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THE ELECTRIC POWERTRAIN MATRIX FROM VOLKSWAGEN Volkswagen has developed a modular matrix with components that enable different electrified powertrains for hybrid and electric vehicles to be built. The matrix is being used for the first time in the new, all-electric cars from Volkswagen, the e-up! and e-Golf.
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MOTIVATION
AUTHORS
and electric vehicles without or with very little modification. Torque and power can be scaled by varying the length of the active components of the electric motor, while adapting the number of windings and phase currents as well as the current-carrying capacity of the power electronics accordingly. With regard to the design of the electric motor and power electronics, a differentiation is made between base modules and project-specific components, ❷. In the electric motor, the base modules include the windings, the laminations, the insulation, the rotor concept and the rotor position sensor. In the power electronics, they include the power modules, the driver board, the control circuit board, the DC/DC converter and the capacitors. For application in a vehicle, the motor housing, the cooling interface, the highvoltage (HV) and low-voltage (LV) interfaces and the EMC components are adapted as necessary in accordance with the project requirements. Important mechanical assemblies in the transmission, such as the gearbox housing, bearing and lubrication concepts, remain the same, as does the parking lock. Modularisation can reduce the complexity of components and achieve a substantial reduction in development time and effort, thus significantly cutting costs. On the basis of this modular strategy, a comprehensive matrix of components is derived that enables a variable degree of electrification to be applied across all vehicle classes.
Reducing fuel consumption and CO2 emissions is a key objective in the development of new vehicle powertrains. The ongoing development of internal combustion engines and transmissions will offer further potential for improving fuel efficiency in the future, while the electrification of powertrain systems in hybrid and fully electric vehicles will enable additional significant reductions in CO2 emissions to be achieved, ❶. The Volkswagen e-up! was launched in autumn 2013 as the first fully electric series-production vehicle on the market, and it will be followed by the e-Golf in spring 2014. These vehicles feature electric powertrains that are developed and produced by Volkswagen [1, 2, 3], and they are described in more detail in t he following report.
DIPL.-ING. HANNO JELDEN is Head of the Main Department for Powertrain Electronics, Powertrain Development for Volkswagen AG in Wolfsburg (Germany).
DIPL.-ING. PETER LÜCK is Head of the Department for HEV Components, Powertrain Development for Volkswagen AG in Wolfsburg (Germany).
MODULAR MATRIX FOR ELECTRIC POWERTRAINS
Customer requirements concerning electrified and conventional vehicles are basically comparable, and when it comes to lifetime, reliability and safety, they are identical. What is more, customers also expect a Volkswagen to be fun to drive and comfortable and to offer good acoustic quality. In order to be successful, new drive systems will need to be economically attractive and deliver low energy consumption, or they must offer a high potential for substituting conventional fuels by the use of electric power. Volkswagen develops powertrain components as modules based on a matrix approach, thus making it possible to apply the components to different hybrid
DIPL.-ING. GEORG KRUSE is Technical Project Manager for Electric Vehicle Projects, Powertrain Development for Volkswagen AG in Wolfsburg (Germany).
DIPL.-ING. JONAS TOUSEN is responsible for E-Motors for Electric Vehicles, Powertrain Development for Volkswagen AG in Wolfsburg (Germany).
ELECTRIC MOTOR
The electric motors for the Volkswagen e-up! and the e-Golf are permanent-mag-
CO2 reduction potential
+
+ Micro hybrid Mild hybrid
+
Full hybrid (HEV)
Touareg Hybrid rid (2010)
❶ Degree of electrification and CO2 reduction potential of electric powertrains
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+
Plug-in hybrid (PHEV)
XL1 (2013) Jetta Hybrid (2012) Degree of electrification
Golf Plug-in Hybrid (2014)
+ Battery electric vehicle (BEV)
e-up! (2013) e-Golf (2013/2014) Fully electric operation
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Base modules: electric motor
Base modules: power electronics
: Winding
: Power modules
: Laminations
: Driver board
: Insulation
: Control circuit board
: Rotor concept
: DC/DC converter
: Position sensor
: Capacitor
Project-specific: : Housing; package : Electro magnetic compatibility (EMC) : Cooling interface : HV/LV interface; HV/LV cables
net three-phase synchronous motors with five pairs of poles and a maximum drive speed of 12,000 rpm. They consist of a number of main assemblies: the motor housing, the stator, the rotor and the low-voltage module, ❸. The motors are produced at Volkswagen’s Kassel plant. The stator contains the winding with its three-phase connections, while the rotor has an internal solid-pole design with permanent magnets made from a Neodymium alloy. The stator and rotor are installed in a cast motor housing. The output end (A-side) has a caston bearing plate while the opposite end (B-side) has a bolted-on bearing plate. The B-side also houses the cable entries and terminal block for the three-phase
❷ Matrix – modularisation of the electric powertrain components
connection, the rotor position sensor, which consists of a signal output module, evaluating electronics and a sensor disc, and the signal plug. The signal plug transmits signals from the rotor position sensor, the winding temperature sensor and the pilot line safety contact to the power electronics. MOTOR HOUSING
The motor housing is made from an aluminium alloy and is produced using the chill casting method. It contains a cast-in cooling jacket with a special honeycomb structure for the liquid coolant that surrounds the stator seat, ❹. This ensures an extremely uniform flow pattern with
a large wetted surface area, resulting in a favourable relationship between heat flux density and pressure loss. The cooling system is connected by threaded stub pipes and flexible hoses. The complete stator is shrink-fitted into the motor housing from the B-side. Special attention was paid to the design of the input housing, and its topology was optimised, taking its deformation properties into account. The structure of the two-piece housing has been made particularly rigid by integrating part of the transmission into the electric motor housing and by additional targeted reinforcement of the material, for example by using circumferential acoustic fins, thus enabling structure-borne noise to be sig-
Low-voltage module incl. rotor position sensor
Stator
Rotor Motor housing
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❸ The main assemblies of the electric motor
❹ Motor housing (left) with integrated cooling structure (right) for the electric motor
❺ Stator (left) and rotor (right) of the electric motor
nificantly reduced. In order to minimise excitation and improve acoustic behaviour, the design and production methods of the shaft bearings and gear teeth were further optimised.
the electric-power UV process. The finished stator is subjected to automated test routines and is also attached automatically to the motor housing by a shrink-fit process.
STATOR
ROTOR
The stator principally consists of the lamination stack and the three-phase winding. The lamination stack is made up of separate, coated sheet-metal stampings with an outside diameter of 220 mm, ❺ and ❻. The sheet metal used to produce the laminations has high magnetic conductivity and an electrically insulating coating on both sides. The complete stack of stator laminations consists of five sub-stacks that are offset during assembly. This minimises the effect of the direction in which the sheet metal was rolled on the homogeneity of the rotating magnetic field. During stamping, the laminations are provided with pre-shaped lugs that engage when the laminations are assembled into stacks. This stamping and assembly process prevents the radial displacement of single laminations. The three-phase winding requires a total of 15 coils, each filling four of the total of 60 grooves in the stator. The coils are wound and pulled into the stator grooves using an automatic process in a special production device. The stator produced in this way is provided with a recess in the winding head for the temperature sensor. For additional insulation, improved heat transfer and strength, the stator is immersed in a bath and impregnated with resin using
The rotor consists of the rotor shaft, the stack of laminations with embedded permanent magnets, the balancing discs and the rotor position-sensing wheel. The rotor lamination stack is made up of six different, coated sub-stacks. The ends of the rotor are closed off by the balancing discs and are held together by five clamping bolts that pass through the laminations. The rotor lamination stack with the embedded magnets is pre-assembled by an automated process and is joined to the rotor shaft using a shrink-fit process, ⑤.
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The rotor shaft is hollow and is assembled by pressing three parts together. It has longitudinal internal splines to connect it to the gearbox input shaft. Both shafts have a triple bearing arrangement with friction-optimised, grooved ball bearings. This layout further reduces mechanical losses. LOW-VOLTAGE MODULE
To enable the electric motor to be controlled by the power electronics, the position of the rotor must be detected to allow the three-phase winding in the stator to be switched accordingly. For this purpose, a rotor position sensor consisting of evaluating electronics and a sensing wheel, ❼, is attached to the B-side of the electric motor. The low-voltage module additionally acts as a support for the safety contact of the pilot line in the bearing plate cover and as a cable duct. It holds and locates the wires for the safety contact, the rotor position sensor evaluating unit and the temperature sensor, as well as supplying their signals to the signal plug and making contact with the wire screens through a clip with a screw on the surrounding motor housing. POWER ELECTRONICS
The power electronics are connected to the electric motor via the three-phase cables and with the high-voltage battery via two traction lines. The maximum permissible range for the DC voltage is 250 to 430 V. The e-up! uses 296 to 418 V and the e-Golf 255 to 360 V, depending
Circumferential winding head with interconnected coils W V
Three-phase connection
U Winding bandage Phase insulation Groove insulation Coil heads Coated lamination package with grooves Star point (inside the insulation hose) Motor housing
❻ Stator mounted in the motor housing
Recess for temperature sensor
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on the battery voltage. During motor operation, the power electronics use high-performance transistors to convert direct current into three-phase alternating current with variable frequency and amplitude. In generator mode, the alternating current is converted into direct current to charge the high-voltage traction battery. The maximum phase current of the power electronics is 450 A. In the e-up!, this is limited to 380 A and in the e-Golf to 430 A. The DC/DC converter for the isolated supply of the 12 V vehicle electric system with up to 170 A is integrated into the power electronics. The key technical data of the electric drive systems for the e-up! and e-Golf are summarised in, ❽. POWERTRAIN CHARACTERISTICS
The first electric powertrain to be produced in-house by Volkswagen was configured specifically to meet the requirements of a series-production electric vehicle. The transmission has one fixed gear ratio and, in addition to the differential, also incorporates the mechanical parking lock. The overall gear ratio is achieved in two stages via an intermediate shaft with a spur-gear drive. For the e-up!, an overall gear ratio of 8.16 was chosen in the interests of the car’s acoustic characteristics. As a result, an electric motor speed of only approximately 10,000 rpm is required for the car to reach its top speed of 130 km/h. This arrangement also reduces the superimposed excitation of the natural frequencies of the electric motor and the transmission, thus further optimising the acoustic characteristics of the powertrain. For the drivetrain of the e-Golf, the higher mass of the electric motor, the different number of gear teeth and therefore the different natural frequencies made it possible to use a maximum speed of 12,000 rpm for the electric motor while still maintaining equally good acoustics. With its overall gear ratio of 9.76, the e-Golf reaches a top speed of 140 km/h. In order to minimise acoustic excitation, a three-point bearing was chosen for the rotor and transmission drive shafts. This rules out the possibility of mechanical tension occurring in the installation of the shafts. To reduce acoustic effects generated by eccentricity of the spline between the rotor shaft and the transmission drive shaft, the
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❼ Rotor position sensor, low-voltage module and sensor wheel Cable duct with safety contact
Sensor wheel with five-fold symmetrical sine contour
Signal plug Cable duct for plug-in temperature sensor Senor with integrated evaluation electronics
assembly tolerances were reduced, which meant that all bearing seats and the stator volume were finished in an assembled condition in a single set-up. Special attention was also paid to the gearing. Gear meshing was optimised and gearing quality was improved accordingly. DRIVING ENJOYMENT AND EFFICIENCY
In motor mode, the electric motor in the Volkswagen e-up! develops a maximum torque of 210 Nm and a maximum power output of 60 kW, while the e-Golf develops 270 Nm and 85 kW. Compared to conventional powertrains, acceleration is impressive, as the car’s superior pullaway performance is already available from a motor speed of 0 rpm due to the high torque of the electric motor. A high level of reluctance torque ensures that maximum power can be
maintained from rated speed up to maximum speed. The drive system has also been configured to provide its full performance across the entire voltage range, even if the traction battery has a low state of charge. This ensures reproducible driving characteristics, for example for overtaking manoeuvres on the open road. The drive system layout was derived from a detailed evaluation of the energy flows in the electric motor map for various driving cycles. As an example, ❾ shows the results for the so-called Braunschweig cycle. Compared with other electric motors with a comparable power output, the phase currents at the same axle torque are around 10 % lower, which in turn also reduces the ohmic losses accordingly. The numbers of pole pairs were already selected during the magnetic circuit design process to ensure that the cars have an efficiency advantage in inner-city driving compared
Technical data
e-up!
e-Golf
Electric motor Permanent-magnet synchronous motor Power electronics
Output [kW]
60
85
Torque [Nm]
210
270
10,000
12,000
8.16
9.76
Voltage range [V]
296-418
255-360
Max. current [A]
380 (450)
430 (450)
Max. speed [rpm] Transmission Gearbox
Two-stage spur-gear drive Gear ratio Power electronics
Electric motor
❽ Technical data of the electric powertrain for the VW e-up! and VW e-Golf
Frequency [kHz]
9 or 10
60 kW 200
Op Operating points in the BS cycle: IIncreasing frequency
Torque [Nm]
150
100
50
0
0
2000
4000
6000
8000
10,000
Efficiency [%] 94 93 92 91 90 88 86 84 82 80 75 70 60
12,000
Rotational speed [rpm]
❾ Operating points in the Braunschweig (BS) cycle in the efficiency map of the Volkswagen e-up! electric powertrain
to other configurations. The efficiency figures lie in a broad, customer-relevant map area significantly above 90 %. This and other efficiency measures on the vehicle enable the cars to deliver attractive performance in customer operation and to offer a high degree of driving enjoyment coupled with a low energy consumption that is groundbreaking for the entire market. In competition with the electric vehicles available on the market, the e-up!, with its energy consumption of 11.7 kWh/100 km, is the efficiency world champion.
ditions, snow, rippled surfaces, cobblestones). An anti-bucking control model in the power electronics evaluates the speed of the electric motor and the signals from the ESP control unit to calculate a corresponding damping torque that is superimposed on the set-point torque value. The anti-bucking function, which is an innovative feature for electric vehicles, successfully suppresses drivetrain vibrations, thus preventing oscillation of the drivetrain. This results in reduced torsional loads on the drivetrain and therefore to a significant increase in driving comfort.
POWERTRAIN CONTROL
The powertrain control system in the Volkswagen e-up! and e-Golf also has a modular design and was developed on the basis of standardised function packets which are used throughout the entire Volkswagen Group. The control system takes into account all torque demands made by the driver, the transmission control unit and the operating strategy, including energy recuperation. Furthermore, it also coordinates torque input from the vehicle assistance systems and the brake control system. The scalability of the software enables it to be used in all new powertrains. This means that both conventional vehicles and those with different levels of electrification can be operated on the same basis by using the appropriate modules. A specially developed anti-bucking control system absorbs possible drivetrain vibrations, especially those that can occur with high wheel torque and unfavourable road surface conditions that offer low friction (e.g. wet road con02I2014
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If Eco or Eco+ mode is selected, the driver can use the kick-down function of the drive pedal to reactivate maximum drive performance at any time, for example for overtaking. The driver also has the option of using the selector lever to set the recuperation torque in four stages when the foot is not on the drive pedal. This allows the deceleration effect to be adapted to match the individual driving style.
DISPLAY AND OPERATING CONCEPT
The display and operating concept in the Volkswagen e-up! is intuitive to use. Unlike in the current conventionally powered up!, the tachometer is replaced by a power meter, and instead of showing the fuel level, the display shows the state of charge of the high-voltage battery. Furthermore, the navigation system features an energy flow display with trip data as well as a user interface for the charging functions. The driver has the option of selecting individual settings via the user interface. For example, a driving profile can be selected to influence the vehicle’s energy consumption. The following modes can be set: : Normal: maximum drive performance, consumption-optimised operation of the air-conditioning/heating system : Eco: reduced drive performance, reduced climate control performance : Eco+: significantly reduced drive performance, air conditioning/heater deactivated.
SUMMARY
The electric powertrains developed by Volkswagen consist of a highly efficient permanent-magnet synchronous motor, a friction-optimised single-speed transmission, highly compact power electronics and an innovative drivetrain control system. In combination with a lithiumion battery, the drive system in the new Volkswagen e-up! provides an electric driving range of 160 km, while the e-Golf has an electric range of 190 km. The vehicles also exhibit dynamic and reproducible driving performance. The Volkswagen e-up! accelerates from 0 to 100 km/h in 12.4 s and achieves a top speed of 130 km/h, while the e-Golf accelerates from 0 to 100 km/h in less than 11 s and reaches a top speed of 140 km/h. The drive systems in the Volkswagen e-up! and e-Golf are part of a modular matrix, the components of which enable different levels of electrification for hybrid and electric vehicles to be applied. The control software for the new powertrains is also derived from a modular approach. The matrix for electrified powertrains represents the systematic continuation of the modularisation approach for new vehicles from Volkswagen. It offers a significant reduction in effort and costs, thus making it a key prerequisite for increasing the market penetration of hybrid and electric vehicles. REFERENCES [1] Tousen, J.; Jelden, H.; Lück, P.; Alonso, G.; Kruse, G.: The Modular Electric Drive of the Volkswagen e-up! 22 nd Aachen Colloquium Automobile and Engine Technology, 2013 [2] Zillmer, M.; Neußer, H.-J.; Jelden, H.; Lück, P.; Kruse, G.: The Electric Drive of the Volkswagen e-up! – a step towards modular electrification of the powertrain. 34th International Vienna Motor Symposium, 2013 [3] Hadler, J.; Neußer, H.-J.; Jelden, H.; Lück, P.; Tousen, J.: Golf Blue-e-Motion – The Electric Volks wagen. 33 rd International Vienna Motor Symposium, 2012
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