Bentley to reveal alt-fuel car at Geneva

  • 25-Feb-2009 10:32 EST
Bentley Supersports 1_1.jpg
Bentley's new Continental Supersports is the company's first biofuel-capable production model.

A lighter weight biofuel Bentley will be presented at the Geneva Motor Show next month. Engineering Director, Dr. Ulrich Eichhorn, describes it as being not only the greenest Bentley ever to enter production but also the most powerful, fastest, and most extreme: “It is the first biofuel supercar.”

The combination of very high output—463 kW (621 hp) and 800 N·m (590 lb·ft)—provides the two-seat Bentley Continental Supersports with performance figures that include a 0-100 km/h (0-62 mph) time of 3.9 s and a 329-km/h (204-mph) top speed. CO2 emissions have not been certified yet but are expected to be sub-400 g/km, representing a cut of about 2% compared to the GT Speed. “Yes, it is a small reduction, but we are getting a significant improvement in performance yet still achieving that reduction,” stressed Eichhorn.

It has taken the company less than two years to develop the flex-fuel car (it can run on E85, straight gasoline, or any mix of the two), although R&D biofuel programs by Bentley, and particularly the Volkswagen Group (of which it is a part), have been in place for many years.

Eichhorn insists that the Supersports is not just a technology demonstrator but also a “serious production car” although it has only two seats and will be priced about 20% above the current Continental GT Speed.

It represents a step toward Bentley’s stated aims of achieving a reduction of overall tailpipe CO2 of at least 15% by 2012 through improved technologies for current powertrains, new transmission systems and drivelines, weight reduction, and attention to detail development.

But more is expected. Particularly significant is Bentley’s plan to introduce a new powertrain, within about three years, capable of providing a 40% improvement in fuel economy. By then, all new cars from the company will be compatible with renewable fuels.

Bentley is placing future emphasis on well-to-wheel measurement of emissions rather than tailpipe figures. “With CO2, it is the global concentration that matters,” said Eichhorn. “It doesn’t matter if it comes out of the power station or the car.”

The Supersports’ engine has some 50 component changes compared with the Continental GT Speed unit, with intercooler and turbocharger efficiencies improved to increase power output and provide an extra 50 N·m (37 lb·ft) of torque. Subsystem changes include a sensor to monitor fuel composition; E100 is achievable, but cold start below -7°C (+19°F) may be difficult. Also, there are different fuel system gaskets, pipework, and pumps to achieve chemical compatibility; and the tank filler neck is changed.

Because the energy density of ethanol is about 30% below that of gasoline, higher pressure, variable-flow pumps of increased diameter are necessary. Fuel-injection pressure needs to be modulated between 4 and 6 bar (58 and 87 psi). With E85 in its tank, fuel consumption of the Supersports is about 25% higher than when burning only gasoline.

In addition to working with the Volkswagen Group on development of the flex-fuel engine, Bentley was also involved with other companies including Lotus Engineering.

Another significant technology development for the Supersports is the use of a quick-shift system for its ZF 6HP26 transmission. Software specialists have achieved significant improvements, and Bentley claims a reduction of some 200 ms (about 50%) for shift times, with the car able to accelerate through the shifts.

Fuel and ignition cutoffs during upshifts have also been introduced, and the resultant torque reduction is improving shift quality and durability. Gearbox and engine controllers interact to provide positive torque during downshifts via throttle “blipping.” The transmission also incorporates a strengthened planetary gearset.

As with the Continental GT coupe and Flying Spur sedan, the new car is all-wheel drive but has a 40:60 front/rear torque split bias instead of 50:50. Damping has been retuned and the steering has reduced friction.

Reduced weight was an essential element of the Supersports program, and the result is a car 110 kg (220 lb) lighter than the Continental GT Speed.

It was weight reduction potential that led to the Supersports and its development into biofuel capability. “When we were doing the GT Speed project, some members of the team thought it would be cool to create an even sportier car,” explained Eichhorn. Paring interior equipment to a minimum and taking out the two rear seats achieved a reduction of 200 kg (440 lb).

But that was deemed too radical; a “basic” Bentley is not what the marque is about; customers are used to having carpets and headliners! But two seats instead of four, the rear becoming a stowage area with a carbon-fiber load retention bar, was acceptable. The bar adds some torsional stiffness. Seats are lightweight and incorporate carbon fiber in their structure.

Of the 110 kg (240 lb) saved for the production Supersports, front seating saved 45 kg (99 lb); interior trim reduction using Alcantara (the appearance and tactile feel of which is similar to that of suede) instead of leather, lighter substrates, and rear-seat deletion, 26 kg (57 lb); SGL carbon brakes (standard), 20 kg (44 lb); wheels, 10 kg (22 lb); and chassis components, 9 kg (20 lb).

Changes to the car’s exterior include larger air intakes to feed the intercoolers as well as louvers on the hood to extract hot air. Extensive use of computational fluid dynamics was used to achieve required airflow for cooling. A chin spoiler is fitted, the rear track widened by 50 mm (2 in) via different offset for the wheels, and the rear side panels are slightly wider. The 9.5J x 20 in alloy wheels, produced by Washi Beam, save 2.5 kg (5.5 lb) each.

Bentley has introduced PVD (physical vapor deposition) to give a shiny, smoked steel finish effect to stainless-steel surfaces of the new Bentley. Eichhorn believes it is the first automotive application of dark PVD. Relatively costly, it is highly scratch resistant.

Bentley has already announced that it aims to reduce overall tailpipe CO2 by at least 15% by 2012 through improved technologies for current powertrains, new transmission systems and drivelines, weight reduction,­ and attention to detail development.

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