ISSUE 012 Winter 2021 Sigma Powertrain EMAX transmission dossier l In conversation: David Hudson l 48 V systems focus l 2021 Battery Show North America and Cenex-LCV reports l Everrati Porsche 911 digest l Switching insight l Motor laminations focus

W hile governments have not made any noises about banning aircraft that burn hydrocarbon fuels, as they have with road and other vehicles, aviation is nonetheless under a lot of pressure to decarbonise (writes Peter Donaldson). As the automotive industry is further along with vehicle electrification than the aviation sector, it is natural to wonder whether the future of electric aircraft might mirror the recent past and the present of electric cars, with hybrids flourishing first but being caught up and passed by battery EVs as battery technology improves, and with hydrogen fuel cell cars – so far – being little more than curiosities. There are, however, strong indications that electric aviation’s path will be different. Fundamentally, aircraft are much more sensitive to weight than ground vehicles, so the relative energy densities of fuels and batteries is even more important, as is the additional weight of ancillaries needed to support energy storage and conversion. Known as the balance of plant, this is a particular consideration with fuel cell systems. Power failures in aircraft pose far greater risks than in ground vehicles, and electric power is particularly well-suited to distributed propulsion systems that provide redundancy through multiple motors without suffering the inefficiencies as the same approach using IC engines. At the lighter end of general aviation, the fixed-wing basic training aircraft market for example could be disrupted by purpose-built battery-electric aircraft (BEA) such as the eFlyer 2 from Bye Aerospace, thanks largely to an hourly cost claimed to be half that of an equivalent avgas-fuelled trainer. The nascent urban air mobility sector is also dominated by battery-powered eVTOL machines. Even though VTOL is very energy-intensive, zero local emissions are a fundamental selling point, while range and endurance requirements are modest, so batteries are a good solution. Hybrids offer much greater ranges, along with zero local emissions if they take off and land under electric power, and small or negligible carbon footprints if they burn ‘green’ aviation fuel derived from biomass. However, emissions en route are likely to be subject to increasingly stringent environmental regulations sooner rather than later, so the window of opportunity for aircraft with fuel- burning hybrid powerplants is likely to be relatively narrow. Nonetheless, much innovative thinking is in evidence with programmes such as the Faradair Bio-Electric Hybrid Aircraft, billed as an STOL workhorse sized to carry 18 passengers or 5 tons of cargo on regional routes up to about 1150 miles and capable of battery-only flights of up to 15 minutes. Some analysts conclude that hydrogen fuel cells have good long- term prospects in large fixed-wing aircraft, perhaps better than in ground vehicles. Large aircraft have more volume and weight-lifting capacity to accommodate a bulkier, heavier and more complex powerplant, enabling fuel cell electric aircraft (FCEA) to take advantage of the energy density of compressed hydrogen gas or even cryogenic liquid hydrogen to achieve three times the range of an equivalent pure BEA. In a pure BEA, the inventory of primary components can be as short as a battery, inverter and motor, while FCEA powerplants are more akin to hybrids and similarly characterised as either series or parallel in architecture, both of which include a battery to meet rapid changes in power demand, as fuel cells are relatively slow to respond. When they do respond though they can develop a lot of power. ZeroAvia, for example, is looking to use a hydrogen fuel cell powerplant capable of delivering over 3 MW, and plans to offer it in a 19-seat Dornier 228 regional airliner by 2024, scaling up to a 76-seat Dash 8 Q400 by 2026. The company is also working with Mitsubishi on a zero- emissions powerplant for regional jets, both for new aircraft and as retrofits, to be available by about 2028. Batteries cannot be ruled out for long-range aircraft, however. Elon Musk, for example, has said that a specific energy of 400 Wh/kg would make batteries competitive with kerosene, and that they could be available as soon as 2025. PS | Electric and hybrid aircraft Hybrids lourished irst for electric cars but electric aviation’s path will be di erent 74 Winter 2021 | E-Mobility Engineering