24/5/11

Overview: Porsche Boxster E

Porsche demonstrated both rear- and four-wheel drive electric Boxsters at the Michelin Challenge Bibendum in Berlin.

The Boxster E concept cars — which is all they are at present — are being used as part of the large-scale ‘Model Region Electro-Mobility Stuttgart’ trial to explore the everyday practicality of all-electric vehicles and how they are used, especially in terms of driving and battery charging.

Porsche Boxster E.

Propulsion in the case of the all-wheel drive variant is provided by two independent electric motors with single-speed reduction gears that act on the front and rear axles. The rear drive unit comprises the motor and transmission, together with the power electronics for controlling the motor; this is located where the manual transmission and exhaust system would be in the conventional Boxster. At the front, the space vacated by the 64-litre fuel tank is occupied by the second drive unit, with power electronics for the front motor in the adjacent luggage compartment. A central electric control unit looks after the synchronisation of the two electric motors and controls the drive torque distribution to the front and rear axles. Kerb mass is around 1600kg.

The space normally occupied by the flat-six petrol engine is used for the lithium ion phosphate battery-pack. It is anchored to the same points on the body structure as the combustion engine in the Boxster S, so the load paths in the event of a collision are all the same. The nominal cell voltage is 3.3V, and each cell has a capacity of 20Ah. Based on the NEDC, the combined energy from the 440 individual cells is good for a little over 100 miles of driving.

With an overall mass of 341kg, the battery’s total capacity is is 29kWh, of which for inherent physical reasons approximately 26kWh are available for use. Its maximum power output is 240kW, or 60kW more than the all-wheel drive Boxster E draws under full load.

The battery and power unit components are connected to two water cooling circuits. As with the conventional power unit, the heat exchangers are accommodated behind the front air intakes.

As with the combustion engine in the production car, the traction battery is installed in the Boxster E from below, and can be swapped out very quickly if necessary. This is facilitated partly by special motor-sport fasteners that enable the battery to be disconnected from the cooling circuit without air getting into the system.

The charger is accommodated at the front of the car. Its charging capacity is a maximum of 3.3kW, so a full charge takes about nine hours, but there is a rapid charging function. There is enough space left over for luggage in the front storage compartment despite the electronic components; the rear luggage compartment of the Boxster E remains unchanged.

The two electric motors deliver 244PS and 540Nm between them; their maximum shaft speed is 12,000rpm. As the power units are electric motors, maximum torque is available from the first revolution.

Porsche quotes a standing-start acceleration time of 5.5s to 100km/h; maximum speed is limited to 124mph. The high power output of the two motors offers a second advantage: it means that they can recover more energy during deceleration and braking.

Rear-wheel drive Boxster Es are simply the same thing without the front drivetrain. Maximum outputs are therefore 122PS and 270Nm.

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