30 November – 1 December 2021, Estrel Hotel Berlin, Germany

20 Years CTI – Transmission Improvement by Architecture and Components

Prof. Dr Robert Fischer

Executive Vice President, Engineering and Technology Powertrain Systems, AVL List GmbH

Plenary Speaker at CTI SYMPOSIUM on 30 November 2021 at 08:45 am

The last twenty years have been characterized by a continuous evolution of conventional transmission architectures (MTs, ATs, CVTs) but have also seen the introduction of new types like the dual-clutch transmission (DCT) in 2003 or the dedicated hybrid transmission (DHT) in 2015. Costs, installation space, weight, drivability as well as a reduction in acoustics and fuel consumption are part of the development targets. Since the reduction of CO2-emissions has been one of the top priorities due to stricter legal requirements to face climate change there has also been a shift to more environmentally friendly technologies.

Reducing CO2 happens indirectly and directly through the transmission and its effectiveness. The indirect part (= conversion quality) results from shifting the combustion engine’s operating points towards its optimum operating range, which is often a result of the interaction with an electric motor. Although the indirect part is often the dominant one, the continuous improvement of internal combustion engines and the increased use of electric motors makes the direct part (= transmission efficiency) more and more important. The architecture of the transmission mostly follows the requirements for the conversion quality – this sometimes leads to a deterioration in efficiency, which is often overcompensated by optimizing the components.

Besides giving an overview of the technical aspects and market shares of passenger car transmissions over the past twenty years Mr Fischer´s plenary speech at CTI SYMPOSIUM 2021 focuses on determining how and where efficiency improvements in transmissions have been achieved. Furthermore, the impact of different architectures and enhanced components are compared and evaluated using simulation data. Improving those components contribute a significant part to the overall reduction of CO2-emissions.

BorgWarner’s Integrated Drive Module (iDM)

Matthew Vanderlip, iDM Solutions Architect & Manager, System Strategy & Platform Engineerings, BorgWarner

Efficient system integration for the EV market

With an ever-growing trend towards electric propulsion there is also an increasing demand for flexible electrification components that meet the requirements of different vehicle architectures. By permanently enhancing the portfolio through development of innovative technologies and acquisitions that contribute to the company’s expertise, BorgWarner has positioned itself to accelerate its growth in electrification products and drivetrain solutions.

The latest generation of BorgWarner’s integrated Drive Module (iDM), for example, is easy to fit, tested and correlated, and it supports automakers in their design of efficient hybrid and electric vehicles. Based on common architecture, it combines power electronics, electric motor, and mechanical components within one single eAxle unit enabling the fast development of primary and secondary propulsion applications and helping manufacturers to comply with strict sustainability demands and emissions rules.

Developing and improving the solution

With its capability to design, test and manufacture complete electrification systems for passenger cars as well as commercial vehicles, BorgWarner can perform motor, power electronics, drivetrain, and software design in-house to develop fully optimized and highly integrated system solutions for a wide range of applications.

BorgWarner’s first iDM system platform, the iDM200 developed in 2018, utilizes the 200mm outer diameter hairpin motor HVH200 with an interior permanent magnet (IPM) rotor, which was designed to target performance requirements ranging from 80 to 150kW. The next step was the development of a new iDM system for larger, more powerful vehicle applications. The iDM220 platform is intended for operations in the 150 to 250kW power range when utilized in 400V systems, or up to 500kW in 800V form. For this, a new HVH220 (220mm outside diameter) machine was designed. The new HVH220 concept right-sizes the conductors to bring the current density back to an ideal range to match the iDM220’s innovative oil cooling strategy. This enabled an average power consumption reduction of 250W over the WLTC (World harmonized Light-duty vehicles Test Procedure) drive cycle, representing a 22% improvement.

Cooling system a question of performance requirements

Another important aspect of the iDM system development is its cooling design as it can be leveraged to reduce overall costs by allowing the use of smaller, power-dense motors and electronics. System cooling is critical to extracting the most performance out of the iDM while keeping cost under control. Therefore, BorgWarner has developed both water-cooled and oil-cooled solutions.

Lowest system costs – while still complying with performance and package requirements – are achieved by combining the inverter and motor cooling circuit. Typically, the motor is part of the water ethylene-glycol (WEG) circuit after the power electronics and cooled via a water jacket which encircles the stator. This cooling approach is limited by the surface area of the stator’s outer diameter, relying on conduction from the heat source through the stator laminations to the WEG mixture.

When a higher performance is requested, direct oil cooling becomes a more interesting solution for thermal management. Utilizing the gearbox oil to cool the motor can improve heat transfer and therefore the total heat rejection of the system, thus increasing its performance. However, an electric pump is needed to achieve the flow rates required for effective oil cooling, which adds to the system cost and complexity. Also, the oil is now being used as a sink for more heat therefore an oil cooler is often added to keep oil temperatures within operating range. But the cost of these components can be balanced against savings on the electric machine. Furthermore, oil cooling solutions include novel design features to minimize system losses through the elimination of high-speed seals and gear-set churning. This approach also utilizes direct rotor oil cooling to extract heat from the core of the electric motor thus enabling higher torque and power density. These benefits lead to vehicle-level improvements in range or opportunities to reduce battery size resulting in a net benefit.

Flexible platform

By using a building-block approach, each iDM system is designed to be flexible in terms of its motor winding configuration and stack length, power module selection, gear ratio and cooling. This allows solutions to match customer performance and packaging needs without having to start from a blank sheet, resulting in faster execution and reduced cost. In addition to the base functionalities, system add-ons have been developed and can be optionally integrated including park, disconnect and torque-vectoring systems.

Electronics portfolio expansion

With the acquisition of Delphi Technologies in 2020, BorgWarner has strengthened its position in electrification by expanding its product portfolio. Among these new technologies, the Viper family of power devices enable significant advances in BorgWarner’s system development capability. The Viper silicon or silicon carbide versions offer system designers a solution to cover nearly all power levels in both 400V and 800V systems. The devices take advantage of dual-sided cooling to maximize silicon utilization. Dual-sided cooling enables up to 50% reduction in semiconductor area for the same performance. Viper modules also eliminate the need for wire bonds, which improves product yield and reliability. The power modules are physically the same size, which enables interchangeability within a given packaging space. This allows system solutions to be developed using common building-blocks that can be tailored to match the required system performance. Among the Viper product family are SiC MOSFETS with both 750V and 1200V ratings. These devices enable over 50% inverter loss reduction compared to their Silicon IGBT counterparts for those applications demanding the highest efficiency.

System integration

By developing the new IPM motor solution with a 220mm outside diameter, combining that with the appropriate Viper power module and integrating it into a gearbox utilizing a low-loss parallel layshaft gear reduction with a high efficiency oil cooling and lubrication strategy system, designers are able to reduce the average WLTP system power consumption by nearly 700W at 60°C. The combination balances system performance as well as costs and allowed to successfully build and deliver the first two system variants of BorgWarner’s new iDM220 drive unit. The first system with a peak power of 170kW and the second system with a peak output of 225kW.

NVH improvements

With performance targets achieved as described above, specific tools were developed and implemented into the design, analysis, and testing processes to optimize features crucial for the NVH behavior. Thus, lightweight design objectives could be met while housing deflection and dynamics were improved at the same time. At the transmission level, unique design solutions securing gear strength and durability simultaneously minimize gear mesh transmission errors for better NVH. Furthermore, the design process resulted in an optimization of electromagnetic torque ripple in the e-machine and reduced radial forces along with enhanced performance in total output torque and efficiency.

Summary

By integrating a state-of-the-art transmission along with electric motor and custom power electronics, BorgWarner’s iDM can support the automotive industry in its development of cleaner and more efficient passenger cars and commercial vehicles. The modular approach to system integration and development means that BorgWarner can offer customers a solution that will fit nearly any vehicle need, providing compact, lightweight, and efficient components. Each application can draw from the sub-system portfolio to be customized while still using tested building blocks to reduce cost, risk, and time-to-market. The iDM is an innovative solution that helps automakers reduce their electric vehicles’ energy consumption and increase pure electric reach per battery charge.

Fluid thinking: Changing requirements in lubricant and coolant technologies have led to a versatile, multipurpose solution

The drive toward electrification is impacting every single area of drivetrain development, and lubrication is no exception. In the case of TotalEnergies, the company has been developing a range of dedicated EV fluids, under the brand Quartz EV-Drive MP, able to act as both a transmission lubricant and motor coolant.

Hakim El Bahi, a research engineer who specializes in electric and transmission fluids at TotalEnergies, summarizes the challenge of creating such a versatile fluid: “It’s the art of formulation, of balancing properties that sometimes may sound contradictory. It is a new concept.”

There are many benefits to a multirole approach to cooling and lubrication, from both a mechanical and electrical efficiency perspective. “With most EVs currently on the market, the motors are cooled indirectly, using a water jacket,” notes El Bahi. This approach has its limitations when it comes to removing heat from hot spots within the motor; increasingly an issue as manufacturers push smaller motors to higher performance levels.

El Bahi highlights the reasons behind this trend toward more power dense, higher RPM motors. “You can increase the torque or you can increase the speed, but if you do want to increase the torque you have to increase the size of your machine. So, instead of increasing the torque, manufacturers increase the speed. But by increasing the speed you’re increasing heat loss and with that come problems in cooling.” The answer lies in being able to feed the coolant directly to areas such as the motor windings; using the transmission fluid, which is a dielectric, seemed an ideal solution. As El Bahi outlines, “If you can put the coolant in direct contact with the hot spots of the e-motor, you can increase the heat extraction. “We thought, let’s use the transmission fluid. It is a dielectric fluid so you can send it to the motor without any fear of short circuits or electrical damage. Now, the fluid is not only a lubricant, it is a coolant as well. That is why we are shifting the vocabulary from lubricants to fluids.“

The result is a fluid that is fully capable of lubricating transmission gears and bearings, and electric motor bearings – which, in some vehicle applications, would traditionally rely on dielectric greases – as well as cooling the motor windings. Friction reduction in the electric motor and transmission, combined with greater cooling potential for the motor – enabling higher power outputs to be achieved – will ultimately lead to an increase in overall vehicle efficiency and thus range.

Of course, creating a fluid that can meet these opposing requirements to the correct standards was not easy. Considerations such as the heat capacity of the fluid, its viscosity, wear reduction properties and compatibility with the various components it meets all had to be balanced. Of course, every manufacturer will have its own specific motor and transmission architectures, each requiring a different cooling and lubrication strategy. El Bahi explains that there is scope for customization of the latest product: “The idea with Quartz EV Fluid was to design a kind of platform because we know the direction of trends in electric vehicles for lubrication and cooling purposes. However, when working with OEMs, we can start with this platform and design a tailor-made solution, because each will have their own specific requirements.” It is efforts such as TotalEnergies’ that will drive the efficiency of next-generation EVs, and the company is already looking at areas in addition to motors and transmissions where its EV Fluids can be deployed.

Total Energies believes that it can take the multirole approach to transmission and e-motor cooling and lubrication even further “The fluid is not only a lubricant, it is now a coolant as well” Hakim El Bahi, research engineer, electric and transmission fluids, TotalEnergies torque you have to increase the size of your machine. So, instead of increasing the torque, manufacturers increase the speed. But by increasing the speed you’re increasing heat loss and with that come problems in cooling.” The answer lies in being able to feed the coolant directly to areas such as the motor windings; using the transmission fluid, which is a dielectric, seemed an ideal solution. As El Bahi outlines, “If you can put the coolant in direct contact with the hot spots of the e-motor, you can increase the heat extraction. “We thought, let’s use the transmission fluid. It is a dielectric fluid so you can send it to the motor without any fear of short circuits or electrical damage. Now, the fluid is not only a lubricant, it is a coolant as well. That is why we are shifting the vocabulary from lubricants to fluids.”

Copyright details: © 2021 UKIP Media & Events. All rights reserved.
Citation details: Article published by kind permission of Transmission Technology International 2021.
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Video: Digital and electric – En route to the climate neutral mobility of the future

Watch Hildegard Müller´s, President, Verband der Automobilindustrie e.V. (VDA) video with her statement on the climate-neutral mobility of the future.

Learn more in her plenary speech at the CTI SYMPOSIUM Berlin