🔍 Key Concepts

• Fuel oil changeover systems on marine diesel engines (switching between heavy fuel oil (HFO) and diesel oil (DO)) • The function of an interlocked three‑way valve in a fuel supply/return system • Why it is important that fuel returned from the engine goes back to the correct service/day tank during changeover

💭 Think About

• Trace on the diagram where the three‑way valve is located: Which tanks and lines is it connected to? Supply, return, or both? • If the vessel is operating on HFO and then changes to DO (or vice versa), what must happen simultaneously to both the supply line and the return line to avoid contaminating tanks? • Which option best matches a function that requires an interlock (two things moving together), rather than just a simple open/close or flow‑rate control?

✅ Before You Answer

• Verify which answer choices mention both changing between heavy and light oil and directing oil to the proper tank • Eliminate any option that only talks about shutting off fuel, since an interlock usually coordinates more than just on/off • Check the diagram to confirm whether the valve is in a bypass/warm‑up loop or in the changeover/return routing, and choose the option that fits that location.