Xtrac (www.xtrac.com) and Zytek (www.zytek.co.uk) have joined forces to create a diesel-electric hybrid drive system for a next-generation small family car. The dual-mode hybrid prototype is housed in a SMART ForFour, and includes a 1.5-liter three-cylinder diesel mated to a torque-split gearbox sandwiched between a DC starter/generator in the bellhousing, and an AC traction motor at the other end of the transmission. The gearbox weighs just 77 lb., and is 1.6-in longer than the SMART’s standard manual transmission.
From 0 to speeds as high as 45 mph, the car relies on the electric traction motor—a 50-kW brushless AC permanent magnet unit providing up to 105 Nm of torque—for power. From there, it shifts power to the diesel engine, and depends on proprietary software to perform torque blending as the electric motor’s participation is reduced to zero. Under hard acceleration, the DC motor assists the diesel engine. This allows a high input ratio to be engaged at low engine revs to keep the diesel operating in its most efficient range. The electric motors engage the drive unit when a barrel within the gearbox is electronically rotated to move the selector fork. This automatically matches the input and gearbox layshaft speeds for seamless zero torque interruption shifts.