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Hybridising a brand
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01/09/2007
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Porsche is one of the strongest brands in the automotive sector, so how does it go about adopting hybrid technology without affecting driver perceptions? Ian Adcock took a ride in its Cayenne hybrid to find out
A decade ago the words “Porsche” and “hybrid” in the same sentence would have been as unthinkable as “honest” and “politician”. But such is the shift in attitudes, especially in the USA, that the German high performance sports car specialist has developed a hybrid version of its Cayenne SUV.
Although the production version will not appear for at least two years, Porsche has opened the doors of its otherwise top secret Weissach R&D centre to reveal its hybrid technology.
Plans to develop a hybrid SUV were revealed at the Frankfurt motor show two years ago in conjunction with its SUV partners, VW and Audi. The concept Q7 displayed featured a 4.2 litre FSI V8 mated to a 32kW electric motor. While Porsche refused to comment on its partners’ plans it did not deny that the modular hybrid technology could be used on a production V8, although Dr Ing Michael Leiters, general project manager for the Cayenne hybrid, told EAD it would be “a very tight fit”.
Leiters would not detail how the trio had split development between them beyond saying that Porsche was always the lead on the Cayenne/Touareg/Q7 programme and that VW and Audi engineers would be responsible for ensuring their versions reflected their core brand values.
In the interests of improving fuel economy, rather than enhancing performance, Porsche has opted for the more compact, direct injection 3.6 litre, 213kW V6 with the Aisin six-speed automatic gearbox introduced earlier this year.
Porsche claims that in real-life driving conditions there will be a 25% reduction in fuel consumption. Currently the prototype Cayenne hybrids are returning 9.8 litres/100km in the New European Driving Cycle, although Leiters is confident of achieving 8.9 litres/100km by the time the vehicle comes to market.
The SUVs seen at Weissach were anything but rough prototypes and looked like finished products right down to the instrument panel with an additional battery condition gauge and displays showing running conditions. Revealing the car so early allows Porsche to test on open roads unencumbered by disguise, possibly something of an advantage given the rumours about cooling issues.
As might be expected from Porsche, its hybrid does not follow the route taken either by Lexus or Honda.
Unlike the Lexus system, which Porsche describes as an “output-branched hybrid”, its is a parallel hybrid with the electric motor located between the engine and transmission. Packaging the additional 12cm into the vehicle proved a challenge, but has been achieved without any body-in-white changes, says Porsche.
The nickel metal hydride battery stack is beneath the rear floor where the spare wheel usually sits, raising the floor level about 3cm. The 69kg stack is surrounded by its own 7kg steel alloy crash box to protect it in the event of a rear impact. Separate fans draw air from the passenger compartment through and around the battery stack to maintain the –30°C to +40°C operating range.
In common with many other the systems on the car, negotiations are still being conducted with suppliers about component sourcing. However, one definite technology partner is Bosch, which worked with Porsche on developing the 34kW, 285Nm three-phase synchronous electric motor.
The Cayenne can be driven in three modes, electric only, electric and petrol or petrol only depending on conditions and driver demands. This intelligent drive control is managed by a 32-bit hybrid manager ECU dealing with 20,000 parameters, compared with 6,000 for a petrol-engined Cayenne.
The biggest challenge, said Leiters, was developing a software strategy that would allow seamless integration of the engine and electric motor as the car is moving. The software developed to allow this integration is the subject of one of several patents on the vehicle Porsche holds or has pending, either jointly with suppiers or independently.
The system has a disengagement clutch between the flywheel and electric motor and a converter with lock-up clutch between the electric motor and transmission. When needed, the electric motor engages the clutch and fires up the engine in milliseconds. This is aided by a new sensor that continuously monitors crankshaft position to determine which piston is in the optimum position to initiate ignition. The torque converter with its own lock-up clutch balances the torque of the electric motor and the V6 until they’re in harmony when it engages to produce a seamless mating between the two power units.
In practice this means the Cayenne can start under electric power, or in urban stop-go conditions operate purely as an electric vehicle. When the driver demands more power the petrol engine seamlessly integrates to provide more torque and greater acceleration. Porsche will not disclose performance figures beyond saying there is a “significant” improvement over the standard V6.
The electric motor will eventually cut out leaving the Cayenne powered by the V6. However, judicious use of the throttle when cruising means the engine cuts out leaving the Cayenne “sailing” ,in Porsche terms, on electric power at speeds up to 120km/h (75mph).
The electric motor eliminates the need for both the alternator and starter, but running on electric power only means the air-conditioning system, oil pump, brake vacuum and power steering are also electrically operated. Porsche clams the electro-hydraulic power steering is a first in this class of vehicle.
One engineer who preferred to remain anonymous said these power saving technologies would migrate to other models in the future.
The hybrid Cayenne is expected to only account for some 4,000 to 6,000 global sales a year and has been made financially viable by Porsche’s partnership with VW and Audi and the fact that the drive will also be in the Panamera four-seater coupe, although not at launch.
In the debate about hybrid versus diesel, Porsche argues that a low revving diesel is the very antithesis of what it stands for in terms of brand DNA. But one senior engineer admitted “they weren’t yet convinced” that diesel would be preferred to hybrid in the future. Coming from Porsche, this is the nearest we are likely to get to an admission that diesel has hove into view on its horizon.
Ride reactions
I am not allowed to drive the prototype Cayenne hybrid, just be a front-seat passenger. Apart from the garish paintwork, a battery charge gauge on the instrument panel and visuals on the command screen in the centre stack and ahead of the driver showing how the power is being generated are the only indications that this is a hybrid… Until the ignition key is turned. There’s complete silence.
‘Drive’ is selected and we’re off in a car that sounds as exciting as a golf buggy. Then my driver guns the throttle and the V6 instantly rasps into life, surging the car forward onto Weissach’s test track. Even though Porsche won’t reveal any hints as to how much quicker the hybrid is at low acceleration, having an additional 285Nm added before the V6 reaches its peak torque of 385Nm at 3,000 rev/min certainly makes its presence felt.
Cruising round the test track at 100km/h the driver backs off the throttle and we start “sailing” ,as Porsche likes to term it. The V6 is silent, just the electric motor whirring softly in the background accompanied by rustling wind round the A posts.
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Author
Ian Adcock
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