Luxury car manufacturers around the world are devising multiple solutions for improving fuel economy and emissions while maintaining the high-performance multi-cylinder engines their customers have traditionally expected. One of the those solutions is cylinder deactivation.
Bentley already has two methods of cylinder deactivation on its 6.75-L and 4.0-L V8 engines but, as Rolf Frech, Member of the Board for Engineering, explained at the 2015 Geneva Motor Show, the complexity of the 6.0-L W12’s layout (essentially two narrow-angle V6s on a common crankshaft) and cylinder firing demanded a third solution.
“It was clear that we couldn’t use either of the solutions we currently employ as that would have demanded a new cylinder head," he said. "So we asked how can we do that with an intelligent arrangement?”
The challenge is that when cylinders are switched off they cool down, as does the catalytic converter, resulting in spikes of exhaust emissions. “The complication was to develop intelligent management just to switch the cylinders off and switch them back on with the right timing and change to the other six,” he said.
In essence, the engine control unit simply deactivates one bank of six cylinders while leaving the valves to operate as normal.
“The big advantage of this is not only about reducing CO2 on the normal test, but in daily use,” Frech explained. The significant gains are under part-throttle cruising conditions.
“If you make it into a real six-cylinder, you don’t have any problems with balance," he noted. "There was something of an issue with exhaust boom, but we have countered that with some modifications to the exhaust.”
By doing it in this manner, the inlet and exhaust sensors determine when to switch between each bank, ensuring the cylinders remain hot. It also fulfills U.S. OBD requirements.
The engine also receives a power increase, from 567 to 582 hp (423 to 434 kW), with rated torque also rising from 516 to 531 lb·ft (700 to 720 N·m). The greater output is accompanied by claimed improvements to fuel economy of up to 5% (combined fuel economy is 16.6 U.S. mpg) and lower CO2 emissions of 329 g/km.
Looking to future applications, Frech revealed that there would be “other opportunities” with W12-powered vehicles, including possible individual cylinder deactivation and electric-motor assistance for the forthcoming Bentayga SUV due later this year.