World’s first affordable diesel hybrid powertrain

Battery and electrical systems

The battery, mounted on the boot floor flush with the loading lip, is a lithium ion unit supplied by Gaia. Compared with today’s Nickel metal hydride (NiMH) batteries, this chemistry offers very high energy density at a potentially lower production cost. “We surveyed all available technologies, including NiMH cells and electrochemical supercapacitors, and concluded that the Gaia Li-Ion solution offers the best combination of weight, cost and packaging volume for hybrid vehicles,” says Neil Cheeseman, project manager for the ULCCC hybrid smart forfour. “Our research has shown it has considerable potential for production applications.”

Battery cells are cooled with cabin air blown through the battery compartment, exiting through new vents in the rear of the vehicle. As the vehicle is a plug-in hybrid, an on-board charger has been fitted that can recharge the hybrid battery in under seven hours from 0-100% SOC (state of charge). The charging system is mounted in the spare wheel well under the battery and should not need servicing. The other key power component, the inverter, is mounted under the bonnet, close to the electric motors to minimise electrical losses. It’s innovative, software-managed current control system is used for both DC and AC machines, providing vehicle manufacturers with substantial design flexibility and the possibility of component sharing across different architectures.

As well as supplying the 300V three phase power for the motors, Zytek’s inverter supplies 1,800W of 12V power for the vehicle’s electrical systems.

The battery can be charged in three ways: via regenerative braking to efficiently capture the vehicle’s kinetic energy instead of dissipating it as heat; via the engine driving the starter/alternator to generate electricity; and by plugging the vehicle in to a conventional domestic 13Amp electrical socket using a connection hidden under the rear numberplate. Two additional safety pins on the plug, which is carried with the vehicle, disable the powertrain while it is connected to the mains supply.

Further fuel consumption savings have been achieved by replacing parasitic systems with more efficient and controllable electrically driven systems. When the engine is running, the flow of coolant is much more closely controller via use of an electrically-driven water pump, only consuming power when necessary. The brake vacuum pump has been supplemented with an electric pump so braking assistance can be provided even when the engine is off.

There is also a 6kW electric water heater to provide cabin warmth when the vehicle is operating as an electric vehicle, and a 5kW electric air conditioning compressor to provide all the cabin cooling requirements.

The electric water heating element is also used to dissipate energy when the batteries are full and the vehicle is slowing using regenerative braking. This helps the engine and cabin heating system warm up more quickly and provides a more consistent brake feel. “An important objective for this vehicle is that it is totally ready for production,” says Cheeseman. “It has the design detail and sophistication needed to enter the market immediately.”

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