Method and control for a drive system having four-wheel drive

Abstract

A method is provided for operating a drive train comprising an internal combustion engine or an electric machine as a primary drive and an electric machine as a secondary drive, wherein the electric machine is detachably coupled, together with an inverter and controller, on one of the vehicle axles. The electric machine and at least one switchable element is actuated in order to minimize drag losses of the electric machine and to provide a defined connection time for the electric machine. The electric machine is stationary and decoupled during a first speed range. The electric machine is actuated at a preset speed during a second speed range, where a defined connection time is not possible if the electric machine were stationary. The electric machine is coupled to the axle and rotates at the vehicle speed in the third range, when losses while coupled are lower than if uncoupled.

Claims

1. A method for operating a drive train comprising an internal combustion engine as a primary drive and an electric machine as a secondary drive, wherein the electric machine of the secondary drive is detachably mounted, together with an inverter and a controller, on a vehicle axle of a vehicle, the method comprising: actuating the electric machine of the secondary drive and at least one switchable element in order to minimize drag losses of the electric machine of the secondary drive during operation of the vehicle by way of the internal combustion engine over an entire vehicle speed range of the vehicle and to provide a defined connection time for the electric machine of the secondary drive, dividing the vehicle speed range of the vehicle into multiple ranges, wherein a first range of the multiple ranges starts from the stationary state of the vehicle and the electric machine of the secondary drive, and wherein the at least one switchable element is actuated differently and the electric machine of the secondary drive is selectively coupled or uncoupled from the vehicle axle depending on which of the multiple ranges the vehicle is operating in, wherein the first range of the vehicle speed extends up to a defined speed of the vehicle in accordance with a WLTC cycle, the defined speed being a vehicle speed at which the defined connection time is possible, wherein the electric machine of the secondary drive is uncoupled from the vehicle axle within the first range; wherein, in a second range of the multiple ranges, the electric machine of the secondary drive is actuated such that it rotates at a preset speed, wherein the electric machine of the secondary drive remains uncoupled from the vehicle axle while in the second range, and wherein in a third range of the multiple ranges of the vehicle speed, which includes an overall maximum speed of the vehicle, the electric machine of the secondary drive is coupled to the vehicle axle and rotates at the speed of the vehicle axle.

2. The method of claim 1, wherein the electric machine of the secondary drive is part of an electric axle, wherein a four-wheel drive is implemented together with the internal combustion engine of the primary drive.

3. A controller for carrying out the method as claimed in claim 1, the controller comprising: a control module, the control module including a coupling and decoupling controller which couples or uncouples the electric machine as a drive element in a state-dependent manner.

4. The method of claim 1, wherein the first range is adapted to the vehicle.

5. The method of claim 1, wherein operating the electric machine at the preset speed in the second range reduces connection time relative to stationary and with reduced drag losses relative to a coupled condition with the axle.

6. The method of claim 1, wherein the electric machine is an asynchronous electric machine.

7. The method of claim 6, wherein the inverter is actuated such that the electric machine rotates and the switchable element remains open in the second range.

8. The method of claim 1, wherein in the first range, power loss reduces at a lower rate as vehicle speed increases relative to rate of power loss when the electric machine is coupled to the vehicle axle.

9. The method of claim 1, wherein in the second range the defined connection time cannot be implemented without actuating the electric machine at the preset speed.

10. The method of claim 9, wherein a threshold vehicle speed that defines a transition between the first range and the second range is based on the defined connection time.

11. The method of claim 9, wherein in the second range with the electric machine actuated at the preset speed, losses are created in the electric machine and in the inverter, wherein the losses are less than a passive coupled mode between the electric machine and the axle and define reduced losses.

12. The method of claim 11, wherein the reduced losses provide a shorter connection time of the electric machine.

13. The method of claim 1, wherein the third range is defined as a range of speed where losses in the electric machine while maintaining a specified speed via the electric machine are higher than drag losses at the corresponding speed of the axle when coupled.

14. The method of claim 1, wherein in the first range the electric machine is decoupled from the axle and in a stationary state, the switchable elements are open, and the inverter is in a ready state.

15. The method of claim 14, wherein in the second range the electric machine is decoupled and rotates at the preset speed that provides the defined connection time, the inverter is activated and controls the speed of the electric machine, and the switchable elements are open.

16. The method of claim 15, wherein in the third range the electric machine is coupled to the axle and rotates at the vehicle speed, the inverter is in the ready state, and the switchable elements are closed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. In the drawings:

(2) FIG. 1 shows a schematic drive train,

(3) FIG. 2 shows a graph of the power loss with respect to the rotation speed of the vehicle axle,

(4) FIG. 3 shows an illustration of the connection times with respect to the vehicle speed,

(5) FIG. 4 shows ranges of the vehicle speed with different states of the controller, and

(6) FIG. 5 schematically shows a controller.

DETAILED DESCRIPTION

(7) FIG. 1 shows a highly schematic illustration of a motor vehicle comprising a drive train 1 with two drive motors, specifically an internal combustion engine VM and an electric machine EM. The internal combustion engine VM is coupled to the front axle, comprising two propeller shafts 4a and 4b with the front wheels 5a and 5b, via a manual transmission 2 with a coupled differential gear 3. The electric machine EM is coupled to the differential 6 via a transmission, not illustrated, and said differential 6 is coupled to the rear wheels 7a and 7b via propeller shafts 8a and 8b, also referred to as the vehicle axle 8a, 8b. The electric rear axle drive can be used at low speeds, e.g. <30 km/h, as a single drive of the vehicle or can be connected to the front axle drive in order to implement all-wheel drive. A shift clutch, not illustrated, is used for this purpose. A controller 9 controls the electric machine and the shift clutch.

(8) However, the present disclosure is not limited to configurations according to FIG. 1.

(9) FIG. 2 shows a graph plotting the power loss of a drive train 1 with respect to the rotation speed of the electrified vehicle axle. The curve denoted 10 shows the increase in the power loss as the vehicle axle speed increases. If the electric machine EM is decoupled, the situation improves dramatically, as illustrated by curve 11.

(10) FIG. 3 shows how the switching times change with respect to the speed of the vehicle axle and in relation to the second scale of vehicle speed.

(11) The curve 12 shows the synchronization time, which is dependent on the peak power of the electric machine.

(12) Curve 13 describes the purely mechanical connection time, which is specified by the clutch and its maximum engagement speed.

(13) By way of example, connection times 14 of 250 ms and 300 ms are drawn as horizontal dashed lines.

(14) Depending on the definition of the threshold for the connection times, a connection operation of the electric machine from the stationary state up to 128 km/h or up to 145 km/h is possible in the exemplary illustration, depending on the connection time.

(15) Therefore, if a connection time of 300 ms is selected, the range of the vehicle speed from 0 to clearly above 140 km/h is covered, this corresponding to the entire WLTC speed range. The Worldwide harmonized Light duty driving Test Cycle WLTC is the test method for vehicles that is specified by the UNECE.

(16) In order to achieve an optimum, as illustrated in FIG. 4, the entire speed range of the vehicle, in this case of from 0 to 200 km/h, is divided into 3 ranges. In the first range B1, the electric machine EM is decoupled from the vehicle axle 8a, 8b by the controller 9 via the switchable elements and the losses change in accordance with the curve 11.

(17) In the second range B2, the defined connection times would no longer be able to be implemented. Therefore, the controller 9 switches the electric machine EM to active and rotates the electric machine EM at a low rotation speed, without connecting it to the vehicle axle 8a, 8b. This creates drag losses and electrical losses in the electric machine and in the inverter, but not to such an extent as in the case of purely passive coupled motion of the electric machine EM. In addition, the connection times of the electric machine EM can be shortened as a result.

(18) In the third range B3 above 190 km/h, the electric machine EM is coupled and rotates at the speed of the vehicle axle 8a, 8b. The range B3 is defined when the losses in the electric machine EM while maintaining a particular specified speed are higher than the drag losses of the corresponding speed of the vehicle axle 8a, 8b. The individual ranges described herein are simulated for a vehicle by way of example. The ranges have to be adapted depending on the vehicle.

(19) The controller 9 controls both the switchable elements and the electric machine EM. The 3 control states are: From 0 km/h up to 145 km/h in the first range B1, the electric machine EM is decoupled and in the stationary state, the switchable elements are open, and the inverter is in a ready state. From 145 km/h to 190 km/h in range B2, the electric machine EM is decoupled and rotates at a predetermined speed in order to achieve the required connection time. The inverter is activated and controls the speed of the electric machine, and the switchable elements are open. From 190 km/h up to the maximum speed in range B3, the electric machine EM is coupled and rotates at the vehicle speed since maintaining the predetermined speed is less efficient than the drag losses at the corresponding speed. The inverter is once again in a ready state and the switchable elements are closed, it being possible for this to be implemented with an asynchronous machine.

(20) The availability of decoupling is considerably expanded, specifically from the first to the second range, via such control in ranges. This optimizes the total efficiency of the vehicle and maintains the total time of the decoupling process provided that the dynamic connection time of the electric machine of 300 ms or for the total speed range up to the maximum speed is not exceeded.

(21) The multi-stage control is advantageous since it leads to improved WLTC results in the speed range up to 135 km/h, that is to say to energy savings for the drive train.

(22) FIG. 5 shows, by way of example, the controller 9 with its modules. Module 9A is the control region in which the ranges B1, B2, B3 are stored and in which the decision is made as to which of the ranges is present and whether the electric machine EM should be coupled.

(23) To this end, the vehicle speed v, the driving mode DM, and the available torque T are used as input variables. The connection mode ZM is output as a result.

(24) The module 9B describes the control of the connection operation of the electric machine EM for a four-wheel drive. Here, the vehicle speed, the requested torque, road conditions, longitudinal accelerations, lateral accelerations, and yaw factor are queried at the input end. The connection operation CR1 is then requested at the output.

(25) A dynamic controller of the vehicle can likewise request the connection operation CR2 via another input.

(26) The requests CR1 and CR2 reach the module 9C which coordinates the torque and the driving mode. The module 9C may then output a signal CC in order to close the switchable elements. Furthermore, signals CEM for controlling the electric machine EM and the speed request VR are output to the electric machine EM.

(27) FIG. 5 is a schematic illustration, the individual models 9A, 9B and 9C are illustrated by way of example and do not have to be present with such a division or arrangement. The individual control tasks can be run in different physical controllers.