Overview: Porsche’s hybrids

Manufacturers of large and high-performance vehicles can say what they will about thermal efficiency: the squeeze is on outright mpg figures, and companies like Porsche are feeling it. Sportscar manufacturers at this level depend on glamour and image, even if these dubious and evanescent qualitites are spinoffs from the grittier stuff of raw engineering. It’s the headline performance figures that give a sportscar company its status. So the manufacturers are stuck between the public’s expectations of glamour and speed, and the rightful demand of legislators to pare carbon emissions to the bone.

Diesel engines haven’t been widely used in sportscars. Audi’s R8 TDI sadly vanished before it ever made it onto a price list, and all of the fast diesels we have are sports saloons. Emotions and perceptions are very powerful in this part of the market; the exhaust note matters. Never mind that Peugeot achieved a one-two finish at le Mans last year with diesel-engined 908 HDis, selling on image is another matter entirely. Diesel works, but it’s sensible and unemotional.

So if we want to make an ‘efficient’ sportscar for the more rarefied end of the market, we need to look at making it a series-parallel hybrid. This means that it can run on electric power alone for some distance — though probably not far, unless we want a very heavy battery pack; it can run on its petrol engine alone, for ‘normal’ motoring; and it can use petrol and electric power together to exploit the torque of the electric motor, achieving brief but spectacular acceleration.

But Porsche’s interest in hybrids goes beyond the show-room. Of the triumvirate of new hybrids announced by the Company at the Geneva motor show in March, one of them is a competition car. Even allowing for the vehicle’s immediate function as a test-bed, this shows considerable faith in hybrid technology.

Here we take a look at the three Geneva hybrids in turn: the Cayenne, the 918 Spyder, and the 911 GT3 R.

Cayenne S Hybrid

Porsche’s new generation of Cayenne SUV débuted at Geneva in March, with the hybrid model in the lineup from the start. The new cars are lighter than their predecessors — the Cayenne S, for example, loses 180kg — and feature a new eight-speed epicyclic automatic transmission with Tiptronic manual control. The new gearbox incorporates an automatic stop-start function on all models; the engine coolant and transmission oil are now thermally managed — that is, they are kept out of their respective cooling circuits until a pre-determined temperature has been achieved; and kinetic energy capture is used to charge batteries on all versions, not just the hybrid.

If ever there were a Porsche that could comfortably use a diesel engine without losing its cool, the Cayenne is it. And in fact there is a Cayenne diesel: it uses a three-litre V6 that delivers 240PS and 550Nm, achieving a combined 38.2mpg without the help of a heavy and complex hybrid system. The Cayenne S Hybrid also uses a three-litre V6, but it’s a supercharged petrol engine of 333PS. Its companion electric motor delivers 47PS. The two power unit working together provide 580Nm at 1000rpm (one thousand — that’s not a typo). A clutch between the electric motor and the V6 allows the two units to function independently.

Porsche Cayenne S Hybrid.

Porsche does not quote a range under electric power, only an approximate maximum speed ‘approaching 40mph’.

The electric motor operates to boost the V6 when pulling away from rest, when the electric power pack is at its most efficient. Under light loads at speeds up to 97mph, the piston engine can be disengaged completely from the driveline: Porsche calls this ‘sailing mode’.

The Cayenne Hybrid returns a combined fuel economy figure of 34.4mpg with CO2 emissions of 193g/km. The much slower diesel model achieves 38.2mpg and 195g/km — though a hybrid based on the diesel powertrain would obviously be more economical still. (It would also provide even better torque than the petrol-electric combination, to the extent that electronically limiting the combined drivetrain torque might prove necessary to keep the transmission in one piece.)

Porsche Cayenne
S Hybrid
Body type 5-door
SUV
Cylinders 6V
Valves 4
Bore/stroke 84.5/89.0
Swept volume 2995cc
Compression
ratio
10.5:1
PS/rpm
E-motor
Combined
333/5500
47
380/5500
Nm/rpm
E-motor
Combined
440/3000
300
580/1000
High V battery:
Type
NiMH
High V battery:
Nominal voltage
288
Capacity 6Ah
Maximum speed 150
0-100km/h 6.5s
Urban MPG
(l/100km)
32.5
(8.7)
Combined MPG
(l/100km)
34.4
(8.2)
CO2 g/km 193
Transmission A8
Driven wheels All
Torque F:R 40:60
Wheels
Tyres
8.0Jx18
205/55 XL
Brakes: front
Brakes: rear
360x36
330x28
Kerb mass* 2240
Length 4846
Width 1939
Height 1705
Wheelbase 2895
Cd 0.36
CdA 1.008
Track: front
Track: rear
1655
1669
Fuel tank 85l
* DIN. EU kerb mass = DIN + 75kg.

Drivetrain aside, the Cayenne Hybrid is similar to its siblings. It has a unitary, fully-galvanised all-steel bodyshell. Unequal-length double wishbones in combination with struts are used to locate the front wheels, with a multi-link arrangement at the back. The Hybrid’s petrol engine is an all-alloy unit with two camshafts for each cylinder bank; the timing of the intake camshafts is variable. The cylinders are directly fuelled, and supercharged by means of a mechanical com­pressor. A single three-way catalytic converter serves both banks of cylinders, though two lambda sensors are used on each side. The ignition system uses six undividual coils.

:Next

Porsche Cayenne S Hybrid: 1. Nickel metal hydride battery. 2. Air supply duct. 3. Power electronics. 4. Hybrid module. 5. V6 petrol engine.
Text and design copyright © Under the Skin 2010. We recommend Firefox.