In ignition timing, the spark occurs before the piston reaches TDC.

• A. Before TDC
• B. At TDC
• C. After TDC
• D. BDC

Answer: (A)

Ignition timing is about when the spark plug fires relative to the piston's position. The goal is to start combustion early enough that the pressure rises as the piston is near the top of its stroke, so that the expanding gases push the piston with maximum effective force. Because burning takes a finite amount of time, the spark is set a small number of degrees before the piston reaches TDC. This gives the flame time to propagate and pressure to peak close to the moment the piston is at or near TDC, maximizing power and efficiency.

If the spark occurred exactly at TDC, the flame would be just starting as the piston is at its peak, so the pressure would peak later in the stroke and less work would be delivered. If it fired after TDC, the peak pressure would come too late, further reducing power and potentially causing rough running or knocking. Firing at bottom dead center would be far out of sync with the compression-to-power event.

So, the spark must occur before TDC.