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

Rory Jackson explains how this modular all-electric gearbox can combine two motors to drive any size and weight of EV For the common good H igh-voltage electric motors produce torque across a wide range of their rpm bands, enabling most electric cars on the road these days to achieve reasonably high power efficiencies with single-speed gear reductions. However, as discussed with David Hudson at ePropelled (see page 16), a more efficient kind of powertrain is needed if the world is to switch to all-electric propulsion. That is because current e-motors suffer losses outside their peak efficiency spot, particularly at their top speed and maximum torque outputs, where the drops can be 10-25%. They also rarely generate enough torque at low speeds for any significant controllability or launching power, especially for heavy commercial EVs such as cement trucks, city buses and so on. These issues create the potential for wasting considerable battery energy, and in the longer term place unnecessary burdens on maintenance technicians and national grids. Some EV makers try to compensate by relying on one very large, heavy e-motor and a single reduction, but as has been shown by companies such as Designwerk (in EME 8, Winter 2020), using multiple small motors in combination via a gearbox will match the power output of one big one, while weighing and costing much less. Clearly, gearboxes can improve weight, energy and cost efficiencies by allowing EVs to use smaller motors, and by shifting gears to create a much broader peak operating zone – as they do in IC-engined vehicles – across their drive cycles. Choosing the right gearbox is key to optimising a powertrain’s efficiency. For instance, using traditional, analogue- controlled geartrains makes little sense given that EVs are sold increasingly to commercial fleet owners on the promise of greatly reduced noise, parts counts and potential points of failure compared with IC powertrains. Also, automated manual transmissions (among other automatic and semi- automatic types of transmission) suffer parasitic losses from their hydraulic actuation systems that cancel out their actual efficiency gains. Seeking to resolve these drawbacks is Michigan-based Sigma Powertrain (SPT). Its patented EMAX transmission is designed principally to integrate two electric motors of any size and combine their power through an all-electric four- speed gearbox. The gearbox measures 18 in (45.7 cm) long, and forgoes hydraulics and mechanical control levers. All the actuation is electric, and controlled electronically, with parasitic losses and noise minimised by careful selection of gears, shafts and electrical components. It is cylindrically shaped Sigma Powertrain’s EMAX uses a summing gearset to combine two electric motors into a sinNle output sOaft anK Tultiple OiNO eɉciency gear modes for Class 6-9 EVs (0TaNes courtesy of :iNTa 7o^ertrain) 20 Winter 2021 | E-Mobility Engineering