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

our 911 coupe,” says Williams. “In fact, our coupe exceeds the power output of the original 911 by some margin, yet we found over time that the classic coupe chassis handles power really well. “On top of that, you’ll see many new things throughout the car, such as LED lights, uprated suspension, heated seats and heated windscreens, all aimed at addressing the ownership problems that the old 911 had. Most resto-modders focus on just aesthetically moving it backwards, but we figured that since no-one’s going to find their driveways covered in oil first thing in the morning any more, we’d put in a ton of other changes that really make the car so much more reliable and driveable.” To hit the fundamental project targets, as well as maintaining the inertia and resonances of the vehicle, Kerr needed to use the original 911 chassis without modifications. That strictly defined the packaging volumes and hence the mass distribution and architecture for the powertrain he and his team needed to design. “The batteries for example are split into two packs: a smaller one at the front and a bigger one at the back, with the rear pack coupled with the OEM drive unit we use,” Kerr explains. “Both are mounted on the chassis using the original Porsche 911’s transmission and engine mounts, but a bit further forward for a slightly improved polar moment of inertia. “The power electronics are packaged in the front of the vehicle. There you’ll find the onboard charger and the cabin DC-DC converter, and it’s worth noting that we’ve managed to maintain enough luggage space for a flight suitcase in the front as well. “Meanwhile, both battery packs are kept well outside the original vehicle’s crumple zones, which really enables us to fast-track the certification and hence production of our vehicles.” That has resulted in an overall weight of 1440 kg, versus the 1450 kg of the original 911 Turbo, but with the same front-to-rear mass distribution of about 39/61%. Battery architecture The 360 V battery packs collectively comprise 24 modules, eight in the front and 16 in the rear. Each module stores 2.2 kWh, has a capacity of 150 Ah and supplies a nominal 14.5 V, with 12 prismatic lithium-ion cells arranged in a 4S3P configuration. Kerr comments that the electrical architecture and construction have been designed for compliance with R100 regulations, with certification underway to Part 1 of the standard for production purposes. “The pack split was done for weight distribution, not safety, but each pack does have its own contactors and HVIL for safety reasons,” he says. The packs share a single liquid- cooling circuit. Bespoke aluminium cooling plates are installed at the pack level, along with silicon-based thermal transfer pads at the module level. Each module has a temperature sensor, and has been simulated to identify both its cooling capacity respective to performance limits and where hotspots have the potential to form. “Our battery assemblies have undergone extensive cooling plate characterisation testing, at both the module and pack level, to validate a safe performance envelope, with further structural assessments of the mechanical housings and battery frames undertaken via CAE,” Kerr adds. BMS and charging The battery energy can be replenished through Type 2 AC charging or DC fast charging, via a CCS2 combined charging port seated at the original fuel filler bowl’s location (a CCS1 charger will be made available in the first quarter of next year). For the former option, a 6.6 kW onboard charger from Electric Conversions is installed to provide a maximum continuous recharge of 6 kW, with a maximum AC charge duration of 7.5 hours. The latter option takes 40 minutes at most, with a 70 kW expected charging power input, although that will vary with the battery voltage. Kerr adds that the BMS software is programmed with progressive limitation and control of charge/discharge speeds when the packs are at a low SoC, with a curve that drops sharply outside 10% and 90% charge levels. The BMS also encompasses a temperature controller for managing the separate liquid-cooling circuits, pumps and fans for the battery and the motor and inverter. It also monitors and protects each module independently for temperature, voltage, current, internal resistance and faults. In addition to isolating any faults, it will carry out smart cell balancing to equalise the SoC across all modules. Traction and suspension Everrati’s drive unit uses an OEM- validated design containing a three- phase AC induction motor, producing 328.1 kW on the standard 911 and 372.8 kW on the Signature edition. Control comes from an integrated inverter, and as indicated a second A large battery pack is installed in the rear, with a smaller one in the front, for 52.8 kWh in total 52 Winter 2021 | E-Mobility Engineering