Diesel Engine Fuel Injectors

Mechanical injectors consist of two main parts—an injector body, or nozzle holder, and a needle (Fig. 5-21). The injector body secures the assembly to the cylinder head, and includes connections for fuel inlet and return lines.

A spring-loaded needle (“nozzle valve”) in the drawing, controls fuel flow through the injector. At NOP fuel pressure exerts sufficient force on the raised shoulder near the tip of the needle to overcome spring tension. The needle retracts clear of the orifice and injection commences. Once the needle lifts, its full cross-sectional area—the shoulder and tip—comes under fuel pressure. Consequently, less pressure is required to hold the needle open than to unseat it. This feature holds the nozzle open as fuel pressure drops in response to injection. When the pump plunger descends and the delivery valve closes, fuel pressure drops abruptly. The return spring forces the needle down to seal the orifice and injection ceases.