The ignition system fires a spark plug before the piston reaches which position?

• A. Bottom dead center (BDC)
• B. Top dead center (TDC)
• C. Mid-stroke
• D. Exhaust stroke

Answer: (B)

Spark timing is ahead of the piston reaching the top of its compression stroke. The ignition system fires a few degrees before the piston reaches top dead center to give the flame front time to propagate through the compressed air–fuel mixture. This timing makes the combustion produce peak pressure as the piston is near TDC, maximizing the force on the piston during the power stroke. If ignition occurred at or after TDC, there wouldn’t be enough time for full combustion to develop pressure when it can do the most work, reducing power and efficiency and potentially causing knock. The other choices don’t fit because igniting during the intake or exhaust strokes, or mid-stroke, wouldn’t place the peak pressure at the right point in the cycle to maximize the power stroke.