Direct injection

Port fuel-injection systems long ago replaced carburetors in cars because of their efficiency and lower maintenance requirements. But now automakers are moving toward the even more efficient direct injection. With port fuel injection, gasoline is sprayed into the intake manifold, where it mixes with air, and then is sucked down into the cylinders. Direct injection places an injector on each cylinder, spraying gasoline into the cylinder itself.

Because of the multiplication of injectors, direct-injection systems are more expensive, and have also been associated with increased engine noise. But direct injection leads to more-efficient engines, as the gasoline-air mixture burns more completely. The individual injectors also ensure that each cylinder gets the same amount of fuel, and the spray can be more precisely timed.

Audi embraced direct injection early this century, but most automakers are starting to roll out new engines using the technology.

Examples: 2012 Audi A6, 2011 Nissan Juke