To focus on reducing emissions of single-aisle, twin-engine aircraft, which make up a sig-nificant proportion of all air travel, NASA has conducted several studies to estimate how the various interconnected systems on an aircraft might benefit from full, or at least partial, electrification [24–26]. The primary figure of merit is the fuel burn rate, which is a factor of both the weight of the aircraft (more thrust is necessary to keep a heavier aircraft aloft) and efficiency of the propulsion system (which allows travel of greater miles per gallon, as it were). Lower efficiency also means more fuel is required for a given flight, and is thus coupled to the aircraft weight. These studies produced key performance parameters for the electric drivetrain and identified a target design space – in spite of the series of energy conversions involved in a hybrid architecture (chemical to mechanical to electrical back to mechanical) – where a drive designed beyond the break-even frontier would lead to an increase in aircraft propulsive efficiency upwards of 10%. This frontier is defined as a trade-off between efficiency and gravimetric power density (lightweight, yet still supplying the required output power), and culminated in program targets specified in Table 1.1. The minimum targets correspond
Table 1.1: NASA key performance parameters for an electric drivetrain [27].
Minimum Target Stretch
Specific Power 12 kW/kg 19 kW/kg 25 kW/kg
Efficiency 98.0% 99.0% 99.5%
to the break-even frontier; technology here would demonstrate the concept is plausible, while meeting the reach goal would indicate propulsive efficiency gains are indeed achievable.
In addition to these targets that directly drive vehicle efficiency, two higher-level con-straints must also be met. First, a sophisticated electrical distribution system is required for any more electric aircraft, and the weight and efficiency of this system must also be optimized. Several studies considered both ac and dc bus systems up to 6 kV, where higher voltages can be traded against lower currents (and therefore lower conductor mass) at the same power level [28, 29]. A synchronous, high-voltage, ac grid had been shown to be the most efficient implementation, but the synchronous nature tightly couples the generator and propulsor speeds and removes a significant degree of freedom in system design. Conversely, a high-voltage, dc bus, with independent rectifiers and variable speed drives providing power conversion as needed, but trades some efficiency for added flexibility [30]. Nonetheless, flex-ibility was preferred for the designs pertaining to this work, so a dc grid was selected as the lowest mass option, illustrated in Fig. 1.1c. While dc voltages of up to 3 kV will continue to provide system level benefits, 1 kV represents the knee of the performance curve relating input voltage to power density [28] and was established as an appropriate target for this work in [27].
Finally, the high specific power (HSP) motors for this application also pose unique chal-lenges: lightweight, “iron-less” (typically permanent magnet) machines have very low induc-tance and require a low total harmonic distortion (THD) drive current, while brush-less dc and switched reluctance variants require a carefully shaped current waveform. Some inverter designs in Section 1.2 have made exciting progress towards addressing this application and these motors. However, unconventional hybrid architectures may ultimately provide the great leaps in performance necessary for electric flight.
Recently, the U.S. Department of Energy launched a similar research program to ac-celerate electrified propulsion through the Advanced Research Projects Agency – Energy (ARPA-E) program titled ASCEND (Aviation-class Synergistically Cooled Electric-motors with iNtegrated Drives) [31]. This aggressive program demands similar performance to that defined by NASA, but also provides a target flight profile to benchmark against (as done in Chapter 2). Additionally, this program further requires compliance with DO-160, a standard covering everything from environmental conditions to electromagnetic emissions. While the former is outside of the scope of this work, the latter is addressed in Chapter 4.
In the past few years, there has also been a similar push to advance electric vehicle tech-nologies. To cater to diverse consumer preferences and use cases, and to forge a path towards energy efficiency and reliability of transportation choices, the U.S. DRIVE (Driving Research
Table 1.2: Key 2025 targets in the US Drive 2025 electric powertrain roadmap [32]. 100 kW/L 100-200 kW 55-110 kW >98 % 900-1200 V 2 kHz
and Innovation for Vehicle efficiency and Energy sustainability) consortium has sought to foster collaboration between government organizations, private companies and academic in-stitutions to establish a vision for vehicle research and development [33]. Through regular consortium meetings facilitated by the Vehicle Technologies Office in the United States De-partment of Energy, an Electrical and Electronics Technical Team Roadmap was produced to guide such efforts [32]. This document ties many of the high level goals of emissions reductions, vehicle range and peak power, price point and lifetime to key requirements of future electric drivetrains, several of which are listed in Table 1.2.
The first target of note is the power density. While gravimetric power density and vol-umetric power density are both important objectives to maximize, the volvol-umetric density is more heavily weighted for automotive applications. This is because the space in automo-biles is limited; where it is not used for passenger or cargo, it would otherwise be used for batteries to extend the range of the vehicle. The high power density of 100 kW/L represents an order of magnitude increase compared to what has been possible with extant automotive technologies. The related goal of a high output power is also necessary to ensure desirable ac-celeration characteristics as well as satisfy towing and other high-torque applications served by conventional utility vehicles. Additionally, the powertrain must be highly efficient to get the most range out of each charge.
The remaining milestones are again, slightly more nuanced. For instance, while current-generation electric vehicles typically host a 400 V dc bus, higher voltages are increasingly more appealing – especially with the advent of wide-bandgap power semiconductors that support these voltage ratings [34]. Although there are system and energy density benefits gained by moving to higher bus voltages, the targets upwards of 800 V dc are actually in support of fast dc charging; there is less loss “at the pump,” as it were, when pushing charge rates above 350 kW [35]. That said, high-voltage drives have also shown performance boosts in heavy-duty vehicles – with a recent demonstration by John Deere of a 200 kW industrial loader featuring a dc bus over 1 kV [34]. As for the motors, most high-specific-power machines have tended to feature a high pole count, and therefore high electrical frequency – upwards of several kilohertz [36–38]. As such, the inverter system must provide the corresponding modulated waveform – necessitating either a much higher switching frequency or additional harmonic filtering.