Fuel pump works

The oil absorption and pressure process in a fuel injection pump involves the reciprocating motion of the plunger within the sleeve. When the plunger is at its lowest position, two oil ports on the sleeve are open, allowing fuel to be drawn into the chamber through the pump body’s oil passage. As the cam rotates, it pushes the roller, causing the plunger to rise. During this upward movement, the oil ports are gradually closed by the upper part of the plunger, marking the beginning of the pre-stroke phase. At this stage, the fuel inside the chamber is compressed but not yet injected. Once the plunger fully blocks the oil ports, the pressure within the chamber begins to rise sharply. This is the actual pressure phase, where the fuel is forced through the nozzle once the pressure overcomes the spring force of the fuel valve. The point at which the oil inlet port is completely sealed by the plunger is known as the theoretical start of fuel supply. As the plunger continues to move upward, the pressure remains high until the helical groove on the plunger aligns with the return oil hole in the sleeve. At this moment, high-pressure fuel is redirected back to the pump’s oil passage, causing a rapid drop in pressure. The fuel valve then closes, stopping the injection. Even though the plunger is still moving upward, no further fuel is being supplied. This point, when the return oil hole is opened, is referred to as the theoretical end of the fuel supply. Throughout the entire plunger stroke, only a portion—known as the effective stroke—is responsible for the pressure and fuel delivery. The rest of the movement does not contribute to fuel injection. To adjust the fuel supply, the engine's load requirements must be met by varying the amount of fuel delivered from maximum (full load) to zero (idle or shutdown). This adjustment is achieved by rotating the plunger using a rack and sleeve mechanism. While the timing of fuel supply initiation remains unchanged, the end of the fuel supply is altered based on the plunger’s rotation angle. A larger rotation angle means the plunger remains in the pressure phase longer, increasing the fuel quantity. Conversely, a smaller rotation angle causes earlier cutoff, reducing the fuel amount. To stop fuel supply entirely, the longitudinal groove on the plunger can be aligned with the return oil hole, allowing all fuel to bypass the pressure chamber and return to the pump body. In this case, no fuel is injected. This method of adjusting fuel supply by changing the end of the pressure phase is called the "end-of-injection" adjustment. Other methods involve modifying the shape of the plunger’s bevel, allowing for different fuel delivery characteristics. The figure below illustrates three common plunger profiles used in various fuel control strategies.

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