A unit injector is a combined pump and injector – think miniature bicycle pump filled with liquid. Imagine pinching the outlet nozzle with your fingers, pushing the plunger in to pressurize the liquid, then releasing your grip on the tube, allowing the fluid to escape. The unit injector works in a similar way and on Volkswagen’s two-valve diesels a single camshaft operates both the inlet and exhaust valves as well as the unit injectors.
The main benefits of unit injectors are high pressure for power, efficiency and accuracy. On the downside, cylinder heads must be designed from scratch to accommodate the relatively bulky injectors and lack flexibility. In a common rail there is more freedom in the injection timing.