TUBE 3255871 - Caterpillar

Refer to Illustration 9. The injector is in the phase of pre-injection when the engine is running and the injector is between firing cycles. Plunger (10) and the intensifier piston (8) are at the top of the piston bore. The cavity below the plunger is full of fuel.In the upper end, the armature (3) and the seated pin (4) are held down by the armature spring (2). High pressure actuation oil flows into the injector. The oil then flows around the seated pin to the top of the check piston (16). This provides a positive downward force on the nozzle check (19) at all times when fuel is not being injected.The spool valve (6) is held in the top of the bore for the spool valve by the spool spring (5). In this position, the spool valve blocks actuation oil from reaching the intensifier piston. Actuation pressure is felt on both the top and bottom of the spool, so hydraulic forces on the spool are balanced. The spool valve is held in the up position or the closed position by the force of the spool spring.Pilot Injection
Illustration 10 g01103154
Cross section of pilot injection cycle
(1) Solenoid
(3) Armature
(4) Seated pin
(6) Spool valve
(7) Check ball for intensifier piston
(8) Intensifier piston
(10) Plunger
(15) Nozzle spring
(16) Check piston
(19) Nozzle check
(20) Nozzle tip
(21) Drain Refer to Illustration 10. Pilot injection occurs when the ECM sends a control current to the solenoid (1). The current creates a magnetic field which lifts the armature (3) and the seated pin (4). The seated pin has a lower seat and an upper seat. When the seated pin is lifted by the armature, the upper seat closes off the flow of actuation pressure to the check. The lower seat opens. This allows the actuation oil on top of check piston (16) to flow to drain (21). Actuation oil that is trapped below spool (6) will also flow to drain (21). The actuation oil drains through a vent hole in the side of the injector.The drop in pressure under the spool causes a hydraulic difference that acts on the spool. The spool moves into the open position when hydraulic pressure acts on the top of the spool. This hydraulic pressure forces the spool downward. The downward movement of the spool is stopped when the spool and the pin force the check ball (7) for the intensifier piston onto the ball seat in the closed position. This prevents any actuation pressure from escaping from the cavity for the intensifier piston (8). This drop in the actuation pressure also removes the downward force on the check piston.Actuation oil now flows past the open spool and to the top of the intensifier piston. The downward movement of the piston and plunger (10) pressurizes the fuel in the plunger cavity to the nozzle tip (20). Pilot injection begins when the injection pressure increases in order to overcome the force of the nozzle spring (15) which lifts the nozzle check (19).Pilot injection will continue if the following