Ignition timing in a reciprocating engine occurs when?

• A. Top dead center
• B. 15 degrees after top dead center
• C. About 15 degrees before top-dead-center
• D. 30 degrees before bottom dead center

Answer: (C)

Ignition timing is defined by how many crankshaft degrees before the piston reaches top dead center (TDC) the spark plug fires. The flame needs time to start and grow, so starting the burn a little before TDC lets the combustion pressure build while the piston is near the top of its travel. The goal is to have peak pressure occur just after TDC to push the piston downward during the power stroke.

That’s why firing about 15 degrees before TDC is the best fit in many engines: it lines up the rapid pressure rise with the piston position that gives the most efficient conversion of combustion energy into work. If the spark happens at TDC or after TDC, the peak pressure occurs too late, reducing power and efficiency. If the spark is fired far before TDC, the burn may finish before the piston reaches TDC, wasting energy and potentially causing rough operation or knocking.

So the correct timing is roughly 15 degrees before top dead center.