Method for determining a load distribution, control unit, powertrain and motor vehicle

Abstract

A method for determining a load distribution in a powertrain of a motor vehicle, whereby the powertrain has at least two drive machines, whereby the first drive machine is provided for a front-wheel drive and the second drive machine is provided for a rear-wheel drive, whereby the method comprises: determining a load distribution characteristic map that is based on a first efficiency characteristic map of the first drive machine and on a second efficiency characteristic map of the second drive machine.

Claims

1. A method for determining and controlling a load distribution in a powertrain of a motor vehicle, whereby the powertrain has at least two drive machines, wherein the first drive machine is provided for a front-wheel drive and the second drive machine is provided for a rear-wheel drive, wherein the method comprises: determining a load distribution characteristic map that is based on a first efficiency characteristic map of the first drive machine and on a second efficiency characteristic map of the second drive machine; and controlling the load distribution between the first drive machine and the second drive machine on the basis of the load distribution characteristic map; wherein the load distribution characteristic map is based on a first and a second axle efficiency characteristic map, the first axle efficiency characteristic map being determined on the basis of the first efficiency characteristic map and the second axle efficiency characteristic map being determined on the basis of the second efficiency characteristic map, and wherein at least one of the first and the second axle efficiency characteristic map is determined taking into account at least one of: a differing gear ratio, and a bearing loss.

2. The method according to claim 1, wherein the load distribution characteristic map is based on the calculation of linear combinations of the first axle efficiency characteristic map and the second axle efficiency characteristic map as a function of a drive torque of the first drive machine and of the second drive machine.

3. The method according to claim 2, further comprising determining linear combinations of the first axle efficiency characteristic map and the second axle efficiency characteristic map at which the load distribution in the powertrain of the motor vehicle is optimized.

4. The method according to claim 2, wherein the linear combinations are determined for a plurality of prescribed rotational speeds.

5. The method according to claim 1, wherein the load distribution characteristic map is determined for a plurality of prescribed torques and for a plurality of prescribed rotational speeds.

6. The method according to claim 1, wherein the first drive machine and the second drive machine are controlled on the basis of the load distribution characteristic map.

7. The method according to claim 1, wherein at least one of: the first efficiency characteristic map is determined by measuring the first drive machine; and the second efficiency characteristic map is determined by measuring the second drive machine.

8. A powertrain for a motor vehicle, wherein the powertrain has at least two drive machines, wherein the first drive machine is provided for a front- wheel drive and the second drive machine is provided for a rear-wheel drive, and wherein the powertrain has a control unit that is configured to: determine a load distribution characteristic map that is based on a first efficiency characteristic map of the first drive machine and on a second efficiency characteristic map of the second drive machine; and, control the load distribution between the first drive machine and the second drive machine on the basis of the load distribution characteristic map; wherein the load distribution characteristic map is based on a first and a second axle efficiency characteristic map, the first axle efficiency characteristic map being determined on the basis of the first efficiency characteristic map and the second axle efficiency characteristic map being determined on the basis of the second efficiency characteristic map, and wherein at least one of the first and the second axle efficiency characteristic map is determined taking into account at least one of: a differing gear ratio, and a bearing loss.

9. The powertrain according to claim 8, further comprising a front axle and a rear axle, wherein the first drive machine is configured to power the front axle and the second drive machine is configured to power the rear axle.

10. The powertrain according to claim 9, wherein at least one of: the first drive machine is an electric machine; and the second drive machine is an electric machine.

11. A motor vehicle having a powertrain, wherein the powertrain has at least two drive machines, wherein the first drive machine is provided for a front-wheel drive and the second drive machine is provided for a rear-wheel drive, and wherein the powertrain has a control unit that is configured to: determine a load distribution characteristic map that is based on a first efficiency characteristic map of the first drive machine and on a second efficiency characteristic map of the second drive machine; and, control the load distribution between the first drive machine and the second drive machine on the basis of the load distribution characteristic map; wherein the load distribution characteristic map is based on a first and a second axle efficiency characteristic map, the first axle efficiency characteristic map being determined on the basis of the first efficiency characteristic map and the second axle efficiency characteristic map being determined on the basis of the second efficiency characteristic map, and wherein at least one of the first and the second axle efficiency characteristic map is determined taking into account at least one of: a differing gear ratio, and a bearing loss.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described by way of example, making reference to the accompanying drawing, in which:

(2) FIG. 1 schematically shows an embodiment of a motor vehicle with a powertrain;

(3) FIG. 2 schematically shows an embodiment of a control unit;

(4) FIG. 3 schematically shows a flow diagram of an embodiment of a method for load distribution;

(5) FIG. 4 shows the linear combinations of axle efficiency characteristic maps;

(6) FIG. 5 shows optimal distributions for a load distribution;

(7) FIG. 6 shows an area of a load point shift in the depiction of FIG. 6;

(8) FIG. 7 shows a characteristic map for the capacity utilization of a rear-wheel drive;

(9) FIG. 8 shows a characteristic map for the capacity utilization of a front-wheel drive;

(10) FIG. 9 shows a characteristic map for the torque portion of the rear-wheel drive;

(11) FIG. 10 shows a characteristic map for the torque portion of the front-wheel drive.

DETAILED DESCRIPTION OF THE INVENTION

(12) An embodiment of a motor vehicle 1 with a powertrain 2 is schematically shown in FIG. 1.

(13) The powertrain 2 has a first drive machine 3, which is an electric machine and which is configured as a front (axle) drive. The first drive machine 3 powers a front axle 5 of the powertrain 2 via a first transmission 4.

(14) The powertrain 2 also has a second drive machine 6, which is an electric machine and which is configured as a rear (axle) drive. The second drive machine 6 powers a rear axle 8 of the powertrain 2 via a second transmission 7.

(15) An electric energy storage device 9, which is configured as a lithium-ion high-voltage battery, is connected to the first drive machine 3 and to the second drive machine 6 via a high-voltage on-board network 10. The electric energy storage device 9 has, for example, 96 lithium-ion cells and an integrated battery management system. Moreover, a first electric power converter 11 is coupled between the first drive machine 3 and the electric energy storage device 9, and a second power converter 12 is coupled between the second drive machine 6 and the electric energy storage device 9. For example, a charging socket with a charging device can be connected to the second power converter 12 in order to externally charge the electric energy storage device 9.

(16) FIG. 2 schematically shows a control unit 15 that is designed to control the motor vehicle 1 and that is configured to carry out the method described here.

(17) The control unit 15 is connected to the individual components of the motor vehicle 1 that are to be controlled, namely, to the first drive machine 3, to the second drive machine 6, to the electric energy storage device 9, to the electric power converters 11 and 12, and to other components, which have been omitted here for the sake of simplification.

(18) Below, a method 20 for controlling the motor vehicle 1 or its powertrain 2, or for determining the load distribution is described, which serves to bring about the described load distribution between the first drive machine and the second drive machine, whereby the method 20 is explained with reference to the motor vehicle 1 of FIG. 1, without restricting the present invention thereto.

(19) In Step 21, a first efficiency characteristic map and a second efficiency characteristic map are created on a test bench for the first drive machine 3 and for the second drive machine 6 in that the appertaining power losses for the appertaining torques and rotational speeds of the appertaining drive machine are determined. Moreover, in each case, the maximum torque capacity M.sub.max for the first drive machine and for the second drive machine is determined. The efficiency characteristic map can also take into account losses that occur, for example, due to an inverter or due to other components. As a result, the various optimal characteristic map ranges for the first drive machine 3 and for the second drive machine 6 can be determined as a function of the operating point.

(20) Then, in Step 22, a first axle efficiency characteristic map based on the first efficiency characteristic map is determined for the first drive machine 3 and a second axle efficiency characteristic map based on the second efficiency characteristic map is determined for the second drive machine 6 in that, for the first axle efficiency characteristic map, the gear ratio and the efficiency of the appertaining transmission 4 are taken into account, and in that, for the second axle efficiency characteristic map, the gear ratio and the efficiency of the appertaining transmission 7 are taken into account. The first or the second axle efficiency characteristic map then applies on the axle level in terms of the axle rotational speed and wheel torque of the appertaining axle. Here, the axle efficiency characteristic maps are also power loss characteristic maps since they represent the losses that occur on the path between the first drive machine and the second drive machine on the path to the axle or on the path to the wheel.

(21) In Step 23, linear combinations of the first and second axle efficiency characteristic maps are determined at a prescribed rotational speed, as a result of which a sum characteristic map is obtained:
M.sub.G=X.sub.HA*M.sub.HA,max+X.sub.VA*M.sub.VA,max
wherein M.sub.G stands for the total axle torque at the prescribed rotational speed that is delivered by the first drive machine 3 and by the second drive machine 6, X.sub.HA is a number between 0 and 1, and it represents the portion of the total axle torque M.sub.G that is contributed by the maximum torque M.sub.HA,max of the second drive machine 6 (rear axle), and X.sub.VA is a number between 0 and 1, and it represents the portion of the total axle torque M.sub.G that is contributed by the maximum torque M.sub.HA,max of the first drive machine 3 (front axle). In other embodiments, the determination is carried out in the opposite order, that is to say, to start with, all of the rotational speeds are computed in order to determine the efficiency characteristic maps and the linear combinations at a prescribed torque, and then the efficiency characteristic maps and linear combinations are determined for the various torques.

(22) FIG. 4 shows such a linear combination 30, whereby FIG. 4 shows the total axle torque M.sub.G in Nm on the abscissa, and the power loss L.sub.V in watts is shown on the ordinate for the first drive machine 3 and for the second drive machine 6 combined, as is obtained from the first and second axle efficiency characteristic maps.

(23) Each linear combination 30, whereby FIG. 4 only shows a few of them, then represents the power loss curve at a certain rotational speed and for appertaining different total axle torques M.sub.G, whereby all of the available axle torques are computed for the first drive machine and for the second drive machine.

(24) In Step 24, the optimal linear combinations (that is to say, minimal losses in the total drive) are now determined, as is also shown in FIG. 4 by the curve 31, which (at a prescribed rotational speed), in each case, shows the linear combination with the lowest total power loss L.sub.V for a total axle torque M.sub.G, and thus with the best (optimal) efficiency.

(25) In Step 25, in each case at the prescribed rotational speed, the optimal distribution is determined for X.sub.VA and X.sub.HA, as is also shown in FIG. 5. FIG. 5 shows the total axle torque M.sub.G on the abscissa and values between 0 and 1 on the ordinate.

(26) The solid line in FIG. 5 represents the values for X.sub.VA, and the broken line represents the values for X.sub.HA, as can be seen from the appertaining curve 31. Thus, in FIG. 5, in each case for a given total axle torque M.sub.G, the appertaining optimal values for X.sub.HA and X.sub.VA at the prescribed rotational speed are shown, whereby X.sub.HA and X.sub.VA represent the appertaining load portions for the second drive machine and for the first drive machine.

(27) FIG. 6 corresponds to the depiction of FIG. 5, except that in FIG. 6, a diagonally shaded area represents an area of the load point shift at which the load is shifted from the rear axle to the front axle. After all, for instance, if one were to simply linearly continue the curve of X.sub.HA (solid line), then one would obtain the curve shown in FIG. 6 (dot-dash line in FIG. 6) until a maximum value of X.sub.HA would be reached, which results from the maximum power of the second drive machine (straight course of the dot-dash line in FIG. 6).

(28) The shaded area in FIG. 6 shows that, as far as the total efficiency is concerned, it makes more sense to lower the load of the second drive machine and to add power to the first drive machine.

(29) In Step 26, the total calculation for all of the (available) rotational speeds is carried out so that a load distribution characteristic map is obtained for all of the axle torques and rotational speeds and thus, in Step 27, the load distribution characteristic map is determined accordingly. In Step 28, the powertrain 2 is controlled according to the determined load distribution characteristic map in that, for example, for the first and second drive machines, an appertaining actuation characteristic map is derived that indicates the proportional load for the appertaining drive in each case.

(30) FIGS. 7 through 10 then present examples of load distribution characteristic maps, whereby the abscissa shows the axle rotational speed D in rpm and the ordinate shows the appertaining axle torque M in Nm. In each of FIGS. 7 through 10, there is a scale on the right-hand side indicating the torque portion, whereby the uppermost shading (dots) shows the torque portion from 0.75 to 1, the diagonal shading shows the torque portion from 0.5 to 0.75, the wavy shading shows the torque portion from 0.25 to 0.5, and the checkered shading shows the torque portion from 0 to 0.25. Moreover, in each case, a vertical line shows an axle rotational speed, which here is, for instance, 750 rpm and which applies to FIGS. 5 and 6. In other embodiments, the scale can, of course, be finer and the coarse scale here serves only to simplify the depiction.

(31) Purely by way of example, FIG. 7 shows the capacity utilization of the second drive machine (rear axle), in other words, M.sub.HA/M.sub.HA,max. As one can see from the characteristic map of FIG. 7, for example, the capacity utilization of the second drive machine at very low axle rotational speeds (e.g. below 100) and at very low torques (e.g. below 100) is almost zero (area with the checkered shading), that is to say, the second drive machine is practically not being used, in contrast to which at very high axle rotational speeds, for example, at the vertical line and at a very high axle torque, for example, at about 3000, the capacity utilization of the second drive machine is 100% (area with the dotted shading).

(32) Purely by way of example, FIG. 8, like FIG. 7, shows the capacity utilization of the first drive machine (front axle), in other words, M.sub.VA/M.sub.VA,max. As one can see from the characteristic map of FIG. 8, especially in comparison to FIG. 7, there is a large area (checkered shading) in which the torque portion of the first drive machine is small or practically zero, that is to say, in that area, the axle torque is being delivered essentially only by the second drive machine.

(33) Purely by way of example, FIG. 9 shows the torque portion of the total axle torque, in other words, M.sub.axle,HA/M.sub.axle, total, contributed by the second drive machine (rear axle). As one can see from the characteristic map of FIG. 9, in the area with the dotted shading, the portion contributed by the second drive machine is high (almost or equal to one), so that in that area, essentially only the second drive machine is delivering the torque.

(34) Purely by way of example, FIG. 10 shows the torque portion of the total axle torque, in other words, M.sub.axle,VA/M.sub.axle, total, contributed by the first drive machine (front axle). As one can see from the characteristic map of FIG. 10, in the area with the checkered shading, the portion contributed by the first drive machine is small (almost or equal to zero), so that, in that area, the first drive machine is essentially being carried along and is not delivering any torque.

(35) In general, in some embodiments, not all of the values for characteristic maps are measured or calculated, and any intermediate values that might be required are determined, for example, by means of interpolation, as is generally known and usual for characteristic maps.

LIST OF REFERENCE NUMERALS

(36) 1 motor vehicle 2 powertrain 3 first drive machine 4 first transmission 5 front axle 6 second drive machine 7 second transmission 8 rear axle 9 energy storage device 10 high-voltage on-board network 11 first electric power converter 12 second electric power converter 15 control unit 20 method 21 determining the first and second efficiency characteristic maps 22 determining the first and second axle efficiency characteristic maps 23 calculating the linear combinations of the first and second axle efficiency characteristic maps 24 determining optimal linear combinations 25 determining optimal distribution 26 expanding to all rotational speeds 27 determining the load distribution characteristic map 28 controlling the powertrain 30 linear combinations 31 curve showing lowest total loss