- The Washington Times - Friday, December 28, 2001

Cruising in the Vortec V-8-powered GMC Sierra test pickup seemed comparable to other Sierras I've experienced. Despite this feeling of normalcy, this engine was a next-generation, displacement-on-demand, small-block V-8, the forerunner to engines set to make grand appearances on General Motors pickups and sport utility vehicles in 2004.
This technology improves fuel economy via the ability to switch back and forth from eight- to four-cylinder operation. On the test drive, a General Motors powertrain engineer was using a computer to monitor transitions between eight- and four-cylinder operation. In a short trip of mainly city driving, he said the shift occurred hundreds of times.
"The whole reason for doing this is to literally give the customer the best of both worlds," said John Juriga, GM assistant chief engineer for small-block engines. "It offers them a vehicle that has V-8 performance but gets fuel economy they really want on a day-to-day basis." The benefit of a displacement-on-demand engine is that larger sport utilities and trucks purchased with towing in mind aren't penalized in daily commutes, when they're not towing.
"The average person is going to spend a good 50 to 60 percent of the time in four-cylinder mode," Mr. Juriga said. Fuel savings depend on the individual's driving habits and can vary greatly. GM reports the customer should benefit from a 7 to 9 percent fuel economy improvement over a conventional V-8. That estimate includes combined highway and stop-and-go city traffic, but steady highway driving could produce savings as high as 20 percent. For instance, with a conservative driver behind the wheel, a Chevy Silverado pickup with no trailer could travel from Washington, D.C., to Cincinnati roughly 509 miles on a single tank of gas.
There's an astounding aspect to the technology that erases a mind-set that's existed for decades. It calls for a new paradigm: With the new technology, a larger-displacement engine is more likely to get better fuel economy than a smaller-displacement engine.
"The larger-displacement engine can operate in a four-cylinder mode more often than a smaller-displacement engine," Mr. Juriga said. The engine starts in eight-cylinder mode and remains in that mode until the computer determines it would be better off operating as a V-4. At that time, the computer seamlessly shuts off operation of four cylinders. Every other cylinder is nullified which ones depend on the firing order at the time of the change.
The transition process, which occurs in about 100 milliseconds, is smooth because engine torque output requirements remain equal whether four or eight cylinders are working. The computer matches torque output from eight- to four-cylinder modes, and vice versa.
A typical fuel-saving situation occurs when the engine is operating at a relatively light load, such as highway cruising at 50 to 60 mph with no trailer in tow. It will shift from eight- to four-cylinder operation because "it doesn't need anywhere near the power capability of the engine," Mr. Juriga said. When additional performance is required, the computer reacts as the driver increases the throttle. The engine responds with a transition into eight-cylinder mode.
Eight pistons move up and down at all times, but in four-cylinder mode half are "nullified" and aren't doing any work. The trick to displacement on demand comes with the design of the lifters, which hydraulically engage and disengage. Lifters are split into two pieces, and disengagement occurs when one section collapses or telescopes into the other. A lower half constantly moves up and down with the camshaft, and the top portion is disengaged to shut off cylinder operation via a hydraulic system. When a lifter disengages, it closes the exhaust valve first, then the intake valve. When engaged, the lifter's upper half acts as a solid part and moves to drive the valvetrain.
Disengaging a lifter results in "trapping a charge," said Mr. Juriga. Fuel to the cylinder is cut off and air is all that's left. This air compresses, expands, and acts as a spring so loss of efficiency is minimal. All cylinders are activated again when a driver puts his or her foot on the throttle. Valves activate, fuel is injected into the cylinders and operation resumes in eight-cylinder mode.
GM proposes to build more than 150,000 V-8s with displacement-on-demand technology. Production is set to accelerate over the next several years to nearly 1.5 million units in 2007.

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