Spark ignition engine with at least one ignition device per cylinder

The engine has at least one ignition device (14) per cylinder. At least one piston (4) has a recess (8) in the piston head. The recess in plan view onto the piston head deviates from the circular to form a heart shape. In the plan view, the shape of the recess has a precise axis of symmetry (10). This shape may be a non-convex one.