You are assigned to an OSV fitted with main...

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Question 1 of 2066636eef28f7522a1c51f70a

You are assigned to an OSV fitted with main propulsion diesel engines operating on the cycle represented in the polar timing diagram shown in the illustration. When do the exhaust valves open and close respectively? Illustration MO-0084

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Question 1 of 20

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🔍 Key Concepts

• 4-stroke diesel cycle basics – know which stroke the exhaust valve is open on, and when it usually opens and closes relative to TDC/BDC. • Valve lead and lag – exhaust valves generally open before BDC on the power stroke and close after TDC on the exhaust/intake overlap period. • Reading the polar timing diagram – follow the direction of rotation, find the colored arc for the exhaust valves, and note its start (opening) and end (closing) angles relative to TDC and BDC.

💭 Think About

• On a 4‑stroke diesel, during which stroke is the piston approaching BDC just before the exhaust valve opens? Use this to match the wording in the choices. • Looking at the diagram, where does the exhaust arc begin in degrees before or after BDC, and where does it end relative to TDC? Compare those two numbers and their reference strokes to each answer choice. • Which choices describe exhaust valves opening and closing exactly at BDC or TDC with no lead or lag? Is that realistic for a high‑speed diesel engine used on an OSV?

✅ Before You Answer

• Identify on the diagram which colored ring or sector represents exhaust valve timing (not fuel or intake). • Verify the direction of crankshaft rotation shown by the arrow, then trace the arc from its beginning (valve opens) to its end (valve closes). • Confirm the exact angles labeled near BDC and TDC on that exhaust arc (e.g., __° before BDC on power and __° after TDC on intake/exhaust overlap) and eliminate any answer whose numbers or strokes don’t match both.

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