Which term describes the piston's position at the bottom of its stroke?

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

Answer: (A)

Pistons move between two extreme positions inside the cylinder, called top dead center and bottom dead center. The position at the bottom of the stroke is Bottom Dead Center, the lowest point the piston reaches as it travels downward. At that moment the piston’s velocity is momentarily zero as it reverses direction to begin the next stroke. In a four-stroke engine, Bottom Dead Center occurs at the end of the intake stroke (and again at the end of the exhaust stroke in the cycle), while Top Dead Center is the highest point where compression and ignition prep occur. Mid-stroke is just a point somewhere between the extremes, not a defined endpoint, and idle position isn’t a standard term for piston location.