While Nothing New for Mercedes-Benz, GM Looks Back to the Future to Improve Fuel Economy

Do you remember Cadillacs V-8-6-4 engine from the early 1980s? If it didnt show off the brands forward thinking engineering skills it probably would be best to leave its memory hiding in some dark recess of automotive history as just another mechanical nightmare.

The engine shut off fuel to specific cylinders when maximum power wasnt needed to curb usage, and then fed fuel to all cylinders when the driver got on the gas. While the idea was conceptually brilliant, technical gremlins made its execution less so and it was quickly discontinued.

But Cadillacs innovative cylinder deactivation system had merit, and now with todays reliable computerized technology this best-of-both-worlds combination of 4-cylinder fuel efficiency and V8 power is about to make a serious comeback.

GM is bringing its Delphi developed Displacement on Demand (DOD) system to market later this year, first in its gasoline powered pushrod and overhead cam (OHC) port-fuel-injected V8 engines, and then after in its midsize six-cylinder engines.

“With two valves per cylinder, only two actuators per cylinder are needed,” said Sam Winegarden, GM Powertrain chief engineer of Vortec V8 engine. “The system is intelligently simple and compact.”

Winegarden then added, “By contrast, while Displacement on Demand certainly has merit for overhead cam multi-valve engines, four valves per cylinder require four actuators per cylinder. Additionally, the electro-hydraulic (or other) control network must be packaged within the tight confines of the cylinder head assembly, adding both complexity and cost.”

With complexity and cost in mind DOD was first shown in Cadillacs V12 Cien concept car, and will be available in the V12 powered Escalade. Basically the system uses GMs eMotion powertrain control system to close the intake and exhaust valves on specific cylinders, effectively turning off half the cylinders, which improves fuel economy by around 8 to 10 percent.

That might sound like little gain for the high level of R & D capital needed to develop the system, but combined with other fuel saving technologies it could go a long way to both reducing the brands CAFE rating and improving green customer relations.