Control system and control process in an internal combustion engine

A control system for an internal combustion engine in which a valve timing is switched between a low-speed (LO-V/T) or high-speed (HI-V/T) valve timing within a lean-burn control range established in accordance with the operational state (an intake pipe internal absolute pressure (PBA) and an engine revolution number (Ne)) of the engine, wherein an air-fuel ratio of an air-fuel mixture supplied to the engine is enriched for a predetermined time when the valve timing is switched (b) from the low-speed valve timing to the high-speed valve timing while carrying out a lean-burn control. The ignition timing (¸IG) may be changed from a value suitable for one valve timing to a value suitable for the other valve timing via an intermediate value. The air-fuel ratio may be enriched for a set period when going from a lean-burn or fuel-cut condition to a stoichiometric condition when there is a ternary catalyst in the exhaust.