Electric Drive Unit (EDU)

Battery Electric Vehicle Transmission

6. November 2018 | Engineering Service

The development towards battery electric vehicles will continue steadily in the coming years. A main reason for this is the need to meet future fleet fuel consumption and emission targets. Through the progress to be expected regarding battery charging infrastructure, battery capacity, sufficient driving range, vehicle weight and cost, the obstacles which may hinder this development will be eliminated.
This article deals with transmissions within the powertrain of battery electric vehicles. While 1-speed transmissions are common and sufficient today for many vehicles, there are also applications for which the use of 2-speed transmissions is advantageous. Reasons for this will be presented. Furthermore, a modern 2-speed transmission family will be introduced which ensures both load shifting capability and sailing operation in addition to recuperation.

Requirements for EDU transmissions and development trends

The trend towards battery electric vehicle will continue or even accelerate in the future, as these concepts will make a significant contribution to meet the future targets of fleet fuel consumption and vehicle emissions. To be successful, these new concepts require modern and intelligent solutions for their powertrain and in particular for their drive units. To find the best possible solution, the characteristics of different applications such as small or bigger passenger cars, light, medium or heavy duty trucks or off-road equipment have to be considered carefully. With this multitude of possible applications, the variety of transmission concepts has increased significantly.

Many other criteria define the current development trends and influence the selection of the right concept. As an example, the number of transmission speeds has a high impact on function, complexity and therefore cost of the vehicle. As single-speed transmissions are sufficient for many vehicles in many applications, 2-speed transmissions can increase both range and top speed while maintaining good drive-away performance. However, 2- or multi-speed transmissions in battery electric vehicles require power-shift capability to realize the smooth acceleration without interruptions of the tractive force which is characteristic of electric vehicles.

The amount of gears has a direct impact on system performance and required E-Motor size, and it is thus heavily influencing total system cost. In particular, multi speed solutions are beneficial in vehicles with high wheel torque and relatively low power requirement such as delivery trucks of the 3.5 t class with emission-free operation in city centers. Such vehicles face the challenge that both sufficient launch torque and a reasonable maximum speed on the highway should be achieved. These commercial vehicles will quickly become established in vehicle fleets for the distribution of goods in cities and suburban areas. The vehicle load spectrum and the comparatively simple setup of a charging infrastructure – each day the vehicles return to a distribution center – favor the introduction of vehicles propelled exclusively by an electric machine. The same applies to small municipal vehicles such as road sweepers or forward-tipping dumpers, which are used primarily in inner-city areas.

>> TO BE SUCCESSFUL NEW CONCEPTS REQUIRE MODERN AND INTELLIGENT SOLUTIONS FOR THEIR POWERTRAIN AND IN PARTICULAR FOR THEIR DRIVE UNITS

In addition, the high market pressure forces all manufacturers to develop electric drive unit concepts already with high maturity, short time to market and development cost. Therefore, all selected main subsystems such as electric machine, inverter and, if applicable, clutch including actuation system should already have the highest possible degree of maturity.

Particularly with the widely spread permanent magnet synchronous motors, operation mode sailing should be introduced in addition to recuperation during coasting or braking. To achieve sailing operation, the electric motor will be decoupled from the driving wheels in a “neutral function”. This leads to a reduction in the drag losses of the electric motor, which are essential and disadvantageous for efficiency, especially at high vehicle speed and associated high shaft speeds.

There is a clear trend towards higher integration levels, too. The electric drive unit, consisting of inverter, electric motor, transmission and heat exchanger merge to form an assembly extending far beyond the sub frame providing advantages regarding package, weight and cost. This very elaborate complete system, tested “End-of-Line” in advance, will be supplied directly to the vehicle manufacturing plant for installation in the vehicle.

Considering these mentioned development trends, FEV has developed solutions for the drive units of battery electric vehicles. These drive units are described later in this article. The next chapter will describe when multi-speed transmission should be utilized and why.

Selection of number of transmission speeds

Fig. 1: Determination of Continuous Power Demand

Figure 1 shows the power demand of different vehicle types for constant driving at an inclination of 3 percent. Using this diagram, the required continuous power of an electric drive unit can be determined based on the top speed requirement of the vehicle. Additionally, the secondary x-axis on top allows to directly convert the vehicle speed (km/h) into wheel speed (rpm) for a given tire radius. For a typical D-class vehicle, a continuous power of 140 kW is required to reach a vehicle top speed of 220 km/h or 1.705 rpm at the wheels respectively.

Fig. 2: Determination of Launch Torque Demand Based on Acceleration

Besides the continuous power and wheel speed requirement derived from the previous figure, peak power and peak axle torque requirements can be determined using Figure 2 based on a desired vehicle acceleration from 0 to 100 km/h. The diagram is based on standardized acceleration simulations under consideration of wheel slip limits. For a typical D-class vehicle, a peak axle torque of 5.000 Nm and a peak power of 280 kW is required to achieve a 0 to 100 km/h time of 5.5 seconds.

Fig. 3: Ratio of Achievable Peak Power Versus Continuous Power

Figure 3 is based on benchmark data of various electric motors and displays the ratio of achievable peak power (for at last 30 seconds) versus continuous power. Most available motors do lie on or below the 2:1 line, while some newer designs of the 800 V class do lie above. Using this figure, it can be assessed if the defined requirements for continuous and peak power can be realistically delivered by a single electric motor. In this example, 140 kW continuous and 280 kW peak are well in reach for available motor designs. In case the requirements are outside the typical range, either the vehicle level performance requirements must be re-considered or the electric motor must be upgraded in terms of either peak or continuous power.
Using the previous figures, target values for continuous power, peak power, peak axle torque and maximum wheel speed have been defined for given vehicle performance targets.

By multiplying maximum wheel speed and required peak axle torque, a reference value called “spread” can be calculated. This reference value can be directly compared to the “spread” of electric motors which can be calculated by multiplying their peak torque and maximum speed values. This direct comparison of vehicle-level required and electric motor-level available “spread” values is possible because torque and max speed can be adjusted and traded-off using a single speed transmission with a fixed gear ratio. It is not important whether the electric motor provides a large peak torque and rather low maximum speed or vice versa, as long as the spread value matches the target value of the vehicle. The gear ratio will be used to convert the electric motor values into the values required at the wheels.

Fig. 4: Number of Transmission Speeds Based on Vehicle Performance Demands

Figure 4 does display the spread value of different electric motors over their continuous power. It is visible, that the achievable spread values do depend on the motor technology. The trend line for axial flux motors does lie significantly below the one for radial flux machines due to the limited max speed of axial flux motors. However, by introducing multi-speed transmissions, the available spread can be increased by multiplying the spread of the motor with the ratio steps of the multi-speed transmission. Taking the target values defined before, it becomes visible that the combination of 140 kW continuous power with a spread requirement of 8.525 kNm*rpm can be covered by either a radial flux machine paired with a single-speed transmission or an axial flux machine paired with a 2-speed transmission. Using this kind of diagram, the necessity of a 2-speed transmission can be easily determined based on the performance requirements of a new application.

Electric Drive Unit concept utilizing a 2-speed dual clutch transmission

Considering the described development trends FEV has realized different 2-speed concepts for EDUs (Electric Drive Units).

As a first concept, FEV has designed and built a prototype of a 2-speed dual clutch transmission, as seem in FIgure 5, with just a single pair of fixed gear wheels per sub-transmission. It utilizes mature series production components and sub-systems, including electric machine, inverter, dual clutch unit, and actuation system [3]. With this approach, the complexity and therefore the production cost could be kept strictly within limits. In addition, this “off-the-shelf” approach helps to considerably reduce development risk and time-to-market and allows to create a business case also for applications with lower volumes and more conservative launch scenarios, as no significant development effort has to be spent on complex sub-systems.

Fig. 5: Electric Drive Unit with 2-Speed Power-Shift Electric Drive Unit from FEV

Figure 5 shows a photograph of the electric drive unit in hardware and a stick diagram of the gearset. In this gearset, the first gear G1 with the driving wheel on the input shaft IS1 is assigned to clutch C1. The second gear G2 with the driving wheel on the input shaft IS2 is assigned to clutch C2. The output shaft OS carries both driven gear wheels, which are implemented as fixed wheels. If a park lock function is required, then a park lock wheel P can be added to the output shaft. Due to the implementation as a lay-shaft transmission, the transmission ratios are selectable within a wide range. In this example the ratio step was set to 1.6 in order to achieve best shift comfort and smoothness, as it is expected from a pure electric vehicle. The value is similar to the ratio step between 1st and 2nd speed of an automatic transmission for the conventional drivetrain. The selected gear ratio step of 1.45 lies below this limit value.

The maximum input torque of the transmission unit is 300 Nm, slightly lower than the theoretical short time peak torque of the electric machine. The torque-limiting component in this concept is the dry-running dual clutch as it has been carried over from existing large-series production. This dual clutch also ensures the required powershift capability of the electric drive unit. The energy entry into the clutches is lower compared to a combustion engine application, because they are primarily used as gear-shift clutches and the shifting frequency is lower. With regard to the thermal load, dry-running clutches are sufficient across all vehicle classes. Also, the drag torques are considerably lower compared to wet-running clutches. This is very important, as no gear can be disengaged in a concept with only two speeds and no gear-shift actuating elements in the sub-transmissions.

Fig. 6: Electric Drive Unit Specifications

Figure 6 shows the main specifications of the electric drive unit.
With no possibility of gear disengagement, attention must be paid to the permissible rotation speeds of the clutch discs. Also, the input speed capability must be matched to the maximum speed of the intended electric machine. Available dual clutch systems have mostly been designed for combustion engine powertrains and are typically not designed for the input speeds of power-dense, high-spinning electric machines. One exception is the P400 S motor from company YASA [1], which is based on axial flux technology and which, both in terms of speed range and form factor, is a perfect match to the chosen dual dry clutch system. The main technical data of the machine is shown in Figure 7.

Fig. 7: YASA P400 S Electric Machine [1]

With a battery voltage of 400 V, the output power of the YASA P400 S electric machine is 90 kW peak and 70 kW continuous.
The inverter belongs to the latest-generation from company SEVCON [2]. The AC interface has been slightly modified in order to establish a short, direct (plug-)connection between the inverter and the electric machine, thus avoiding any separate phase cables. The inverter is capable of being the master control unit for the entire drive system and can also integrate and execute 3rd party code, e.g. the shift control software of a 2-speed transmission. It communicates with the actuation motors through a local CAN bus. In this way, no additional control unit is required in the system Figure 8 contains additional information about the inverter.

Fig. 8: Latest Generation: SEVCON Gen5 Size9 Inverter [2]

Electric Drive Unit concept utilizing a Ravigneaux Planetary Gearset

Fig. 9: FEV High-Performance 2-Speed EDU

In order to be able to cover the demands of high performance applications as well, a second powershift capable 2-speed concept was developed [5]. Figure 9 shows different views of the drive unit. To deal with higher electric machine speeds and higher torque requirements, a Ravigneaux planetary gearset (Figure 10) is used instead of the dual clutch transmission architecture.

Fig. 10: 2-Speed Concept on the Basis of a Ravigneaux Set

In conjunction with this simple, combined planetary gearset, two brakes B1 and B2 are sufficient to realize two speeds. The small sun gear serves as the input. The power is output via the ring gear. The planetary gear carrier is fixed by brake B1 or alternatively the large sun is fixed by brake B2. With this gear set concept, the differential speeds at open gear-shift elements can be reduced thus significantly decreasing clutch drag losses. In addition, the thermal capacity of brakes can be scaled via the thickness of their (stationary) steel lamellae without negatively affecting rotary mass moments of inertia. As opposed to clutches, brakes avoid the use of rotary joints or engagement bearings to actuate the gear-shift elements and are thus significantly cheaper. The exclusive use of brakes was therefore an important criterion in the selection of the concept. The combination of brake B1 with a one-way clutch enables the brake itself to be designed smaller, thus further reducing drag losses.
Both brakes are actuated via an existing series-production, on-demand actuator from LuK. The unit, also known as HCA (hydrostatic clutch actuator [4]), operates with a brushless electric motor for each gear-shift element, which actuates a hydraulic master piston via a spindle. Because of the leakage-free seals this system is very efficient. Alternatively, electromechanical actuation concepts could be used thanks to the good axial accessibility of the brakes.
The electric motor and the inverter form a compact unit and are bolted to the transmission. Both subsystems – inverter and electric motor – are very well advanced in their series development. Therefore, they enable a short development period for the entire drive unit and thus rapid market introduction. Oil cooling is required due to the special design of the electric motor as an axial flux machine. Using a dedicated EDU fluid, the motor, inverter and transmission part use a common oil system. An electric oil pump draws oil out of the transmission sump and feeds it via an oil/water heat exchanger to the inverter. From there the oil flows through the electric motor and subsequently back into the transmission, where the volumetric flow is divided. One part is fed into the main shaft of the gear wheel set, from where it not only lubricates the wheel set, but also cools the brakes as required. The remainder is not drained into the sump, but buffered in a storage tank inside the transmission. From here, further components are lubricated via various channels, including the gear meshes and the bearings of the intermediate shaft. An intelligent oil pump control strategy allows the level of the storage tank and thus the oil level in the transmission to be varied, which makes a large contribution to a reduction in churning losses and thus an increase in efficiency.

Transmission family and construction kit

For economic and many other practical reason, it make sense to introduce a transmission family for battery electric vehicles, too. Such families with two to three transmissions are already realized in conventional, transversely-mounted internal combustion engine drivetrains covering the required torque range up to 600 Nm input torque. However, these transmissions sometimes contain different base technologies, because the characteristics of some technologies are not suitable for all vehicle classes. For instance, dry-running clutches as drive-away elements in automatic transmissions are more suitable for lighter vehicles due to their lower thermal capacity, whereas wet-running clutches are more frequently used for heavy vehicles.

Fig. 11: Internal Views of the 2-Speed Power-Shift Transmission

FEV proposes a transmission family or construction kit with a low variety of components that covers different variants, characterized by the number of speeds, the implementation of the parking lock, the implementation of the differential gear and power-shift capability.
In conjunction with the YASA P400 electric motor and its maximum speed of 8,000 rpm, the first transmission concept with power-shift capability and a neutral function covers the requirements of powertrains with lower speed of the electric motor and torques up to 300Nm. However, the transmission is not suitable for higher-speed electric motors on account of the burst speeds of dry-running friction discs. In addition, users would like a solution for higher torques exceeding 300 Nm. For these two reasons, motor speed strength and torque capacity, FEV decided to develop the second 2-speed concept for the new transmission construction kit.
This construction kit consists in total of four derivatives: two torque classes with maximum electric motor torques of 300 Nm and 600 Nm, each with one or two speeds. The 2-speed variants are based on the described EDU concept with a Ravigneaux gear set. By simplifying this design the two variants with only one speed are realized focusing primarily on low cost. For example a neutral function to enable sailing is omitted. Figure 12 briefly describes the structure of the construction kit.

Fig. 12: Concept of a Transmission Construction Kit

If necessary inside the transmission, the parking lock is realized independently of the torque class. It is implemented inexpensively and identically with a large locking wheel for all applications. The parking lock is actuated electromechanically as “park-by-wire” system which are becoming the standard and allow greater flexibility in the design of the man-machine interface in comparison with the purely mechanical system.
Thanks to the compact unit comprising electric motor and inverter as well as the transmission architecture tailor-made for it, electric drive units with a high power density can be realized within the construction kit presented. This amounts to an outstanding 0.6 kg/kW continuous for the most powerful 2-speed variants. The single-speed variants with their simpler structure are even better. At the same time the use of proven standard components, especially in the actuation and cooling areas, enables short development times.

Outlook and summary

In comparison with internal combustion engine drivetrains, purely electrically propelled vehicles require more simple transmissions. Product complexity will decrease. The requirements for individual attributes, in particular the transmission acoustics and high input speeds, are increasing noticeably.
Single-speed transmissions will be adequate for most electric vehicles. Only the need for a particularly high launch torque or a higher top speed justifies the investment in a 2-speed transmission. Out of today’s view, 3-speed transmissions will be in niche applications, such as sports cars.
A 2-speed transmission needs to have power-shift capability, because an interruption in the traction force when shifting gear in electrically powered vehicles will not be accepted by customers. When investing in a 2-speed transmission, the neutral function – where the electric motor is decoupled from the wheels – must be planned for right from the start. The proposal presented in this article satisfies both requirements.
FEV has been able to combine available, mature components such as electric machine, inverter and dual clutch unit with a newly developed 2-speed transmission to create an electric drive unit which is suited for different kinds of vehicles. It can easily be integrated into both existing and new vehicle platforms and enables manufactures to enter the market of pure electric vehicles quickly. Compared to 1-speed transmissions, this solution is superior in both performance and system efficiency. Together with the YASA P400 S axial flux electric machine [1] and the inverter from SEVCON [2], the concept forms a very short and compact drive unit.
With the proposed concepts, four derivatives can be realized which are sufficient for the vehicle volume segment of the transverse-mounted drive such as single-speed transmission up to 300 Nm, single-speed transmission up to 600 Nm, 2-speed transmission up to 300 Nm and 2-speed transmission up to 600 Nm.
These four derivatives are also suitable for ERAD systems (Electric Rear Axle Drive) of P4 hybrid vehicles. The volume per derivative can thus be increased further.
A parking lock system, if integrated in the transmission, must be “park-by-wire”-capable. The implementation of the parking lock is not influenced by different vehicle torques or performances. A uniform design for all four derivatives is sufficient.

REFERENCES:
[1] YASA-P400 Series Product Information, YASA Motors Limited, Abington, UK
[2] SEVCON Gen4 Size 10 AC Motor Controller Product Information, Tyne and Wear, UK
[3] G. Hellenbroich, P. Janssen, H.-P. Lahey, I. Steinberg, Integrated Electric Drive Units including up-to 2 Speeds, Aachen Colloquium China, Beijing, 2017
[4] 10th Schaeffler Colloquium 2014, Solving the Powertrain Puzzle, Transmission Actuators
[5] I. Steinberg, G. Hellenbroich, J. Nowack, Efficient transmission kit for battery electric vehicles – Trends and solutions International Vienna Motor Symposium, Vienna, 2018

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