Which of the following is NOT a piston crown type?

• A. Convex
• B. Concave
• C. Dome
• D. Flat

Answer: (D)

Piston crown shapes control how the combustion chamber is formed at the top of the piston and, in turn, affect mixture motion, flame propagation, and compression characteristics. A concave crown creates a bowl in the piston surface, promoting turbulence and efficient mixing. A convex crown bulges outward to modify the chamber shape and can alter the compression and flame path. A dome crown rises in the center to tailor the chamber geometry for specific performance and knock characteristics.

The flat top, on the other hand, doesn’t contribute a curved boundary to the combustion chamber; the chamber shape in that case is defined mainly by the cylinder head and bore, not the piston crown itself. In this classification, a flat top isn’t considered a piston crown type, making it the correct choice.