World’s first affordable diesel hybrid powertrain

Zytek’s novel solution is a dual-mode hybrid that eliminates these compromises by using sophisticated control strategies to replace clutches and complex epicyclic gearsets. In addition to the many advantages of dual-mode operation, this reduces component count, eliminates wearing friction surfaces, reduces packaging volume and substantially reduces manufacturing costs. The design also allows each motor to be optimised for its function within the drivetrain.

An affordable diesel hybrid transmission

To take the substantial step in efficiency needed to exceed the ULCCC emissions target, Zytek decided that its new hybrid should be built around a small, very high efficiency diesel engine. With common rail injection systems and sophisticated aftertreatment, the latest diesels provide quiet operation and exceptional fuel economy, combined with very low tailpipe emissions. They also work well in hybrid transmissions because the electric drive can be used to disguise turbo lag and give exceptional driveability.

The company already had a long heritage of electric and hybrid vehicle engineering programmes with DaimlerChrysler’s smart brand, so the 1.5 litre, three cylinder smart forfour CDi was the natural choice of base vehicle. DaimlerChrysler was extremely supportive and provided a vehicle and technical data for the project.

The biggest challenges for Zytek were packaging and cost. The help solve these, programme manager Neil Cheeseman decided to develop a bespoke hybrid transmission that would directly replace the existing gearbox without any changes to the vehicle body or powertrain mounting points. This novel dual-mode technology would also completely eliminate the usual licensing issues associated with the use of hybrid vehicle epicyclic gearboxes. To keep cost and engineering effort down, the new transmission would use largely proven production components.

The dual mode architecture has two motors: a starter/generator permanently engaged with the engine and a traction motor. Both are specifically designed for their application and engaged with the drivetrain when required via a dog synchromesh system similar to that found in automotive manual transmissions. A highly sophisticated control system, a key Zytek strength, takes the place of the synchro cone by automatically matching the speed of the input shaft and the lay shaft to allow seamless shifts with zero torque interruption. The technology uses experience from Zytek’s highly successful A1 Grand Prix engine and gearbox, which last season completed more than one million clutchless gearchanges at race speed, without a single failure.

The starter/generator is a 43kW DC brushless permanent magnet device capable of operating at speeds up to 3,600rpm. It is integrated into the gearbox bellhousing and sized to allow engine cranking at temperatures down to -40oC. It also provides torque assist when required to boost acceleration. Supplementing drive torque in this way allows a single very high gear to be engaged at low revs (1,600rpm) to keep the engine operating in its most efficient range. Cooling is provided by means of a water jacket integrated within the diesel engine’s cooling system.

The traction motor is a 50kW AC brushless permanent magnet device mounted on the end of the gearbox and capable of operating at speeds up to 11,500rpm. Cooling is provided by a dedicated system shared with the inverter. Testing of the traction motor has shown a peak efficiency of more than 97 percent. Although it shares the same fundamental architecture as the starter/generator, Zytek has chosen a different internal construction to optimise the device for low speed, high torque operation.

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