Miss those tasty gobs of smooth, unfettered torque from an American V-8? Thank the Corporate Average Fuel Economy mandate enacted by Congress in 1975. The law set minimum fuel economy standards for new cars beginning in 1978, before doubling them to 27.5 mpg by 1985. The law wasn’t enough to kill the V-8, which also survived the arrival of the 2007 Energy Independence and Security Act, which dictated automakers deliver a combined average fuel economy of 35 mpg for all cars and trucks by 2020.
In response, manufacturers are using every technological trick to meet the standard. Many are replacing V-8s with forced-induction V-6s or straight-sixes, while others opt for cylinder deactivation, which shuts off some engine cylinders under light loads in order to save fuel. And like so much in the auto industry, it’s far from a new idea.
The first known car to feature cylinder deactivation is one you’ve probably never heard of—the Boston-built 1905 Sturtevant. The driver could cut power to three of the Sturtevant six cylinders by turning off one of the magnetos and lifting their exhaust valves.
The concept reared its heads once again with the debut of the 1917 Enger Twin-Unit Twelve. Uniquely, this car’s 60-degree V-12 engine had a cylinder deactivation lever on the steering column that held the exhaust valves open on the left side of the engine block while closing the intake manifold on the same side. The company claimed this allowed the car to return 35 mpg in tests on the Indianapolis Motor Speedway. But financial difficulties, coupled with the untimely suicide of company founder Frank Enger, led to the company’s demise.
The notion didn’t sprout up again until 1979 during the second OPEC oil embargo. As Americans were waiting in line for rationed quantities of gasoline while listening to Abba, Barry Manilow, and Led Zeppelin, executives at Cadillac—purveyor of the 221-inch long Fleetwood—had a problem. How could it indulge Americans’ love of land yachts while returning better fuel economy?
The company started by downsizing its 7.0-liter V-8 to 6.0 liters and adding throttle-body fuel injection to enhance fuel economy. Dubbed the L61, it wasn’t fuel efficient enough. Then engineers had an idea. Under certain conditions, such as cruising the interstate, cars don’t need eight cylinders to sustain speed. So why keep them all running?
General Motors engineer Chris Meagher, working with automotive supplier Eaton Corporation, came up with the L62 engine. Like the L61 it displaced 6.0 liters, but the L62 automatically deactivated cylinders when they weren’t needed, reducing power and fuel consumption to that of a 4.5-liter V-6 or a 3.0-liter V-4. The system used engine speed, EGR (exhaust gas recirculation) position, intake manifold air pressure, coolant temperature, exhaust, and air pump operation to tell the Computer Command Module whether to deactivate two or four cylinders by disengaging the appropriate rocker arms, and closing the intake and exhaust valves on two or four cylinders. Cadillac estimated fuel economy improvements of as much as 30 percent in highway driving.
The company was so confident in its new powerplant, it became standard on all Cadillacs except the Seville, which got an Oldsmobile-sourced diesel V-8. In a 12-page salesman’s pocket brochure explaining the new engine, the company urged customers to “sit behind the wheel of a V-8-6-4 Cadillac to see firsthand what a remarkable advancement it really is.”
Once they did, however, problems ensued. While the system sounded good, the microprocessors scrambled to activate, deactivate, and then reactivate cylinders as driving conditions quickly changed, causing the car to hesitate, buck, and stall. As GM issued 13 updates—none of which worked—dealers disabled the system on their customers’ cars, leaving them permanently in V-8 mode. GM pulled the engine after a single year, although work continued on cylinder deactivation at GM and Eaton, as well as other automakers, although not a single system reached production.
By the turn of the century, as gas prices started climbing again, automakers were looking to protect the vehicles that were making huge profits: pickup trucks and SUVs. At GM, Meagher, now assistant chief engineer for GM's small-block engines, pitched reviving the idea of cylinder deactivation system.
"We knew that we were going to have to attack truck fuel economy. We started asking ourselves, 'How can we do that?' " said Meagher in a 2006 Automotive News interview. "You go right back to this whole idea of operating on only what you need vs. what the engine is capable of."
Remarkably, GM Powertrain Chief Tom Stephens liked the idea, with a caveat: the new system had to work seamlessly. Time and improved computing power allowed GM to successfully roll out Displacement on Demand on its 2005 mid-size SUVs. The system was renamed Active Fuel Management when it appeared on the redesigned Chevrolet Tahoe and GMC Yukon a year later (as early 2007 models), along with the Chevrolet Impala SS and Pontiac Grand Prix GXP. These days, even the C7 Corvette can cruise the highway with four active cylinders that can yield 30 mpg.
Chrysler, which had been working on the same idea since the 1990s, beat GM to the market by six months. Other brands, such as Mercedes-Benz, Honda, and Toyota also had cylinder deactivation systems in the works, although none had the tarnished legacy of Cadillac's 1981 attempt.