|The 918 Spyder. Click to enlarge.|
Porsche has introduced the 918 Spyder plug-in hybrid sports car, the first of six or more plug-in hybrids coming from the Volkswagen Group. (Earlier post.) Porsche had unveiled the concept version of the 918 Spyder Plug-in Hybrid at the Geneva Motor Show in 2010. (Earlier post.)
The 918 Spyder features a novel all-wheel drive concept with a combination of a 608 hp (453 kW) V8 combustion engine, 115 kW electric motor on the rear axle and ~95 kW electric motor on the front axle. Combined system power is 887 hp (661 kW). It is based on knowledge gained by Porsche during motor races with the 911 GT3 R flywheel Hybrid (earlier post).
V8. The main source of propulsion is the 4.6-liter V8 derived directly from the power unit of the successful 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 fiber reinforced polymer.
Further extensive 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.
The V8 no longer supports any auxiliary systems, there are no external belt drives and the engine is therefore particularly compact. Weight and performance optimizations achieve a power output per liter of approx. 133 hp/l—the highest power output per liter of a Porsche naturally aspirated engine—which is significantly higher than that of the Carrera GT (106 hp/l).
|Tailpipe terminate in upper part of rear end. Click to enlarge.|
The tailpipes terminate in the upper part of the rear end immediately above the engine; no other production vehicle uses this solution. The top pipes’ greatest benefit 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 channeling 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 68 and 104 degrees Fahrenheit. Consequently, less energy needs to be used for active cooling of the battery.
Hybrid modules. The V8 engine is coupled to the hybrid module—the 918 Spyder is designed as a parallel hybrid as are the current hybrid 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 jointly.
A seven-speed Doppelkupplung (PDK) transmission handles power transmission to the rear axle. The high-performance transmission is the sportiest version of the successful PDK; it has undergone a complete redesign for the 918 Spyder and has been further optimized for high performance.
To ensure a low mounting position for a low center of gravity of the entire vehicle, the gear unit was turned “upside down" by rotating it 180 degrees about its longitudinal axis, in contrast to other Porsche series. If no power is required on the rear axle, the two motors can be decoupled by opening the decoupler and PDK clutches. This is the action behind the Porsche hybrid drive’s typical “coasting" with the combustion engine switched off.
The front axle is equipped with 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 decouples the electric motor at high speeds to prevent the motor from over-revving. Drive torque is independently controlled for each axle. This makes for very responsive all-wheel drive functionality that offers great potential in terms of traction and driving dynamics.
Li-ion battery. The 6.8 kWh, liquid-cooled lithium-ion battery pack comprises 312 individual cells and delivers 220 kW maximum power. The battery of the 918 Spyder has a performance-oriented design in terms of both power charging and output, so that it can fulfill the performance requirements of the electric motor.
The power capacity and the operating life of the lithium-ion traction battery depend on several factors, including thermal conditions. The global warranty period for the traction battery is seven years.
Porsche developed a new system with a plug-in vehicle charge port and improved recuperation potential. This vehicle charge port in the B-pillar on the front passenger side lets users connect the storage battery to an electrical supply at home to charge it. The charge port is standardized for the country of purchase.
The on-board charger is located close to the traction battery. It converts the alternating current of the household electric supply into direct current with a maximum charge output of 3.6 kW. Using the supplied Porsche Universal Charger (AC), the traction battery can be charged with a conventional wall plug in seven hours from a 10A-rated, fused power socket a US 110 Volt household electrical supply, for example.
Furthermore, the Porsche Universal Charger (AC) can be installed at home in the garage using the Charging Dock. It enables rapid and convenient 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.
Five operating modes. The Porsche developers defined five operating modes for the 918 Spyder that can be activated via a “map switch" on the steering wheel, just as in motorsport cars. On the basis of this pre-selection, the 918 Spyder applies the most suitable operating and boost strategy without driver intervention, thus allowing the driver to concentrate fully on the road.
E-Power. E-Power mode is the default operating mode at start-up, as long as the battery is sufficiently charged. In ideal conditions, the 918 Spyder can cover approximately 18 miles (29 km) on battery power. In pure electric mode, the 918 Spyder accelerates from 0 to 62 mph in 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.
Hybrid. 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 optimized for greater 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 and an especially sporty driving style. The combustion engine is chiefly used under high load, and charges the battery when the driver is not utilizing its maximum output. Again, the electric motors provide additional support in the form of boosting. Furthermore, the gear-shifting program of the PDK is set up for even sportier driving.
he electric motors are used up to the maximum power output limit to deliver the best possible performance for the race track. In this mode, the battery charge state is not kept constant, rather it 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.
Weight. The entire load-bearing structure is made of carbon fiber reinforced polymer (CFRP) for extreme torsional rigidity. Additional crash elements at the front and rear absorb and reduce the energy of a collision. The car’s unladen weight of approximately 3,715 lbs. (3,616 lbs. with “Weissach" package), a low weight for a hybrid vehicle of this performance class, is largely attributable to this concept, Porsche said.
The drivetrain components and all components weighing above 110 lbs. (50 kg) are located as low and as centrally as possible within the vehicle. This results in a slightly rear-end biased axle load distribution of 57% on the rear axle and 43% on the front axle, combined with an extremely low center of gravity at approximately the height of the wheel hubs, which is 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 center 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 Porsche 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. Basically, 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.
This makes cornering even more direct, faster and more precise, and it reduces the turning circle. At higher speeds, the system steers the rear wheels in the same direction as the front wheels. This significantly improves the stability of the rear end when changing lanes quickly. The result is very secure and stable handling.
Porsche Active Aerodynamic (PAA). Porsche Active Aerodynamic (PAA), a system of adjustable aerodynamic elements, ensures 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 somewhat, 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" 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 round off the adaptive aerodynamic system. When the vehicle is stationary and 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 81 mph (130 km/h) or when cooling requirements are higher.