Method for removing sulfur from a NOx storage catalyst

Abstract

Method for removing sulfur from a NOx storage catalyst (4) arranged to receive an exhaust gas stream from a power engine (2) of a mild hybrid vehicle (10) which has an electric machine driven by the power engine (2), the method comprising: Holding the mild hybrid vehicle (10) stationary in a vehicle service workshop while the following steps are carried out: Connecting an electrical system of the mild hybrid vehicle (10) to an external load, selecting a neutral gear in a transmission of the vehicle, operating the electric machine as a generator to load the power machine (2), increasing a torque requirement to the power machine (2) to generate an exhaust flow to the NOx storage catalyst (4) that is substoichiometric, and using the external electrical load to absorb the electrical energy generated by the electric machine.

Description

[0001] The present invention relates to the removal of sulfur impurities from a NOx storage catalyst of a motor vehicle and in particular to a method for removing sulfur impurities when the vehicle is stationary.

[0002] Motor vehicles with exhaust aftertreatment systems are known, for example, from US 2005 / 0034451 A1, DE 60119653 T2, and DE 102007013588 A1. It is known to equip a motor vehicle with a lean NOx trap aftertreatment system (LNT aftertreatment system, LNT - lean NOx trap / NOx storage catalyst) to control NOx emissions into the environment. It is also known to equip a motor vehicle with an electric machine connected to the engine, such as an integrated starter-generator, which can be used to provide a brief torque boost to the engine during vehicle acceleration and stop-start events.Such a vehicle is often referred to as a mild hybrid electric vehicle (MHEV) because electrical power can be used to assist the engine but is not used to power the vehicle on its own.

[0003] Furthermore, it is known that the LNT gradually becomes contaminated with sulfur during use, which is a byproduct of combustion. The effect of such sulfur contamination is that the active sites in the LNT are reduced and, with increasing contamination, can no longer trap NOx. Ultimately, such contamination causes the vehicle to which the LNT is attached to no longer meet emissions regulations.

[0004] If a vehicle is identified as having an emissions failure, it must be taken to a vehicle service center (a workshop) for repair. However, the vehicle service center cannot usually apply a sufficient load to the engine to remove the sulfur from the LNT (Low Nitrogen Transmittance) while the vehicle is in the workshop. This is because the engine needs to operate substoichiometrically to remove the sulfur, and this can only be achieved by supplying the engine with a high torque.

[0005] One solution to this problem is for the vehicle service to drive the vehicle through a defined driving cycle to provide sufficient torque to enable the generation of a rich exhaust gas. However, this is often problematic because traffic, traffic signals, and other uncontrolled events can prevent the successful completion of the driving cycle, resulting in incomplete removal of sulfur from the LNT (Low-Temperature Fuel).

[0006] One object of the present invention is to provide a method for removing sulfur from a NOx storage catalyst that overcomes the problems associated with the prior art.

[0007] According to the invention, a method for removing sulfur from a NOx storage catalyst according to claim 1 is provided. Advantageous embodiments of the invention are specified in the dependent claims and the following description.

[0008] The external electrical load can be a battery that has sufficient capacity to absorb the electrical energy generated by the sulfur removal process.

[0009] Furthermore, the method may include connecting the mild hybrid vehicle to an external electronic control system arranged to control the operation of the electric machine during the sulfur removal process.

[0010] Alternatively, the method may also include connecting the mild hybrid vehicle to an external electronic control system arranged to control the torque requirement to the engine during the sulfur removal process.

[0011] As a further alternative, the method may also include connecting the mild hybrid vehicle to an external electronic control system arranged to control the operation of the electric machine and the torque requirement to the engine during the sulfur removal process.

[0012] Furthermore, the procedure may include monitoring the composition of the exhaust gas stream downstream of the NOx storage catalyst and using the monitored gas stream composition to decide when to stop the sulfur removal process.

[0013] Alternatively, the procedure can also include using a sulfur removal model to decide when to stop the sulfur removal process.

[0014] Furthermore, the procedure may include carrying out the sulfur process in response to a warning provided by an exhaust gas monitoring system of the mild hybrid vehicle.

[0015] The load generated by the electric machine can be varied during the period in which sulfur is removed from the NOx storage catalyst to produce alternating rich and non-rich exhaust gas flows.

[0016] The exhaust gas flow can alternate between substoichiometric and superstoichiometric.

[0017] On Fig. 1 Referring to, a mild hybrid electric vehicle 10 is shown in schematic form.

[0018] The vehicle 10 has a diesel engine 2 which is connected to an electric machine in the form of an integrated starter-generator 3, which in this case is an integrated belt-driven starter-generator.

[0019] Exhaust gas from engine 2 flows via an exhaust channel 6 to an aftertreatment device in the form of a NOx storage catalyst 4 and from the NOx storage catalyst 4 to the atmosphere via a downstream exhaust channel 7. The exhaust channels 6 and 7 together form an exhaust system for engine 2.

[0020] The vehicle 10 further comprises a central electronic control unit 5, which in this case controls both the operation of the engine 2 and the operation of the integrated starter-generator 3. It is understood that separate electronic control units could be used for the engine 2 and the integrated starter-generator 3, and that the invention is not limited to use with an electronic architecture comprising only a single electronic control unit.

[0021] The integrated starter-generator 3 is operatively connected to a battery 9. The battery 9 is arranged to be charged by the integrated starter-generator 3 when the integrated starter-generator 3 is operated as a generator, and to provide electrical power to the integrated starter-generator 3 when the integrated starter-generator 3 is operated as a motor to provide torque amplification for the vehicle 10. Unlike a full hybrid or plug-in hybrid vehicle, the integrated starter-generator 3 is not used to power the vehicle on its own; it is used only to provide torque amplification for the power unit 2; and the storage capacity of the battery 9 is therefore significantly smaller than would be required if the integrated starter-generator 3 were needed to provide drive power to the vehicle 10 for any duration.

[0022] As mentioned previously, a particular problem with a NOx storage catalyst (LNT) is that it is exposed to sulfur in the exhaust gas from engine 2 during operation. The sulfur in the exhaust gas causes aging of the LNT and also reduces the number of available NOx storage sites in the LNT, so that the LNT will eventually be degraded to such an extent that a sulfur removal process (also known as "desulfurization" or "deSOx") will be required to flush the adsorbed sulfur compounds out of the LNT.

[0023] To carry out such a deSOx process, the engine must be operated in such a way as to provide a relatively rich exhaust mixture; that is, the mixture has a fuel surplus compared to the amount of oxygen available for combustion for a certain period of time. Furthermore, the deSOx process requires an elevated exhaust temperature, such as an exhaust temperature of at least 600°C. To generate such a combination, it is necessary to operate the engine under a relatively high load. If a rich exhaust mixture and an elevated exhaust temperature can be provided for a significant period, any sulfur compounds trapped in the LNT (liquid nitrogen) are removed from the LNT and flow out through the vehicle's exhaust system.

[0024] As previously mentioned, deSOx requires that the engine 2 be operated in such a way as to produce an exhaust mixture that is substoichiometric and has an elevated exhaust temperature of approximately 600-700°C; and during normal operation of the vehicle 10, the opportunities for such engine operation may be limited, especially if the vehicle is primarily operated in an urban or local environment where the opportunities to use high torque for any extended period of time are very limited.

[0025] One way to generate the required torque from a power unit during normal use is to utilize the integrated motor / generator as a generator, applying a braking load to the power unit and increasing its torque output without increasing the vehicle's speed or acceleration. To achieve this, however, the electrical energy generated by the integrated starter-generator must be dissipated somehow. This could be done by deliberately discharging the vehicle's battery and then using the electrical energy generated by the integrated starter-generator during deSOx to recharge the battery.

[0026] This technology is practical for use with a full hybrid vehicle or a plug-in hybrid vehicle, as the batteries used for such vehicles have a very high storage capacity.

[0027] In the case of a mild hybrid vehicle, such as vehicle 10, the storage capacity of the battery 9 is relatively small and is not sufficient to allow the use of the integrated motor / generator 3 as a generator for a sufficient period of time to carry out a full deSOx of the LNT 4.

[0028] Therefore, according to the present invention, a method is provided which overcomes the problems associated with carrying out deSOx, especially when the vehicle has a limited battery capacity.

[0029] Vehicle 10 is equipped with an exhaust gas monitoring system that includes several sensors (not shown) for monitoring emissions from engine 2. These sensors are used to monitor the composition of the exhaust gas stream downstream of LNT 4 and upstream of LNT 4. In this example, the difference between these measurements is used to determine when to start and when to stop the deSOx process.

[0030] If the signals from these sensors, or alternatively a model of LNT impairment over time, indicate that a deSOx is required, and if in some cases it is also determined that the operating cycle of the vehicle 10 has been unsuitable for performing such a deSOx, a user will be notified of this fact either by the illumination of a warning light or by an alphanumeric message. In some cases, a message could be sent to a vehicle service center indicating that the vehicle requires attention, and the vehicle service center could then contact the user to schedule a suitable appointment for the vehicle 10's maintenance. Regardless of the type of warning device used, a deSOx is normally performed in response to the warning, and a user will take their vehicle to an authorized vehicle service center.

[0031] When the vehicle 10 is in the workshop of the vehicle service and is stationary, an employee of the vehicle service (a mechanic) performs numerous steps that constitute the method according to the present invention.

[0032] First, the vehicle 10 is connected to several external devices required to carry out the deSOx.

[0033] One of these external devices is an external load, such as a large-capacity battery 20, connected to the power side of an electrical system of the vehicle 10. The large-capacity battery 20 is discharged before use to ensure that it has sufficient capacity for use in removing sulfur from the LNT 4. It is understood that the invention is not limited to the use of a large-capacity battery and that any external load capable of absorbing the electrical energy generated by the integrated starter-generator 3 during the deSOx process could be used.Furthermore, it is understood that the battery 9 could be disconnected or isolated from the integrated starter generator 3 to prevent its overcharging during deSOx, or the external load could be connected in parallel, such as when a vehicle is started via jumper cables.

[0034] A second of these external devices is an external electronic control unit 30, which is arranged to control the operation of the integrated starter-generator 3 during the deSOx process. This can be achieved by connecting the external control unit to an electronic system of the vehicle 10, such as a control unit of the integrated starter-generator, which forms part of the electronic control unit 5.

[0035] A third of these external devices is an external electronic control 30, which is arranged to control the torque requirement to the power machine 2 during the deSOx process. This can be achieved by connecting the external control 30 to an electronic system of the vehicle 10, such as a power machine management control, which forms part of the electronic control 5.

[0036] At the in Fig. In the example shown, the external electronic control 30 is arranged to control both the operation of the integrated starter-generator 3 and the torque requirement to the power machine 2 during the deSOx process.

[0037] A fourth of these external devices is an exhaust gas extraction system 25, which is used to direct exhaust gas from the downstream outlet channel 7 to the atmosphere during deSOx.

[0038] To start the deSOx process, the vehicle service employee starts engine 2 and places a gearbox (not shown) of vehicle 10 into neutral.

[0039] In this example, the deSOx process is automatic. The vehicle service employee starts a pre-programmed routine provided in the external electronic control unit 30. The external control unit 30 is configured to operate the integrated starter-generator 3 as a generator to load the engine 2 and increase the torque demand on the engine 2, thus generating a substoichiometric exhaust flow to the LNT 4. The connected external load, in the form of the high-capacity battery 20, is used to absorb the electrical energy generated by the integrated starter-generator 3.

[0040] The external electronic control 30 is configured to maintain the application of the load to the power unit 2 for a sufficient duration to remove sulfur from the LNT 4. This duration can be based on measurements of the exhaust gas composition, a preset time, or a model predicting sulfur removal. It is understood that a temperature measurement of the LNT 4 can be provided to the external electronic control 30 by one or more temperature sensors to assist the external control 30 in reaching the required temperature for deSOx.

[0041] The electronic control unit 30 is configured to reduce the torque demand on the power unit 2 and to reduce the load applied by the integrated starter-generator 3 until the power unit 2 reaches an idle state, once it has been determined that the deSOx process is complete. The external electronic control unit 30 can provide a message to the vehicle service technician indicating that the deSOx process is complete.

[0042] After the deSOx process is complete, the vehicle service employee switches off the engine 2 and disconnects the high-capacity battery 20, the exhaust extraction system 25, and the external electronic control unit 30 from the vehicle 10. Then the process is finished.

[0043] In an alternative deSOx method, no external electronic control is used to control the deSOx. In such an embodiment, the vehicle 10 is equipped with a human-machine interface that can be used by the vehicle service employee to switch the integrated starter-generator 3 into a generator mode independently of the charging requirements of the battery 9, which, as mentioned above, is normally disconnected or isolated from the integrated starter-generator 3.

[0044] As before, vehicle 10 is placed in neutral, and the high-capacity battery 20 and the exhaust extraction system 25 are connected before the deSOx is started. The vehicle service technician manually controls the torque requirement to the engine 2 by depressing the accelerator pedal of vehicle 10 to generate the required engine speed. A countdown timer can be provided on vehicle 10, or some other means can be used to measure the time the vehicle service technician continues with the deSOx. When the deSOx is complete, the vehicle service technician releases the accelerator pedal, switches the control of the integrated starter-generator 3 back to normal operating mode using the human-machine interface, and disconnects the high-capacity battery 20 and the exhaust extraction system 25 from vehicle 10.Battery 9 is then reconnected to the state it was originally disconnected. The process is then complete.

[0045] In another alternative deSOx method, no external electronic control is used to control the deSOx. In this embodiment, the electronic control unit 5 of the vehicle 10 is provided with a deSOx routine that can be selected by a vehicle service employee. The deSOx routine automatically switches the integrated starter-generator 3 into generator mode, independent of the charging requirements of the battery 9. Either the battery 9 is disconnected, and an external load, such as the load 20, which can be a high-capacity battery or any other device capable of dissipating the electrical energy generated by the integrated starter-generator 3, is connected in its place, or the external load 20 is connected in parallel with the battery 9, as is done when a motor vehicle is jump-started.

[0046] As before, the vehicle 10 is placed in neutral, and the external load and an exhaust extraction system, such as the exhaust extraction system 25, are connected before the deSOx process is started. The vehicle service technician starts the routine on the electronic control unit 5, which can be done via an external diagnostic device connected to the vehicle 10 or via a human-machine interface that forms part of the vehicle 10. The routine automatically controls the torque requirement to the power unit 2 and the operation of the integrated starter-generator 3 to generate the required conditions for the deSOx process.

[0047] When the deSOx process is complete, the vehicle service employee returns the electronic control unit 5 to normal operating mode, disconnects the external load 20, and disconnects the exhaust extraction system 25 from the vehicle 10. Then, if the battery 9 was originally disconnected, it is reconnected.

[0048] It is understood that the external load could be used to discharge the vehicle's battery before starting the deSOx, and the integrated starter-generator could then either recharge the vehicle battery during the deSOx, or the vehicle battery, in combination with an external load or battery, could be used during the deSOx to absorb the electrical energy generated by the integrated starter-generator.

[0049] Furthermore, it is understood that the composition of the exhaust gas flowing to the NOx storage catalyst could be varied between substoichiometric and superstoichiometric by varying the load applied to the electric machine during deSOx. That is, the electric machine could be switched between generator and non-generator modes, or the load applied to the electric machine could be varied between maximum and minimum altitudes. Maximum altitude is used to produce the rich exhaust mixture, and minimum altitude is used to produce the lean exhaust mixture. For example, and without being limited to this, the operating mode of the electric machine could be switched between generator mode and off mode every 5 to 10 seconds.The total required rich time for deSOx depends on the amount of sulfur to be removed, the exhaust gas mixture provided, and the temperature prevailing in the NOx storage catalyst, ranging from 50 to 150 seconds.

[0050] It is obvious to the person skilled in the art that although the invention has been described by way of example with reference to one or more embodiments, it is not limited to the disclosed embodiments and that alternative embodiments could be conceived without deviating from the scope of protection of the invention as defined by the attached claims.

Claims

[1] A method for removing sulfur from a NOx storage catalyst (4) arranged to receive an exhaust gas stream from an engine (2) of a mild hybrid vehicle (10) having an electric machine driven by the engine (2), the method comprising: Keeping the mild hybrid vehicle (10) stationary in a vehicle service workshop while the steps are carried out: Connecting an electrical system of the mild hybrid vehicle (10) to an external load, selecting an idle gear in a transmission of the vehicle, operating the electric machine as a generator to load the engine (2), increasing a torque request to the engine (2) to generate an exhaust gas flow to the NOx storage catalyst (4) that is substoichiometric, and using the external electrical load to absorb the electrical energy generated by the electric machine. [2] The method of claim 1, wherein the external electrical load is a battery (20) having sufficient capacity to absorb the electrical energy generated by the sulfur removal process. [3] The method of claim 1 or claim 2, wherein the method further comprises connecting the mild hybrid vehicle (10) to an external electronic controller (30) arranged to control operation of the electric machine during the sulfur removal process. [4] The method of claim 1 or claim 2, wherein the method further comprises connecting the mild hybrid vehicle (10) to an external electronic controller (30) arranged to control the torque demand to the engine (2) during the sulfur removal process. [5] The method of claim 1 or claim 2, wherein the method further comprises connecting the mild hybrid vehicle (10) to an external electronic controller (30) arranged to control the operation of the electric machine and the torque demand on the engine (2) during the sulfur removal process. [6] A method according to any one of claims 1 to 5, wherein the method further comprises monitoring the composition of the exhaust gas stream downstream of the NOx storage catalyst (4) and using the monitored gas stream composition to decide when to terminate the sulfur removal process. [7] The method of any one of claims 1 to 5, wherein the method further comprises using a sulfur removal model to decide when to terminate the sulfur removal process. [8] The method of any one of claims 1 to 7, wherein the method further comprises performing the sulfurization process in response to a warning provided by an exhaust monitoring system of the mild hybrid vehicle (10). [9] A method according to any one of claims 1 to 8, wherein the load generated by the electric machine is varied during the period during which sulfur is removed from the NOx storage catalyst (4) to produce alternately rich and non-rich exhaust gas streams. [10] A process according to any one of claims 1 to 9, wherein the exhaust gas flow alternates between substoichiometric and superstoichiometric.

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