Cylinder deactivation works on the same principle as variable valve timing and lift: An engine needs different amounts of fuel for different types of work.
Take a V-8 engine, for example. A V-8 engine has eight cylinders. Whenever the engine is on, all eight of those cylinders are working, burning up fuel and air. Most cars, trucks and SUVs with V-8 engines have them because, generally, the more cylinders an engine has, the more powerful it is. That's why you typically see V-8 engines in sports cars or heavy-duty trucks. But, while people like V-8 engines because they go fast and can pull heavy loads, most people aren't doing those things all the time.
That's where cylinder deactivation comes in. Cylinder deactivation shuts down a number of the engine's cylinders when they're not needed. That means that when a car or truck is maintaining a constant speed and not accelerating, some cylinders aren't in use. Since they aren't in use, they aren't getting any gas either -- and that saves fuel. Cylinder deactivation technology has been used on V-8-powered cars and trucks for the past few years, but some car makers are starting to add it to their six-cylinder engines, too. It's estimated that cylinder deactivation technology can improve engine efficiency by 7.5 percent [source: U.S. Department of Energy].