Method for operating a powertrain of an electric vehicle

Abstract

Method for operating a powertrain of an electric vehicle, wherein the powertrain has at least one primary axle and at least one secondary axle, wherein a first drive unit has a first efficiency and transmits a first torque to the primary axle, and a second drive unit has a second efficiency and transmits a second torque to the secondary axle, wherein the first efficiency is greater than the second efficiency. wherein in a first operating mode the first moment transmitted to the primary axis and the second moment transmitted to the secondary axis have opposite signs and thereby the primary axis is braced against the secondary axis, in a second operating mode, during a load change in the drivetrain, the first drive unit is controlled in such a way that the first torque transmitted to the primary axle changes sign and a sum of the first torque and the second torque corresponds to a recorded driver request, where, after the sign change of the first moment, the magnitude increases and the magnitude of the second moment transferred to the secondary axis decreases and During the load change, the first drive unit is controlled in such a way that the first moment changes sign in such a way that an acoustically optimized clearance cycle of the primary axle takes place.

Description

[0001] The invention relates to a method for operating a drive train of an all-wheel drive electric vehicle, wherein the method increases the overall efficiency of the drive train and thus the range that can be traveled by the electric vehicle.

[0002] An electric vehicle's powertrain connects a drive unit, in the form of an electric motor, to at least one output unit, such as a wheel or an axle connected to the wheels. Electrically powered all-wheel-drive vehicles typically have multiple drive units, each connected to a wheel or, in particular, an axle. In such cases, the vehicle's powertrain consists of several sub-drivetrains.

[0003] Since such a partial drive train is not a rigid structure but rather contains moving parts, such as gear connections, through which torque is transmitted from the drive unit to the driven unit, the drive train exhibits a certain amount of backlash. This backlash must be overcome before torque can be transmitted between the drive unit and at least one driven unit. In other words, when a load is applied, i.e., when torque is applied, this backlash must first be overcome before power transmission between the drive unit and the driven unit can occur.

[0004] It has been defined that a torque curve from the drive unit to the output unit (traction operation) is referred to as positive torque, while a torque curve from the output unit to the drive unit (deceleration operation) is referred to as negative torque. Positive and negative torque act in precisely opposite directions. Accordingly, when positive torque is applied, the corresponding axle is said to be on the tension flank, while when negative torque is applied, it is on the deceleration flank. The torque reversal, i.e., the zero crossing, is also referred to as load change. In other words, load change is the transition between braking and accelerating the vehicle, or vice versa. During this load change, the direction of the torque applied to the drivetrain also changes.This change in the direction of torque requires that the inherent backlash in the drivetrain be overcome before an opposing load can be transmitted through the drivetrain. When transitioning from a positive to a negative torque, the corresponding drive shaft must therefore be disengaged from the tension flank and moved onto the shear flank. This process is called backlash.

[0005] This process of using up or bridging the backlash until the drivetrain components engage the tension or thrust flank can lead to delays in response. For example, if the driver wants to accelerate a vehicle from braking, during which the drivetrain experiences negative torque and is operating on the thrust flank, the backlash must first be bridged until the drivetrain engages on the tension flank and can transmit the positive torque to the output unit. This delay caused by the backlash can be perceived as detrimental by the driver. Furthermore, the change in flank, i.e., the axle's backlash, can cause a so-called load shock, which is audible and perceptible, reducing vehicle comfort and is therefore undesirable.

[0006] In two-axle all-wheel-drive vehicles, it is known that both axles can transmit torque between a corresponding drive unit and a corresponding driven unit. To avoid backlash and its associated disadvantages, the prior art involves pre-tensioning the two axles against each other, as shown, for example, in patent document DE 102017210075 A1. Pre-tensioning the axles means that both axles are subjected to opposing torques, so that one of the drive axles is always in tension, while the other drive axle is always in thrust, as described in [reference to...]. Fig. 1 will be explained in more detail below.

[0007] Fig. Figure 1 shows two diagrams. The upper diagram illustrates an example of a driver's desired acceleration (solid line) and deceleration (dashed line) of the vehicle over time. The lower diagram shows the actual torques (i.e., torques applied to the axles) corresponding to the driver's desired torque at the different axles. It should be noted that in electric vehicles, during braking, the vehicle's kinetic energy can be recuperated by switching the electric motor to generator mode. This allows a portion of the braking energy to be converted back into electrical energy and stored in the vehicle's energy storage system.

[0008] In the example shown, one axle is designated as the primary axle and the other as the secondary axle. While the primary axle always exerts at least a slightly positive drive torque on the vehicle, regardless of the driver's input, the secondary axle always exerts at least a slightly negative drive torque on the vehicle, regardless of the driver's input. Consequently, the primary axle is permanently biased on the tension flank and the secondary axle permanently biased on the thrust flank, thus creating the tension between the two axles. The drive torque acting on the vehicle is therefore the sum of the individual drive torques applied via the primary and secondary axles. If the driver expresses neither a deceleration nor an acceleration request, the two applied torques of the primary and secondary axles cancel each other out.

[0009] If the driver requests acceleration, the drive torque of the secondary axle is set to a minimal negative torque, while the primary axle is set to a significantly higher positive torque. If the driver requests deceleration, a slightly positive torque is applied to the primary axle to maintain the preload between the axles, while a significantly increased negative torque is applied to the secondary axle to brake the vehicle, and the corresponding drive unit on the secondary axle is used to recuperate braking energy. This change from acceleration to deceleration represents a load change in the drivetrain and is indicated by the left vertical dashed line in the diagram.It should be noted that the torques used to maintain tension should be kept as low as possible to avoid excessive energy loss during both acceleration and recuperation, i.e., braking.

[0010] The efficiency of the drive units used depends on the design of the respective electric motors and the materials used in the power electronics. Efficiency is generally understood as the ratio between useful power output and input power. Therefore, the efficiency of a drive unit is determined by comparing the power delivered to the output unit to the electrical power supplied to the drive unit. Drive units with high efficiencies are significantly more expensive than those with lower efficiencies. To save costs, the prior art uses different drive units for different axes, with the drive unit of a primary axis having a higher efficiency than the drive unit of a secondary axis.Especially in sports cars, the primary axle is the rear axle and the secondary axle is the front axle, whereby, due to the sportier driving characteristics, the primary axle is the drive axle for accelerating the vehicle and the secondary axle is used accordingly for recuperation during braking.

[0011] If, as in the example shown, the primary axle always provides positive torque, this results in the vehicle's acceleration via the primary axle being highly efficient, while the recuperation during braking via the secondary axle using the drive unit is less efficient. Therefore, less energy can be recovered during recuperation via the secondary axle than would be technically possible if recuperation were performed using the primary axle's drive unit.

[0012] The object of the present invention is therefore to provide a method for operating the powertrain of an electric vehicle, by means of which the efficiency of the powertrain can be increased without increasing the cost of the powertrain. This object is achieved by the subject matter of the independent claim. Advantageous embodiments of the invention are contained in the dependent claims.

[0013] The drivetrain of an electric vehicle, which is to be operated according to the method according to the invention, has at least one primary axle and at least one secondary axle. A first drive unit with a first efficiency transmits a first torque to the primary axle, and a second drive unit with a second efficiency transmits a second torque to the secondary axle, the first efficiency being greater than the second efficiency. According to the invention, in a first operating mode, the two torques transmitted to the respective axles have opposite signs. The term "opposite sign" means that the two torques act in opposite directions of rotation. Thus, while a positive torque rotates the output unit in one direction, a negative torque, i.e., a torque with the opposite sign, rotates the output unit in the other direction.The opposite signs cause the primary and secondary axes to be preloaded against each other. The operating mode refers to a specific operating requirement expressed by the driver's request. This could, for example, be the driver's request to accelerate the vehicle.

[0014] According to the invention, during a load change in the drivetrain, which defines a second operating mode, the first drive unit is controlled such that the first torque transmitted to the primary axle changes sign and thus corresponds to that of the second torque. In this second operating mode, both torques are therefore directed in the same direction, thereby eliminating the tension between the two axles. The sum of the first and second torques corresponds to a driver input. This input is preferably expressed, as is common in prior art vehicles, by the pedal position (accelerator and brake pedals) in the cockpit.

[0015] By changing the sign of the torque transmitted to the primary axle during the load change from acceleration to deceleration of the vehicle, the torque at the primary axle can now be adjusted so that recuperation occurs via the primary axle and thus with the first, and therefore more efficient, degree of efficiency. This allows a greater amount of energy to be recuperated during braking, thereby improving the overall efficiency of the drivetrain and increasing the vehicle's range. It should be noted that the invention is not limited to load changes from acceleration to deceleration, but can also be applied to load changes from deceleration to acceleration. Furthermore, the primary axle can readily be either the front or rear axle of a vehicle and can be preloaded with either a positive or negative torque.According to the invention, after the sign change of the first moment, its magnitude increases and the magnitude of the second moment transmitted to the secondary axis decreases.

[0016] According to the invention, the first drive unit is controlled during load changes such that the initial torque changes its sign, i.e., its direction, in such a way as to achieve an acoustically optimized clearance cycle of the primary shaft. An acoustically optimized clearance cycle is understood to mean the torque control is designed to enable the smoothest possible clearance cycle. In other words, the clearance cycle is executed in such a way that the noise generated when the torque moves from the tension flank to the thrust flank, or vice versa, is minimized, resulting in minimal load shock. The transition is therefore not abrupt, but smooth. In this way, the comfort-reducing side effects of the clearance cycle can be avoided.

[0017] This embodiment is particularly advantageous when the magnitude of the second moment transmitted to the secondary axis increases during the primary axis's travel time. Because the acoustically optimized travel time is slow, a high moment cannot be applied directly to the primary axis when it transitions from the tension flank to the thrust flank or vice versa. To nevertheless enable the appropriate moment to be applied directly during the load change, i.e., when a change in driver input is detected, the moment at the secondary axis is increased while the primary axis is still in its travel time and cannot apply the moment itself. This ensures full functionality despite the slower travel time.

[0018] It is further advantageous if the first torque transmitted to the primary axle is increased during the drive cycle, and the second torque applied to the secondary axle is reduced by the same amount. "By the same amount" means that the increase in the first torque corresponds to the decrease in the second torque. This maintains a constant total torque of the drivetrain. Therefore, if the driver has a constant deceleration or acceleration request, this is met by a constant total torque of the drivetrain.

[0019] In an advantageous embodiment of the invention, in which the magnitude of the second torque transmitted to the secondary shaft initially increases during the load change and then decreases again, the torque is reduced to zero and the second drive unit is engaged in electric freewheeling mode. When the second drive unit is in electric freewheeling mode, its power electronics do not cause any switching losses and therefore consume no energy.

[0020] In an advantageous embodiment of the invention, upon a subsequent load change in the opposite direction, the first moment transmitted to the primary axis again reverses its sign, wherein, in particular, the second moment transmitted to the secondary axis is non-zero and opposite to the first moment, i.e., has the opposite sign. In this embodiment of the invention, the tension state of the primary and secondary axes of the first operating mode is thus restored with reversed signs as soon as a subsequent load change occurs. This embodiment is particularly preferred when the first moment reverses its sign, i.e., its direction, in such a way that an acoustically optimized clearance cycle of the primary axis also takes place.

[0021] In a further advantageous embodiment of the invention, the second torque transmitted to the secondary axle also changes sign after the sign change of the first torque transmitted to the primary axle, so that both torques again have opposite signs and are oriented in opposite directions to each other. If, in such an embodiment, the first drive unit is controlled in such a way that the first torque changes sign upon a subsequent load change, the torque requested by the driver can be supplied via the secondary axle by the second torque during the primary axle's travel time until the primary axle's travel time is complete, and the first drive unit of the primary axle can supply the requested torque corresponding to the driver's request in an efficiency-optimized manner.If the value of the first moment is increased, the value of the second moment is decreased accordingly, so that the total equals the requested moment. This prevents any delay in the response time.

[0022] The computer-implemented method according to the invention for increasing the efficiency of an electric vehicle comprises the method steps from one of the preceding claims.

[0023] Advantageous aspects and embodiments of the method according to the invention are explained in more detail below with reference to the accompanying drawings. They show: Fig. Figure 1 shows two diagrams illustrating a method for operating a drive train according to the state of the art. Fig. Figure 2 shows two diagrams to illustrate a method according to the invention for operating a drive train.

[0024] On the Fig. Point 1 has already been addressed in relation to the description of the state of the art, therefore a further description is omitted here.

[0025] Fig. 2 shows, as does Fig. 1. Two diagrams are shown, the upper diagram illustrating the driver's desired acceleration and deceleration of the vehicle, and the lower diagram illustrating the actual torque (actual torque) applied to a primary and a secondary axis over time. As in Fig. In a first operating mode, the two axes are also preloaded, which is achieved by a positive actual torque on the primary axle and a simultaneously applied slightly negative actual torque on the secondary axle. The drive unit of the primary axle has a higher efficiency than the drive unit of the secondary axle. In the illustrated embodiment, the primary axle is defined as the rear axle and the secondary axle as the front axle. The dashed vertical lines in the drawings indicate different points in time (1 to 6) of the depicted operation of the drive unit and are intended to simplify orientation within the diagrams.

[0026] As in Fig.In this process, the driver initially requests acceleration by pressing the vehicle's accelerator pedal in a region prior to time 1. This results in a higher positive torque on the primary axle compared to the negative torque applied to the secondary axle, such that the sum of the torques across both axles is positive, and the vehicle accelerates. The vehicle is then driven by the applied positive torque on the primary axle, which has a higher efficiency than the secondary axle's drive unit, resulting in an all-wheel drive distribution optimized for minimal power loss.

[0027] Between times 1 and 2, the driver's acceleration request decreases continuously until it reaches zero. Corresponding to the decreasing acceleration request, the actual torque applied to the primary axis also decreases. If neither an acceleration nor a deceleration request is detected, the primary and secondary axes are only subjected to the respective minimum actual torques necessary to maintain the tension. These torques act in opposite directions (i.e., they have opposite signs) and, in the illustrated embodiment, are set such that they cancel each other out.

[0028] Time point 2 marks a negative load change, i.e., the transition from traction to deceleration mode of the drive system. From this point onward, a deceleration request is expressed by the driver pressing the brake pedal in the cockpit, as illustrated by the dashed line in the upper diagram. When a load change is detected, the first operating mode is exited, and the tension on both axles is released. This occurs because the applied torque on the primary axle changes sign, so that it is no longer a positive but a negative torque. Between times points 2 and 3, a shift (flank transition) of the primary axle takes place, during which the primary axle transitions from the traction flank to the thrust flank. As described earlier, such shifts can cause load shocks, which significantly reduce comfort.Thus, in the illustrated embodiment, the game cycle is performed as quickly as possible, but as slowly as necessary to prevent any audible or perceptible load shock. The game cycle, or flank transition, is therefore acoustically optimized. The flank transition does not necessarily have to proceed at a constant speed, as suggested in the diagram. It is quite conceivable to use a higher speed at the beginning of the flank transition than at the end.

[0029] Due to the acoustically optimized flank shift, the primary axle cannot directly implement a desired actual torque corresponding to the driver's requested deceleration. However, to avoid impairing the responsiveness of the entire drivetrain, the driver's initial deceleration request is taken over by the secondary axle. This results in the second drive unit applying a corresponding negative actual torque to the secondary axle, ensuring the vehicle's deceleration.

[0030] Once the cycle is complete at time 3, the negative torque corresponding to the desired deceleration is built up on the primary axle, while the negative torque on the secondary axle is reduced by the same amount, so that the sum of both torques on the axles corresponds to the detected deceleration request. At time 4, the torque on the secondary axle has returned to its level before the load change, and the required braking torque is largely provided by the primary axle, with energy recuperation occurring on the primary axle in an efficiency-optimized manner.

[0031] During deceleration, electrical energy can be recuperated due to the negative torque applied to the drive units. Initially, recuperation can occur through the secondary axle's drive unit during the transition of the primary axle's torque. Once the primary axle, which has a higher efficiency, also exhibits a negative torque, efficiency-optimized recuperation then takes place. The transition of the torque applied to the primary axle, and thus the change in sign, enables the vehicle to be braked using the primary axle's drive unit. Due to its higher efficiency, more energy can be recuperated for the same braking force. In this way, the overall efficiency of the powertrain can be increased.

[0032] At time 5, the driver releases the brake pedal, thus preventing any further deceleration request from being registered. To ensure that a positive torque can be applied to the primary axle without delay upon a subsequent acceleration request, the primary axle undergoes another play cycle, causing the applied torque to reverse its sign once again. Once this play cycle is complete, the two axles again exhibit oppositely oriented and canceling torques, as was the case between times 1 and 2. This play cycle is also preferably acoustically optimized, thus eliminating any load shock. From time 6 onward, another acceleration request is registered, and a corresponding positive torque is generated on the primary axle.

[0033] It is also conceivable to implement the invention in which the actual torque on the secondary axle is reduced to zero between times 4 and 5 after the braking torque on the primary axle has fully built up, and the drive unit of the secondary axle is commanded into electric freewheel mode. In this way, energy consumption on the secondary axle is minimized.

[0034] Furthermore, an embodiment of the invention is conceivable in which the actual torque of the secondary axis is not only reduced to zero, but the secondary axis also performs a backlash cycle, i.e., a flank shift, in order to subsequently apply a positive actual torque to the secondary axis. In this way, the secondary axis would again be preloaded against the primary axis, since, in contrast to the first operating mode before time 2, the secondary axis would now have a positive actual torque and the primary axis a negative actual torque. Such an embodiment is particularly advantageous because, in the case of a rapid positive load change from a desired deceleration to a desired acceleration, the initial acceleration request can be made directly by the secondary axis without a delayed response, while the primary axis is still in the backlash cycle of the positive load change between times 5 and 6.Such rapid positive load changes, i.e., from a desire to decelerate to a desire to accelerate, can occur particularly during sporty driving, especially for drivers using both feet.

[0035] In this embodiment, it is also conceivable that after the deceleration request ends (i.e., after time 5), the primary axis does not immediately undergo another flank transition. This is particularly advantageous if, after time 5, another deceleration request is detected and, instead of an acceleration request as shown, another deceleration request follows. Thus, the primary axis could directly provide the corresponding negative actual torque without having to undergo another flank transition. An acceleration request detected as shown (see from time 6 onwards) could initially be implemented, as described, by an actual torque provided by the secondary axis. In this way, unnecessary flank transitions of the primary axis could be avoided, and a permanently instantaneous response behavior could be ensured.

[0036] It should be noted that embodiments are also conceivable in which the primary axle is defined as the front axle of a vehicle and, accordingly, the secondary axle as the rear axle of a vehicle.

Claims

[1] Method for operating a powertrain of an electric vehicle, wherein the powertrain has at least one primary axle and at least one secondary axle, wherein a first drive unit has a first efficiency and transmits a first torque to the primary axle and a second drive unit has a second efficiency and transmits a second torque to the secondary axle, wherein the first efficiency is greater than the second efficiency, wherein in a first operating mode the first moment transmitted to the primary axis and the second moment transmitted to the secondary axis have opposite signs and thereby the primary axis is braced against the secondary axis, in a second operating mode, during a load change in the drivetrain, the first drive unit is controlled in such a way that the first torque transmitted to the primary axle changes sign and a sum of the first torque and the second torque corresponds to a recorded driver request, where, after the sign change of the first moment, the magnitude increases and the magnitude of the second moment transferred to the secondary axis decreases and During the load change, the first drive unit is controlled in such a way that the first moment changes sign in such a way that an acoustically optimized clearance cycle of the primary axle takes place. [2] Method according to the preceding claim, wherein during the acoustically optimized game run of the primary axis the magnitude of the second moment transferred to the secondary axis increases. [3] Method according to one of the preceding claims, wherein the increase in the magnitude of the first moment corresponds to the decrease in the second moment, such that the sum of the two moments remains constant. [4] Method according to the preceding claim, wherein the second torque transmitted to the secondary axis is reduced to zero and the second drive unit is commanded into an electric freewheel. [5] Method according to the preceding claim 3, wherein the second moment transferred to the secondary axis also changes sign after the sign change of the first moment transferred to the primary axis, so that subsequently the two moments again have opposite signs. [6] Method according to one of the preceding claims, wherein, upon a subsequent load change, the first moment transferred to the primary axis again changes sign and, in particular, the second moment transferred to the secondary axis is not equal to zero and is opposite to the first moment. [7] Computer-implemented method for increasing the efficiency of a powertrain of an electric vehicle, comprising the method steps from any of the preceding claims.

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