referring to fig1 illustrating the first embodiment of the present invention , reference numeral 1 denotes a cylinder block ; 2 , a cylinder ; 3 , a piston ; and 4 , a cylinder head . a main combustion chamber 6 is formed between the inner surface of the cylinder head 4 and the top surface of the piston 3 at its upper dead point , and is provided with a suction valve 5 and an exhaust valve ( not shown ). within the cylinder head and in the vicinity of the top portion of the main combustion chamber 6 is formed a recess 7 into which is fitted an auxiliary chamber element 8 consisting of parts 8a and 8b . within the auxiliary chamber element 8 is defined an auxiliary combustion chamber 9 . the reasons why the auxiliary chamber element 8 consists of two parts 8a and 8b are ( a ) that the inner surface of the auxiliary combustion chamber 9 may be smoothly machined so as to increase its resistance to heat and ( b ) that part 8b through which a passage 10 intercommunicating between the main combustion chamber 6 and the auxiliary combustion chamber 9 is formed may be made of a material capable of sufficiently withstanding the high temperature in the passage 10 . the passage 10 consists of a first passage 10a extending through the part 8b from the auxiliary combustion chamber 9 to the outer surface of the part 8b and a second passage 10b extending through the part 8b from the main combustion chamber 6 to the outer surface of the part 8b . a spark plug 12 is screwed into a hole 11 formed in the cylinder head 4 communicated with the passages 10a and 10b . the spark gap 12a of the spark plug 12 is placed in the vicinity of the intersection between the passages 10 a and 10 b ; that is , the sharply bent portion 10c in the passage 10 . the lean air - fuel mixture drawn into the cylinder 2 through the suction valve 5 in the suction stroke is pushed into the auxiliary combustion chamber 9 through the passage 10 from the main combustion chamber 6 as the piston 3 is lifted in the compression stroke . the combustion products produced in the previous combustion stroke which have remained in the passage 10 are carried away into the auxiliary combustion chamber 9 by the flow of the lean air - fuel mixture through the passage 10 . thus no post combustion gas remains around the spark gap 12a of the spark plug 12 at the moment of the ignition which takes place immediately before the completion of the compression stroke . therefore the misfire due to the residual post combustion gas may be completely eliminated . according to the present invention , very important features and advantages may be attained due to the fact that the sharply bent portion 10c is formed in the passage 10 . that is , in order to attain the high combustion speed as well as the effective combustion , the combustion gases from the auxiliary chamber 9 must be widely scattered through the passage 10 into the main combustion chamber 6 . to this end , the length of the passage 10 must be reduced as practically as possible . however , in the prior art , a straight passage 10 &# 39 ; is formed as shown in fig2 and its length is inevitably increased because the spark - plug gap of the spark plug 12 must be placed within the passage 10 &# 39 ;. as a result , the flow of the combustion gases flowing through the long straight passage 10 &# 39 ; becomes almost a laminar flow so that the combustion gases cannot be widely scattered into the main combustion chamber 6 , thus resulting in the decrease in combustion efficiency . moreover the combustion gases injected through the straight passage 10 &# 39 ; concentrate on a limited portion on the top of the piston 3 , thereby causing local heating thereof . however , according to the present invention , the passage 10 is bent sharply at 10c as described above so that the flow of the combustion gases is much disturbed and becomes turbulent . because of this turbulent flow , the combustion gases may be widely distributed in the main combustion chamber 6 . thus the combustion efficiency may be remarkably increased while the local heating of the piston 3 may be positively prevented . since the gap 12a is placed at the bent portion 10c in the passage 10 , the flow of the lean air - fuel mixture flowing from the main combustion chamber through the passage 10 into the auxiliary combustion chamber 9 is suitably decelerated so that the misfire due to the extinction of the spark caused by the air - fuel mixture flowing at a high speed through the gap , may be prevented . the second embodiment shown in fig3 is substantially similar in construction to the first embodiment shown in fig1 except that the passage 10b is diverged toward the main combustion chamber 6 so that the turbulent flow of the combustion gases produced at the bent portion 10c in the passage 10 may be more smoothly and widely scattered into the main combustion chamber 6 . thus , the combustion efficiency may be further increased while local heating of the piston 3 may be more positively prevented .