Methods for extracting energy from an exhaust stream and motor vehicles

Abstract

Motor vehicle with an internal combustion engine (10) from which an exhaust gas stream is discharged, and with a Clausius-Rankine cycle in which a working fluid is conveyed in a closed system from a pump (14) to an evaporator (16) in which the exhaust gas stream supplies heat energy to the working fluid, from the evaporator (16) is conveyed to an expansion machine (18) in which electrical energy is generated, from the expansion machine (18) to a condenser (22) and from the condenser (22) back to the pump (14), wherein the pump delivery rate is adjustable, characterized in that the adjustment of the pump delivery rate is effected by control signals from a control device (24) which is designed toto control the transport capacity of the pump as a function of at least one quantity determining the heat energy transferable per unit time in the evaporator (16) by the current exhaust gas flow, and the control device is coupled to a source (28) that specifies the position of an accelerator pedal (30).

Description

[0001] The invention relates to a motor vehicle with an internal combustion engine from which an exhaust gas stream is discharged.

[0002] The exhaust gas from an internal combustion engine has a relatively high temperature, and it would be desirable to be able to utilize the thermal energy contained in the exhaust gas. It would seem sensible to generate electrical energy using the thermal energy in the exhaust gas. A suitable approach to this would be to use techniques familiar from power plants. This process uses a so-called Rankine cycle: a working fluid, usually water, is alternately evaporated at high pressure and condensed at low pressure. The high pressure is reduced in an expansion machine, particularly a turbine, releasing work, thus generating electrical energy. After condensing in a condenser, the now liquid working fluid is pumped to the evaporator.In the evaporator, the working fluid receives heat from another fluid via so-called indirect heat transfer, in which the other fluid is separated from the working fluid by a partition. In this case, it is conceivable that said other fluid could be precisely the exhaust gas flow from the combustion engine. However, difficulties in implementation have so far arisen due to the fact that the exhaust gas mass flow is variable and depends in particular on the current power of the combustion engine. It is therefore not sensible to move the working fluid uniformly in the Clausius-Rankine cycle. Control of the movement of the working fluid has already been considered, which should be dependent on measured values ​​​​obtained from the working fluid, e.g., depending on the temperature, mass flow, or pressure of the working medium.However, due to the inertia of such a control, which is determined by the ratio of the internal volume of the system to the mass flow, a stable control of the type mentioned has proven to be unfeasible.

[0003] EP 1 333 157 A1 actually describes how the exhaust gas stream from an internal combustion engine is used as an energy source for a Clausius-Rankine cycle. This involves two cycle systems in which the working fluids differ in their boiling points. The pumping capacity is variably adjustable in at least one of the cycles. EP 1 333 157 A1 states that this allows the efficiency of heat energy recovery from the exhaust gas stream to be maximized.

[0004] FR 2 884 555 A1 discloses a method for extracting energy from an exhaust gas stream from an internal combustion engine, wherein the exhaust gas stream is fed to an evaporator in which a working fluid is converted from the liquid state to the gaseous state due to indirect heat transfer from the exhaust gas stream. The gaseous working fluid is fed to an expansion machine in which the energy is extracted, and furthermore the gaseous working fluid is fed to a condenser in which it is converted from the gaseous state back to the liquid state, and furthermore the liquid working fluid is fed back to the evaporator by means of a pump. The transport capacity of the pump is controlled as a function of at least one variable determining the thermal energy transferable per unit of time in the evaporator by the current exhaust gas stream.

[0005] US 2005 / 0 050 909 A1 discloses a method for extracting energy from an exhaust gas stream from an internal combustion engine, wherein the exhaust gas stream is fed to an evaporator in which a working fluid is converted from the liquid state to the gaseous state due to indirect heat transfer from the exhaust gas stream. The gaseous working fluid is fed to an expansion machine in which the energy is extracted, and furthermore the gaseous working fluid is fed to a condenser in which it is converted from the gaseous state back to the liquid state, and furthermore the liquid working fluid is fed back to the evaporator by means of a pump. The transport capacity of the pump is controlled as a function of at least one variable determining the thermal energy transferable per unit of time in the evaporator by the current exhaust gas stream.The transport capacity of the pump is adjusted as a function of at least the exhaust gas mass flow and the variable(s) determining the operation of the combustion engine is / are determined and the exhaust gas mass flow is deduced from a characteristic curve or a characteristic map.

[0006] The object of the present invention is to provide a motor vehicle for recovering energy from an exhaust gas stream from an internal combustion engine, which is practically implementable but simplified compared to the prior art.

[0007] The problem is solved by a motor vehicle having the features according to patent claim 1.

[0008] To extract energy from an exhaust stream, a Rankine cycle is actually used, whereby thermal energy from the exhaust stream is added to the working fluid in an evaporator. The pump's transport capacity is then controlled as a function of at least one variable that determines the thermal energy transferable per unit of time in the evaporator by the current exhaust stream, thus being adjusted to vary over time.

[0009] Control based on internal circuit variables can be dispensed with if the amount of liquid working fluid transported to the evaporator is adapted to the current exhaust gas flow. If the exhaust gas flow provides more thermal energy, the pump's transport capacity can be higher, and if the exhaust gas flow provides less thermal energy, the pump's transport capacity can be lower. The thermal energy provided is directly proportional to the exhaust gas mass flow, so that the pump's transport capacity is preferably adjusted as a function of at least this. In particular, a characteristic curve for a manipulated variable determining the pump's transport capacity as a function of the exhaust gas mass flow can be used when adjusting the pump's transport capacity. The exhaust gas temperature is also taken into account.In this case, a characteristic map is preferably used that represents a control variable that determines the pump's transport performance as a function of exhaust gas mass flow and exhaust gas temperature.

[0010] It can be provided to simply measure the exhaust gas mass flow and feed the measurement signal to a control unit that activates the pump. However, the exhaust gas mass flow does not have to be the current measured variable. The exhaust gas mass flow is thus defined by the operation of the internal combustion engine. Therefore, one or more variables that determine the operation of the internal combustion engine can be determined, and a characteristic map can be provided for the internal combustion engine (a characteristic curve if only one variable is used) that shows the dependence of the exhaust gas mass flow on this variable. Therefore, based on a characteristic map, the exhaust gas mass flow can be deduced from the determined variable(s), and the pump's transport capacity can be determined depending on the exhaust gas mass flow thus estimated.

[0011] The transport capacity of the pump is usually determined directly by the pump speed.

[0012] The invention provides, for the first time, a motor vehicle with an internal combustion engine that uses a single Clausius-Rankine cycle. This is made possible by the fact that the pump's transport capacity is adjustable. According to the invention, the pump's transport capacity is adjusted by control signals from a control device, and this control device is designed to control the pump's transport capacity as a function of at least one variable that determines the heat energy that can be transferred per unit of time in the evaporator by the current exhaust gas flow.

[0013] Here, too, the exhaust gas mass flow, and if necessary also the temperature, is preferably used as the input variable for the control.

[0014] If the exhaust gas mass flow is inferred based on a characteristic map, the control device must receive signals that indicate the variables that determine the operation of the internal combustion engine. The operation of the internal combustion engine is determined, on the one hand, by the amount of fuel injected into it and, on the other hand, by the air supplied. Both variables, in turn, depend on the setting of an accelerator pedal. According to the invention, the control system is coupled to a source of a signal that indicates the position of an accelerator pedal. In the case of an electronically operated accelerator pedal, this source can be a sensor that directly detects the position of the accelerator pedal, with the signals from the sensor then also being fed to a control device that controls the internal combustion engine (or the fuel and air supply to it).In a mechanical system, however, the position of the accelerator pedal can be determined based on gas flows, so that the above-mentioned source of the signals can include a measuring device for the gas flow.

[0015] A preferred embodiment of the invention is described below with reference to the drawing, in which the only figure is a schematic representation of the components used in a motor vehicle according to the invention for implementing a method according to the invention.

[0016] The motor vehicle according to the invention comprises an internal combustion engine 10 in which fuel is burned, producing exhaust gas which is discharged via an exhaust line 12. The exhaust gas in the exhaust line 12 is then used as an energy source for a Clausius-Rankine cycle. In such a cycle, a working fluid, preferably water in this case, is conducted in a closed system. The water is pumped in liquid state by a pump 14 to an evaporator 16. In the evaporator 16, heat energy is transferred from the exhaust gas to the water, causing it to evaporate and reach a high pressure. The steam is fed to a turbine 18, which is coupled to a generator 20. The steam expands as it passes through the turbine 18, and the work performed in this process is converted into electrical energy by the generator 20.After passing through the turbine 18, the steam is condensed into water in a condenser 22, where heat is also transferred to a suitable coolant. Like the working fluid, this can be water, but is separated from the working fluid by a partition in the condenser 22. This water can be routed through the motor vehicle's radiator. The working fluid water is then fed from the condenser 22 back to the pump 14.

[0017] Now, for a given exhaust gas mass flow and a given exhaust gas temperature, a maximum mass flow of the working fluid can be determined that can be evaporated in the evaporator 16 under these conditions. It is therefore not advisable to run the pump 14 at a constant speed, because in one case (with a low exhaust gas mass flow or a not too high exhaust gas temperature), a portion of the liquid working fluid supplied by the pump 14 to the evaporator 16 would not be evaporated at all, and in the other case (with a high exhaust gas mass flow and a relatively high exhaust gas temperature), the maximum possible energy would not be extracted. Therefore, it is provided that the speed of the pump 14 is variably adjustable, and it is controlled by a control device 24. The control device 24 is intended to ensure that as much thermal energy as possible is transferred from the exhaust gas flow to the working fluid.In the present case, this occurs as a function of the exhaust gas mass flow and exhaust gas temperature. The control device 24 can, in particular, use a characteristic map that describes the pump speed as a function of the exhaust gas mass flow and exhaust gas temperature. Since the control takes place as a function of the exhaust gas mass flow and exhaust gas temperature, the control device 24 must be supplied with corresponding information signals. A suitable measuring device 26 can be provided in or on the exhaust line 12, which measures the exhaust gas mass flow and also the exhaust gas temperature. A sensor 28 can also detect the position of an accelerator pedal 30 of the motor vehicle, via which the amount of fuel supplied to the internal combustion engine 10 via an injection line 32 and the amount of air supplied to the internal combustion engine 10 via a line 34 are determined.The position of the accelerator pedal 30 therefore determines the operation of the internal combustion engine 10, and the exhaust gas mass flow is directly dependent on the position of the accelerator pedal 30, and in part also on the exhaust gas temperature. Based on the signals from the sensor 28, the control device can determine the exhaust gas mass flow or the exhaust gas temperature. For this purpose, suitable characteristic curves or characteristic maps are stored in the control device 24, which represent a relationship between the position of the accelerator pedal 30 and the exhaust gas mass flow. A practical solution is to infer the exhaust gas mass flow from the position of the accelerator pedal 30, and to determine the temperature of the exhaust gas flow by the sensor 26.

[0018] The system according to the invention ensures that the Clausius-Rankine cycle runs very stably, especially during transient processes. The system can also be easily integrated into an existing control solution system; for example, the control device 24 can be identical to the engine control unit 10.

Claims

[1] Motor vehicle with an internal combustion engine (10) from which an exhaust gas stream is discharged, and with a Clausius-Rankine cycle in which a working fluid is guided in a closed system from a pump (14) to an evaporator (16), in which the exhaust gas stream supplies heat energy to the working fluid, is guided from the evaporator (16) to an expansion machine (18) at which electrical energy is generated, is fed from the expansion machine (18) to a condenser (22) and from the condenser (22) back to the pump (14), wherein the pump transport capacity is adjustable, characterized bythat the pump transport capacity is adjusted by control signals from a control device (24) which is designed to control the transport capacity of the pump as a function of at least one variable determining the thermal energy that can be transferred per unit of time in the evaporator (16) by the current exhaust gas flow, and the control device is coupled to a source (28) for signals indicating the position of an accelerator pedal (30).

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