Early Stage Design of an Electric Drive Train for a Heavy Duty Transport Vehicle

Early Stage Design of an Electric Drive Train for a Heavy Duty Transport Vehicle Erling Gjeset Institute of Electrical Engineering Karlsruhe Institute of Technology Karlsruhe, Germany [email protected] Martin Bremer Institute of Electrical Engineering Karlsruhe Institute of Technology Karlsruhe, Germany [email protected] Pia Lindh Laboratory of Electrical Drives Technology Lappeenranta University of Technology Lappeenranta, Finland [email protected] Martin Doppelbauer Institute of Electrical Engineering Karlsruhe Institute of Technology Karlsruhe, Germany [email protected] Abstract—This paper presents design considerations for the creation and optimization of the electric drive train (EDT) of an electric heavy duty vehicle (HDV). An analytical sizing tool is also created that takes into account the requirements and guidelines for HDVs, a comparison of state-of-the-art electric trucks, and an estimation of the physical demand of a sample long-haul transport drive cycle (DrC). As an EDT evaluation criterion, compliance with the ideal DrC of the vehicle is analyzed and a rough design for an axial flux machine is developed. By this method, the groundwork is laid to carry out a multi-domain optimization of the entire EDT in the future. Index Terms—Electric trucks, electric drive trains, vehicle electrification I. INTRODUCTION Due to an increasing focus on more environmentally friendly transport, electric light duty vehicles such as passenger cars and vans have seen a distinct growth in design, production and sales. Meanwhile, electric heavy duty vehicles (HDVs), such as medium and heavy trucks, do not sell as well as their lighter counterparts, claiming only 1.2% of the global truck sales share in 2022 [1]. This is despite the fact that HDVs today account for more 35% of direct CO2emissions from road transportation, although they make up fewer than 8% of the total number of road vehicles. The reasons for the lack of sales are manifold, but chief among these are cost, carrying capacity and range. HDVs with electric drive trains (EDTs) are currently not considered lightweight enough to compete with their internal combustion engine (ICE) counterparts. Electric HDVs are also limited by new, unproven and costly technology, making potential buyers worry about price, performance and reliability [2]. There is a need for advancement within the field of EDTs optimized for the operational requirements of heavy duty This is the accepted version of a paper published in International Conference on Electrical Machines (ICEM), IEEE, 2024. The final version is available at IEEE Xplore: 10.1109/ICEM60801.2024.10700106. This research received funding from the EU Horizon Europe MSCA Doctoral Network [HIPO], Grant Agreement No. [101072580]. transport. Increasing the EDT efficiency or reducing its mass would both extend the range, reduce transport cost and leave more space for cargo, thus making the electric HDV a more viable option on the market. This paper outlines the initial design considerations for the drive train of an electric semi trailer and presents a sizing methodology for an electric motor. By this method, the groundwork is laid in order to carry out a multidomain optimization of the entire EDT in the future to achieve high torque/power density. II. TRUCK REQUIREMENTS To design the HDV EDT, it is first necessary to define the vehicle it will power and its main operating parameters. This can be done by analyzing governmental requirements and/or guidelines, creating a benchmark from state-of-the-art electric HDVs, and review drive cycles (DrC) of typical transport missions the vehicle will undergo. A. Regulations and Guidelines for Heavy Duty Transport There are regulations governing both the transport vehicles and the roads on which they travel. These must be taken into account in the design choices of the drive train. The proposed vehicle will operate in Europe, so the guidelines to be followed are those laid out by the EU and/or local governments. TABLE I REGULATIONS AND GUIDELINES GOVERNING HDV DESIGN Parameter Value Regulation/ Governing Source Guidelineaagency Max GCWR 40 t Reg. EU [3] Max truck speed 90 km/h Reg. EU [4] Max highway inclination 6% Gui. GER [5] Start ability 12% Rec. – [6] aReg.: Regulation, Gui.: Guideline, Rec.: Recommendation. The most important aspects to consider are summarized in Table I. These parameters will govern the gross combined weight rating (GCWR), the maximum speed and torque that the EDT must produce. Also included in this list is a recommendation found in [6] on the start ability, i.e. the maximum inclination the HDV should be able initiate from. B. Existing and Future Electric Truck Models Although the market for electric HDVs is not large, there are some models already available, either in development or for sale. Table II shows a selection of current and proposed electric HDVs, along with their EDT power ratings. It can be seen from Table II that the spread in power ratings is quite large. The Tesla Semi and Nikola TRE are rated highest at 800 kW peak, while most current models range between approximately 350 and 500 kW. Comparatively, high power HDVs with ICE are able to produce up to 600 kW [7]. TABLE II EXISTING OR FUTURE ELECTRIC TRUCK MODEL COMPARISON Brand Model GCWR Power Source (t) (kW) Scania BET 64 450b[8] BYD 8TT 48 360a[9] Volvo FH/FM 44 490b[10] Renault E-Tech T 44 490a[11] Iveco HD BEV 44 480b[12] MAN eTGX 44 400a[13] DAF XF BEV 42 350a[14] Mercedes eActros 40 600a[15] Volvo VNR 37 340a[16] Nikola TRE 37 800a[17] Tesla Semi 37 800c[18] aPeak value (or unspecified). bContinuous value. cPeak value estimated from available performance data. C. Drive Cycle Analysis Although the design regulations and current models provide insight into what can be required of a HDV, analyzing an actual cargo transport mission will provide realistic operating conditions for the vehicle. Considering the wide range of variables such as road conditions, transport lengths, target speeds, etc., it is necessary to define a standardized course as an average representative. The European Commission has a number of predefined DrCs intended for use as a reference for HDV operation. These are listed in the Vehicle Energy Consumption calculation TOol (VECTO), and vary depending on vehicle use cases [19]. For long-range transport, the most appropriate DrC is the ”long-haul” mission profile, which lists a series of data points that contain information on the vehicle’s target speed, elevation grade, and road length, shown in Fig. 1, with vehicle velocity of approximately 80-85 km/h on a road with varying inclination of up to 5.7 %. In the VECTO DrC, the target speed is considered constant, as the DrC does not take into account the influence of the driver. Fig. 1. VECTO ”long-haul” mission profile. D. Rigid Body Motion In order to translate the DrC into EDT requirements, one must first describe the external loads that must be overcome. According to Newton’s second law, the vehicle driving force, Fv, the air resistance, Fd, the rolling resistance, Fr, and gravity, Fgacting on a rigid body on wheels, as shown in Fig. 2, will result in an acceleration, a, of the total mass, m. Fv−Fd−Fr−Fg=ma (1) Equation (1) does not take into account wheel slip or inertia losses in the transmission of forces within the drive train. The truck itself is also considered as a point mass in order to simplify wheel weight distribution and moment of inertia effects. While these factors may not be negligible in the final analysis, these simplifications are made in order to facilitate the early design stage. The driving force, Fv, is produced by the motor torque, Tm, and transmitted through the power train to the surface of contact of the wheels as follows: Fv=2 Dw ηgTmγ(2) Where Dw,ηgand γrefer to wheel diameter, transmission efficiency, and gear ratio, respectively. The counteracting forces are created by air resistance, Fd, rolling resistance, Fr, and gravity, Fg. F v F g Fd F rα Fig. 2. Forces acting on a truck in motion on an incline. Fd=1 2CdρAv2(3) Fr=µrmg cos(α)(4) Fg=mg sin(α)(5) Where Cdand µrare the air and surface resistance coefficients, ρis the air density, Ais the truck frontal surface area, vis the vehicle speed, gis the constant of gravitational acceleration and αis the angle of inclination of the road. The relationship between the rotational speed on the motor shaft, nm, in min−1, and the vehicle speed, v, in m/s, is: nm=60 ·v Dwπγ(6) And finally, the motor power, Pm, is defined as: Pm=Tm 60 ·nm 2π(7) In the subsequent DrC analysis, most of the physical variables in (1)–(6) are considered constant and are listed in Table III. TABLE III CALCULATION CONSTANTS Parameter Symbol Value Gravitational constant g9.81 m/s2 Air density ρ1.204 kg/m3 Wheel/asphalt resistance coeff. µr0.007 Front surface A9 m2 Air resistance coeff. Cd0.704 Wheel diameter Dw0.9924 m Total gear ratio γ1:8 Transmission efficiency ηg90% III. THE ELECTRIC DRIVE TRAIN Typically, an EDT system will consist of: •Mechanical power transmission (gears, axles, etc.) •Electric drive unit •Power electronics units for motor control and charging •Energy storage (and fuel cells in case of hydrogen) System configurations, as well as energy sources, may vary, but the components and their functions will remain the same. The following sections will discuss some design choices important for the EDT design for HDV applications. A. Mechanical Power Transmission From the electric motor output shaft, the torque is transmitted through gears and differential until it reaches the wheel contact surface. Gears are especially influential in EDT design, since, by selecting a suitable machine/gearbox combination, it is possible to: •Reduce required maximum torque, ˆ T, of the machine, potentially minimizing size and cost •Increase machine speed, thus increasing its power density •Shift the motor operation to points of higher machine efficiency However, the gearbox leads to additional mechanical losses, mass, cost and maintenance of the supplementary components, leading to a multi-objective optimization problem. As seen in (7), torque, power and speed are directly related. Thus, it is possible to reduce machine output torque, and therefore size and weight, when increasing rotational speed. This is appliccable to even the most extreme cases, such as in [20], where very high speeds of up to 50 krpm were tested, and it was found that the combined mass of machine and gearbox was continuously decreasing within the analysis. The configuration of the mechanical transmission system will also influence the design of the machine. Options include distributed or concentrated electric machines, single or multiple speed gearboxes, number of motors and combinations thereof. Comparisons of different architectures are made in [6], [21] from both a qualitative perspective and cost analysis. Both agree that distributed layouts have the most benefits, including low weight, efficiency, modularity, freedom of control, redundancy and ease of service. Using multiple smaller machines also opens for the possibility of torque vectoring, such as in [22], where machines with different efficiency maps may cooperate to achieve lower overall DrC energy consumption. As a result of the many interdependencies in the design of the machine and gearbox, the best option in EDT design is to adapt both subsystems to each other, as in the case of [23], [24]. To begin this iterative process, an initial configuration should be chosen. Since DrC efficiency is not yet considered, it will suffice to select an overall total gear ratio, regardless of choice between single or multiple speed. B. Electric Drive Unit Electrical machines for e-mobility need to be lightweight, reliable, reasonably priced and produce high starting torque and constant power over a wide speed range [25]. Many machine types may be designed to match these criteria, such as: •Permanent magnet synchronous machines (PMSMs) with high efficiency and power density •Switched reluctance machines (SRMs) with high speed capabilities and lower cost •Induction machines (IMs) with lower cost and reliability This study does not intend to draw any conclusion on which type is better suited for EDTs. The individual design should consider which aspects to focus on and find the optimum solution therein. The machine topology selected for this project is an axial flux PMSMs (A-PMSMs). Recent development has shown this particular machine type to be power dense [26] and has provided commercially available machines suited for high torque, low speed applications such as HDVs [27]. The geometry also lends itself to modular designs, such as stacking multiple rotors/stators [26] and using interchangeable parts [28]. Regardless of topology, the EDT has to provide relatively constant power for extended periods of time. This, combined with the drive towards higher power density, demands highperforming cooling systems in order to maintain acceptable temperatures within the machine and increase reliability. An extensive review of different strategies may be found in [29], and particular systems for A-PMSMs are studied in [27]. As a general note, the cooling system performance will set the limit for continuous power output of the machine, regardless of its electromagnetic performance. With well-designed thermal management, these may almost overlap [30], but it is more common to have discrepancies between peak and continuous power between 20–50%. This must be taken into account when considering the power requirement of the EDT. C. Power Electronics The power electronics (PE) of the EDT comprise of battery management and charging systems, as well as the motor controller and signal processing unit. A general trend when designing PE for electrical machines is to increase the integration between the two subsystems, shortening the physical distance between the parts [31]. While this approach has potential of reducing transmission losses and increasing power density, the co-design process demands more custom solutions and component adaptations, requiring more resources for design and production. Meanwhile, by bringing sensitive PE components in proximity to the vibrations and temperatures of the rotating machine, proper precautions should be taken to ensure reliability of the system [32]. For this project a three-phase traction inverter with 450A SiC Half Bridge Modules from Wolfspeed® is selected as a sample PE module for the EDT [33]. The specifications of the unit are listed in Table IV. TABLE IV INVERTER SPECIFICATIONS [33] Parameter Symbol Value Power rating P300 kW DC link voltage Udc 800 V Phase current IRMS 360 A Switching Frequency fSW,max 80 kHz D. Energy Storage For e-mobility in general, the size and weight of the energy storage is one of the major concerns. This is especially the case for cargo transport, since it will directly affect both range, freight capacity and economic viability. Multiple studies have reported on the state-of-the-art and technology roadmaps of energy storage solutions [34], [35]. Meanwhile, on an EDT system level, it is important to evaluate the subsystem interdependencies when sizing the HDV EDT. The required energy storage capacity is dependent on the consumption of the EDT, which in turn is influenced by machine efficiency and control strategy. Numerous research efforts have explored the relationship between machine sizes, gear ratios, and the battery capacity [24], [24], [36]. IV. TRUCK AND DRIVE TRAIN SIMULATION The following analysis presents a method of using the regulations, DrC and equations presented in Section II in order to evaluate the EDT requirements and set initial parameters for EDT design. The analysis is for long range cargo transport, but the method easily lends itself to other electric vehicles of light and medium duty. The peak torque and top speed are defined by the start ability and maximum velocity of the vehicle. These values are presented in table V. The EDT peak power requirement, ˆ Pm, is determined by the operating point with the highest vehicle velocity at the steepest inclination. When considering the highest possible values found in either the governmental guidelines, or the most extreme demand from the DrC (The maximum value found in Fig. 3), the EDT should be rated for 700–800 kW. However, this is beyond what most of the trucks listed in Table II, including the most powerful ICE HDVs, are capable of. Meanwhile, when evaluating the instantaneous values of Pm, shown in Fig. 3, it becomes clear that the required power at only rarely exceeds 400 kW. An alternative sizing method should instead be used in order to avoid unnecessary high EDT power ratings. If the vehicle reduces its velocity during points of extreme inclinations and target speeds, (3) and (7) indicate that the resulting power requirements are reduced. This control strategy will reduce the average DrC velocity, ¯v, but allow for a smaller and more lightweight EDT. By applying ndpoints of deceleration where DrC demand exceeds EDT peak power, it becomes possible to simulate the HDV performance within a range of possible power ratings comparable to those listed in Table II. The comparison, shown in Table VI, takes into account the HDV average velocity, ¯v, DrC completion time, maximum velocity at 6% incline and maximum incline at top rated speed, cmax. TABLE V REGULATION/GUIDELINE EDT DEMAND GCWR Top speed Start Max Peak torque ability rotational speed 40 t 90 km/h 12 % 4000 rpm 3500 Nm Fig. 3. Power requirement to maintain constant speed during the ”long haul” mission profile. TABLE VI EVALUATED EDTS ˆ P nd¯vDriving v@6% cmax (kW) (km/h) time (min)a(km/h) (%) 300 17 81 74 36 1.4 400 6 82 73 42 2.4 500 3 83 73 52 3.3 600 2 83 72 70 4.2 800 0 83 72 90 6.0 VECTO – 83 72 – – aNot accounting for stops. As can be seen in Table VI, EDT power rating has negligible impact on the driving time of the transport mission. Although it is beneficial for highway traffic to maintain high speed, it is also important to consider the additional size, weight, and cost of a more powerful system. Mass and efficiency are crucial when calculating energy consumption and range, and should be given more consideration when sizing a HDV EDT. Limits are also imposed by the control system and EDT architecture. When all of these factors are taken into account the highestpower system may not be the ideal option. As a compromise between power and size, a 500 kW peak EDT is selected. Considering the results in Table VI, a lower power rating would also be possible, but given the potential continuous power constraints mentioned in Section III-B, it is advisable to have some margin. The EDT will have one powered axle driven by dual motors, following to the positive evaluation made in [6], [21], [22], consisting of two 250 kW motors, to provide modularity, flexibility, and redundancy to the system. V. ELECTRIC MACHINE SIZING Given the EDT requirements calculated in the previous section, an analytical analysis can be performed to estimate the dimensions of the electric motor. This project proposes a multi-stack A-PMSM as a potential candidate for electric HDVs, as they promise high torque density and performance capabilities [37]. The machine will consist of two identical stators in a yokeless and segmented armature (YASA) design with concentrated winding and three permanent magnet rotors. The design concept is shown in Fig. 4. As an initial design, a concentrated winding scheme with a high number of slots Fig. 4. Principle exploded view of an axial flux machine with double stator and triple rotor (DSTR) TABLE VII START PARAMETERS OF THE MACHINE DESIGN Description Symbol Value Number of slots N36 Number of poles 2p34 Number of phases m3 Efficiency ηe0.95 Power factor cos φ0.9 Amplitude of the air gap flux density ˆ Bδ1.05 T N= 36 is considered. With a number of pole pairs p= 17, the design rule 2p±2 = Nfor single-tooth windings is applied [38], [39]. Designs with fewer slots and poles will also be investigated in the later stage design process. Various simplifications are assumed for the first design; initially, only the fundamental wave is considered. Therefore, the induced voltage is estimated in order to determine the internal power. The induced voltage caused by the main flux can be calculated using (8) [37], [40]. Eh=1 √2ωe(wξ)ˆ Φh(8) The induced voltage in (8) depends on the angular frequency ωe, the amount of winding turns w, the fundamental winding factor ξas well as the amplitude of the main flux: ˆ Φh=2 πˆ Bδτplseg (9) The pole pitch in the middle of the segments can be calculated with: τp=π 2pDm(10) In (9), (10), the segment length lseg refers to the radial length of the tooth segments and is thus comparable with the active length of the radial flux PMSM. Together with the mean diameter, Dm, these parameters constitute the main geometric dimensions, as illustrated in Figure 5. Maximum internal power can be determined on the basis of the number of phases, the induced voltage, and the possible inverter current [40]. Substituting provides (11) Psi =mEhI(11) The resulting internal power produced by the machine should exceed the required output power by a margin, considering 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 A A B B C C D D E E F F 1 A2 Status Änderungen Datum Name Gezeich Kontroll Norm Datum Name 28.02.2024 yk8466 ØD_m l_seg Fig. 5. Axial view of a partial stator, with the sizes of segment length lsegand mean diameterDm 150 200 250 300 350 400 450 500 550 600 650 700 750 Mean diameter Dmin mm 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 Number of coil windings wcoil 240.1 226.7 249 231.2 244.5 222.3 240.1 249 217.8 249 240.1 222.3 244.5 231.2 249 266.7 266.7 266.7 280.1 280.1 266.7 266.7 266.7 220 230 240 250 260 270 280 Induced voltage Ehin V NaN Fig. 6. Induced voltage of various feasible machine designs losses in the machine and the drive system. The output is thus obtained by multiplying the internal power Psi by the estimated efficiency ηeand the power factor cos φ. Psiηecos φ≥Ptarget = 250 kW (12) Eh≤1 2√2Udc = 282,843 V (13) By varying the number of coil windings wsp and the stator mean diameter Dm, possible machine sizes may be evaluated using (8)–(13). In Fig. 6, applicable designs are highlighted in blue, marked with the corresponding induced voltage, while unsuitable combinations are grey. The design combinations in the upper right are not possible due to a too high induced voltage, all designs below indicate too little power for the previously defined use case. In order to validate the analytical design, a simplified FEA simulation was set up in Ansys Maxwell. A machine with a mean diameter of Dm= 350 mm, a segment length of lseg = 50 mm and a wsp = 8 was simulated. The induced voltage was calculated to be Eh,FEA = 258.2 V, compared to Eh,precalc = 248.9 V in Fig. 6. The resemblance shows that the analytical equations can be used as a rough draft. In order to improve accuracy, an analytical model will be created that takes harmonic and slot effects into account. In addition, a two-dimensional multi-layer equivalent linear motor simulation method will be implemented for a detailed optimization of the motor design. VI. CONCLUSION AND FURTHER WORK This article has highlighted important design considerations when constructing electric drive trains (EDTs) for heavy duty vehicles (HDVs). In order to make electric cargo transport a more viable option, the EDT subsystems should be cooptimized, since the components have significant impact upon each others’ size, performance, cost and reliability. In order to start such an iterative process, an analytical model has been constructed to analyze HDV EDT power requirements. A drive train specification has been set and a first estimation of the size of an axial flux permanent magnet synchronous machine (A-PMSM) has been calculated based on requirements and guidelines, a comparison of the stateof-the-art electric trucks, and an estimation of the physical demand of a sample long haul transport mission. 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BIOGRAPHIES Erling Gjeset received his M.Sc. degree in mechanical engineering from the Norwegian University of Science and Technology (NTNU) in 2020 with a specialization in thermodynamics and heat transfer. He is currently working towards his Ph.D. degree at the Karlsruhe Institute of Technology (KIT) in the laboratory of hybrid electric vehicles (HEV) in the field of cooling systems and mechanical design of axial flux machines. Martin Bremer obtained a B.Sc. and M.Sc. degree in Mechatronics and Information Technology from the Karlsruhe Institute of Technology (KIT). With the end of his studies in 2022 he joined the Electrotechnical Institute (ETI) at KIT as a research associate. His research interests include analytical modeling of electric machines in combination with noise and vibration mitigation techniques. Pia Lindh received her M.Sc. degree in energy technology in 1998 and her D.Sc. degree in electrical engineering in 2004 from Lappeenranta University of Technology (LUT). She is currently serving as an associate professor at the Department of Electrical Engineering in LUT Energy, Lappeenranta, where she is engaged in teaching and research of electric motors and electric drives. Her research work focuses on permanent magnet motors, especially concentrated winding machines and new cooling methods. Lately, her several projects have been focused on mobile working machines, bus applications, and other traction motors. Martin Doppelbauer is a full professor since 2011 at the Institute of Electrical Engineering (ETI) at the Karlsruhe Institute of Technology (KIT) in Karlsruhe, Germany, where he holds a chair of Hybrid Electric Vehicles. Martin Doppelbauer is also the head of the New Drive Systems (NAS) group at the Fraunhofer Institute for Chemical Technology (ICT) in Pfinztal, Germany. Prior to his career in reasearch, he worked in industry for 15 years, most recently as head of electrical machine development at SEW Eurodrive GmbH in Bruchsal. Martin Doppelbauer is also active in national and international standardization. He is the chairman of IEC Technical Committee 2 Rotating Electrical Machines