste heat recovery system based on estimated stored heat associated with the at least one heat source and a predicted mass flow of the at least one heat source. The stored heat associated with the at least one heat source will affect the temperature of the at least one heat source. The stored heat associated with the at least one heat source and the mass flow will thus determine the amount of energy entering the waste heat recovery system and thus the amount of energy that will be recovered by the waste heat recovery system. The control unit is thus adapted to estimate the amount of energy that will be recovered by the waste heat recovery system during the downhill slope. The at least one heat source is suitably exhaust gases in an exhaust system and the control unit is thus suitably adapted to determine the amount of energy that will be recovered by the waste heat recovery system based on estimated stored heat in the exhaust system upstream of the evaporator and the predicted exhaust gas mass flow. The exhaust system of the vehicle may comprise exhaust pipes and conduits leading the exhaust gases from the combustion engine via an exhaust gas aftertreatment system and the waste heat recovery system to the environment.

The control unit is suitably adapted to control the vehicle speed based on the amount of energy that can be recovered from the system. The vehicle suitably comprises various cruise control systems for controlling the vehicle speed. Different aspects, such as fuel consumption, comfort and time are considered when controlling the vehicle and different systems use different strategies for controlling the vehicle. When the vehicle comprises a waste heat recovery system which provides mechanical work to the powertrain, it is therefore important that the waste heat recovery system is considered when planning the operation of the vehicle. This way, the vehicle can be operated with optimal energy utilization.

According to an aspect of the invention the control unit is adapted to decrease the vehicle speed prior to the predicted downhill slope, wherein the decrease depends on the determined amount of energy that can be recovered from the waste heat recovery system during the downhill slope. The control unit is suitably adapted to decrease the vehicle speed at the crest of the predicted downhill slope. The control unit may be adapted to ensure that the combustion engine is connected to the rest of the powertrain, ensure that a gear is engaged and cut off the fuel supply. This way, the combustion engine is driven by the driving wheels of the vehicle. Since the expander of the waste heat recovery system is connected to the powertrain of the vehicle the additional torque provided by the expander will act on the driving wheels and thereby help propelling the vehicle. By decreasing the vehicle speed prior to the downhill slope braking of the vehicle can be avoided despite the additional torque provided by the waste heat recovery system.

According to an aspect of the invention the control unit is adapted to, when the vehicle is driving down the slope, disconnect the combustion engine from the rest of the powertrain and ensure that the combustion engine is operated with an engine speed not exceeding a standard idling speed by means of the energy recovered by the waste heat recovery system. The control unit is suitably adapted to, when the vehicle is driving down the slope, disconnect the combustion engine from the rest of the powertrain and ensure that the combustion engine is operated with a standard idling speed by means of at least partly, the energy recovered by the waste heat recovery system. With a vehicle comprising a waste heat recovery system according to the invention, additional torque will be provided to the powertrain when the waste heat recovery system is active. Additional torque means torque in addition to the torque provided by combustion in the combustion engine. This additional torque must be considered when controlling the vehicle speed. The expander of the waste heat recovery system is suitably connected to the combustion engine, such that the additional torque will act on the crankshaft of the combustion engine. This way, the additional torque will help maintaining the idling speed of the combustion engine. By disconnecting the combustion engine from the rest of the powertrain and ensuring that the combustion engine is operated with a standard idling speed by means of the energy recovered from the waste heat recovery system, the fuel consumption is reduced and the vehicle is operated in an energy optimal way.

According to an aspect of the invention the control unit is adapted to, when the vehicle is driving down the slope, disconnect the combustion engine from the rest of the powertrain, control the combustion engine to an engine speed lower than the standard idling speed and ensure that the waste heat recovery system provides additional torque to the powertrain. Since the control unit is adapted to ensure that torque is provided by the waste heat recovery system, the engine speed of the combustion engine can be decreased to a speed lower than the standard idling speed. Normally, a too low idling speed of a combustion engine will result in vibrations caused by excited resonant frequencies from the combustion engine. However, since the waste heat recovery system provides torque to the combustion engine, less torque is needed from combustion in the combustion engine for achieving the engine speed. The excitation amplitude from the combustion engine is thereby decreased and the engine speed can be decreased below the standard idling speed with maintained comfort.

According to an aspect of the invention the control unit is adapted to turn off the combustion engine and control the working fluid in the waste heat recovery system to bypass the expander, when the vehicle is driving down the slope.

The cooling circuit connected to the condenser may be part of the combustion engine cooling system or a separate cooling system. The cooling fluid cooling the condenser may thereby be circulated in the cooling circuit by a cooling pump, driven by the combustion engine or by an electric machine.

The waste heat recovery system may comprise one or more evaporators/ heat exchangers. The waste heat recovery system may for example comprise a recuperator arranged to pre-heat the working fluid before entering the evaporator. The waste heat recovery system may also comprise one or more condensers, such that cooling of the working fluid may be performed in multiple steps. Furthermore, the system may comprise one or more expanders. The expander is suitably a fixed displacement expander, or turbine. The expander may be mechanically connected directly to the combustion engine or it may be mechanically connected to the gearbox or other components of the powertrain.

The vehicle may be a hybrid vehicle. Such hybrid vehicle comprises an electric machine for propulsion, in addition to the combustion engine. The control unit may be the engine control unit or may comprise a plurality of different control units. The control unit may be a part of a cruise control system or a vehicle control system. A computer may be connected to the control unit.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following details, and also by putting the invention into practice. Whereas the invention is described below, it should be noted that it is not restricted to the specific details described. Specialists having access to the teachings herein will recognise further applications, modifications and incorporations within other fields, which are within the scope of the invention.

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 drawings, and in which: Figure 1 schematically illustrates a vehicle according to an embodiment of the invention; Figure 2 schematically illustrates a waste heat recovery system according to an embodiment of the invention; Figure 3 schematically illustrates a flow chart for a method for controlling a vehicle according to an embodiment of the invention; and Figure 4 schematically illustrates a control unit or computer according to an embodiment of the invention.

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

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

The waste heat recovery system 10 is suitably based on an organic Rankine cycle. The working fluid WF is thus suitably organic, such as ethanol or acetone. The waste heat recovery system 10 is configured such that the liquid working fluid WF is pumped from low pressure to high pressure and enters the evaporator 14. The working fluid WF is thereby heated by the at least one heat source 24 connected to the evaporator 14 and the working fluid WF is thus evaporated. The vapour is then expanded in the expander 16 whereby mechanical work is produced and transferred to the powertrain 3, whereby the temperature and the pressure of the vapour is decreased. The vapour thereafter enters the condenser 18 where condensation through heat exchange between the vapour and the cooling fluid of the cooling circuit 26 brings the working fluid WF back to its initial liquid state. Thus, the heat source 24 is providing the energy entering the waste heat recovery system 10 and the energy is leaving the waste heat recovery system 10 as mechanical work via the expander 16 and as heat via the cooling circuit 26 cooling the condenser 18.

The vehicle 1 comprises a control unit 30 arranged in communication with the waste heat recovery system 10. The control unit 30 is adapted to predict a downhill slope which will not require braking of the vehicle 1 ; determine the amount of energy that will be recovered by the waste heat recovery system 10 during the downhill slope; and to control the vehicle 1 based on the determined amount of energy that will be recovered by the waste heat recovery system 10. A computer 32 may be connected to the control unit 30.

Only vapour should enter the expander 16 and the waste heat recovery system 10 therefore comprises a bypass arrangement 34, such that the working fluid WF can be controlled to bypass the expander 16 through the bypass arrangement 34. The control unit 30 may be adapted to control the bypass arrangement 34 such that the working fluid WF is bypassing the expander 16.

The expander 16 is suitably a fixed displacement expander, such as a piston expander, or a turbine. The expander 16 may be mechanically connected directly to the combustion engine 2 or to the gearbox 4. The at least one heat source 24 connected to the evaporator 14 may be exhaust gases from the combustion engine 2, an exhaust gas recirculation system (EGR), the cooling fluid of the combustion engine 2, the combustion engine 2 itself or any other hot component associated with the combustion engine 2. The at least one heat source 24 is herein illustrated as a medium passing through the evaporator 14. The at least one heat source 24 is herein illustrated as arrows and may be exhaust gases originating from the combustion engine 2. The at least one heat source 24 is suitably part of an exhaust system 36 of the vehicle 1. The exhaust system 36 is here illustrated as exhaust pipes/ducts but may also comprise an exhaust gas aftertreatment system (not shown), an exhaust gas recirculation system etc. The waste heat recovery system 10 may comprise a plurality of heat sources 24. The evaporator 14 is suitably a heat exchanger connected to the at least one heat source 24 and the working fluid circuit 12. The waste heat recovery system 10 may comprise one or more heat exchangers 14. The waste heat recovery system 10 may for example comprise a recuperator arranged to pre-heat the working fluid before entering the evaporator 14. The waste heat recovery system 10 may also comprise one or more condensers 18, such that cooling down of the working fluid WF may be performed in multiple steps. Furthermore, the system 10 may comprise one or more expanders 16.

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

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

Figure 3 shows a flowchart for a method for controlling a vehicle 1 in connection with a downhill slope. The vehicle 1 is configured as described in Figure 1 and 2. The vehicle 1 comprises a powertrain 3 with a combustion engine 2 and a waste heat recovery system 10 associated with the combustion engine 2, the waste heat recovery system 10 comprising a working fluid circuit 12; an evaporator 14; an expander 16; a condenser 18; a reservoir 20 for a working fluid WF and a pump 22 arranged to pump the working fluid WF through the circuit 12, wherein the evaporator 14 is arranged for heat exchange between the working fluid WF and at least one heat source 24, wherein the waste heat recovery system 10 further comprises a cooling circuit 26 arranged in connection to the condenser 18, and wherein the expander 16 is mechanically connected to the powertrain 3. The method comprises the steps of predicting s101 a downhill slope and determining if the predicted downhill slope fulfils a predetermined requirement; determining s102 the amount of energy that will be recovered by the waste heat recovery system 10 during the downhill slope; and controlling the vehicle 1 based on the determined amount of energy.

The predetermined requirement is suitably that the gradient of the downhill slope is such that braking of the vehicle 1 will not be required in order to maintain the vehicle speed below a predetermined vehicle speed (not exceed a predetermined vehicle speed) while driving down the predicted downhill slope. The predetermined vehicle speed may be a desired speed requested by the operator of the vehicle, it may be a speed set by a control system or it may be a speed limit.

The step of predicting s101 a downhill slope suitably comprises to predict a downhill slope which will not require braking of the vehicle 1. The step of predicting s101 a downhill slope suitably comprises to predict a downhill slope which will not require braking of the vehicle 1 to maintain the vehicle speed below a predetermined vehicle speed.

The step of predicting s101 a downhill slope suitably comprises to predict a downhill slope where the gradient is such that braking of the vehicle 1 is not required in order to not exceed a predetermined vehicle speed. Such predetermined vehicle speed may be a desired speed requested by the operator of the vehicle, it may be a speed set by a control system or it may be a speed limit.

A downhill slope which will not require braking of the vehicle 1 may be predicted based on road inclination, friction, length of the slope or similar. Such road data is available in the vehicle 1 and may be determined according to conventional methods by means of navigation systems, sensors and/or cameras arranged in the vehicle 1. The step of predicting s101 a downhill slope which will not require braking of the vehicle 1 may further be based on vehicle characteristics, such as vehicle speed and the weight/load of the vehicle 1.

The step of determining s102 the amount of energy that will be recovered by the waste heat recovery system 10 during the downhill slope suitably involves determining the additional torque that the waste heat recovery system 10 will provide during the downhill slope. The method may comprise the step of determining the amount of energy that will be recovered by the waste heat recovery system 10 during a predetermined period of time ahead. By determining the energy that can be recovered by the waste heat recovery system 10 during the downhill slope the operation of the vehicle in connection with the downhill slope can be planned in an energy optimal way.

The amount of energy that will be recovered by the waste heat recovery system 10 during the downhill slope is suitably determined based on estimated stored heat associated with the at least one heat source 24 and a predicted mass flow of the at least one heat source 24. The stored heat associated with the at least one heat source 24 and the mass flow will determine the amount of energy entering the waste heat recovery system 10 and thus the amount of energy that will be recovered by the waste heat recovery system 10. The stored heat associated with the at least one heat source 24 will affect the temperature of the at least one heat source 24. The at least one heat source 24 is suitably exhaust gases in an exhaust system 36 of the vehicle 1 and the energy recovered by the waste heat recovery system 10 is suitably determined based on the estimated stored heat in the exhaust system 36 upstream of the evaporator 14 and the predicted exhaust gas mass flow. The temperature of the exhaust gases 24 entering the evaporator 14 thus depends on the stored heat in the exhaust system 36 upstream of the evaporator 14. The exhaust system 36 may comprise an exhaust gas aftertreatment system which typically stores a lot of heat.

The step of controlling s103 the vehicle 1 based on the determined amount of energy that will be recovered by the waste heat recovery system 10 suitably comprises to control the vehicle 1 in connection to the predicted downhill slope based on the determined amount of energy. The step of controlling s103 the vehicle 1 may comprise to control the vehicle speed based on the amount of energy that can be recovered by the waste heat recovery system 10.

The step of controlling s103 the vehicle speed may comprise to decrease the vehicle speed prior to the downhill slope, wherein the decrease depends on the determined amount of energy that can be recovered by the waste heat recovery system 10 during the downhill slope. One strategy of operating a vehicle 1 in a short downhill slope is to keep the combustion engine 2 connected to the powertrain 3 with a gear engaged and the fuel supply cut off, such that the combustion engine 2 is driven by the driving wheels 6 of the vehicle 1. This strategy improves the fuel consumption and braking of the vehicle 1 is avoided. With a waste heat recovery system 10 according to the invention, additional torque will be provided to the powertrain 3. This additional torque must be considered when controlling the vehicle speed if braking of the vehicle 1 is to be avoided. Since the expander 16 of the waste heat recovery system 10 is connected to the powertrain 3 the additional torque will act on the driving wheels 6 and thereby help propelling the vehicle 1. By decreasing the vehicle speed prior to the downhill slope braking of the vehicle 1 can be avoided despite the additional torque provided by the waste heat recovery system 10. The decrease depends on the determined amount of energy that will be recovered by the waste heat recovery system 10 during the downhill slope. The more energy that will be recovered by the waste heat recovery system 10 the more should the vehicle speed be decreased prior to the downhill slope.

The step of controlling s103 the vehicle 1 may comprise to, when driving down the slope, disconnect the combustion engine 2 from the rest of the powertrain 3 and ensure that the combustion engine 2 is operated with an engine speed which does not exceed a standard idling speed by means of the energy recovered by the waste heat recovery system 10. The step of controlling s103 the vehicle 1 may comprise to, when driving down the slope, disconnect the combustion engine 2 from the rest of the powertrain 3 and ensure that the combustion engine 2 is operated with a standard idling speed by means of the energy recovered by the waste heat recovery system 10. One strategy of operating a vehicle 1 in a short downhill slope is to disconnect the combustion engine 2 from the rest of the powertrain 3 and control the combustion engine 2 to a standard idling speed. With a waste heat recovery system 10 according to the invention, additional torque will be provided to the powertrain 3. The expander 16 of the waste heat recovery system 10 is suitably connected to the combustion engine 2, such that the additional torque will act on the crankshaft of the combustion engine 2. This way, the additional torque will help the combustion engine 2 maintaining the idling speed. By operating the vehicle 1 with the combustion engine 2 idling during the downhill slope, the waste heat recovery system 10 will add energy to the combustion engine 2 and less fuel is needed to achieve the idling speed of the combustion engine 2. By disconnecting the combustion engine 2 from the rest of the powertrain 3 and ensuring that the combustion engine 2 is operated with a standard idling speed by means of the energy recovered from the waste heat recovery system 10, the fuel consumption is reduced and the vehicle 1 is operated in an energy optimal way.

The step of controlling s103 the vehicle 1 may comprise to, when driving down the slope, disconnect the combustion engine 2 from the rest of the powertrain 3, control the combustion engine 2 to an engine speed lower than the standard idling speed and ensure that the waste heat recovery system 10 provides additional torque to the powertrain 3. Since torque is provided by the waste heat recovery system 10, less torque is needed from combustion and the engine speed of the combustion engine 2 can be decreased to a speed lower than the standard idling speed. Normally, a too low idling speed of a combustion engine 2 will result in vibrations caused by excited resonant frequencies from the combustion engine 2. However, since the waste heat recovery system 10 provides torque to the combustion engine 2, less torque is needed from combustion in the combustion engine 2 for achieving the engine speed. The excitation amplitude from the combustion engine 2 is thereby decreased and the engine speed can be decreased below the standard idling speed with maintained comfort. This way, the fuel consumption is decreased further.

The step of controlling s103 the vehicle 1 may comprise to, when driving down the slope, turn off the combustion engine 2 and control the working fluid WF in the waste heat recovery system 10 to bypass the expander 16. The working fluid WF is suitably controlled to bypass the expander 16 via the bypass arrangement 34. One strategy of operating a vehicle 1 in a short downhill slope is to turn of the combustion engine 2. Normally when the combustion engine 2 is turned off, the waste heat recovery system 10 is turned off since the at least one heat source 24 is associated with the combustion engine 2. However, if the combustion engine 2 is turned off in a short downhill slope where braking of the vehicle 1 is not required, there is no need to turn off the waste heat recovery system 10. Instead the waste heat recovery system 10 is kept active but the torque provided by the expander 16 will not be needed since the combustion engine 2 is turned off. By controlling the working fluid WF in the waste heat recovery system 10 such that it is bypassed the expander 16, no mechanical work is generated by the expander 16.

The method steps are suitably performed by means of a control unit 30 connected to the waste heat recovery system 10. The predetermined vehicle speed is suitably stored in the control unit 30.

Figure 4 schematically illustrates a device 500. The control unit 30 and/or computer 32 described with reference to Figure 2 may in a version comprise the device 500. The term “link” refers herein to a communication link which may be a physical connection such as an optoelectronic communication line, or a nonphysical connection such as a wireless connection, e.g. a radio link or microwave link. The device 500 comprises a non-volatile memory 520, a data processing unit 510 and a read/write memory 550. The non-volatile memory 520 has a first memory element 530 in which a computer program, e.g. an operating system, is stored for controlling the function of the device 500. The device 500 further comprises a bus controller, a serial communication port, I/O means, an A/D converter, a time and date input and transfer unit, an event counter and an interruption controller (not depicted). The non-volatile memory 520 has also a second memory element 540.

There is provided a computer program P which comprises routines for a method for controlling a waste heat recovery system 10 associated with a combustion engine 2 of a vehicle 1 according to the invention. The computer program P comprises routines for predicting a downhill slope and determining if the predicted downhill slope fulfils a predetermined requirement. The computer program P comprises routines for predicting a downhill slope which will not require braking of the vehicle. The computer program P comprises routines for determining the amount of energy that will be recovered from the waste heat recovery system during the downhill slope. The computer program P comprises routines for controlling the vehicle based on the determined amount of energy that will be recovered from the waste heat recovery system. The computer program P comprises routines for controlling the vehicle speed based on the amount of energy that can be recovered by the waste heat recovery system. The computer program P comprises routines for decreasing the vehicle speed prior to the downhill slope, wherein the decrease depends on the determined amount of energy that can be recovered from the waste heat recovery system during the downhill slope. The computer program P comprises routines for disconnecting the combustion engine from the rest of the powertrain and ensuring that the combustion engine is operated with a standard idling speed by means of the energy recovered from the waste heat recovery system. The computer program P comprises routines for controlling the combustion engine to an engine speed lower than the standard idling speed. The computer program P comprises routines for turning off the combustion engine and controlling the working fluid in the waste heat recovery system to bypass the expander. 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 preferred embodiments of the present invention is provided for illustrative and descriptive purposes. It is not intended to be exhaustive or to restrict the invention to the variants described. Many modifications and variations will obviously be apparent to one skilled in the art. The embodiments have been chosen and described in order best to explain the principles of the invention and its practical applications and hence make it possible for specialists to understand the invention for various embodiments and with the various modifications appropriate to the intended use.

Claims

1. A method for controlling a vehicle (1) in connection with a downhill slope, the vehicle (1) comprising a powertrain (3) with a combustion engine (2); and a waste heat recovery system (10) associated with the combustion engine (2), the waste heat recovery system (10) comprising a working fluid circuit (12); an evaporator (14); an expander (16); a condenser (18); a reservoir (20) for a working fluid (WF) and a pump (22) arranged to pump the working fluid (WF) through the circuit (12), wherein the evaporator (14) is arranged for heat exchange between the working fluid (WF) and at least one heat source (24), wherein the waste heat recovery system (10) further comprises a cooling circuit (26) arranged in connection to the condenser (18), and wherein the expander (16) is mechanically connected to the powertrain (3), characterized by the steps of: - predicting (s101) a downhill slope and determining if the predicted downhill slope fulfils a predetermined requirement that the slope will not require braking of the vehicle; - determining (s102) the amount of energy that will be recovered by the waste heat recovery system (10) during the downhill slope; and - controlling (s103) the vehicle (1) based on the determined amount of energy that will be recovered by the waste heat recovery system (10).

2. The method according to claim 1 , wherein the predetermined requirement is that the gradient of the downhill slope is such that braking of the vehicle 1 will not be required.

3. The method according to claim 1 or 2, wherein the amount of energy that will be recovered by the waste heat recovery system (10) is determined based on estimated stored heat associated with the at least one heat source (24) and a predicted mass flow of the at least one heat source (24).

4. The method according to any of claims 1-3, wherein the step of controlling (s103) the vehicle comprises to control the vehicle speed based on the amount of energy that can be recovered by the system (10).

5. The method according to claim 4, wherein the step of controlling (s103) the vehicle speed comprises to decrease the vehicle speed prior to the downhill slope, wherein the decrease depends on the determined amount of energy that will be recovered by the waste heat recovery system (10) during the downhill slope .

6. The method according to any of claims 1-3, wherein the step of controlling (s103) the vehicle (1) comprises to, when driving down the slope, disconnect the combustion engine (2) from the rest of the powertrain (3) and ensure that the combustion engine (2) is operated with an engine speed which does not exceed a standard idling speed by means of the energy recovered by the waste heat recovery system (10).

7. The method according to any of claims 1-3, wherein the step of controlling (s103) the vehicle (1) comprises to, when driving down the slope, turn off the combustion engine (2) and control the working fluid (WF) in the waste heat recovery system (10) to bypass the expander (16).

8. A vehicle (1), comprising a powertrain (3) with a combustion engine (2); and a waste heat recovery system (10) associated with the combustion engine (2), the waste heat recovery system (10) comprising a working fluid circuit (12); an evaporator (14); an expander (16); a condenser (18); a reservoir (20) for a working fluid (WF) and a pump (22) arranged to pump the working fluid (WF) through the circuit (12), wherein the evaporator (14) is arranged for heat exchange between the working fluid (WF) and at least one heat source (24), and wherein the waste heat recovery system (10) further comprises a cooling circuit (26) arranged in connection to the condenser (18), and wherein the expander (16) is mechanically connected to the powertrain (3), characterized in that the vehicle (1) comprises a control unit (30) adapted to predict a downhill slope and determine if the predicted downhill slope fulfils a predetermined requirement that the slope will not require braking of the vehicle; determine the amount of energy that will be recovered by the waste heat recovery system (10) during the downhill slope; and to control the vehicle based on the determined amount of energy.

9. The vehicle according to claim 8, wherein the predetermined requirement is that the gradient of the downhill slope is such that braking of the vehicle 1 will not be required.

10. The vehicle according to claim 8 or 9, wherein the control unit (30) is adapted to determine the amount of energy that will be recovered by the waste heat recovery system (10) based on estimated stored heat associated with the at least one heat source (24) and a predicted mass flow of the at least one heat source (24).

11. The vehicle according to any of claims 8 -10, wherein the control unit (30) is adapted to control the vehicle speed based on the amount of energy that will be recovered by the waste heat recovery system (10).

12. The vehicle according to claim 11 , wherein the control unit (30) is adapted to decrease the vehicle speed prior to the predicted downhill slope, wherein the decrease depends on the determined amount of energy that will be recovered by the waste heat recovery system (10) during the downhill slope.

13. The vehicle according to any of claims 8-10, wherein, when the vehicle (1) is driving down the slope, the control unit (30) is adapted to disconnect the combustion engine (2) from the rest of the powertrain (3) and ensure that the combustion engine (2) is operated with an engine speed which does not exceed a standard idling speed by means of the energy recovered by the waste heat recovery system (10).

14. The vehicle according to any of claims 8 -10, wherein, when the vehicle (1) is driving down the slope, the control unit (30) is adapted to turn off the combustion engine (2) and control the working fluid (WF) in the waste heat recovery system (10) to bypass the expander (16).

15. A computer program (P), wherein said computer program comprises program code for causing an electronic control unit (30; 500) or a computer (32; 500) connected to the electronic control unit (30; 500) to perform the steps according to any of the claims 1-7.

16. A computer program product comprising a program code stored on a computer-readable medium for performing the method steps according to any of claims 1-7, when said computer program is run on an electronic control unit (30; 500) or a computer (32; 500) connected to the electronic control unit (30; 500).