When thinking about the onset of engine electrification, some people initially reference the engine control unit (ECU), as well as the more recently introduced transmission control unit (TCU), since these ushered the era where electronics literally took control of engine operation. Today, beyond these “smarts,” there is a growing emphasis on the utilization of electric drive technology in engines and related peripheral automobile systems. While hybrid and electric vehicles have gotten a lot of well-deserved press due to the notable technical achievements that have been realized, there is also a lot more opportunity for utilizing electrification throughout the automobile. These areas include fuel injectors, valvetrain systems, exhaust recovery, superchargers, power steering systems and other accessory motors, and, of course, generators, starter-generators, and hybrid motors. Below we explore a few examples of recent, past, and future applications, as well as motor technology developments.
Power Steering
Electric power steering was at the vanguard of the recent wave of electrification under the hood because of the significant mileage gain resulting from its use. As the new technology was developed, a relatively minor concern known as torque ripple, became a significant problem when drivers reported feeling it in the steering wheel. Although it has no impact on performance, it could be a distraction or a concern for a customer accustomed to a smooth hydraulic steering system, thus negatively impacting customer acceptance.
Motor technology quickly focused on permanent magnet (PM) brushless motors as a key solution. In principle, a synchronous PM machine with a sinusoidally-magnetized rotor produces a flat torque. But, to make this principle a reality, considerable engineering was needed. The solutions included careful machine design, from optimally shaped and appropriately magnetized magnets to a special stator-tooth design, for instance, with phantom slots.
In phantom slot designs, small indents on the tooth pole multiply the frequency of this kind of torque by a factor of two, making the natural filtering by the mechanical system more effective. It was also necessary to develop a deeper understanding of the controller’s role in producing torque ripple, due in part to commutation issues or to the discrete nature of controllers. In the latter case, a trade-off is critical between finer steps and ripple minimization. In the process, the practical issue of the accurate measurement of increasingly small torque pulsations came to the forefront. Manufacturing processes and tolerance variations were also examined carefully, and six sigma and other Taguchi tools came to shine in identifying parameters key to minimizing the impact of manufacturing variations on performance.
High-Speed Motors
Superchargers and turbochargers pressurize the intake air into the engine, enabling a significant reduction in engine size, and increase in efficiency. For superchargers, an electric drive has advantages over its mechanical counterpart. In particular, it allows the use of centrifugal pumps over positive-displacement systems. Positive-displacement units work at lower speeds, and, thus, can be driven by the engine belt, while centrifugal pumps require much higher speeds that preclude a belt drive, but are compatible with an electric motor.
Turbo systems use the energy otherwise wasted in the exhaust stream primarily to pressurize the air intake, but they can be electrified as well. There is considerable work on turbines using Rankine cycles in the exhaust stream to power a generator to charge batteries. Such systems face similar issues concerning design for high-speed motors, with experimentation taking place with both switched reluctance and axial PM motors. Turbo generators can also be used in conjunction with a supercharger, with the electrical energy produced out of the exhaust typically being more than that needed to power the supercharger.
Winding Technology
Two important developments in terms of winding technology of particular interest to automotive applications have taken place recently. Concentrated stator windings, where the coils are wound around individual poles, are becoming increasingly popular in automotive applications and have multiple benefits, including lower manufacturing cost (since the coils can be wound individually before assembly of the motor), higher slot-fill factor, and shorter end turns. At the same time, research has shown ways to design these machines to mitigate or even overcome their limitations compared to distributed-winding motors, at least in brushless PM machines.
Separately, the introduction of bar windings, also called hairpin windings, to the smaller motors typical of the automotive world is leading to unprecedented high fill factors that make for much better heat dissipation and more compact designs. Whether with concentrated or distributed windings, with round wires of bar windings, electric motors are currently considered for pretty much any under-the-hood function, replacing current belt-driven units for oil pumps, air conditioning compressors, cooling pumps, and other devices.
Starters and Generators
Over the last 20 years, the Lundell generator has been vastly improved to allow more power to be generated in both absolute terms and per volume—magnets were placed between the claws on the rotor, mainly to reduce leakage flux. Additionally, new windings with pre-formed bars were used in the stator, to allow better slot fill, heat removal, and thus power density. Water-cooling was introduced for the same reasons.
All these improvements have delayed, although probably not prevented, the replacement of the Lundell machine topology with induction or PM machines as generators, especially when the generator is tasked with also starting the engine, in “starter-generator” mode. Interestingly, the use of PM brushless and induction configurations for hybrid and electric vehicles has generated significant developments for these motors, facilitating their introduction in starter-generator applications, not the least of which was engineers being less reluctant to use higher voltages.
The automotive industry continues to be a key driver for innovation in advancing electric drive technologies. It will be interesting to see how this technology further develops and is implemented across a variety of challenging applications, each with its own unique set of requirements. What’s clear is that electrification in automobiles will continue to increase, and much of the new and emerging technology will be applicable for a number of different electrification use cases in other industries as well.