When designing the 918 Spyder, the task faced by the Porsche development team was to create a sports car for the next decade with a highly efficient and powerful hybrid drive. Indeed, the entire car was designed around the hybrid drive and the German car maker has gone so far as saying that the 918 Spyder will act as the “gene pool” for all of its future sports cars.
A great deal of insight gained from the development of Porsche race cars for the Le Mans 24 hours in 2014 has been integrated into the 918 Spyder – and vice versa. The structural concept of the 918 Spyder with a rolling chassis as its basis – a basic vehicle that can be driven even without a body – is an accepted race car tradition at Porsche. The concept of the V8 engine originates from the LMP2 RS Spyder race car. The load-bearing structures, the monocoque and subframe, are made of carbon fibre reinforced polymer (CFRP).
The 918 Spyder is an all-wheel drive concept that combines a combustion engine with two electric motors – one on the rear axle and a second one on the front axle. It is based on knowledge gained by Porsche racing the 911 GT3 R Hybrid.
The current lap time for the Nürburgring-Nordschleife is 7:14 minutes. This time was achieved in the presence of international journalists during test drives in September 2012 – more than a year before start of production. The 918 Spyder prototype was therefore approximately 20 seconds quicker than the Porsche Carrera GT. More test drives on the Nürburgring will follow.
An even more important factor is that the 918 Spyder surpasses previous models and competitors in its efficiency as well. As a plug-in hybrid vehicle, it combines the performance of a racing machine with over 880 hp and low NEDC fuel consumption, which at about three litres fuel per 100 km is better than that of most small cars today.
The entire load-bearing structure is made of carbon fibre reinforced polymer for torsional rigidity. Additional crash elements at the front and rear absorb and reduce the energy in a collision. The unladen weight of approximately 1,640 kg is largely attributable to this concept. The drivetrain components, and all components weighing over 50 kg, are located as low and as centrally as possible within the vehicle. This results in a slightly rear-biased axle load distribution of 57 per cent on the rear and 43 per cent on the front, combined with a low centre of gravity at approximately the height of the wheel hubs: ideal for driving dynamics. The central and low position of the traction battery directly behind the driver not only supports efforts to concentrate masses and lower the centre of gravity, it also provides the best temperature conditions for optimum battery power capacity.
Chassis with rear-axle steering
The multi-link chassis of the 918 Spyder is inspired by motorsport design, complemented by additional systems such as the Porsche Active Suspension Management (PASM) adaptive shock absorber system and rear-axle steering. This incorporates an electro-mechanical adjustment system at each rear wheel. The adjustment is speed-sensitive and executes steering angles of up to three degrees in each direction. The rear axle can therefore be steered in the same direction as the front wheels or in opposition to them. At low speeds, the system steers the rear wheels in a direction opposite to that of the front wheels. At higher speeds, the system steers the rear wheels in the same direction as the front wheels. This improves the stability of the rear end when changing lanes quickly.
Porsche Active Aerodynamic (PAA)
Porsche Active Aerodynamic (PAA), a system of adjustable aerodynamic elements, allows for variable aerodynamics. Its layout is automatically varied over three modes ranging from optimal efficiency to maximum downforce and is tuned to the operating modes of the hybrid drive system.
In ‘Race’ mode, the retractable rear wing is set to a steep angle to generate high downforce at the rear axle. The spoiler positioned between the two wing supports near the trailing edge of the airflow also extends. In addition, two adjustable air flaps are opened in the underfloor in front of the front axle, and they direct a portion of the air into the diffuser channels of the underbody structure. This also produces a ‘ground effect’ at the front axle.
In ‘Sport’ mode, the aerodynamic control system reduces the attack angle of the rear wing, which enables a higher top speed. The spoiler remains extended. The aerodynamic flaps in the underfloor area close, which also reduces aerodynamic drag and increases attainable vehicle speeds.
In ‘E-Power’ mode, the control is configured entirely for low aerodynamic drag; the rear wing and spoiler are retracted and the underfloor flaps are closed. Adjustable air inlets under the main headlights complete the adaptive aerodynamic system.
When the vehicle is stationary or in ‘Race’ and ‘Sport’ mode, they are opened for maximum cooling air intake. In ‘E-Power’ and ‘Hybrid’ modes, they close immediately after the car is driven off in order to keep aerodynamic drag to a minimum. They are not opened until the car reaches speeds of approximately 130 km/h or when cooling requirements are higher.
From comfortable to race-ready: five modes for three motors
The core of the 918 Spyder concept is its distribution of propulsive power among the three power units, and their co-operation is controlled by an intelligent management system. To best exploit these different approaches, Porsche engineers defined five operating modes that can be activated via a ‘map switch’ on the steering wheel, just like a race car.
When the vehicle is started up, ‘E-Power’ is the default operating mode as long as the battery is sufficiently charged. In ideal conditions, the 918 Spyder can cover over 18 miles (30 km) on purely electric power. Even in pure electric mode, the 918 Spyder accelerates from 0 – 62 mph in less than seven seconds and can reach speeds of up to 93 mph. In this mode, the combustion engine is only used when needed. If the battery’s charge state drops below a set minimum value, the vehicle automatically switches to hybrid mode.
In ‘Hybrid’ mode, the electric motors and combustion engine work alternately with a focus on maximum efficiency and minimum fuel consumption. The use of individual drive components is modified as a function of the current driving situation and the desired performance. The Hybrid mode is typically used for a fuel economy-oriented driving style.
• ‘Sport Hybrid’
In more dynamic situations, the 918 Spyder selects the “Sport Hybrid” mode for its power sources. The combustion engine now operates continuously and provides the main propulsive force. In addition, the electric motors provide support in the form of electric boosting or when the operating point of the combustion engine can be optimised for better efficiency. The focus of this mode is on performance and a sporty driving style at top speed.
• ‘Race Hybrid’
‘Race Hybrid’ is the mode for maximum performance. The combustion engine is chiefly used under high load, and charges the battery when the driver is not utilising its maximum output. Again, the electric motors provide additional support in the form of boosting. In this mode, the battery charge state is not kept constant, but instead fluctuates over the entire charge range. In contrast to ‘Sport Hybrid’ mode, the electric motors run at their maximum power output limit for a short time for better boosting. This increased output is balanced by the combustion engine charging the battery more intensively. Electric power is thus available even with several very fast laps.
• ‘Hot Lap’
The ‘Hot Lap’ button in the middle of the map switch releases the final reserves of the 918 Spyder and can only be activated in ‘Race Hybrid’ mode. Similar to a qualification mode, this pushes the traction battery to its maximum power output limits for a few fast laps. This mode uses all of the available energy in the battery.
Main propulsion: the race car’s eight cylinder engine
The main source of propulsion is the 4.6-litre, eight cylinder engine that produces 612 hp of power. The engine is derived directly from the power unit of the RS Spyder. Like the race engine of the RS Spyder, the 918 Spyder power unit features dry-sump lubrication with a separate oil tank and oil extraction. To save weight, components such as the oil tank, the air filter box integrated into the subframe and the air induction are made of carbon fibre reinforced polymer. Further lightweight design measures have resulted in such features as titanium connecting rods, thin-wall low-pressure casting on the crank case and the cylinder heads, a high-strength lightweight steel crankshaft with 180 degrees crankpin offset and the extremely thin-walled alloy steel/nickel exhaust system.
Interesting features of the V8 are that it no longer supports any auxiliary systems, there are no external belt drives and the engine is therefore particularly compact. Weight and performance optimisations achieve a power output per litre of approx. 132 hp/l – the highest power output per litre of a Porsche naturally aspirated engine – which is significantly higher than that of the Carrera GT (106 hp/l).
Unique race car design heritage: top pipes
It isn’t just this engine’s performance, but also the sound it makes that stokes the emotional appeal of the 918 Spyder. This is attributable first and foremost to the so-called exhaust ‘top pipes’: the tailpipes terminate in the upper part of the rear end immediately above the engine. No other production vehicle uses this solution.
The greatest benefit of the top pipes is optimal heat removal, because the hot exhaust gases are released via the shortest possible route, and exhaust gas back pressure remains low. This design requires a new thermodynamic air channelling concept. With the HSI engine, the hot side is located inside the cylinder V, the intake channels are on the outside. There is another benefit as well: the engine compartment remains cooler. This is especially beneficial to the lithium-ion traction battery, as it provides optimum performance at temperatures between 20 and 40 degrees Celsius. Consequently, less energy needs to be used for active cooling of the battery.
In parallel in the drivetrain: hybrid module
The V8 engine is coupled to the hybrid module, since the 918 Spyder is designed as a parallel hybrid: exactly like the current hybrid production models from Porsche. Essentially, the hybrid module comprises a 115 kW electric motor and a decoupler that serves as the connection with the combustion engine. Because of its parallel hybrid configuration, the 918 Spyder can be powered at the rear axle either individually by the combustion engine or electric motor, or via both drives combined. As is typical for a Porsche sports car, the power pack in the 918 Spyder has been placed in front of the rear axle, and does not have any direct mechanical connection to the front axle.
Upside-down for a low centre of gravity: Porsche Doppelkupplung (PDK)
A seven-speed Porsche Doppelkupplung (PDK) transmission handles power delivery to the rear axle. The transmission is a sporty version of the PDK gearbox. It has undergone a complete redesign for the 918 Spyder. To ensure a low mounting position for a low centre of gravity of the entire vehicle, the gearbox was turned upside down by rotating it 180 degrees about its longitudinal axis, in contrast to other Porsche road cars. If no power is required on the rear axle, the two motors can be de-coupled by opening the decoupler and PDK clutches. This is the action behind the Porsche hybrid drive’s typical ‘coasting’ feature, with the combustion engine switched off.
Independent all-wheel drive: front axle with electric motor
On the front axle, there is another independent electric motor with an output of approximately 95 kW. The front electric drive unit drives the wheels at a fixed ratio. A decoupler de-couples the electric motor at high speeds to prevent the motor from over-revving. Drive torque is independently controlled for each axle.
Lithium-ion battery with plug-in charging system
The electric energy for the electric motors is stored by a liquid-cooled lithium-ion battery comprising 312 individual cells with an energy content of about seven kilowatt hours. To supply it with energy, Porsche developed a new system with a plug-in vehicle charge port. This vehicle charge port in the B-column on the front passenger side lets users connect the storage battery to a mains supply at home and charge it.
Using the supplied Porsche Universal Charger (AC), the traction battery can be charged within four hours from a ten ampere rated, fused power socket on the German 230 Volt mains supply, for example. Furthermore, the Porsche Universal Charger (AC) can be installed at home in the garage using the Charging Dock. It enables charging within approximately two hours, irrespective of regional conditions. The Porsche Speed Charging Station (DC) is available as an optional extra. It can fully charge the high-voltage battery of the 918 Spyder in just 25 minutes.