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Spark timing in a four-stroke petrol (SI) engine: identify the usual ignition timing. Choose the standard practice for when the charge is ignited relative to top/bottom dead centre (TDC/BDC).

Difficulty: Easy

30° before top dead centre
30° after top dead centre
30° before bottom dead centre
30° after bottom dead centre

Correct Answer: 30° before top dead centre

Explanation:

Concept/Approach
To allow flame development and complete combustion near TDC, spark timing is advanced. Typical advance is of the order of 10°–30° before TDC depending on speed/load.

Final Answer
30° before top dead centre (representative textbook value).

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