Method for controlling an externally excited electric machine to support the regeneration of a NOx storage catalyst

Abstract

Method for controlling an externally excited electrical machine to assist regeneration of a NOx storage catalyst (121) of an arrangement (100) comprising an internal combustion engine (110), the externally excited electrical machine (130) which is connected to the internal combustion engine (110) in a torque-transmitting manner and which has a rotor (131) with a rotor winding (L) and a stator (132), and the NOx storage catalyst (121) which is arranged in an exhaust system (120) connected downstream of the internal combustion engine (110), wherein the rotor winding (L) of the electrical machine (130) is supplied with a pre-excitation current as a function of an operating state of the NOx storage catalyst (121).

Description

[0001] The present invention relates to a method for controlling an externally excited electric machine to support the regeneration of a NOx storage catalyst, as well as a computing unit and a computer program for carrying out the method. State of the art

[0002] To further reduce the fuel consumption of internal combustion engines and thus further reduce carbon dioxide emissions, they can be operated with an excess of oxygen (lean burn operation). However, this produces other undesirable and harmful exhaust gases, such as nitrogen oxides (NOx). So-called NOx storage catalysts, in which the nitrogen oxides are stored, can be used for exhaust gas purification. However, since these only have a finite storage capacity, they must be emptied, i.e. regenerated, regularly (e.g. in motor vehicles approximately every few minutes). For this purpose, the internal combustion engine is operated substoichiometrically with a rich mixture (i.e. lack of air), so that the gases produced by incomplete combustion (e.g. carbon monoxide (CO) and hydrocarbons (HC)) are available in the NOx storage catalyst to convert the released nitrogen oxides into nitrogen.

[0003] DE 10 2004 058 231 A1 discloses a method for controlling an internal combustion engine in a hybrid vehicle that more reliably reduces a torque difference that occurs in association with a change in a combustion air-fuel ratio to the rich state in rich control. The method comprises the steps of: estimating the generated or absorbed torque and the absorbable torque of the motor-generator and estimating the increased torque of the internal combustion engine due to the air-fuel ratio being shifted to the rich state, judging, based on the estimated torques, whether the increased torque cannot be fully absorbed by the motor-generator, and performing control in this case to reduce the torque generated by the internal combustion engine in accordance with the excess when performing rich control.

[0004] DE 10 2011 078 958 A1 shows a method for operating an electrical machine coupled to an internal combustion engine in a motor vehicle, wherein the electrical machine has a stator winding, a rotor winding, a field regulator associated with the rotor winding and a power converter connected downstream of the stator winding with controllable switching elements, wherein an excitation current through the rotor winding is predetermined as a function of an operating mode of the electrical machine, wherein the electrical machine is operated as a generator in a first generator operating mode in order to brake the motor vehicle, wherein the braking energy recovered in this process is stored. Disclosure of the invention

[0005] According to the invention, a method for controlling a separately excited electric machine to support the regeneration of a NOx storage catalyst, as well as a computing unit and a computer program for implementing the method, are proposed, having the features of the independent patent claims. Advantageous embodiments are the subject of the subclaims and the following description. Advantages of the invention

[0006] The invention proposes an improved method for regenerating a NOx storage catalyst, in which interruptions in an ongoing regeneration of the NOx storage catalyst caused by a shift of the operating point of the internal combustion engine towards lower loads can be reduced or completely avoided.

[0007] As explained, regeneration of the NOx storage catalyst requires the engine to operate with a rich mixture. However, this requires a certain minimum load, e.g., due to high-speed driving. Reducing the load may make it impossible for the engine to operate with a rich mixture, and regeneration may be aborted. This may result in emissions limits being exceeded.

[0008] To prevent this, an electric machine can be switched on in order to shift the operating point of the internal combustion engine back towards higher loads. The more electrical energy is generated by the electric machine, the more the internal combustion engine has to perform. However, starting up an electric machine does not happen suddenly. Particularly with the separately excited electric machines commonly used in motor vehicles, it takes a relatively long time (several 100 ms) after the rotor is energized until the desired excitation field is built up. This can sometimes be too long to maintain regeneration during this time, since the regeneration usually has to be supplied with the correct gases again within 50 ms - 200 ms. The invention now uses the measure of energizing the rotor winding with a pre-excitation current, i.e. pre-exciting it, depending on the operating state of the NOx storage catalytic converter.If a load is then actually required by the electrical machine, the desired excitation field can be built up quickly starting from the pre-excitation current and an interruption in regeneration can be avoided. This means that the desired torque can be achieved more quickly. If the torque build-up itself is fast enough, the time for torque build-up can be kept the same, which leads to a lower torque gradient and thus to less mechanical stress on the components involved. Pre-excitation enables a slower torque increase, since the time that would otherwise be required to excite the rotor winding is now available for torque build-up (i.e., energizing the stator winding). A slower torque increase reduces overshoots in the drive train, particularly in the area of auxiliary drives, and thus the load on all components within it, such asV-belts, belt tensioners, decoupling pulleys, overrunning pulleys if necessary, crankshaft, dual-mass flywheel and auxiliary units such as water pumps, air conditioning compressors, etc. The service life is increased.

[0009] The pre-excitation current is preferably less than 50%, for example, approximately 30% of the maximum excitation current. In particular, the pre-excitation current is a few amperes, e.g., less than 10 A, e.g., 1-4 A. The control variable for the excitation current is usually the duty cycle of a switch used to energize the rotor winding, which switches the on-board voltage to the rotor winding on and off.

[0010] The electric machine operates torque-free during pre-excitation. Stator and phase windings, in particular, are not energized during pre-excitation, or at most, are energized to the extent that the electric machine does not tap any torque from the internal combustion engine. This minimizes energy loss during pre-excitation and essentially limits it to the power loss of the rotor winding, which amounts to only a few watts. Since regeneration is completed within a few seconds, this power loss is not noticeable in the overall energy balance.

[0011] In order to be able to react to a load reduction from the outset, pre-excitation is expediently initiated together (especially simultaneously within the scope of normal running and processing times) with or shortly (e.g., a few ms, especially less than a rotor time constant) after the start of regeneration of the NOx storage catalyst. It is also expediently terminated together with or shortly before the end of regeneration of the NOx storage catalyst in order to minimize power loss.

[0012] The operating state of the NOx storage catalyst, depending on which pre-excitation is performed, can be defined by the regeneration mode, i.e., whether the internal combustion engine is operated with a rich mixture for regeneration of the NOx storage catalyst or not. Alternatively or additionally, it can also be defined by a loading state of the NOx storage catalyst, as described below.

[0013] According to a preferred embodiment, the pre-excitation is performed as a function of the loading state of the NOx storage catalyst as the operating state. In particular, the rotor winding is only pre-excited when the loading state of the NOx storage catalyst reaches at least a loading threshold value. The loading threshold value can be, for example, 50% or more, e.g., 70%.

[0014] This can occur dependently or independently of regeneration, meaning that pre-excitation can always be initiated when the load threshold is reached, or it can be initiated only together with or during regeneration when the load threshold is reached. This further reduces power loss. In the first case, a slightly higher load threshold is advantageously selected, e.g., around 70%; whereas in the second case, a slightly lower load threshold is advantageously selected, e.g., around 50%.

[0015] Preferably, during regeneration of the NOx storage catalyst, the electric machine begins to operate as a generator, in particular if a load reduction occurs to such an extent that the operating point of the internal combustion engine is shifted to such an extent that regeneration of the NOx storage catalyst would have to be aborted. For generator operation of the electric machine, the excitation winding is energized with a desired excitation current and a power converter connected downstream of the phase winding is controlled accordingly, i.e. usually in a rectifying manner. This allows regeneration to be carried out completely and without interruption. The electrical energy is preferably stored in an electrical energy storage device of an on-board electrical system, such as a battery or an accumulator, and can then be reused later.Preferably, the torque tapped from the electric motor is built up so quickly that regeneration of the NOx storage catalyst does not have to be interrupted. As previously explained, this can occur as quickly as possible, i.e., faster than without pre-excitation, or it can occur as slowly as possible, i.e., comparable to or just as quickly as without pre-excitation, but with less wear and tear on the material.

[0016] The electric machine is in particular a starter generator (e.g. belt-driven starter generator, BSG) or the electric machine of a BRS (also known as a boost recuperation machine, BRM). Starter generators (SG) are electrical machines that can be operated in a vehicle as an electric motor or as a generator, as required. As a generator, starter generators must be able to perform all the tasks that are traditionally assigned to the alternator, namely the electrical supply to the on-board electrical system and charging the vehicle battery. As an electric motor, starter generators must quickly bring the crankshaft to the required starting speed when the internal combustion engine is started. With a sufficiently high rated power, the electric machine can support the internal combustion engine in driving mode, e.g. during acceleration in so-called boost mode and to compensate for turbo lag.During braking, a portion of the braking energy can be recovered (recuperated) through generator operation. Such drives are called hybrid drives, and corresponding systems are called boost recuperation systems (BRS).

[0017] A computing unit according to the invention, e.g. a control unit of a motor vehicle, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0018] Implementing the method in the form of a computer program is also advantageous, as this results in particularly low costs, especially if an executing control unit is also used for other tasks and is therefore already present. Suitable data storage devices for providing the computer program include, in particular, magnetic, optical, and electrical storage devices, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible.

[0019] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0020] The invention is illustrated schematically in the drawing using an embodiment and is described below with reference to the drawing. Short description of the drawings Fig. 1 shows schematically an arrangement of an internal combustion engine, a NOx storage catalyst and an electric machine, as may form the basis of the invention. Fig. 2 shows a simplified circuit diagram of an exemplary electrical machine as can be used within the scope of the invention. Embodiments of the invention

[0021] In Fig. Figure 1 schematically illustrates an arrangement 100, which may form the basis of a preferred embodiment of the invention. The arrangement 100 comprises an internal combustion engine 110, for example, a diesel or gasoline engine, whose exhaust gases are directed into an exhaust system 120. A NOx storage catalyst 121 is located in the exhaust system 120.

[0022] Connected to the internal combustion engine 110, more precisely its crankshaft (not shown), is an electric machine designed here as a boost recuperation machine 130, more precisely its rotor 131 (see Fig. 2), connected to transmit torque. Torque transmission can be achieved, for example, via a belt drive.

[0023] With reference to Fig. 2, in which the electrical machine is shown schematically in an equivalent circuit diagram, the electrical machine comprises the rotor 131, a stator 132, and a converter 133. The rotor 131 has a rotor winding L and is rotatably mounted relative to the stator. To generate an excitation field, the rotor can be supplied with an excitation current I e If the electrical machine is operated as a generator, a voltage can be tapped at terminal B+.

[0024] The arrangement 100 further comprises a computing unit 140 programmed to implement a preferred embodiment of the invention. The computing unit, or another computing unit in data communication with this computing unit, is connected to a sensor 122 on the NOx storage catalyst 121, which serves to measure the loading state of the NOx storage catalyst 121 as a possible operating state depending on which the pre-excitation can be carried out. Furthermore, the computing unit 140 is in data communication with the electric machine 130 in order to control it.

[0025] Another possible operating state, depending on which the pre-excitation can be carried out, is the regeneration mode, ie the internal combustion engine is operated with a rich mixture to regenerate the NOx storage catalyst.

[0026] At this point, several preferred combinations of the aforementioned operating states are possible. Pre-excitation can always occur during an ongoing regeneration, or only when a specific load threshold is reached or exceeded. Alternatively, pre-excitation can always occur when a specific load threshold is reached or exceeded, regardless of an ongoing regeneration.

[0027] According to a preferred embodiment of the invention, the rotor winding L of the electric machine 130 is energized with a pre-excitation current during an ongoing regeneration of the NOx storage catalyst 121, regardless of the load state, in the example shown. During pre-excitation, the electric machine is operated torque-free, i.e., the phase windings of the stator 132 are not energized or, at most, are energized to such an extent that the electric machine does not tap any torque from the internal combustion engine.

[0028] Pre-excitation allows the excitation field in rotor 131 to be generated quickly in the event of a torque request, allowing torque to be tapped from the engine just as quickly. This maintains regeneration in the event of a power drop that would otherwise interrupt regeneration. In an alternative embodiment, this allows the torque increase to be implemented with a lower gradient, thus protecting the components involved.

Claims

[1] Method for controlling an externally excited electrical machine to assist regeneration of a NOx storage catalyst (121) of an arrangement (100) comprising an internal combustion engine (110), the externally excited electrical machine (130) which is connected to the internal combustion engine (110) in a torque-transmitting manner and which has a rotor (131) with a rotor winding (L) and a stator (132), and the NOx storage catalyst (121) which is arranged in an exhaust system (120) connected downstream of the internal combustion engine (110), wherein the rotor winding (L) of the electrical machine (130) is supplied with a pre-excitation current as a function of an operating state of the NOx storage catalyst (121). [2] Method according to claim 1, wherein the energization with the pre-excitation current is carried out as a function of a loading state of the NOx storage catalyst (121) as an operating state. [3] Method according to claim 2, wherein the rotor winding (L) of the electric machine (130) is only supplied with the pre-excitation current when the loading state of the NOx storage catalyst (121) reaches at least one loading threshold value. [4] Method according to claim 3, wherein the rotor winding (L) of the electric machine (130) is only supplied with the pre-excitation current when the internal combustion engine (110) is operated with a rich mixture for regenerating the NOx storage catalyst (121). [5] Method according to one of claims 1 to 3, wherein the rotor winding (L) of the electric machine (130) is supplied with a pre-excitation current, while the internal combustion engine (110) is operated with a rich mixture for regenerating the NOx storage catalyst (121). [6] Method according to one of claims 4 or 5, wherein, while the internal combustion engine (110) is operated with a rich mixture for regenerating the NOx storage catalyst (121), the electric machine (130) is started to operate as a generator. [7] Method according to claim 6, wherein a torque of the electric machine is built up so quickly that the regeneration of the NOx storage catalyst does not have to be aborted. [8] Method according to claim 6 or 7, wherein a torque of the electric machine is built up as quickly as possible. [9] Method according to claim 6 or 7, wherein a torque of the electric machine is built up as slowly as possible. [10] Computing unit configured to carry out a method according to any one of the preceding claims. [11] A computer program which causes a computing unit to carry out a method according to any one of claims 1 to 9 when executed on the computing unit. [12] A machine-readable storage medium having stored thereon a computer program according to claim 11.

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