WO2020177833A1

WO2020177833A1 - Method for operating an electric vehicle powertrain with a continuously variable transmission

Info

Publication number: WO2020177833A1
Application number: PCT/EP2019/025436
Authority: WO
Inventors: Vital VAN REEVEN; Lucas Hubertus Johannes RÖMERS
Original assignee: Robert Bosch Gmbh
Priority date: 2019-03-06
Filing date: 2019-12-05
Publication date: 2020-09-10
Also published as: WO2020177931A1; CN113544413A; EP3935291A1; CN113646200A; JP2022525003A

Abstract

The invention relates to a method for operating a powertrain for or in a fully electric vehicle, in particular an electric passenger car, which powertrain comprises an electric machine (1), a driven wheel (2) and a gearing (3), which gearing (3) is arranged between the electric machine (1) and the driven wheel (2), providing a driving connection there between, and which gearing (3) includes, at least, a continuously variable transmission unit (40) that provides a variable speed ratio between an input shaft thereof, rotating as one with the electric machine (1), and an output shaft thereof. According to the invention, during operation of the powertrain, the speed ratio of the continuously variable transmission unit (40) is controlled in relation to, at least, a speed of the electric vehicle and a combined efficiency characteristic of, at least, the electric machine (1) and the continuously variable transmission unit (40).

Description

METHOD FOR OPERATING AN ELECTRIC VEHICLE POWERTRAIN WITH A CONTINUOUSLY VARIABLE TRANSMISSION

The present invention relates to a powertrain for or in an electric vehicle, in particular a passenger car, with an electric machine (EM), also known as a motor/generator device, a driven wheel or wheels and with a continuously variable transmission (CVT) that drivingly connects, i.e. rotationally couples, the EM to the driven wheel. It is noted that within the context of the present disclosure the term electric vehicle is to be understood as including only purely electric vehicles, such as battery electric vehicles (BEV) and fuel cell electric vehicles (FCEV). In other words, the presently considered electric vehicle powertrain includes only the electric machine as a prime mover and does, in particular, not include an internal combustion engine that is, or at least can be, connected to the driven wheels in addition to or instead of the electric machine.

The CVT is mostly known from its widespread application in conventional motor vehicles that are powered by an internal combustion engine, but it can be beneficially applied in electric vehicles as well. The known CVT includes a primary pulley and a secondary pulley, as well as a flexible drive element that is wrapped around and in friction contact with the said pulleys. Each such pulley comprises two (frusto-)conical pulley discs arranged on a shaft, whereof at least one pulley disc is axially moveable and can be urged towards the pulley disc by an actuation system of the CVT, such as a set of electronically controlled and hydraulically operated piston/cylinder assemblies. The flexible drive element comes in several types such as a metal push belt, a metal drive chain or a fibre- reinforced rubber pull belt.

During operation of the CVT, the flexible drive element is clamped between the two pulley discs of each pulley by the said actuation system exerting a respective force on the moveable pulley disc towards the other one of the two pulley discs. A rotational speed and an accompanying torque can then be transmitted between the pulleys by means of friction between the flexible drive element and the pulleys. Also by the forces exerted by the actuation system, more in particular by a ratio there between, a radius of curvature of the flexible drive element at each pulley is controlled. In turn, these radii of curvature determine a speed ratio of the CVT, which speed ratio can be controlled by the actuation system to an arbitrary value within a speed ratio range provided by the CVT.

In the conventional motor vehicle, the CVT thus allows the engine to be operated within a continuous range of rotational speeds -as determined by the speed ratio range of the CVT- in relation to a given vehicle speed, i.e. a given rotation speed of the driven wheel. The engine speed can for example controlled towards optimum fuel efficiency of the engine (typically a low engine speed), towards maximum vehicle acceleration (typically a high engine speed) or even towards emulating a geared, i.e. stepped transmission. In case the engine speed is controlled towards optimum fuel efficiency, a desired engine speed is typically determined based on several operating parameters including, at least, the current vehicle speed and an efficiency characteristic of the engine. The CVT speed ratio is then controlled such that the actual engine speed matches the desired engine speed as closely as possible. Typically, however, more sophisticated control methods are employed that rely not only on the current vehicle speed, but also on other parameters, such as the position of the accelerator pedal. In particular, the accelerator pedal position is often interpreted as an acceleration that is instantaneously demanded by the driver (that can be zero). In this case, the engine fuel efficiency characteristic is in the form of a look up table that links the desired engine speed to the engine torque required for realising the demanded acceleration.

Inter alia it is noted that in case the powertrain is electronically operated, such as by a cruise control system or an autonomous driving system, such system will generate an acceleration demand in lieu of the driver.

Of course, the former, conventional engine speed control method can be easily converted to a vehicle that is powered by an EM, by replacing the engine fuel efficiency characteristic by a similar efficiency characteristic of the EM. In this case, the said look-up table links a desired EM speed (providing optimum EM electrical efficiency) to a vehicle speed. However, according to the present invention, a significant efficiency improvement can in this case be realized. In particular, the present invention considers that the electrical efficiency of the EM is much higher than the fuel efficiency of the internal combustion engine, to such an extent even that the (mechanical and/or electrical) power efficiency of other components of the powertrain become a significant factor in the overall power efficiency of the powertrain. According to the present invention in an electric vehicle powertrain with CVT, the combined power efficiencies of the EM and the CVT is favourably taken into account when determining a desired EM speed. In particular, an efficiency characteristic, in particular a power efficiency characteristic, is predetermined for the combination of EM and CVT that is applied in the powertrain. Preferably, the mechanical efficiency of a gear train that is applied in the powertrain between the EM and the CVT is also taken into account in the combined efficiency characteristic as well. In this respect it is noted that the mechanical efficiency of any gears between the CVT and the driven wheels can be favourably disregarded. Additionally, the electrical efficiency of a so-called inverter between the EM and a battery of the electric vehicle can be taken into account in the combined efficiency characteristic as well.

In relation to the combined efficiency characteristic it is noted that it can be conveniently determined by mutually multiplying the individual efficiency maps of the powertrain components, such as at least the EM and the CVT, that map the power efficiency of the respective powertrain component against the operating parameters of torque or power, whether generated or transmitted by the respective powertrain component, and output speed of that respective powertrain component. In particular, such combined efficiency characteristic ultimately links any vehicle speed and EM power to a desired speed ratio for the CVT for providing maximum powertrain efficiency.

Moreover, according to the invention, the above, novel control method is preferably also applied during vehicle deceleration, i.e. for torque or power levels that are lower than what is required for constant vehicle speed. In particular, in a vehicle braking condition the EM applies a negative torque and operates as a generator, e.g. to (re-)charge the battery of the electric vehicle (i.e. so-called regenerative braking). Also in such generator mode of the EM, most efficient operating points are available for the powertrain as a whole as a function of the instantaneous (reverse, i.e. braking) torque or power and (forward) speed of the electric vehicle. Effectively, the present invention relies on a predetermined efficiency map that links the input parameters of the speed and torque at the drive wheels (or parameters representative thereof, such as vehicle speed and EM power respectively) to the CVT speed ratio and thus to the EM speed that provides such wheel torque at the highest power efficiency. Thereto, the said predetermined efficiency map is prepared by measurement, or more conveniently by calculating the power loss in the system of, at least, the EM and the CVT, as a function of CVT speed ratio, EM power and vehicle speed.

A further efficiency improvement can be realized in accordance with the present invention, by designing a component of the powertrain with an efficiency map that approaches the efficiency map of another component thereof, such that the said multiplying of the individual efficiency maps will yield the highest values. Additionally, these individual and combined efficiency maps preferably approach the most commonly occurring operating points of the electric vehicle, in particular the operating points corresponding to constant vehicle speed on a level road.

As an example of such mutual tuning of power train component designs, the EM is provided with an individual efficiency characteristic in terms of combinations of EM torque and EM speed providing minimal power consumption, which characteristic is intersected by a steady state torque requirement curve when the CVT is in, so-called, Overdrive (i.e. the most accelerating CVT speed ratio). At the same time, such EM efficiency characteristic lies above the steady state torque requirement curve when the CVT is in, so-called, Low (i.e. the most decelerating CVT speed ratio). In this example, acceleration and deceleration of the electric vehicle is possible with excellent electric efficiency by controlling the CVT speed ratio between Medium, i.e. 1 :1 , and Low, while the vehicle can also be efficiently operated steady state in a CVT speed ratio near Medium, where the component efficiency of the CVT is highest, or more towards Overdrive to achieve the highest vehicle speeds. Preferably in this respect, the EM is provided with an individual efficiency characteristic that is intersected by the said steady state torque requirement curve in Overdrive between 30% and 50%, preferably around 35% of the maximum, i.e. peak, speed of the EM, e.g. as specified or as actually occurring during operation of the electric vehicle.

The invention will be explained in more detail by means of non-limiting, illustrative embodiments thereof and with reference to the drawing, in which:

- figure 1 is a schematic representation of the functional arrangement of the main components of a known electric vehicle powertrain with an electric machine and a continuously variable transmission unit;

- figure 2 is a graph illustrating a torque vs rotational speed characteristic of three differently specified electric machines;

- figure 3 illustrates an aspect of the present invention related to the design of an electric machine in a torque vs. speed graph thereof;

- figure 4 illustrates a first aspect of the present invention related to the design of the powertrain in an torque vs. speed graph of the electric machine;

- figure 5 illustrates a second aspect of the present invention related to the design of the powertrain; and

- figure 6 illustrates a third aspect of the present invention related to the design of the powertrain by way of a map linking vehicle speed and electric machine power to continuously variable transmission speed ratio.

Figure 1 shows a basic example of a known powertrain for an electric vehicle such as a passenger car. The known electric vehicle powertrain comprises an electric machine (EM) 1 , also known as a motor/generator device, two driven wheels 2 of the electric vehicle and a gearing 3 that drivingly connects the EM 1 to the driven wheels 2. The known gearing 3 includes a continuously variable transmission (CVT) unit 40, providing a continuously variable speed ratio between an input shaft and an output shaft thereof, is included therein. The CVT unit 40 is as such well-known, in particular in the form comprising a drive belt 41 that is wrapped around and in frictional contact with both an input pulley 42 on the input shaft and an output pulley 43 on the output shaft of the CVT unit 40. An effective radius of the friction contact between the drive belt 41 and a pulley 42, 43 can be varied in mutually opposite directions between the two pulleys 42, 43 by means of a control and actuation system (not shown) of the CVT unit 40, such that a speed ratio provided by the CVT unit 40 between its input and output shafts can be continuously varied with a certain speed ratio range between a most decelerating CVT speed ratio, i.e. Low, and a most accelerating CVT speed ratio, i.e. Overdrive.

By including the CVT unit 40 in the electric vehicle powertrain several advantages and/or optimisation strategies are unlocked. For example, the take-off acceleration and/or top speed of the electric vehicle can be increased thereby. Alternatively, these performance parameters of the vehicle can be maintained at the same level, but while applying a smaller, i.e. downsized EM 1 . As illustrated in figure 2 such downsizing relative to a reference EM 1 (e.g. fig. 2, dashed line) applied in a powertrain without CVT, can be in terms of the maximum, i.e. peak speed requirement of the EM 1 (e.g. fig. 2, lines EMs), the maximum, i.e. peak torque requirement of the EM 1 (not illustrated) or both (e.g. fig. 2, lines EMt). In either of the two illustrated downsized EMs 1 , the maximum, i.e. peak power rating of the respective downsized EM 1 has remained unchanged relative to the reference EM 1 , even though such is not necessary within the context of the present invention. Moreover, although the absolute values shown in figure 2 are entirely realistic, these are merely exemplary to illustrate the downsizing potential.

In particular according to the present invention, the downsized EM 1 is preferably characterised by an at least essentially constant peak torque as a function of its rotational speed, at least in a predominant part of its speed range, i.e. from 0 up to at least 50% of its peak speed. More preferably, the peak torque of the downsized EM 1 is constant up to at least 80%, more preferably up to at least 90% or even up to 95% of its peak speed. Alternatively or additionally, the downsized EM 1 is preferably characterised by an essentially constant peak speed as a function of its torque, at least in a predominant part of its torque range, i.e. from 0 up to at least 50% of its peak torque. More preferably, the peak speed of the downsized EM is constant up to at least 80%, more preferably up to at least 90% or even up to 95% of its peak torque.

If the EM 1 is downsized in terms of its peak torque, a limitation occurs that is illustrated in figure 3. Namely, as the peak torque of the EM 1 is reduced, a so-called, torque-reserve Tr (as defined by the difference between such peak torque and an, instantaneous, actual torque) at a certain speed of the EM 1 . In particular, the torque reserve Tr relative to an efficiency characteristic curve r|_max of the reference EM 1 in terms combinations of EM torque and EM speed providing minimal power consumption, is reduced considerably. According to the present invention, this limitation can, however, be favourably alleviated by designing the downsized EM 1 with an efficiency characteristic curve H_max-EMt that remains below 50% of the EM peak torque Tmax up to, at least 50% and preferably up to around 70% of the EM peak speed Smax. More preferably, such efficiency characteristic curves q_max-EMt of the downsized EM extends up to and thus includes the EM peak speed Smax.

Alternatively or additionally to such design adaptation of the EM 1 , the CVT unit 40 and the EM 1 can be advantageously adapted mutually, as illustrated in figure 4. In particular, the speed ratio range provided by the CVT unit 40 and the efficiency characteristic curve q_max of the EM 1 are preferably mutually adapted in such a manner that:

a) a steady state, i.e. constant vehicle speed, EM torque requirement in CVT speed ratio Low, indicated in figure 4 by the curve TR_Low, is located below the efficiency characteristic q_max of the EM 1 ; and

b) a steady state, i.e. constant vehicle speed, EM torque requirement in CVT speed ratio Overdrive, indicated in figure 4 by the curve TRo_verdrive, is located partly below and partly above the efficiency characteristic curve q_max of the EM 1. Preferably, these latter two curves intersect between 30% and 50%, preferably around 35% of the EM peak speed Smax.

Alternatively or additionally to such design adaptations of the EM 1 or the CVT unit 40, the respective efficiency characteristics thereof can also be taken into account in the control of the speed ratio of the CVT unit 40 in the electric vehicle powertrain. In particular, for a given vehicle speed and required EM power a combined efficiency characteristic is determined by mutually multiplying the individual efficiency maps of the EM 1 and the CVT unit 40, as is schematically illustrated in figure 5. It is, however, noted that figure 5 contains a simplification, namely that the speed ratio dependency of the CVT efficiency map is not illustrated therein, since that would require a third axis of CVT input speed. Instead, the CVT efficiency map of figure 5 is plotted for the CVT speed ratio providing the highest efficiency, which is Medium for a middle part MP of the CVT output speed range, which gradually changes towards Low for a low part LP of the CVT output speed range and which gradually changes towards Overdrive for a high part HP of the CVT output speed range. The resulting, combined efficiency characteristic links a desired CVT speed ratio to a EM power level providing maximum system power efficiency. In the graph of figure 6 the above concept of the combined efficiency characteristic of powertrain components is illustrated once more, however, including the vehicle speed as an input variable and the CVT speed ratio as an output variable. In figure 6, the three lines marked 2, 1 and 0.5 respectively represent the three CVT speed ratios -numerically defined as CVT input speed divided by CVT output speed- of Low, Medium and Overdrive. Of course, in reality the combined efficiency characteristic is a continuous efficiency map, wherein intermediate CVT speed ratios fill-in the space between the said three lines. Therefore, based on such map and given a vehicle speed and an EM power, a desired CVT speed ratio providing optimum combined power efficiency of the EM 1 and the CVT unit 40 is obtained. In practice such continuous map will be approximated by an essentially, i.e. semi-continuous map, look-up table or (set of) mathematical equation(s) programmed into a logic unit of the electric vehicle.

In principle the top half of the graph of figure 6, i.e. the continuous efficiency map represented thereby, representing positive, i.e. driving EM torque or power levels, can also be applied for negative, i.e. braking EM torque or power levels. However, according to the present invention a further improvement can be obtained by measuring or calculating it specifically for such negative EM torque or power levels. Also in such generator mode of the EM 1 , most efficient operating points are available for the powertrain as a whole as a function of the instantaneous (reverse, i.e. braking) torque or power and (forward) speed of the electric vehicle. In this latter respect it is noted that, surprisingly, the said most efficient operating points are different between positive and negative EM torque or power levels. According to the present invention, this asymmetry can be largely attributed to the CVT unit 40, in particular the presently illustrated belt-and- pulleys-type CVT unit 40.

The present invention, in addition to the entirety of the preceding description and all details of the accompanying figures, also concerns and includes all of the features in the appended set of claims. Bracketed references in the claims do not limit the scope thereof, but are merely provided as a non-limiting example of a respective feature. Separately claimed features can be applied separately in a given product, or in a given process as the case may be, but these can also be applied simultaneously therein in any combination of two or more of such features.

The invention is not limited to the embodiments and/or the examples that are explicitly mentioned herein, but also encompass(es) straightforward amendments, modifications and practical applications thereof, in particular those that lie within reach of the person skilled in the relevant art.

Claims

1 . A method for operating a powertrain for or in a fully electric vehicle, in particular an electric passenger car, which powertrain comprises an electric machine (1 ), a driven wheel (2) and a gearing (3), which gearing (3) is arranged between the electric machine (1 ) and the driven wheel (2), providing a driving connection there between, and which gearing (3) includes, at least, a continuously variable transmission unit (40) that provides a variable speed ratio between an input shaft thereof, rotating as one with the electric machine (1 ), and an output shaft thereof, in which method, during operation of the powertrain, the speed ratio of the continuously variable transmission unit (40) is controlled in relation to, at least, a speed of the electric vehicle and a combined efficiency characteristic of, at least, the electric machine (1 ) and the continuously variable transmission unit (40).

2. The operating method according to claim 1 , wherein the gearing (3) further includes a first speed reduction stage (31 ), providing a fixed first speed ratio between the electric machine (1 ) and the continuously variable transmission unit (40) and a second speed reduction stage (32), providing a fixed second speed ratio between the continuously variable transmission unit (40) and the driven wheel (2), characterized in that the combined efficiency characteristic also includes the first speed reduction stage (31 ), but not of the second speed reduction stage (32).

3. The operating method according to claim 1 or 2, characterized in that the speed ratio of the continuously variable transmission unit (40) is controlled also in relation to a demand for acceleration of the vehicle that can take positive, zero or negative values.

4. The operating method according to claim 1 , 2 or 3, characterized in that the combined efficiency characteristic is determined based on, in particular by the multiplication of an individual efficiency characteristic of the electric machine (1 ), of the continuously variable transmission unit (40) and/or of the first speed reduction stage (31 ), which individual efficiency characteristics respectively link the parameter of an output speed of the respective powertrain component (1 ; 40; 31 ) to a mechanical torque generated or transmitted by that respective powertrain component (1 ; 40; 31 ) with the highest achievable efficiency and vice versa.

5. The operating method according to claim 4, characterized in that the electric machine (1 ) has an individual efficiency characteristic that lies above a steady state torque requirement curve of the electric vehicle with the continuously variable transmission unit (40) in its most decelerating speed ratio and that is intersected by a steady state torque requirement curve of the electric vehicle with the continuously variable transmission unit (40) in its most accelerating speed ratio.

6. The operating method according to claim 5, characterized in that the individual efficiency characteristic of the electric machine (1 ) is intersected by the said steady state torque requirement curve of the electric vehicle with the continuously variable transmission unit (40) in its most accelerating speed ratio, at between 30% and 50%, preferably at around 35% of a maximum speed of the electric machine (1 )

7. The operating method according to a preceding claim, characterized in that the electric machine (1 ) can generate a maximum torque that is independent of a rotational speed thereof in a predominant part of a range of rotational speed that can be achieved by the electric machine (1 ), in particular in the range between zero and 80% or more of a maximum rotational speed thereof, preferably up to and including such maximum.

8. The operating method according to a preceding claim, characterized in that the electric machine (1 ) can achieve a maximum rotational speed that is independent of a torque generated thereby in a predominant part of a range of torques that can be generated by the electric machine (1 ), in particular in the range between zero and 80% or more of a maximum torque thereof, preferably up to and including such maximum torque.

9. The operating method according to claim 7 or 8, characterized in that an individual efficiency characteristic (q_max-EMt) of the electric machine 1 , linking a rotational speed thereof to a torque generated thereby with the highest achievable efficiency, remains below 50% of the said maximum torque thereof up to at least 50%, preferably up to at least 70% of the said maximum rotational speed thereof.

10. The operating method according to claim 9, characterized in that the said individual efficiency characteristic (q_max-EMt) of the electric machine (1 ) extends up to the said maximum rotational speed thereof.

1 1 . The operating method according to a preceding claim, characterized in that the combined efficiency characteristic is programmed into a logic unit of the electric vehicle in the form of an essentially continuous map, look-up table or (set of) mathematical equation(s) linking a desired speed ratio value for the continuously variable transmission unit (40) as an output parameter to a range of achievable vehicle speeds, or a parameter representative thereof, and a range of achievable driving torques generated by the electric machine (1 ), or a parameter representative thereof, as a set of input parameters.

12. The operating method according to claim 1 1 , characterized in that for a specific negative, i.e. braking torque applied by the electric machine (1 ), the inverse, i.e. positive value of such braking torque, i.e. the driving torque or a parameter representative thereof, is used as one of the said input parameters, in particular the said driving torque of the said electric machine (1 ), to determine the desired speed ratio value for the continuously variable transmission unit (40) as the output parameter.

13. The operating method according to claim 1 1 , characterized in that the said essentially continuous map, look-up table or (set of) mathematical equation(s) extends to negative, i.e. braking torque levels that can be applied by the electric machine (1 ), or a parameter representative thereof, and that for a specific negative torque applied by the electric machine (1 ), the actual value of such negative torque, or a parameter representative thereof, is used as one of the said input parameters to determine the desired speed ratio value for the continuously variable transmission unit (40) as the output parameter.