Future electrified vehicles require compact electric drive units with high specific power. While many current electric drive units consist of an electric machine, inverter and transmission as independent parts, latest developments show a trend to integrated units to meet the substantial specific requirements. In this paper, FEV describes a highly integrated drive unit in coaxial arrangement in which electric machine, inverter and transmission are integrated into one compact housing. This article will focus on a variant with a peak power of 230 kW.
Since 2009, the trend of CO2 emission reduction in Germany has come to a stop at around 900 mn tonne of CO2 equivalent per year, about 20 % above the 2020 target value of the German government . To achieve the corresponding future CO2 fleet targets for the passenger car segment, high growth rates, both in vehicle electrification and in the application of renewable fuels are needed [2, 3].
Currently, vehicle electrification is a megatrend that can be observed in all markets, at varying degrees. (1) shows a prognosis for the global market in 2030. With an expected 20 mn pure-electric vehicles, this will by far be the fastest growing vehicle segment across the globe. Out of these 20 mn vehicles, the Chinese market alone will have a sales volume of close to 10 mn.
Today, most electric vehicles are derived from conventional vehicle architectures. Some models are offered with conventional, hybrid or electric powertrains. This means that the electric drive and battery systems had to be adopted to an already existing vehicle architecture. In such cases, at the cost of vehicle interior and battery space, the electric drive unit is typically placed in the former engine compartment, where available space is relatively big .
For dedicated electric vehicles, the situation is different. Here, the former compartment of the Internal Combustion Engine (ICE) can be drastically reduced or even eliminated, leaving more interior room and space for the battery at the same outer vehicle dimensions. For these new vehicle architectures, also known as skateboard chassis, the drive unit is of less importance.
A compact design with reduced height for underbody, transversal installation at the front and/or rear axle is expected. In such applications, the coaxial architecture with its small height has a key advantage. (2) shows the evolution of electric vehicle architectures.
ARCHITECTURE OF THE ELECTRIC DRIVE UNIT
The overall development targets for the new highly integrated Electric Drive Unit (EDU) can be listed as follows:
:: high compactness and low total height (for underbody applications);
:: mass production-friendly;
:: high integration level;
:: integrated cooling;
:: high power density on system level;
:: excellent NVH behaviour.
:: Based upon these global requirements, the following minimum targets were defined:
:: target vehicle segment C/D, typical mass 2,000 kg;
:: maximum axle torque 3500 Nm;
:: acceleration 0 to 100 km/h < 6 s;
:: top speed 200 km/h (axle speed 1,500 rpm);
:: continuous power 100 kW, sufficient for a cruising speed of 180 km/h with a 3 % road inclination reserve;
:: parking lock and neutral function.
During the concept evaluation process, different architectures and system arrangements were compared and rated. Decisive criteria for the choice of a coaxial solution were the low total height and the challenging NVH targets. The transmission ratio was realised with a planetary gear set integrated into the rotor of the electric machine. To meet the performance demand, a peak power (30 s) of 230 kW was necessary.
GEAR SET & AXIAL PARKING LOCK
The required number of speeds is determined by the axle torque demand and the maximum speed (spread) versus installed (peak) power of the electric motor . For passenger car applications, typically a single-speed design is sufficient. This is caused by the high continuous power demand to cover the vehicle top speed targets, which will typically result in sufficient available peak power to meet all requirements with just one gear ratio. Also in this application, with 230 kW peak power, a single-speed solution has proven to be sufficient to meet the torque and speed requirements .
The planetary-based gear set structure also provides, besides a compact arrangement, excellent prerequisites for low noise radiation. In layshaft structures, systemic torsional vibrations directly lead to excitation forces on the bearings in the housing. In the chosen coaxial gear set with a star shaped planet arrangement supported by a common carrier, these main bearing excitations are mostly eliminated. The result is lower overall excitation, thus resulting in a low overall system level NVH.
The gear set is realised with two gear tracks and without ring gears. The common planet carrier is the input connected to the electric motor. The small sun gear can be grounded to the housing to ensure torque transfer or decoupled to realise a neutral function. The large sun gear provides the output, which is directly connected to the conventional differential. The elimination of the ring gears offers advantages in manufacturing, as the ring gears are the most expensive elements of a planetary gear set. Moreover, this arrangement allows installation of the gear set below the rotor carrier. With this gear set structure, high total ratios can be realised. However, at high ratios, the structure shows high mechanical circulating power, which affects efficiency. At the chosen total ratio of about 7.2, the circulating power is considered acceptable.
The parking lock is realised in a new axial design and arranged concentrically around the small sun gear equipped with its neutral shift function. Due to their close proximity, the two systems use one electromechanical actuator. The actuator is realised with a ramp design to transfer rotary motion into axial position.
The system is designed in such a way that both safety-critical situations (Neutral and Park) are on each end of the actuator stroke. The Drive position is arranged in the middle between Neutral and Park (N-D-P arrangement). The advantage is that both safety-relevant positions can be detected easily by driving into the mechanical end stop of the actuator. The position measurement is realised by simple incremental counting inside the actuation motor.
The highest functional safety requirement on the actuator is thus lowered from the typical ASIL C to QM. In this way, cost savings in the actuator system and in development effort can be achieved. (3) shows the gear set as lever diagram and in 3D.
The parking lock and neutral shift device are equipped with an actuation sleeve that translates the rotary movement of a tooth segment, which itself is driven by the actuator motor, into an axial movement. The actuation sleeve moves the shift sleeve and the parking lock body. Compared to conventional parking lock systems, this parking lock body functionally integrates the cone and park pawl into one part. The N-D sleeve is pushed in directly by the actuator (Direction N → D), and pushed out by the load spring (D → N).
The parking lock sleeve has exactly the opposite arrangement. It is pushed in with a pre-loaded spring (D → P), but pulled out directly by the actuator without using the load spring. Both functions are realised with the same spring. (4) shows the axial parking lock.
ELECTRIC MOTOR & INVERTER
Electric motor plus inverter roughly represent two-third of the value of the entire EDU, and they also cause about 80 % of all losses in the EDU. Therefore, the development of these components is focssed on high efficiency, low current consumption, high package density and cost-effective mass production. These criteria need to be carefully balanced as some conflict with each other.
Driven by the high efficiency demand, a permanent synchronous electric motor was chosen. This electric motor type yields highest peak and real life efficiencies, especially if equipped with a neutral function to avoid high drag losses.
By integrating the gear set into the rotor of the electric motor, a high diameter to length ratio can be realised, which provides a good current to torque ratio, thus keeping the inverter cost low. The high output torque was used to lower the e-motor speed to a maximum of 10,000 rpm. This helps to reduce rotating inertia, enabling better vehicle driving dynamics and minimising the impact on vehicle control systems.
Because of the short axial length, the stator of the electric motor was realised in a segmented design. In addition to a high copper filling factor and a comparatively large effective iron length, this technology also ensures a cost-effective mass production.
The inverter is realised in multi-phase technology. The main advantage of this technology is the high system redundancy. Even with a short circuit fault, the vehicle remains drivable in a “limp home” mode. Further advantages are higher power capability and especially a reduction of rotational speed irregularities.
Good NVH behaviour is an important prerequisite for the market acceptance of EDUs. Multi-phasing systems offer significantly more possibilities in certain operation modes to influence the excitation and the NVH behaviour by controls. Together with the relatively low maximum speed of the electric motor, small rotational speed irregularities and the integrated gear set, good NVH behaviour can be achieved.
SYSTEM DESIGN & COOLING
The EDU is realised as a fully integrated unit, with one common oil circuit for the reduction gear set, the differential and the electric motor. The lubricant has to fulfil various requirements, not only with respect to cooling and lubrication. High temperature and chemical stability, low oil aeration tendency, as well as avoidance of corrosion of copper components and insulation materials are some of the additional requirements.
The cooling is divided into an outer water jacket cooling and an internal oil cooling, (5). The outer cooling is built up conventionally and is designed to absorb the continuous losses. The internal oil cooling circuit was installed to increase the peak power potential, which is essential for the behaviour of acceleration and recuperation. A typical disadvantage of such a cooling system is the high energy demand that is needed to drive the electric or mechanically driven oil pump.
To minimise these losses, a centrifugal oil cooling was installed. This system consists of a centrifugal pump that pumps the oil along the water jacket to the side of the EDU, where it is fed into the central shaft. This ensures a heat exchanger function between oil and coolant. The main advantage of the centrifugal pump is its simple and robust design, which results in low manufacturing costs as well as in a low energy demand of a maximum 20 W. All further needed energy for hydraulic flow is provided through the rotation of the rotor itself. Compared to a conventional pressurised oil cooling system, the power consumption could be reduced by approximately 50 %.
From the central shaft, oil is fed to the gear set to ensure cooling and lubrication. In the rotor, this oil is captured and fed along the sheet metal stack for rotor cooling. At each end of the rotor, oil is captured in the end plate of the rotor and then centrifuged away to cool the winding head of the fractional slot machine. Despite of the very low electric energy demand, a high cooling performance for the complete system can be achieved, because the rotor itself acts as an additional hydraulic pump. By speed control of the centrifugal pump, the oil flow can be adapted to the desired range.
The EDU is the first member of a new generation of electric drive units, which can be used for current and future electric and hybrid vehicle platforms. By smart integration of the components, a high specific power and efficiency was achieved, while at the same time optimising the acoustic behaviour.
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PETER JANSSEN M.Sc. is Leader of the Competence Center Hybrid and Electric Drives at FEV Europe GmbH in Aachen (Germany).
DR-ING. GEREON HELLENBROICH is Department Manager Transmission Design and CAE at FEV Europe GmbH in Aachen (Germany).
DIPL.-ING. HANS-PETER LAHEY is Senior Specialist for Hybrid and Electric Drives at FEV Europe GmbH in Aachen (Germany).