Cylinder-deactivation doesn't work – BMW, Benz

Engine boffins from Mercedes-Benz and BMW have broken with long-running protocol traditions to claim Audi’s breakthrough cylinder-deactivation technology doesn’t deliver its promised benefits.

Speaking on condition of anonymity, a leading Mercedes-Benz engine developer insisted the fuel-saving 'cylinder on demand' system system used by Audi didn’t save its drivers any fuel, even though it can switch off half its cylinders when the car doesn’t need a lot of power.

“We’ve looked at it and we can’t understand what it really offers,” he told motoring.com.au last week.

“If you look at the ranges it operates in, it doesn’t activate usefully during the European test cycles for fuel consumption or emissions. The other significant problem for them is that it can’t work with the turbocharger.

“All small engines work with turbochargers and it is not sophisticated enough to drop out and still use the turbochargers on the active cylinders.”

Audi has cylinder-deactivation technology as a core of its future engine family and has it on sale already in four-cylinder form in the A1 Sportwagon and in V8 format with the S6, the S7 and the S8.

It is also committed to making the technology more widespread in the upcoming all-new A3, which could arrive here with a variable-displacement 1.4 TFSI engine when it goes on sale here next year, before rolling it out across all its models.

Cylinder-deactivation technology has also been seen for a number of years on V8 engines from General Motors and Chrysler.

Yet that hasn’t stopped BMW from insisting the technology is a complicated and expensive way to deliver less than existing valve control systems.

“More than 10 years ago, we looked at cylinder-deactivation. We introduced Valvetronic instead,” BMW Powertrain’s Dr Manfred Klüting pointed out.

“It’s not as complicated as deactivation so we could introduce it sooner to more models.

“The other point is that years ago, we did studies on cylinder deactivation and could only see it working 60-70 percent as often as Valvetronic across the engine’s range and in normal driving situations.”

“From the physics, Valvetronic is controlling the air flow to the cylinder better and more often, because it works to a thousandth of a millimeter for the valve control, and it can always use the turbo,” he insisted.

But Audi has hit back against the claims, insisting the system operates in the European test cycle to deliver a fuel saving of around 10 percent.

“Its best operating condition is when the driver needs only 25-30 percent of the maximum torque and is at a constant speed,” an Audi spokesman retorted.

“With the engine working that level, there is some light turbo pressure, but it’s not light because the system can’t work with it. It’s light because there isn’t a high gas flow because the turbo is not pushing hard.

“There is an element of constant-speed cruising in the fuel cycle and it’s worth about 10 percent when you cruise, so it definitely works on the EU test cycle.

“Besides, Audi’s TFSI engines offer variable-valve timing and lift that’s also accurate to around a thousandth of a millimetre, so that’s not unique to our rivals and means we have the benefits from that and from deactivation, too.”

The Audi system works by switching its valve-activation management to a lump-less cam lobe on half an engine’s cylinders. This closes both the exhaust and inlet valves on the cylinders (2, 3, 5 and 8 for the V8, so it continues operating as a V4) and keeps them closed. It also shuts down the ignition and direct fuel-injection as well.

The system only switches across to deactivation mode if the driving fits a tight set of parameters, including when the V8 driver only needs between 120 and 250Nm of torque, when the coolant has heated to 30 degrees, when the gearbox is in third gear or higher and when the revs are between 960 and 3500rpm.

When the driver needs more power, the sleeping cylinders refire within 300 milliseconds and, during motoring.com.au’s test of the S8, that 0.3 second switchover is lost in the eight-speed automatic transmission’s kickdown anyway.

Audi claims the system brought the S8’s fuel consumption down (in concert with start-stop technology) by close to 15 percent, even with the addition of turbochargers and more power. It also insists it uses 12 percent less fuel at a steady 80km/h.

For the S8 driver, there’s an indicator in the dash which changes colour (to green, naturally) when the car runs as a four-cylinder. It also fits the S8 with both active engine mounts and Audi Noise Control to the cabin to counter unwanted vibrations in V4 mode and then uses noise-cancelling technology to eradicate them.