To increase the injection pressure of a hydraulic fuel injection nozzle, one could ______.

To increase the injection pressure of a hydraulic fuel injection nozzle, increasing compression on the pressure spring is the correct approach. The pressure spring's function is to maintain the necessary force on the nozzle needle, which regulates the fuel flow through the nozzle. By increasing the compression on this spring, the force it exerts is also increased. When the needle is subjected to greater force, it will require a higher pressure to open, effectively increasing the overall injection pressure at which the fuel is injected into the combustion chamber.

This adjustment is important for optimizing engine performance, as proper injection pressure ensures better atomization of the fuel, leading to more efficient combustion and operation. It is vital to maintain the correct balance because excessively increasing the pressure can lead to nozzle failure or engine damage.

While other options may seem plausible in altering fuel delivery, they do not directly affect the fundamental mechanics of how injection pressure is established. For example, increasing fuel oil booster pump pressure affects the system's overall fuel availability but does not inherently change the nozzle's operating pressure without corresponding adjustments to the nozzle or its spring. Similarly, increasing the nozzle orifice size would typically decrease the pressure required to inject fuel, while removing shims from under the nozzle spring might not provide a reliable means of controlling pressure.