Audi and Bentley engineers have co-operated in the development of an all new, highly compact V-8 engine that will see service in Audi's S6, S6 Avant, S7 Sportback and S8 in 2012 and a unique Bentley version for the scheduled to be revealed at the Detroit show in January.
The 4.0-liter twin-turbo V-8 features cylinder deactivation turning it into a V-4 under light load using experience gained by Bentley engineers during the development of the latest Mulsanne V-8.
This lowers fuel consumption, according to the NEDC test cycle, by about 5% and reduces emissions by approximately 16 to 19 g/mile of CO2. Add to this the effect of the engine start-stop system – fitted in the Audi S models – and the reduction in emissions can be as high 38.62 g/mile of CO2.
At 50 mph, for instance, the gain in efficiency on all "S" models is 12%, and even at 80mph a 7% saving is attainable.
Michael Dick, Audi's head of technical development, suggested that American drivers might benefit from fuel savings as high as 11.5% over the outgoing engines.
The fuel savings for Bentley are even more dramatic at 40% over the W12 engine according to director powertrain, chassis and motorsport, Brian Gush who promised there would be "no loss " in Bentley's driving characteristics with more torque at low engine speed than the Audis, adding that when driven "It feels bigger than it is."
Two versions of the V-8 engine will be launched: with an output of 420 bhp and a maximum torque of 405 lb-ft for the S6, S6 Avant and S7 Sportback, the equivalent figures for the S8 being 520 bhp and 479 lbs.-ft. of torque. Bentley will reveal its figures later in the year.
Cylinders are deactivated if the load on the engine is low. The upper limit for the deactivation to take effect, depending on engine speed, is between about 25 and 40% of maximum torque, about 88.5 and 184.4 lbs.-ft. of torque. In this operating range the mean effective pressure rises to eight bar. Coolant temperature must be at least 86°F and third gear or higher selected with the engine running at between 960 and 3,500 rpm.
If these conditions are met, the system closes the inlet and exhaust valves of two cylinders on each bank. The V-8 continues to run as a V-4 with a regular firing order, but with the mixture in only two cylinders instead of four being ignited on each crankshaft revolution. Instead of 1 – 5 – 4 – 8 – 6 – 3 – 7 – 2, the firing order is then 1 – 4 – 6 – 7; efficiency in the active cylinders is increased because the operating points are displaced toward higher loads.
In the deactivated cylinders, the pistons continue to move because they are being driven by the crankshaft; before the valves close the combustion chambers are filled with fresh air with the fuel injection and ignition shut down. This intake of air lowers cylinder pressure acting as an air spring, effectively reducing the energy needed to move the pistons – an important factor for increased efficiency.
"Bentley had a running start on this," explained Gush, adding "We have a team of calibrators that are specialists in cylinder deactivation with Bosch, and they worked with Audi to establish a workshop for them."
When asked if Bentley's W-12 could, one day, switch itself to a W-6 on demand, Gush just smiled and sipped his glass of water.
Bentley's engines will be assembled alongside Audi's, but given Bentley's identity at Crewe, said Gush.
To ensure smooth running as a V-4 switchable electronic active engine mounts are used on an Audi for the first time, while Active Noise Control in the cabin disguises any intrusive engine noise. Bentley, said Gush, had developed its own strategy for ANC which will be revealed later.