reference will now be made in detail to various embodiments of the present invention ( s ), examples of which are illustrated in the accompanying drawings and described below . while the invention ( s ) will be described in conjunction with exemplary embodiments , it will be understood that present description is not intended to limit the invention ( s ) to those exemplary embodiments . on the contrary , the invention ( s ) is / are intended to cover not only the exemplary embodiments , but also various alternatives , modifications , equivalents and other embodiments , which may be included within the spirit and scope of the invention as defined by the appended claims . an exemplary embodiment of the present invention will hereinafter be described in detail with reference to the accompanying drawings . fig1 is a perspective view of a gasoline direct injection engine according to an exemplary embodiment of the present invention . fig2 is a perspective view of a piston head applied to a gasoline direct injection engine according to an exemplary embodiment of the present invention . fig3 is a top plan view of fig2 . fig4 is a cross - sectional view taken along line iv - iv of fig2 . a gasoline direct injection engine according to an exemplary embodiment of the present invention , as shown in fig1 to 4 , includes a combustion chamber , at least a pair of intake ports 100 , an intake valve 101 positioned at the inlet of the combustion chamber in the downstream of the intake port 100 , a piston 550 , a first cavity 501 and a second cavity 502 formed at the intake side on the piston 550 , an injector directly inject fuel from the side of the cylinder into each combustion chamber , a spark plug , an electronic control unit ( hereafter , referred to as ecu ) performing combustion control , at least a pair of exhaust port 200 , and a exhaust valve 201 positioned at the outlet of the combustion chamber in the upstream of the exhaust port 200 . in this configuration , the intake and exhaust valves 101 and 201 and the intake and exhaust ports 100 and 200 are two , respectively , in the drawings , but only one is shown in the present embodiment for the convenience of description . further , in fig3 , reference numeral ‘ sc ’ indicates the center line of the spark plug 400 , reference numeral ‘ pc ’ indicates the center line of the piston 550 , reference numeral ‘ ex . side ’ indicates the exhaust valve 201 side , and reference numeral ‘ in . side ’ indicates the intake valve 101 side . further , the combustion chamber means the space between the first cavity 501 and the spark plug 400 . the injector 300 is formed toward the combustion chamber and the first cavity 501 depressed on the top of the piston 550 is formed with respect to the injection direction of the injector 300 . the intake port 100 is connected to the intake pipe 110 and the exhaust port 200 is connected to the exhaust pipe 210 . the intake valve 101 opening / closing the intake port 100 and the exhaust valve 201 opening / closing the exhaust port 200 are disposed above the cylinder head of the engine , and at an upper side of the combustion chamber . the intake valve 101 and the exhaust valve 201 , for example , are opened / closed by a valve driving mechanism . meanwhile , the engine has the spark plugs 400 and the injectors 300 corresponding to the number of the cylinders , and at least a pair of the spark plugs 400 and the injectors 300 may be provided to the combustion chamber , respectively . the spark plug 400 is mounted to the cylinder head to be positioned at the center portion of the upper portion of the corresponding combustion chamber . further , in the present embodiment , a side injection type is used and the injector 300 is mounted to the cylinder head to be positioned at the downstream of the intake pipe 110 extending to the combustion chamber . that is , the injector 300 directly injects fuel from the side to the corresponding combustion chamber . meanwhile , the first cavity 501 is formed on the piston head 540 , that is , the top of the piston 550 to produce desired gas mixture layer around the spark plug 400 . the first cavity 501 has a substantially circular or elliptical shape and it is preferable that the center is biased at a predetermined distance from the center of the combustion chamber to the intake port 100 . with this configuration , the gas mixture flowing inside from the intake port 100 is returned by the first cavity 501 formed substantially adjacent to the intake port . that is , the first cavity 501 is biased to the intake valve 101 , flat in the flow direction of the fuel injected from the injector 300 and flowing from the intake side to the exhaust side , that is , in the perpendicular direction to the longitudinal direction of the piston 550 , and gradually curved upward at the end portion . further , the inclination of the bottom , the side , and the rear side of the first cavity 501 is determined such that at least some of the fuel injected from the injector 300 returns to the intake valve 101 . a stratified combustion operation can be performed by the flow along the shape of the first cavity 501 . further , a protrusion 510 extending to the front ( to the exhaust valve ) of the first cavity 501 is formed . the protrusion 510 protrudes to separate the first cavity 501 from the second cavity 502 , which is described below . further , the protrusion 510 is biased to the exhaust valve 201 from the center of the first cavity 501 at a predetermined distance from the first cavity 501 while having an arc shape in a plan . that is , the second cavity 502 is positioned to the exhaust valve 201 from the center of the protrusion 510 . in this configuration , the second cavity 502 is lower than the first cavity 501 . since the first cavity 501 positioned to the intake valve 101 and the second cavity 502 positioned to the exhaust valve 201 are different in height , as described above , it is easy to control the volume of the intake valve 101 and the volume of the exhaust valve 201 , such that it is possible to change the volume ratio of the combustion chamber . in an exemplary embodiment of the present invention , distal end portions of the second cavity 502 may be merged to the first cavity 501 to form a junction portion 525 therebetween . for convenience in explanation and accurate definition in the appended claims , the terms “ upper ”, “ lower ”, “ inner ” and “ outer ” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures . the foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description . they are not intended to be exhaustive or to limit the invention to the precise forms disclosed , and obviously many modifications and variations are possible in light of the above teachings . the exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application , to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention , as well as various alternatives and modifications thereof . it is intended that the scope of the invention be defined by the claims appended hereto and their equivalents .