VEHICLE AND SINGLE-CYLINDER FOUR-STROKE ENGINE UNIT

A vehicle on which a single-cylinder four-stroke engine unit which is able to stably detect the oxygen density in exhaust gas is mounted is provided. A single-combustion-chamber main catalyst 39 is at least partially positioned frontward of a crankshaft axis Cr1, and purifies the exhaust gas exhausted from one combustion chamber 29 most in an exhaust path extending from the one combustion chamber 29 to a discharge port 35e. A single-combustion-chamber upstream oxygen detector 37 is provided upstream of the single-combustion-chamber main catalyst 39 in the flow direction of the exhaust gas. The path length from the one combustion chamber 29 to the single-combustion-chamber upstream oxygen detector 37 is longer than the path length from the single-combustion-chamber upstream oxygen detector 37 to the upstream end of the single-combustion-chamber main catalyst 39.