What fuel injection system do modern diesel engines commonly use?

Modern diesel engines commonly use common-rail fuel injection. In this design, a high-pressure pump feeds a shared fuel rail, and electronically controlled injectors deliver fuel to each cylinder.

The shared rail acts as a high-pressure reservoir. Because pressure is stored separately from the injection event, the engine control unit can precisely choose when and how much fuel each injector sprays. Modern systems can use multiple injections during one combustion cycle, including a small pilot injection that helps reduce noise and vibration.

Common-rail systems developed from earlier mechanical injection arrangements. Experimental and industrial versions existed long before passenger cars, but the technology became commercially important in the 1990s. The first successful mass-production vehicle application appeared in Japan in 1995, and the 1997 Alfa Romeo 156 helped establish the system in passenger cars.

Common rail is not a fuel type or a brand name. It describes the injection architecture. Older diesel engines may instead use inline pumps, distributor pumps, or unit injectors, while gasoline engines can also use related high-pressure direct-injection principles.