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© Mahle
Vehicle Concept for the Urban Mobility of Tomorrow The city of the future will be a living space characterized by environmentally-conscious design, and sustainability will play an even greater role than it already does today. This has serious consequences for individual transportation. As a result, compact, maneuverable, highly efficient, and affordable electric vehicles will be in demand. These vehicles will be designed to meet the specific requirements of megacities. Therefore, Mahle has developed an innovative vehicle concept for the urban mobility of tomorrow: the Meet. 18
COST-OPTIMIZED VEHICLE CONCEPT
The vehicle concept named Meet (Mahle efficient electric transport) is designed as a two-seater city car with an electric drive. It is fun to drive, very comfortable, and highly energy-efficient, all at a low cost. The holistic development approach is built on Mahle’s many years of experiexperi ence both in detailed technical solutions and in overall systems, ranging from the powertrain and thermal management to user interface concepts. The objective for implementing the Meet was to interlink the individual technologies and functions in such a way that they work together optimally as a system to generate added value for the customer. From a technical perspective, efficiency is of key imporimpor tance for an electric vehicle, as in the overall system it determines the cruising range of the vehicle and, respectively, the battery size and therefore its cost. In order to be accepted by cuscus tomers as a full-fledged means of transport, howhow ever, an electric vehicle must also be fun to drive by offering rearea sonable agility thanks to good acceleration and a user-appropriate maximum speed, as well as a high level of maneuverability and modern comfort. The technology package for implementing these somesome times contradictory development targets for the Meet consists, among other things, of a drive modmod ule that includes power electronics and an on-board charging system, comfort zone temperature controls with heatable surfaces, a thermoelectric heat pump, and an innovative HMI user interface concept, FIGURE 1. 1. The high efficiency levlev els of the drive and thermal management system give the vehicle a long cruising range, even when the interior is temperature controlled in the summer or winter. In order to transform these requirements into a concrete specification for the electric powertrain, Mahle performed a comprehensive series of experiments, including test drives in a typical city driving cycle, known as the Mahle Stuttgart cycle, FIGURE 2 (left) [1]. The ATZ worldwide
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measurement data obtained were used as inputs for further analyses in a complete vehicle simulation model that reproduced the specific Meet properties. The power requirement determined in this way for the city vehicle concept is shown in FIGURE 2 (right). This diagram shows the time-based frequency distribution of the torque requirement over vehicle speed. This can be used to derive the general requirements for the drive. Within the city driving cycle, two operating ranges dominate. One is the stopstart and coasting mode at lower speeds, and the other is the main driving range with speeds between 40 and 50 km/h. As the experiments showed, a mechanical power output of 20 kW is sufficient to cover all operating points in both driving scenarios. Mahle designed the drive as a modular unit operated at a low voltage of 48 V. This has a cost advantage of up to 25 % compared with high-voltage concepts because it eliminates the need for the extensive safeguards against electrical hazards that are indispensable and required at higher voltages. This voltage level also allows the drive module to be used for multiple purposes, such as for purely electric cars, but also as an electric drive component in hybrid systems for larger vehicles. In coming years, hybrid concepts with 48-V operating voltage will increasingly go into series production, meaning that greater market penetration and the associated high production volumes can be expected. The resulting economies of scale in production will reduce costs for components of the hybrid system and thus for the Mahle 48-V drive module as well.
AUTHORS
Dr. Otmar Scharrer is Vice President Corporate Research and Advanced Engineering at Mahle International GmbH in Stuttgart (Germany).
Dr. Marco Warth is Director Corporate Advanced Engineering Mechatronics at Mahle International GmbH in Stuttgart (Germany).
Dr. Achim Wiebelt is Director Corporate Advanced Engineering Thermal Management at Mahle International GmbH in Stuttgart (Germany).
Daniel Rieger is Project Manager in Corporate Research and Advanced Engineering Mechatronics at Mahle International GmbH in Stuttgart (Germany).
POWERTRAIN DESIGN
The electric powertrain of the Meet, called the 48 V Twin Power, consists of two individual drive modules located at the rear axle of the vehicle, FIGURE 3 [2]. Each motor has a peak power output of 30 kW, which is available for up to 1 min, and reliably cover the requirement for a continuous power output of 20 kW. Each unit contains one electric motor and the power electronics. Due to its modular design, this arrangement provides great flexibility in positioning and can be used in a variety of vehicle concepts. No differential is required, as each of the electric drive units powers one wheel. Using software
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FIGURE 1 Mahle technology package for the Meet (© Mahle)
functions developed specifically for the Meet, the different speeds and power output levels can now be controlled separately for each driven wheel. The electric motor is a Permanent Magnet Synchronous Machine (PMSM). Because the high output power at low supply voltage produces a high reluctance torque during operation, Mahle has integrated a specially tuned rotor design in the motor system. High agility in city traffic, a small turning radius, and easy maneuverability even in tight parking spaces are core features of an urban vehicle concept.
Mahle will therefore implement a function called torque vectoring in the Meet’s 48 V Twin Power powertrain. With targeted distribution of the drive torque to the driven wheels, a steering boost effect is generated that actively influences the yaw angle of the vehicle. The central axle drive has a transmission ratio of 11.0. This ensures sufficient acceleration even on a 30-% grade with a loaded vehicle. With the definition of additional vehicle parameters, TABLE 1, a complete vehicle simulation, including energy consumption in various driving cycles, was carried out. One important result was the
FIGURE 2 Mahle Stuttgart cycle (left) and power requirement of the Meet city vehicle concept (right) (© Mahle)
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electric cruising range of the Meet. The maximum vehicle speed is defined as 100 km/h. This is due to the requirement to make short freeway trips such as from the inner city to the airport. The results of the full vehicle simulation for various driving cycles are shown in FIGURE 4. For cycles that normally address higher speeds (NEDC, WLTC, and RDE), the maximum speed was limited to 100 km/h. Due to the high overall efficiency of the Meet, its energy consumption is very low, at 7.9 kWh/100 km. This means that a small and therefore inexpensive battery with 15.4 kWh capacity is
sufficient for an adequate cruising range of 194 km in city mode (Stuttgart cycle). Assuming an average commute of 20 km per day, the Meet would therefore need to be connected to the power grid for a recharge only every seven days. The driving dynamics simulation for the Meet shows that it is also fun to drive: From a standing start, it accelerates to 16 km/h in 1 s, reaches 50 km/h in just 3 s, and hits 70 km/h after just 5 s. THERMAL MANAGEMENT
The energy requirements of engine accessories are known to have a significant influence on the cruising range of electric vehicles that can be achieved under real operating conditions. One of the main factors is the air conditioning in the vehicle, that is, cooling the interior in the summer and warming it in the winter. Experience from the field, along with test and simulation data, indicates that the cruising range at ambient winter temperatures can drop by 30 to 50 % relative to nominal values [3]. The reason for this is that, while a conventional combustion engine produces sufficient waste heat to heat the interior, the energy losses of the electric motor are not high enough for cabin heating. The thermal energy must be produced by additional components, such as Positive Temperature Coefficient (PTC) heating elements, which drain the battery. Especially in city cars, this has a disproportionately high impact on the cruising range. This is because the energy required for actual driving is relatively low due to the low vehicle mass and low
FIGURE 3 The 48 V Twin Power drive of the Meet (© Mahle)
Property
Value
Frontal area
2.4 m²
Drag coefficient
0.35
Usable battery capacity
15.4 kWh
Maximum speed
100 km/h
TABLE 1 Meet technical data (© Mahle)
average driving speeds in the city. At the same time, however, the energy required for heating the interior remains at a high level, as this function does not depend on the driving profile. Therefore, for a vehicle like the Meet in typical urban traffic, simulated in this case using the Stuttgart city driving cycle with an average speed of 21 km/h, the energy required for heating via a PTC system is greater than that for the driving operation.
To minimize the energy consumption for heating, Mahle has developed an innovative air conditioning concept that provides maximum efficiency without sacrificing comfort. With previous heating concepts, the air flow was heated up and transferred to the interior, and this was supplemented in places by optional seat and steering wheel heaters. This is too inefficient for an electric vehicle, because these various heating methods
FIGURE 4 Results of full vehicle simulation (© Mahle) ATZ worldwide
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FIGURE 5 Individually adjustable, locally effective surface heaters (left) and thermoelectric heat pump (right) (© Mahle)
are not coordinated with each other, neither on the energy side nor with respect to passenger comfort. In contrast, the new approach is based on two systems that complement each other. The energy required for the passengers’ thermal comfort is minimized by using local surface heaters, FIGURE 5. Special heating elements, in the form of thin, flexible foils, are built into the instrument panel, side trim panels, and armrests. Their radiated heat can be controlled flexibly and as required – for example, taking into consideration the number of passengers and their seating positions. They also respond very quickly, which is especially important for short trips where little time is spent in the vehicle. Thanks to direct contact with the heat source, passengers experience a comfortable temperature shortly after activation, with low energy consumption.
This concept does not entirely eliminate the need for the familiar interior heating systems that heat the cabin air, but the proportion of total heating power that they provide is significantly reduced. As always, a warm airflow is still required, for example, to keep the windows fog-free and to ensure the necessary air circulation in the cabin. The energy required to heat the already low interior airflow has been minimized by Mahle for the Meet concept. A thermoelectric coolant-to-coolant heat pump, FIGURE 5, makes the waste heat from the electric motor, the power electronics and the battery available to the heating system. In comparison with refrigerant compression heat pumps, the thermoelectric unit has several advantages. On the one hand, it is a wear-free component with no moving parts; on the other hand, its average Coefficient of Perfor-
mance (COP) is relatively high, with a value of 2. In city traffic situations, the availability of waste heat from the electric drive components is very limited, typically just a few 100 W. This means that an additional heater is necessary to cover the remaining heating demand. For compressor heat pump systems, a separate PTC element is typically used, which further increases the complexity of the heating system. In contrast, the thermoelectric heat pump does not need additional components as it can generate the necessary heat internally, while the waste heat from the powertrain is still pumped to the required temperature level. In certain operating situations, the thermal inertia of the vehicle battery can also be used as a heat storage source for the thermoelectric heat pump. In combination with the air conditioning system, which uses the climate-neu-
FIGURE 6 Substantial increase in cruising range with vehicle air conditioning thanks to innovative thermal management (© Mahle)
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tral, natural refrigerant CO2 (R744), Mahle has implemented a highly efficient heat management and passenger comfort system in the Meet thanks to intelligent, new technologies. The resulting advantage for the battery cruising range of the Meet in comparison with the use of a PTC heater adds up to 30 % with an ambient temperature of 0 °C in the WLTC and to more than 50 % in the urban Stuttgart cycle, FIGURE 6. INTUITIVE USER INTERFACE PHILOSOPHY
Mahle has created an intuitive and easy user interface philosophy to ensure that the driver is not distracted while driving but can still use all the functionality safely, such as operating the navigation system, music, or air conditioning. The concept enables a contact-free, gesture-based control, personalized comfort settings, and the preconditioning of the interior, for example via a smartphone app while the traction
battery is charging. The touch surfaces provide haptic feedback by slightly vibrating at the fingertip when actuated. SUMMARY AND OUTLOOK
With the Meet, Mahle has demonstrated the challenges as well as the potential benefits of the technical implementation of a future urban electric mobility concept. Thanks to the intelligent development of the overall system, ranging from the powertrain and thermal management to the user interface concept, the vehicle concept combines driving pleasure, low costs, a high level of comfort, and maximum energy efficiency. With the modular 48-V electrification approach, a low-cost alternative to high-voltage concepts has been found. As a result, there is no need for the expensive safeguards against electrical hazards that are required by law for voltages above 60 V. This also greatly expands the scope of application for the drive module. For example, it can be used flexibly as an electric drive com-
ponent in a hybrid system for larger vehicles, so that cost savings resulting from high volumes in production can be expected. The high overall efficiency of the vehicle means that a small, low-cost traction battery is sufficient to ensure both an adequate cruising range and a high level of thermal comfort in the vehicle. Mahle is currently developing the drive module, the surface heaters and the thermoelectric heat pump for series readiness.
REFERENCES [1] Fritsch, K.-M.; Schmülling, C.; Wieske, P.: Designed by Power Demand: An Electric Drive System for Urban Mobility. 26 th Aachen Colloquium Automobile and Engine Technology, 2017 [2] Fritsch, K.-M.; Rieger, D.; Warth, M.; Scharrer, O.: 48-V-Antriebsmodul für Elektrofahrzeuge. (48 V drive module for electric vehicles). In: MTZ 79 (2018), No. 1, p. 30-35 [3] Leighton, D.: Combined Fluid Loop Thermal Management for Electric Drive Vehicle Range Improvement. SAE Int. J. Passeng. Cars – Mech. Syst. 8(2): 711–720, 2015 https://doi. org/10.4271/2015-01-1709
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