Summit Point Raceway, WV—Resurrecting a 20-year-old idea, GM plans to boost fuel efficiency in its large trucks and SUVs with an engine that can run on just four of its eight cylinders when it doesn't need the power.
This "displacement on demand" system can deliver a 6 to 12% improvement in fuel efficiency, and GM claims a 25% improvement in certain conditions.
The engine starts with all eight cylinders, then runs with just four cylinders under normal driving conditions, only adding the others to carry heavy loads, pass other cars, or climb steep hills.
GM first tried the idea 20 years ago, in the 1981 Cadillac's six-liter engine, which could switch from four to six to eight cylinders, according to driving conditions. But a "natural vibration" in six-cylinder operation made the car unpopular with drivers, so the company dropped the concept.
Now two recent technology advances have allowed GM to try again. First, that old cable-activated throttle has evolved into modern electronic throttle control, capable of adjusting the gas so smoothly that it's tough to tell when the extra cylinders kick in. Second, the new engine has processing power to burn—today's engine computers are 32-bit, compared to the '81 Cadillac's 8-bit; and have roughly 100 times the memory and 25 times the clock speed.
"You won't know if you're in eight cylinder or four cylinder mode," says Tom Stevens, GM's VP of engineering. "The only thing you know is you'll go farther on a tank of gasoline."
Indeed, in a test drive here, I accelerated and braked the truck in a series of tight turns, and never felt the engine switch modes. Glancing at his telemetry laptop, the GM technician in the passenger seat said it had happened six times in just a half-mile.
When driving loads are light, the engine closes both the intake and exhaust valves for every other cylinder in the firing order, thus cutting off their air and fuel supply.