Intake port structure for internal combustion engines

Claim 1

An intake port structure for an internal combustion engine having a plurality of cylinders each having a combustion chamber, comprising: (a) two-barrel carburetor means including a primary passage with a primary throttle valve connected to a primary intake manifold assembly for supplying an air-fuel mixture into each of said combustion chambers in a full range of load conditions, and a secondary passage with a secondary throttle valve connected to a secondary intake manifold assembly for supplying an air-fuel mixture into each of said combustion chambers under relatively high load conditions; (b) said primary intake manifold assembly being composed of a plurality of primary branch passage extending from a primary common pipe to intake ports defined in a cylinder head communicating respectively with said combustion chambers; (c) said secondary intake manifold assembly being composed of a plurality of secondary branch passages extending from a secondary common pipe to said intake ports defined in said cylinder head communicating respectively with said combustion chambers; (d) said primary branch passages and said secondary branch passages being respectively joined together immediately upstream of said intake ports, each of said primary branch passages terminating in an outlet which opens directly into a respective one of said secondary branch passages upstream of but closely adjacent the respective intake port; (e) said cylinder head defining a surface which extends substantially perpendicularly with respect to the longitudinal axis of each of the respective combustion chambers, each of said primary branch passages having an end portion which terminates in said outlet and is oriented approximately in parallel relationship with said surface, said outlet of each of said primary branch passages being oriented substantially tangentially to the respective combustion chamber, each of said secondary branch passages terminating in an end portion which opens into the respective said combustion chamber in a direction which is transverse to said surface; (f) a wall projecting sidewardly into the end portion of each of said secondary branch passages adjacent to a respective intake valve stem and adjacent to but downstream of the respective outlet for sidewardly deflecting an air-fuel mixture flowing from said primary and secondary branch passages for substantially tangential entry into said combustion chambers to form a circumferential flow of air-fuel mixture in the combustion chambers; (g) each of said secondary branch passages having a groove extending from said outlet along an upstream side of said wall for guiding the air-fuel mixture flowing from the respective primary branch passage into the respective combustion chamber; and (h) said primary and secondary branch passages being dimensioned to meet the relationship of (n-1)S 1 ≦S 2 where S 1 is the cross-sectional area of each of said primary branch passages, S 2 is the cross-sectional area of each of said secondary branch passages, and n is the number of said cylinders.