Variable timing mechanism for a rotary valve

A variable valve timing mechanism for an internal combustion engine, said engine comprising a crankshaft, a cylinder head, a combustion chamber, and at least one rotary valve, said rotary valve having at least two ports terminating as openings in-its periphery, said cylinder head having a bore in which said rotary valve rotates, a window in said bore communicating with said combustion chamber, said openings successively aligning with said window by virtue of said rotation, and a drive mechanism driving said rotary valve, characterised in that said at least two ports comprise an inlet port and an exhaust port, and said drive mechanism is controlled by a control means to vary the angular velocity of said rotary valve, said control means controlling said drive mechanism in response to sensed engine parameters to produce variations of the angular velocity of said rotary valve at least within a portion of at least one engine cycle whilst maintaining an average angular velocity over said at least one engine cycle that has a fixed relation to the average angular velocity of said crankshaft over said at least one engine cycle, and wherein said drive mechanism produces either a symmetric or assymetric angular velocity ratio profile for said rotary valve with respect to said crankshaft.