Porsche’s 918 Spyder prototype is a two-seat mid-engined sportscar. Its drivetrain is a series-parallel petrol-electric plug-in hybrid.
The car’s headline figures are intriguing: spectacular standing-start acceleration, with 100km/h coming up in 3.2 seconds; a maximum speed of 198mph; and a combined cycle fuel return of 94mpg, with a CO2 yield of 70g/km. Obviously it’s not going to deliver all of these figures at the same time, but this is nevertheless a fine all-round performance.
The 918 will apparently lap the Nürburgring Nordschleife in less than seven minutes and 30 seconds: faster than the 911 Carrera GT, though handling that’s better balanced than the 911’s can’t do any harm.
Porsche 918 Spyder.
The 918’s petrol engine is a V8, derived from the unit used in the RS Spyder racing car and developing more than 500PS; its maximum crankshaft speed is 9200rpm. Two electric motors are used — one for each axle — providing four-wheel electric drive, with a combined output of 218PS.
Power from the V8 is transmitted solely to the rear wheels, by way of a seven-speed PDK (Porsche-Doppelkupplungsgetriebe) double-clutch transmission; this gearbox also feeds the power of the rear electric drive motor to the back wheels. The electric traction motor at the front powers the wheels through a single-speed reduction gear.
Energy storage is in the form of a liquid-cooled lithium-ion battery pack behind the passenger compartment. The 918 is a plug-in hybrid, so the pack can be charged from the electricity network. As you would expect, kinetic energy capturing also provides charge during braking.
A button on the steering wheel allows the driver to choose between four different driving modes.
In ‘E-Drive’ mode, the car runs under electric power alone, with a range of up to 16 miles. In ‘Hybrid’ mode, both the electric motors and the combustion engine are used, with the balance between the two power sources determined by driving conditions. ‘Sport Hybrid’ mode again uses both power sources, but
with the focus on performance. Most of the drive torque goes to the rear wheels, with torque vectoring to distribute torque according to the available traction at each wheel.
In addition to the three driving modes that are set up essentially for road use, there is also a ‘Race Hybrid’ mode. Here, the drivetrain offers its maximum performance and driving dynamics on the track, with all of the power-units operating close to their limits.
Porsche exploits the ability of an electric motor to generate very large amounts of torque — albeit briefly — with a ‘press to pass’ button. Its function is to wring maximum torque from the electric motors for the few seconds that the available charge will allow. If the battery pack is depleted, the system is disabled.
The 918 prototype uses a lightweight body structure influenced by Porsche’s motorsport activities. The monocoque is moulded from carbon-fibre reinforced plastic (CFRP), giving very good torsional stiffness; magnesium and aluminium are also widely used. The finished car weighs around 1490kg empty.
Inside, the 918 uses a touch-screen to reduce the number of visible controls, while the steering wheel carries some of the more important switchgear. Unfortunately, Porsche has succumbed to the temptation of having the instrument lighting colour change according to driving mode: green for the more conscientious running modes, red for the performance-oriented programs. Ho hum.
The 918 Spyder is fitted with a ‘range manager’. This uses the map in the navigation system to show the car’s remaining range. In cities, the Range Manager shows whether the driver can reach the pre-set destination on electric power alone. The locations of recharging points will ultimately be included in the mapping data.
911 GT3 R Hybrid
Developed chiefly as a rolling laboratory for live testing on race-tracks, the hybrid 911 is not a workable road car. Not the least reason for this is that there’s a large flywheel assembly, powered by an electric motor, parked next to the driver: a passenger seat would be out of the question.
Like the 918 concept car, the 911 hybrid racer uses two electric motors and separates the electric motive effort from the work done by the combustion engine. But that’s where the similarity ends. Where the 918 has an electric motor for each axle, the 911 uses one for each front wheel: rear-wheel drive is delivered by the flat six, front-wheel drive by electric power.
911 GT3 R Hybrid.
Maximum power for the straights is delivered by the petrol engine. The front-mounted electric motors make little contribution under sustained high-output conditions. They, and their very unusual power source, are intended to deliver short bursts of torque to assist in accelerating the 911 out of bends.
During braking, the Porsche’s electric motors act as generators — a common enough arrangement in itself. But the electrical power they generate is not fed into a battery pack. Rather, it is used to power a third electric motor; this in turn drives a large, heavy flywheel that sits in the cabin next to the driver. The flywheel runs at up to 40,000rpm and stores kinetic (rather than electrical) energy. When the driver demands power, the flywheel’s motor in its turn becomes a generator, delivering 120kW of electrical power to the front traction motors. These deliver torque to the wheels, and the electromagnetic load on the flywheel’s motor-generator slows the flywheel. If the flywheel is spinning at full speed, it stores enough energy to drive the traction motors at full load for between six and eight seconds.
Interestingly, the quoted torque output of the traction motors is very modest in relation to their maximum power output: could it be that they are being regulated to prevent the 911’s lightly-laden front wheels from spinning?
Should race strategy demand fewer pit-stops, the contribution made by the electric motors can be used to reduce fuel consumption rather than boosting acceleration. Alternatively, the fuel load can be reduced, cutting weight.
911 GT3 R Hybrid
Wheels: front Wheels: rear
Tyres: front Tyres: rear
Brakes: front Brakes: rear
* DIN, approximate. EU kerb mass = DIN + 75kg.
Take away the electric drive and its management systems, and the 911 GT3 R Hybrid is similar to the ‘normal’ GT3 R. There’s a good deal of Kevlar in the bodyshell, the underside is closed and flat, and air jacks are built in. The roll cage is welded in place.
Fuelling of the atmospheric flat six is by manifold fuel injection; lubrication is — necessarily — by means of a dry sump arrangement. The highly adjustable suspension system uses McPherson struts at the front and multi-link location at the rear.