Three-stage pumping of imported plunger pumps

During operation, the imported plunger pump relies on the cam on the fuel pump camshaft and the plunger spring to force the plunger to move up and down, thereby accomplishing the pumping function. The pumping process can be divided into the following three stages: when the cam lobe rotates, the plunger moves downward under the action of the spring force, creating a vacuum in the space above the plunger (known as the pumping chamber). As the upper end face of the plunger aligns with the inlet port on the plunger sleeve, diesel fuel from the fuel pump body’s oil passage flows through the inlet port into the pumping chamber. The plunger then continues its downward stroke until it reaches the bottom dead center, completing the intake phase.

During the fuel delivery process of the intake plunger pump, as the camshaft rotates and the cam lobe lifts the roller follower, the plunger spring is compressed, causing the plunger to move upward and pressurizing the fuel. A portion of the fuel then flows back through the oil passage into the fuel pump’s supply chamber. When the upper surface of the plunger covers the upper edge of the sleeve’s supply port, the extremely small clearance between the plunger and the sleeve seals the plunger’s upper pumping chamber, and the plunger continues to rise.

When the oil pressure in the pump oil chamber rises rapidly, the pump oil pressure and the spring of the delivery valve overcome the residual pressure in the high-pressure fuel line, pushing open the delivery valve. High-pressure diesel fuel then flows through the delivery valve into the high-pressure fuel line and is injected into the combustion chamber via the injector. During the oil return process in the plunger pump, the plunger pump delivers fuel upward.

When the groove on the plunger (on the fuel-shutoff side) aligns with the return port in the sleeve, the low-pressure oil passage in the pumping chamber connects to the plunger’s central bore, radial bores, and the groove, causing a sudden drop in oil pressure. Under the action of the spring, the delivery valve closes rapidly, halting fuel delivery. Subsequently, the plunger must rise. As the cam’s convex surface rotates, the plunger is once again forced downward by the spring.

Conclusion: Based on the foregoing discussion, the following conclusion is reached:

The total stroke l of the plunger’s reciprocating motion is constant and determined by the cam’s lift.

The fuel delivery per cycle by the plunger depends on the injection stroke, which is variable in the absence of camshaft control.

The fuel injection start time remains constant regardless of variations in the fuel injection stroke.

Rotating the inlet plunger pump can adjust the fuel delivery cut-off timing, thereby altering the fuel delivery.

In daily use, regular maintenance should be performed to fully maximize its effectiveness.

A plunger pump supplied by the auxiliary oil pump shall be used. After 3,000 hours of operation, operators shall inspect the plunger pump 1–2 times daily to verify that the hydraulic pump is operating with normal noise.

If the hydraulic cylinder speed decreases or the vehicle becomes excessively hot, the make-up oil pump should be disassembled and inspected for scratches on the impeller edges and excessive clearance in the internal gear pump. For self-priming plunger pumps, the hydraulic oil level in the reservoir must not fall below the lower limit specified in the oil specification; sufficient hydraulic oil must be maintained. The higher the cleanliness of the hydraulic oil, the longer the service life of the hydraulic pump.

A critical component of a plunger pump is the bearing. If the bearing exhibits clearance, the proper clearances of the three friction pairs within the hydraulic pump cannot be maintained. Furthermore, this clearance will compromise the hydrostatic film thickness in each friction pair, thereby reducing the service life of the plunger pump’s bearings.

According to data provided by the hydraulic pump manufacturer, the average service life of the bearing is 10,000 hours. If this limit is exceeded, the bearing must be replaced. Without specialized testing equipment, it is impossible to measure the bearing clearance on a removed bearing; only visual inspection is feasible. If the roller surface shows scratches or discoloration, the bearing should be replaced. When replacing the bearing, be sure to note the original bearing’s letter designation and model number.