Spark-ignition engine for lean air-fuel mixture

In a spark-ignition-type internal combustion engine having a main combustion chamber equipped with an intake valve for an air-fuel mixture, an auxiliary combustion chamber, a transfer passage interconnecting said main combustion chamber with said auxiliary combustion chamber, and a spark plug associated with the transfer passage, the spark plug having an annular threaded portion mounted in a wall defining the transfer passage so that the spark plug electrodes project into said passage, whereby the air-fuel mixture is forced during the compression stroke from the main combustion chamber into the auxiliary combustion chamber and is ignited by the spark plug electrodes, the combustion gas in the auxiliary combustion chamber is injected through said passage into the main combustion chamber, and thus the air-fuel mixture in the main combustion chamber is quickly ignited and combusted, comprising the improvement wherein said transfer passage has a first passage portion of enlarged cross-sectional area and at least one additional passage portion of a smaller cross-sectional area, said electrodes being positioned in said first portion of said transfer passage, and said first portion having a length as measured in the flow direction of the passage which is greater than the outside diameter of the threaded portion of the spark plug.