25 and its thermal efficiency is aided through the same coolant channel as that running in the e-motor. Using SiC increases energy efficiency and smooths transients in the motor, although Frauscher and Porsche note that any gains to handling are difficult to sense at the propeller through the stern drive and its transmission. Stern-drive transmission and gearbox Early on in development, the Porsche team was unsure how well they would be able to calibrate the e-motor and inverter for use in water. They therefore wanted to protect the stern drive from the high speeds and dynamics of the motor shaft, which meant the engineers initially tried a friction clutch between the motor and stern drive, with essentially two plates and a friction coating between them, as one would find in a car. That was set for a fixed-torque limit; if exceeded, the clutch would slip, so neither the motor nor the stern drive would take any damage. But when the team first tested it on a lake, with the motor uncalibrated, it failed multiple times and the boat had to be towed back to the harbour. So, the team commenced with calibrations, and the boat no longer uses a friction clutch. Instead, the 858 now has a relatively simple system with an elastomeric claw clutch to prevent accidental slipping. Mercury Racing’s MerCruiser Bravo 3 XR is used as the stern-drive unit, partially due to Frauscher’s familiarity with the brand (MerCruisers are installed on several of its other boats). It features a twin-screw propulsion system, meaning counter-rotating propellers are arranged on a common shaft. These propellers typically have a diameter of 16 in, and are made from heattreated stainless steel, with the internal gear ratio set to either 1.65:1 or 1.81:1, depending on the customer’s use case. “The advantage of this type of drive over a traditional shaft drive is that you can angle the propeller up or down to meet the water line when the boat is planing,” Helmberger says. “This gets the driver higher speeds and efficiencies than if they’re just steering it with, say, a rudder behind the propeller, which would especially reduce performance in tight turns.” Additionally, although Porsche had expected to run the e-motor in one direction for forward thrust and in the opposite direction to reverse, the MerCruiser’s gearbox necessitated shifting for reverse drive; hence, the e-motor always runs in the same direction. It was therefore a challenge for the team to make sure the boat always goes forward when the user wants it to, and never gets stuck in a given gear stage, especially when there are some ECUs in the stern drive that the boat doesn’t need to use. Gear shifting is controlled by reading throttle signals, including the thresholds built into the throttle lever that correspond to the gear level. Hence, when those thresholds are detected, the eVCU sends a signal to an actuator, which pulls a wire to engage or disengage the gear in the stern drive. Helmberger adds: “Many boat manufacturers attempting electrification will run their e-motors on idle for long durations at startup and stops, just to make it easier to shift gears, which is obviously a waste of energy. The team at Porsche instead programmed the eFantom such that the motor idles for a really short amount of time, basically a split second, enhancing overall energy efficiency. To our knowledge, no-one else is doing that in electric boats.” Battery and charging The 800 V (737 V in nominal operations) battery pack in the eFantom contains 12 modules and a cumulative 100 kWh of energy. Each module contains 34 cells and 67 Ah, with the overall architecture of the pack’s cells given as 204s2p. This architecture enables the use of 400 V charging stations via ‘bank charging’. Any EV with the next-gen Macan powertrain automatically switches HV gates in the battery to split the 800 V pack into two packs, each with a nominal 400 V. Each of those can then be charged in parallel by the 400 V station without the need for a HV booster (and the HV ECU can balance the two halves’ SoCs before charging them together, if necessary). Technically, the team limits its output to 400 kW, but like the In addition to the 800 V main bus, the low-voltage network runs from a DC-DC converter that provides 12 V for cabin systems, lights and so on E-Mobility Engineering | March/April 2024
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