Cylinders take turns to deliver proper power

DETROIT -- You've heard of the continuously variable transmission; now, get ready for the continuously variable engine.

An advanced, fast-acting cylinder-deactivation system expected to be launched within five years -- probably by General Motors -- proposes to save fuel by continuously varying the number of cylinders firing. As a result, the engine delivers only the power that is needed.

It's called Dynamic Skip Fire, or DSF. It enables a V-8-powered SUV, such as a GMC Yukon Denali, to cruise smoothly and steadily at highway speeds with the engine firing on as few as two cylinders.

That could boost fuel economy by as much as 21 percent and reduce emissions by nearly the same amount, says Scott Bailey, CEO of Tula Technology Inc., a Silicon Valley software startup that is developing the system along with GM and Delphi Automotive. GM and Delphi, a former division of the automaker, have invested in Tula.

The first vehicles with Dynamic Skip Fire could be on the road by 2020, said Jeff Owens, Delphi's chief technology officer. He said the company already has customers for the technology.

GM spokesman Dan Flores says GM has an exclusive on DSF, which can work on all types and sizes of gasoline engines.

Delphi and Tula officials gave reporters a sneak peek at DSF this month in Detroit and showcased the system last week at the Frankfurt auto show.

Depending on the size of the engine, an automaker's cost to install DSF would be between $300 and $600 per engine, according to estimates provided by Tula and Delphi that use information from the National Research Council.

Today's cylinder-cutoff systems, such as those used on GM's small-block V-8 and V-6 engines and Fiat Chrysler's Hemi V-8 engines, typically shut down half the cylinders on a V-8 and two or three cylinders on a V-6 at highway cruising speeds.

But a big SUV, such as a GMC Yukon Denali with a 403-hp, 6.2-liter V-8, requires only about 30 hp to cruise steadily at highway speeds, Bailey said.

In that scenario, DSF fires two of the engine's eight cylinders, but the active cylinders continuously change. Tula tested the Denali using the EPA's city and highway test protocol to register the 21 percent fuel economy gain, Bailey said.

Because DSF is active on all cylinders and continuously varies those that fire, the engine maintains proper operating temperature and production levels of smoothness, Bailey said.

"DSF is like going from analog to digital in terms of active fuel management. The only concern I have is [noise, vibration and harshness], but with active engine mounts and active noise cancellation available, that probably won't be much of an issue," said Dave Sullivan, a powertrain analyst at AutoPacific.

Another key difference between today's cylinder cutoff systems and DSF is how the engine's throttle system is managed.

Gasoline engines use a flap in the throttle body to regulate the amount of air that enters the intake manifold. The flap's usual position is nearly closed, causing the engine to work harder to ingest air. DSF keeps the throttle flap almost fully open and controls the engine's power by varying the numbers of cylinders that fire.

Delphi and Tula engineers demonstrated the system on a GMC Yukon Denali powered by a 6.2-liter V-8. At idle, and when accelerating, the hulking SUV ran normally, using all eight cylinders.

But once the driver's foot eased up on the throttle, the number of cylinders firing began to drop. At a steady 65 mph, just two cylinders were active, but there was no excess noise or vibration. When power was needed, the other six cylinders were activated immediately with no lag or other side effects.

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