Piston

1. A piston used for an engine that includes:

an ignition plug disposed in a vicinity of a central axis of a cylinder;

an intake port and an exhaust port disposed at positions where the ignition plug is interposed therebetween; and

an injector disposed at a position offset from the ignition plug toward the intake port to inject a plurality of fuel sprays toward a crown surface of the piston,

the piston comprising:

a recess that is formed by recessing the crown surface of the piston with respect to other portions of the crown surface,

wherein the recess includes:

a bottom surface comprising a flat surface formed on a deepest portion of the recess;

a step rising from the bottom surface on an outer peripheral edge over the entire circumference of the recess with respect to the other portions of the crown surface; and

a pair of lateral sides rising from the bottom surface formed straightly so as to extend in parallel to a straight line that connects the injector and the ignition plug when seen in a direction of the cylinder axis.

2. The piston according to claim 1 , wherein the injector is a multiple hole injector that forms a plurality of fuel sprays, and the outer peripheral edge of the recess is disposed adjacently to hitting areas formed at least by the last fuel injection of the plurality of fuel sprays on the inner surface of the recess.

3. The piston according to claim 1 , further comprising a flat area provided at a central portion of the crown surface,

wherein a principal portion of the flat area extends along a plane orthogonal to an axial line of the cylinder bore, and

wherein the flat area is formed by recessing a central portion of a pentroof shape that is formed by intake side bevels and exhaust side bevels.

4. The piston according to claim 3 , wherein the intake side bevels and the exhaust side bevels are disposed so as to match the shape of the pentroof typed combustion chamber and inclined with respect to a flat surface that is orthogonal to a center axis of a bore so as to protrude on the central side of the bore toward the cylinder head.

5. The piston according to claim 3 , wherein the step is disposed at a central portion of the flat area and has a plan view having an arcuate shape disposed so as to protrude on the exhaust port side with a center angle of 180° when seen in a direction of an axial line of a cylinder bore.

6. The piston according to claim 1 , wherein the bottom surface includes a flat shape parallel to a plane that is orthogonal to the axial line of the cylinder bore.

7. The piston according to claim 1 , wherein the step is formed in an inside region of the crown surface of the piston with respect to an outer peripheral edge of the crown surface.

8. The piston according to claim 1 , wherein the step and the pair of lateral sides are coupled to the other surfaces so as to form pointed edges.

9. The piston according to claim 1 , wherein the step constitutes an end of the recess on an ignition plug side.

10. The piston according to claim 1 , wherein the step is disposed so as to include the ignition plug on an inner diameter side thereof when seen in a direction of an axial line of a cylinder bore.

11. The piston according to claim 1 , wherein the pair of lateral sides are provided in parallel to each other so as to linearly couple both ends of the step.

12. A piston comprising:

a recess that is formed by recessing a crown surface of the piston with respect to other portions of the crown surface,

wherein the recess includes:

a bottom surface comprising a flat surface formed on a deepest portion of the recess;

a step rising from the bottom surface on an outer peripheral edge over the entire circumference of the recess with respect to the other portions of the crown surface; and

a pair of lateral sides rising from the bottom surface formed straightly so as to extend in parallel to a straight line that connects an injector and an ignition plug of an engine including the piston when seen in a direction of the cylinder axis.