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Valve timing (four-stroke petrol engine): when does the inlet valve open and close? State the most accurate generic timing description (qualitative) for the inlet valve events. Choose the correct option.

Difficulty: Easy

Opens slightly before top dead centre (end of exhaust) and closes slightly after bottom dead centre (end of suction)
Opens exactly at TDC and closes exactly at BDC
Opens after TDC during suction and closes at BDC
Opens at BDC and closes at the next TDC

Correct Answer: Opens slightly before top dead centre (end of exhaust) and closes slightly after bottom dead centre (end of suction)

Explanation:

Concept
Typical valve timing uses valve lead and valve lag for better volumetric efficiency: the inlet valve usually opens a little before TDC (overlap with exhaust) and closes a little after BDC to take advantage of intake inertia.

Why other options are incorrect
Exact TDC/BDC opening/closing (Option B) is not used in practice; Options C and D misrepresent the standard timing sequence.

Valve timing (four-stroke petrol engine): when does the inlet valve open and close? State the most accurate generic timing description (qualitative) for the inlet valve events. Choose the correct option.

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