eAxle unit for the 918 Spyder. Click to enlarge.

The 918 Spyder has three independent power units that can be controlled separately. The combustion engine and rear electric motor are arranged in series on a common axle and drive the rear axle via the PDK. The second electric drive of the 918 Spyder—i.e., the eAxle—has a mechanical effect on the front axle, independently of the rear wheels. The drive torque is controlled separately for each axle, which is why the hybrid super sportscar features electrical Porsche Traction Management (ePTM).

This system affords the 918 Spyder its unique traction and driving dynamics. Unlike the rear axle, the front axle electric motor is a permanently energized synchronous motor with an internal armature. This type of electric motor concentrates a particularly large amount of power on small volume with high torque and low weight.

The downstream transmission is designed for the best possible performance both in terms of starting up and when taking to the race track. To this end, the constant gear ratio is selected such that high torque is created at low speeds. The electric motor reaches its maximum revolutions at 16,000 rpm—corresponding to a speed of around 265 km/h. At speeds above this, the transmission and the electric motor are decoupled via the integrated jaw clutch. The electric all-wheel drive function is therefore available virtually at all times.

Realizing our vision for a super sports car capable of setting a record lap time of the Nurburgring and of achieving fuel efficiency of three liters per 100 km placed huge demands on our engineers and suppliers. GKN Driveline’s role in the development program and its innovative eAxle module has earned the company Porsche Technology Partner status.

—Porsche Powertrain Manager Christian Hauck

A key target for the 918 Spyder was to have the lowest possible center of gravity and ground clearance. To achieve this required an original engineering solution: GKN Driveline positioned the output overhead, using an innovative lubrication concept to manage the oil flow.

The oil reservoir is positioned above, not below, the geartrain. Oil is gravity-fed down onto the bearings and gears until it reaches the high-speed input at the bottom where it is then circulated back to the oil reservoir at the top. This design minimizes the amount of oil on the input shaft, keeping churning losses to a minimum.

To ensure the critical areas remain lubricated even in disconnect mode, the motor is driven for brief, intermittent intervals to maintain a constant flow of oil.

The Porsche 918 Spyder’s tight packaging also means there is almost no air flow around the transmission and so water cooling was needed to manage the heat generated by the module’s high power density.

To meet the 918 Spyder’s challenging targets for weight, power density, NVH and durability, we used state-of-the-art simulation and analysis tools developed in-house to optimise the geartrain. An eAxle that meets both Porsche’s requirements and the demands of the Nurburgring can deliver in any situation.

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GKN plc is a global engineering group. It has four divisions; GKN Aerospace, GKN Driveline, GKN Powder Metallurgy and GKN Land Systems, which operate in the aerospace, automotive and land systems markets.

GKN Driveline is the world’s leading supplier of automotive driveline components and systems, including CVJ Systems, AWD Systems, TransAxle Solutions and eDrive Systems.