Diesel Engine Solenoid-Actuated Injectors

Figure 6-2 is a schematic of a multi-hole, solenoid-operated injector of the type used on common-rail systems. The upper end of the spring-loaded needle is flat and the large-diameter lower end is shouldered. Hydraulic pressure applied to the flat tends to push the needle down, while pressure bearing against the shoulder would raise it. Movement depends upon the balance of pressures acting on these surfaces and upon spring tension.

A ball-type spill valve stands between high-pressure fuel from the rail and the low-pressure circuit that returns fuel to the tank. When at rest, the solenoid is not energized and the armature spring holds the ball valve closed. No fuel flows to the tank, but both needle working surfaces see the same high pressure. The forces generated—lift from the shoulder and down-force from the flattened top—neutralize each other and spring tension keeps the needle seated.

When the solenoid is energized, the armature retracts and the ball valve opens to the low-pressure return line. Because the orifice is quite small, most of the pressure drop is confined to the area above the valve. Pressure in the nozzle cavity remains high. This pressure lifts the needle to commence injection. When deenergized, the solenoid closes the ball valve, pressures equalize and the spring forces the needle down to end injection.

Even though hydraulics do the heavy work, solenoids require large amounts of electric power. To overcome armature inertia, the drag of heavy fuel, and other operating variables, Bosch solenoids receive a 50V, 20A jolt during the opening phase. Once the armature is retracted, battery voltage holds it in place.

Suspect injectors must be farmed out to specialists for cleaning, repair, and recalibration.