Save Gas: Cylinders Get Smart

GM Chevrolet Camaro vehicle
While a V-8 engine is great when you need maximum horsepower and torque, most of the time vehicles could get by with the output from a four-cylinder engine. The duty cycle of the typical internal combustion engine consists of many minutes or even hours of leisurely production of only a fraction of its rated horsepower and torque. This is interspersed with a few minutes of pedal-to-the-metal output for passing or climbing a steep hill.
Operation like this is wasteful because fuel is continuously injected into all cylinders and combusted even though maximum power is not needed. There are also pumping losses as the fuel-air mixture is compressed in all cylinders, wasting engine output. So why not 'shut off' some of the cylinders when they're not needed? That's the idea behind variable displacement engines.
Old Idea Requires New Technology 
Variable displacement technology has been attempted several times over the internal combustion engine's history, with limited success. The most famous, or really infamous, example is presented by the 'V-8-6-4' engines used in 1981 Cadillacs. Depending on driving conditions, the V-8-6-4 engine ran on four-, six-, or eight-cylinders. It was offered only in 1981 in consumer versions - although it was available on Cadillac limousines through 1984 - because it was quite troublesome. Early 1980s electronics and computer technology simply weren't ready for this complex job.
Multi-Displacement System transforms V8 power to a 4-cylinder powerplant
Working Great Today
General Motors, Chrysler, and Honda now offer variable displacement engines. GM's Active Fuel Management, developed with assistance from Eaton Corp., is used on V-8 powerplants. According to GM, Active Fuel Management provides fuel savings of 8 to 25 percent, depending on driver and driving conditions. Chrysler's similar Multi-Displacement System is used on its 5.7-liter HEMI V-8. Chrysler says MDS reduces fuel use by about 20 percent. Honda's Variable Cylinder Management (VCM) allows its 3.0-liter i-VTEC V-6 engine to run on either six or three cylinders. According to Honda, VCM combines the performance of a 3.0-liter V-6 engine with the fuel economy of a 2.4-liter four-cylinder engine.
Add Hybrid Technology for Even More MPGs
GM and Chrysler are using variable displacement technology with full hybrid systems to synergistically achieve even greater fuel economy. It is used with the two mode hybrid system offered in the Chevrolet Tahoe, GMC Yukon, and Cadillac Escalade SUVs as well as the Chevrolet Silverado and GMC Sierra pickups. It will be available in the 2009 Saturn Vue SUV as well. Chrysler's two-mode hybrid models that use the HEMI with MDS include the Chrysler Aspen and Dodge SUVs and new Dodge Ram pickup. Honda used VCM in its now-discontinued V-6 Honda Accord Hybrid and continues to use it in conventional models like the Accord and Odyssey.
Chrysler Dodge Ram pickup hybrid
Why It Works Now 
Today, computer technology is quite capable of rapidly and seamlessly turning off and on banks of cylinders in V-6 and V-8 engines. Electronic throttles and electronically-controlled transmissions also help. These technologies were not available in the early 1980s, the main reasons that the Cadillac V-4-6-8 failed.