The ship-docking tug to which you are assigned has main engines fitted with injectors similar to those shown in the illustration. What statement is true concerning the operating principle of this type of injector? Illustration MO-0151
• Spring-loaded needle valve operation in a diesel fuel injector • How fuel pressure in the annulus acts on the needle valve shoulder • Which force must dominate to start injection vs. to stop injection
• Look at the drawing: when high-pressure fuel enters the annulus, does it tend to push the needle toward the seat or lift it off the seat? • When injection must end and pressure falls, what force actually moves the needle back onto the valve seat to stop the spray? • In a closed-type injector, before the pump sends fuel, what is physically keeping the needle tight on its seat?
• Be clear which force is the normal (resting) closing force that holds the needle seated with no fuel pressure. • Identify which force increases as fuel pressure rises in the annulus and what direction that force acts on the needle. • Eliminate any option that makes the spring act in the wrong direction compared to how a typical spring-loaded check/relief valve works.
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