Abstract: A piston of an engine has a bowl including: a top end that is a surface inclined toward a center of the piston from an edge portion of a top of the piston; a central projection protruding upward at a center of the piston; a recession extending outwardly from the central projection and inclined downward beneath the top end; and a neck connecting the recession and the top end to each other wherein a minimum diameter of the neck is within 62˜65% of a cylinder bore diameter, a maximum diameter of the recession is within 102˜105% of the minimum diameter of the neck, a maximum depth from the top of the piston to a deepest portion of the recession is within 10˜20% of the cylinder bore diameter, and a center of a radius of curvature of the neck is disposed within 2˜10% of the cylinder bore diameter.