Engineering Bentley’s first ‘FlexFuel’ engine

At the 2008 Geneva Auto Salon, Bentley announced a far-reaching environmental programme to reduce CO2 emissions and improve fuel economy across its model range.

An important element of this strategy is by 2012 to make the entire Bentley fleet capable of running on renewable fuel through the introduction of FlexFuel technology, where engines are capable of running seamlessly on bioethanol, petrol or any mix of the two.

At the same time Bentley declared that this would be achieved without sacrificing its well-known performance and luxury.

This statement of intent has taken shape in the Continental Supersports – a car that balances extreme potency with the pioneering use of biofuel technology in the luxury sector.

The result is a net CO2 reduction of up to 70 per cent on a well-to-wheel-basis, the measurement of CO2 release of a fuel from its production (well) to its combustion or deployment (wheel).

Bioethanol can be derived responsibly from crops such as corn, soybeans, sugarcane and switchgrass. Second generation bioethanol uses the non-food fibres of the plant (biomass), as well as agricultural waste and forestry residues. These are collected and refined into cellulose ethanol.

CO2 is absorbed by plants as part of the photosynthesis process, and cars using biofuel then release that CO2 back into the atmosphere, thus creating an unbroken energy cycle. In addition, biofuels are not derived from fossil fuels, making them a renewable resource ensuring a secure supply into the future.

Bioethanol is often blended with petrol in a variety of percentages ranging from E10 (10% bioethanol) to the most widely known, E85, blended to a ratio of 85 per cent bioethanol to 15 per cent petrol to ensure cold-start capability.

Bioethanol presents unique engineering challenges compared with petrol, due to its chemical composition and characteristics. E85 biofuel has a very high octane rating of 105 but a lower energy content that requires a 30 per cent increase in the engine fuel flow rate.

This necessitated a complete review of the fuel system to identify, and if required replace, components deemed to be insufficiently resistant to the corrosive properties of ethanol.

For the fuel system all O-rings, seals, gaskets and pipes are new. In addition, increased fuel flow is achieved through a returnless fuel system and twin variable flow fuel pumps for the engine. The use of bioethanol also requires new valve coatings and hardened valve seat material for additional lubrication, a new closed loop fuel rail design with pressure sensor for higher fuel flow rates and new spark plugs with a wider heat range for slower combustion.

The W12 6-litre power unit has the technology to run on petrol, E85 or any mix of these two fuels.

A new Fuel Quality Sensor in the fuel supply circuit detects the current mix of petrol and bioethanol in real time. The engine control unit instantly initiates the correct engine mapping to ensure all performance parameters remain balanced and consistent. Power and torque remain constant whatever the ratio of petrol to bioethanol.

Brian Gush, Director, Chassis and Powertrain Engineering:

“The W12 has been re-engineered to run on petrol or a mix of petrol and bioethanol. A sophisticated fuel sensing system ensures that power and torque remain constant irrespective of the fuel ratio to deliver seamless power delivery in the Bentley tradition.”