Method for operating a motor vehicle and motor vehicle

Abstract

Method for operating a motor vehicle driven by an internal combustion engine with a rechargeable battery and a generator for charging the battery, wherein a charging acceptance ΔQ Δt of the battery and depending on the charging acceptance ΔQ Δt a first charging mode in which the generator for charging the battery is driven by the internal combustion engine, or a second charging mode in which the generator for charging the battery is driven by a propulsion-free driving movement of the motor vehicle, is selected, characterized in that a charging urgency measure urg is determined as a function of the state of charge SOC and the charging acceptance ΔQ Δt of the battery and a charging capability opp depending on a propulsion torque T prp of the combustion engine and a brake pressure p brkin a braking system of the motor vehicle and that the generator is activated to charge the battery if the charging urgency level urg is greater than the charging possibility level opp.

Description

The invention relates to a method for operating a motor vehicle driven by an internal combustion engine, having a rechargeable battery and a generator for charging the battery, and to a motor vehicle configured to carry out the method.Batteries are used in motor vehicles for electrical energy storage. The batteries supply energy for starting an internal combustion engine of the motor vehicle, as well as for further consumers which have to be operated when the internal combustion engine is at a standstill, and also additional electrical energy for consumers when the internal combustion engine is operating. The batteries are charged by a generator driven by the internal combustion engine.In order to save fuel, it is known, by appropriate control of the generator during braking of the motor vehicle, to charge the batteries by recovering (recuperation) the kinetic energy of the motor vehicle converted during braking, i.e. to convert kinetic energy into electrical energy. Various methods have been developed for this purpose, which are intended to ensure optimum use of the battery and optimum recovery of the energy. In particular, battery monitoring systems (BMS) have been developed that determine the current state of charge (SOC), i.e., the portion of the current maximum storable charge that is actually currently stored, and the current state of health (SOH), i.e., the portion of the target capacity of the battery that can be currently used, in order to use these for controlling the charging of the battery.In many cases, a temporal integration of the current that has flowed is sufficient for determining the current state of charge SOC, wherein a reset to a value determined in another way should occasionally take place. The available charge capacity SOH decreases over the life of the battery. Methods for determining the state of charge SOC and the charge capacity SOH within the scope of a battery monitoring system are known, for example, from DE 10 2006 001 201 B4, DE 10 2007 050 346 A1 and DE 10 2008 034 461 A1.From DE 41 24 496 A1, which is not of the generic type, it is known that a brake system provided for motor vehicles with an electric drive is designed in the form of a multi-circuit composite brake system which comprises friction brakes acting on the driven or on the non-driven wheels and an electro-regenerative brake system which is directly or indirectly coupled to the brake pedal and acts on the driven wheels. The actuation of the wheel brakes and the distribution of braking force on the front axle and the rear axle are controlled with the aid of an electronic controller. In addition to the signals representing the position of the brake pedal, the controller is supplied with the vehicle speed, the charging capacity of the battery and the information of an accelerator pedal position sensor. From this, the controller calculates the control data for the control electronics of the electric drive motor of the vehicle, which can be switched as a generator for recovering energy, and the contribution that the friction brakes must apply. The charging capacity is in this case set equal to the state of charge of the battery.EP 1 480 848 B1 discloses a method for feeding energy into an energy bus in connection with an energy generating device and a regenerative braking system in an electric hybrid vehicle, in which the power supplied to the energy bus by the regenerative braking system of the vehicle is increased, while the power supplied to the energy bus by the energy generating device is decreased.From JP H05-316 658 A a charging and discharging device for an automobile is known. Here, a control circuit discriminates between a steady state mode in which discharging and charging are repeated, a discharging mode, and a charging mode, and controls an inverter circuit according to the discriminated mode.U.S. Pat. No. 6,225,784 B1 shows a battery control device for a hybrid vehicle having an internal combustion engine, an electric motor for supporting the motor and a battery. The apparatus includes an apparatus for releasing charging when the remaining battery charge is equal to or lower than a predetermined value, and a charge / discharge controller for setting the predetermined value.However, it has been found that neither the state of charge SOC nor the charging capacity SOH are sufficient to enable consumption-optimized control of a charging process of a rechargeable battery of a motor vehicle of the type mentioned at the beginning.It is an object of the present invention to provide a method for operating a motor vehicle driven by an internal combustion engine, having a rechargeable battery and a generator for charging the battery, wherein the charging of the battery takes place in a manner optimized with regard to the fuel consumption. It is a further object of the present invention to specify a motor vehicle set up to carry out the method.This object is achieved by a method for operating a motor vehicle and by a motor vehicle as specified in claims 1 and 8.A motor vehicle to which the method according to the invention relates has an internal combustion engine which serves for driving the motor vehicle and furthermore comprises a rechargeable battery and a generator. The generator is designed to charge the battery and can furthermore be provided for supplying electrical loads of the motor vehicle with electrical energy. The generator can be driven by the internal combustion engine and can for this purpose be connected to the internal combustion engine, optionally via a clutch. The generator can also be driven by a driving movement of the motor vehicle. Such a drive of the generator by a driving movement of the motor vehicle can be effected in particular in that the internal combustion engine is coupled to a drive axle of the motor vehicle and the generator is coupled to the internal combustion engine; if the internal combustion engine does not generate a propulsion torque and the motor vehicle thus performs a propulsion-free driving movement, the generator is driven by the driving movement. In particular, an output voltage of the generator, which it generates when driven, is controllable in such a way that the battery is charged. The generator can also be designed as a starter generator.The battery can be, in particular, a lead-acid accumulator. Such a battery is also referred to as a lead-acid accumulator and can be filled, for example, with acid in unbound form (floating) into which the lead electrodes are immersed, or contain acid bound by a fleece of glass fibers (absorptive glass mat, AGM). In principle, however, the invention is also suitable for other types of rechargeable batteries.According to the invention, it has been recognized that knowledge of the charge acceptance is required for a consumption-optimized control of the charging of the battery of a motor vehicle driven by an internal combustion engine with the aid of a generator of the motor vehicle. Charge acceptance is understood to mean the maximum instantaneous charging current I CA,inst which can be recorded by the battery at the given point in time, or the maximum average charging current I CA,avg, which can be recorded by the battery on average over a given short time interval Δt. The maximum charge AQΔtwhich can be absorbed by the battery in the time interval Δt can also be used as a measure of the charge acceptance. The time interval Δt preferably has a length that is on the order of the duration of driving operations relevant to the charging of the battery, such as braking operations or propulsionless rolling of the motor vehicle. Δt can be, for example, 10 s. In the context of the present invention, the term "charge acceptance" is used in the sense of the maximum charge ΔQΔtwhich can be absorbed by the battery in the time interval Δt. As a rule, however, the specification of the maximum average charging current I CA,avg or also of the maximum instantaneous charging current I CA,inst is equivalent.The charge acceptance depends, in particular in the case of the lead storage batteries widely used in motor vehicles, on a multiplicity of parameters which are associated, for example, with previous charge and discharge phases, and is generally not to be determined solely from the state of charge SOC and / or the charge capacity SOH. A method for determining the charge acceptance is known in particular from German patent application 10 2011 079 469.7, which is incorporated by reference into the present application.According to the invention, a charge acceptance ΔQΔtof the battery is determined and, depending on the determined charge acceptance, a first charge mode is selected, in which the generator for charging the battery is driven by the internal combustion engine, or a second charge mode, in which the generator for charging the battery is driven only or predominantly by a propulsion-free travel movement of the motor vehicle. In the first charging mode, the generator can be activated, so that the battery is continuously charged. In the second charging mode, no continuous charging of the battery takes place, but only or at least predominantly in propulsion-free phases, i.e. during propulsion-free rolling or during braking of the motor vehicle. In the second charging mode, therefore, no charging of the battery takes place in certain driving situations. In particular, in the second charging mode, it can be provided that the generator is not activated for charging the battery as long as the internal combustion engine generates a propulsion torque or the accelerator pedal of the motor vehicle is actuated, and that the charging is waited until a propulsion-free driving situation occurs.By selecting the first or the second charging mode as a function of a charging acceptance of the battery, it can be ensured that the battery is charged in such a way that the electrical power or charge provided can actually be absorbed by the battery and that charging with the aid of the internal combustion engine only takes place if this is necessary on the basis of the ascertained charging acceptance of the battery. An additional consumption of fuel by charging the battery can thus be avoided in many cases and the charging can be carried out largely or solely by recovering kinetic or potential energy from the driving movement of the motor vehicle. This allows consumption-optimized operation of the motor vehicle. This applies in particular to micro-hybrid vehicles, i.e. to motor vehicles with stop / start functionality.Preferably, the first charging mode is selected for a first value of the charging acceptance and the second charging mode is selected for a second value, which is higher than the first value of the charging acceptance. In particular, a first threshold value of the charge acceptance can be predefinable, wherein the first charge mode is selected if the ascertained charge acceptance is below the first threshold value, and the second charge mode is selected if the ascertained charge acceptance is above the first threshold value. Thus, at a low charge acceptance, the battery is charged by the internal combustion engine, in particular a continuous charging, and at a higher charge acceptance, the battery is charged during a propulsion-free travel movement, i.e. by recovering kinetic or potential energy of the motor vehicle. Since braking processes in particular only continue for short periods of time, charging of the battery by recovering kinetic energy during braking is only effective if a high charge acceptance is available. On the other hand, if the battery has only a low charge acceptance, for example, due to aging, charging by driving the generator with the aid of the internal combustion engine is necessary. As a result, charging of the battery that is further optimized with regard to fuel consumption can be achieved.In particular, it can advantageously be provided that a second threshold value for the charge acceptance can be predefined which is greater than the first threshold value. In the case that the ascertained charge acceptance lies between the first and the second threshold value, the generator is activated for charging the battery only or predominantly during a propulsion-free driving movement of the motor vehicle, for example during each propulsion-free driving movement. In the case that the ascertained charge acceptance is above the second threshold value, the generator is actuated for charging the battery only when a service brake of the motor vehicle is actuated. This ensures that charging takes place in a central region of the charging acceptance not only during braking of the motor vehicle, but also during propulsion-free rolling. The battery is thus charged whenever the motor vehicle is running without the internal combustion engine generating a propulsion torque, i.e. when the driver does not actuate the accelerator pedal. Although the speed of the motor vehicle or the route which can be covered in a propulsion-free manner is reduced by removing kinetic energy, this is not undesirable in every case and thus leads to no or only a low additional consumption. If, on the other hand, the charge acceptance has a high value, the battery is charged and thus kinetic energy is only extracted when the motor vehicle is braked, i.e. when the driver wishes to reduce the kinetic energy anyway, so that no additional fuel consumption occurs in this case. In this way, with regard to the charging of the battery, a particularly consumption-optimized operating mode of the motor vehicle can be achieved.The charge acceptance ΔQΔtcan be determined in particular continuously or in short time intervals, which can correspond, for example, to the duration of typical braking processes, for example at intervals of approximately 10 s. The charge acceptance thus preferably represents the respectively current charge acceptance of the battery. As a result, a consumption-optimized operating mode of the motor vehicle can be achieved at any time.A state of charge SOC of the battery is preferably determined and taken into account when selecting the charging mode. In particular, the first charging mode can be selected in the case of a low state of charge or a low charging acceptance ΔQΔtof the battery, while the second charging mode is selected in the case of a medium state of charge and a medium charging acceptance, wherein the battery is charged during propulsionless driving, i.e. both during rolling and during braking. In the other cases, in particular in the case of a high state of charge and a high charge acceptance, the generator is actuated for charging the battery only when the service brake is actuated. In this case, one or more threshold values can also be predefinable for the state of charge. Preferably, the state of charge of the battery is also determined continuously or at short intervals. A joint consideration of the state of charge and the charge acceptance of the battery enables a further consumption-optimized control of the charging of the battery.Furthermore, it is preferred that when the state of charge SOC of the battery exceeds a predefinable threshold value, the battery is discharged. This makes it possible to ensure that the battery always has an optimum state of charge.According to the method according to the invention, a charge urgency measure urg is determined as a function of the state of charge SOC and the charge acceptance AQΔtof the battery, and a charge opportunity measure opp is determined as a function of a propulsion torque T prp of the internal combustion engine and a brake pressure p brk in a brake system or a brake line of the motor vehicle. In particular, the charge urgency measure urg is a predetermined function of the state of charge SOC and the charge acceptance AQΔt, and the charge opportunity measure opp is a predetermined function of the propulsion torque T prp and of the brake pressure p brkThe functions f(SOC, ΔQΔt) and g(T prp, p brk) can be implemented, for example, as characteristic maps or as look-up tables in a storage means. The generator is then controlled to charge the battery and a charging process is initiated if the charging urgency measure urg is greater than the charging opportunity measure opp. Otherwise, the motor vehicle is operated in normal operation, i.e. the generator is not activated to charge the battery. The input variables of the aforementioned functions are preferably determined or evaluated continuously or at short intervals. This makes it possible to achieve a further improvement in the consumption-optimized control of charging processes.Furthermore, it is preferred that the generator is controlled in such a way that the rate of change over time of the output voltage of the generator does not exceed a maximum value. This makes it possible to ensure that the driver of the motor vehicle is not irritating due to brightness fluctuations of headlights or other lamps or speed fluctuations of electrically operated motors, which are associated with changes in the output voltage of the generator.A motor vehicle according to the invention, which comprises an internal combustion engine arranged for driving the motor vehicle, a rechargeable battery and a generator for charging the battery, furthermore comprises a control device for controlling the generator for charging the battery, wherein the control device is designed for ascertaining a charging acceptance AQΔtof the battery and for controlling the generator for charging the battery as a function of the charging acceptance. For this purpose, the control device can comprise processor means for determining the charging acceptance and for selecting a charging mode, and memory means for storing the characteristic maps or look-up tables for determining the charging urgency measure urg and the charging possibility measure opp. In particular, the control device and the motor vehicle as a whole are configured such that the motor vehicle can be operated according to the method described above.The invention is explained in more detail below by way of example with reference to the drawings. The following are shown: FIG. 1 shows a flow diagram of an exemplary embodiment of the method according to the invention, and FIG. 2 shows an example of characteristic maps or look-up tables for determining the charging urgency measure urg and the charging possibility measure opp.In the exemplary embodiment of the method according to the invention schematically illustrated as a flow diagram in FIG. 1, the data necessary for carrying out the method are first ascertained and made available, in particular the battery voltage U batt, the battery current I batt, the state of charge SOC and the charge acceptance AQΔtof the battery, and the propulsion torque T prp generated by the internal combustion engine, and the brake pressure p brk. These values are determined in particular continuously or in short time intervals in relation to the charging processes and thus represent the respectively current operating variables. The battery voltage U batt, the battery current I batt and the brake pressure p brk can be determined directly or by sensors. Propulsion torque T prp of the internal combustion engine may be ascertained, for example, from the current accelerator pedal position, possibly with the aid of a mathematical model of the internal combustion engine or a characteristic map including further variables, such as the current rotational speed of the internal combustion engine. For the determination of the current state of charge SOC of the battery, a battery monitoring system known per se can be used, for example.The current charge acceptance ΔQΔtcan be determined, for example, from a model of the battery, in which the battery is split arithmetically into a predefined number of compartments i, which do not need to correspond to the physical cells or plates of the battery. The input variables of the model are the battery current I batt, the measured voltage of the battery U batt,meas, the charging voltage U ch, the state of charge SOC of the battery, which state of charge SOC of the battery is ascertained, for example, from a battery monitoring system, the charging capacity SOH of the battery and the temperature θ of the battery. For the charge acceptance, the following results in a summary vectorial notation: wherein R i and C i are the resistances or capacitances of the compartments i of the battery. In particular, the resistance values R i are temperature-dependent. B, C avg and D avg are state matrices that depend on C i %0020≅, R i and Δt. U min is the battery voltage in an open circuit when the battery is fully discharged; U max is the corresponding voltage when the battery is fully charged. Hsoc and Hu are feedback factors for correcting SOC and U batt.The result in the sense of the maximum charge current that can be absorbed is the instantaneous charge acceptance I CA,inst and the average charge acceptance I CA,avg, from which the charge acceptance in the sense used in the present invention can be calculated according to the maximum charge that can be absorbed in the time interval Δt.From the input variables determined in the previous step, a charge urgency measure and a charge possibility measure are then determined. These can be, for example, dimensionless variables having values in the range between 0 and 1, which indicate, on the one hand, the urgency of charging the battery and, on the other hand, the possibility or opportunity of charging the battery, which is evaluated on the basis of the current driving variables. The functions in question can be stored, for example, as predetermined characteristic maps or look-up tables in a control device. Examples of such functions are shown in FIG. 2 in a three-dimensional representation (top) or in a contour line representation (bottom; contour lines are shown at intervals of 0.1). In the left two representations, a value of the charge urgency measure urg between 0 and 1 can be read for each value of AQΔtand SOC. Likewise, in the right representations, a value of the charging possibility measure between 0 and 1 can be read for each value of the brake pressure p brk and of the propulsion torque T prp.The values of the charging urgency measure urg and of the charging opportunity measure opp determined in this way are compared with one another in the next step. If the charge urgency measure urg is not greater than the charge opportunity measure opp, the motor vehicle is operated in normal operation. In normal operation, it is first established whether the state of charge SOC of the battery exceeds a threshold value SOC thresh and whether the internal combustion engine is at a standstill.If neither of the two conditions is fulfilled, the output voltage U gen of the generator is regulated in such a way that the battery current I is batt= 0. This means that the electrical energy required by the electrical consumers of the motor vehicle is supplied solely by the generator and the battery is neither charged nor discharged.On the other hand, if either of the two conditions is satisfied, the battery is discharged. If the generator voltage is above a minimum voltage U min, at which the battery is discharged, the generator voltage U gen is reduced. This occurs in particular at the maximum rate of change at which a driver of the motor vehicle still does not notice any change in the brightness of lamps or in the speed of electric motors.On the other hand, when the current value of the charge urgency degree urg is larger than that of the charge opportunity degree opp, the battery is charged. For this purpose, first the maximum voltage U ch,max with which the battery can currently be charged is determined, as is the maximum current I max, which can be used for charging the battery; this is the difference between the maximum current which the generator can deliver and the total current requirement of the electrical loads of the motor vehicle. In the next step, it is determined whether the battery voltage U batt and the battery current I batt are below the aforementioned limits. If this is the case, the output voltage U gen of the generator is increased, in particular at the maximum permissible speed, so that this is not noticeable to the driver by the behavior of the electrical loads. If this is not the case, the output voltage U gen of the generator is lowered; the generator voltage thus fluctuates around the aforementioned limit values in a narrow range.A generator in a passenger car is usually able to supply approximately 2.0-2.5 kW of electrical power. If this power were to be generated by the internal combustion engine of the vehicle, this would lead to an increased consumption of fuel. According to the invention, the corresponding power is taken from the kinetic or potential energy of the vehicle, in particular when the kinetic energy is reduced anyway by braking on account of the driving situation. If neither the brake pedal nor the accelerator pedal are actuated, the motor should likewise as far as possible not be loaded by driving the generator for charging the battery, but this is only possible if the charge acceptance of the battery is sufficiently high. Within a typical time period Δt=10 s, the generator may supply more than 1,000 As. The charge acceptance ΔQ 10s should therefore be of this order of magnitude to allow charge of the battery by recovering the kinetic energy of the vehicle. The smaller the charging acceptance, the more it is necessary to use the internal combustion engine for driving the generator for charging the battery, i.e. also to consume fuel for this purpose. In an intermediate range, it may be necessary to charge the battery even in driving situations in which the vehicle rolls propulsion-free, i.e. without the driver actuating the brake pedal or the brakes. When the charge acceptance of the battery becomes a very low value, continuous charging of the battery is necessary. This results in an increased fuel consumption of the internal combustion engine, but this is relatively low, since the battery accepts only a low charging current in this case.The method described can also be used if the generator is not driven via the engine, but directly via the driving movement, for example if the generator is installed on the transmission. In this case, too, the kinetic energy of the vehicle should be reduced only when the service brake is actuated, provided that the charging acceptance permits this.

Claims

Method for operating a motor vehicle driven by an internal combustion engine, having a rechargeable battery and a generator for charging the battery, wherein a charging acceptance AQΔtof the battery is determined and, as a function of the charging acceptance AQΔt, a first charging mode in which the generator is driven by the internal combustion engine for charging the battery, or a second charging mode in which the generator is driven by a propulsion-free travel movement of the motor vehicle is selected, characterized in that, a charging urgency measure urg as a function of the state of charge SOC and the charging acceptance ΔQΔtof the battery and a charging possibility measure opp as a function of a propulsion torque T prp of the internal combustion engine and a brake pressure p brk in a brake system of the motor vehicle are ascertained, and the generator is actuated for charging the battery if the charging urgency measure urg is greater than the charging possibility measure opp.Method according to Claim 1, characterized in that a first threshold value of the charging acceptance AQΔt can be predefined and in that the first charging mode is selected if the ascertained charging acceptance is below the first threshold value, and the second charging mode is selected if the ascertained charging acceptance is above the first threshold value.Method according to Claim 2, characterized in that a second threshold value of the charge acceptance AQΔt which is greater than the first threshold value can be predefined, and in that, if the ascertained charge acceptance lies between the first and the second threshold value, the generator is actuated for charging the battery during a propulsion-free travel movement of the motor vehicle, and, if the ascertained charge acceptance lies above the second threshold value, the generator is actuated for charging the battery only when a service brake of the motor vehicle is actuated.Method according to one of the preceding claims, characterized in that the charge acceptance AQΔt is determined continuously or in short time intervals.Method according to one of the preceding claims, characterized in that a state of charge SOC of the battery is determined and taken into account when selecting the charging mode.Method according to the preceding claim, characterized in that, if the state of charge SOC of the battery exceeds a threshold value, the battery is discharged.Method according to one of the preceding claims, characterized in that an output voltage U gen of the generator is controlled in such a way that the rate of change of the output voltage does not exceed a maximum value.Motor vehicle having an internal combustion engine arranged for driving the motor vehicle, having a rechargeable battery and having a generator for charging the battery and having a control device for actuating the generator for charging the battery, wherein the control device is designed to determine a charging acceptance AQΔtof the battery and to actuate the generator for charging the battery as a function of the charging acceptance AQΔtaus, characterized in that, a charging urgency measure urg as a function of the state of charge SOC and the charging acceptance ΔQΔtof the battery and a charging possibility measure opp as a function of a propulsion torque T prp of the internal combustion engine and a brake pressure p brk in a brake system of the motor vehicle are ascertained, and the generator is actuated for charging the battery if the charging urgency measure urg is greater than the charging possibility measure opp.

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