What allows fuel flow into the injector cup as the plunger of a mechanical injector moves up?

When the plunger of a mechanical injector moves up, it creates a change in volume within the injection system. This upward movement decreases pressure in the injector cup, allowing fuel from the fuel supply to flow into the injector cup through the metering orifice.

Opening the metering orifice is crucial because it allows fuel to overcome the pressure differential created by the plunger's movement. This ensures that the necessary fuel is available for the next injection cycle. If the metering orifice is closed, fuel flow would be restricted or blocked entirely, thus preventing proper fuel delivery to the injector cup.

In summary, the opening of the metering orifice in this scenario enables the influx of fuel, providing the injector with the fuel it needs to perform efficiently during its operational cycle.