What is common-rail diesel injection?

Common-rail diesel injection is a modern fuel injection technology used on nearly all modern diesel engines. It feeds fuel into the engine from a single high-pressure 'rail' using electrically controlled solenoid valves to control the flow into each injector. This allows for far more precise and efficient combustion than an old-school low-pressure fuel pump, leading to better fuel economy and lower emissions. A common rail injection system tends to run at more than 2,000 bar (29,000 psi) and, because the solenoid valves are electrically controlled, they can be opened and closed with incredible speed and precision. For comparison, a regular petrol engine's fuel pump runs at around four bar (60psi).