Air fuel ratio control system for variable valve timing type internal combustion engines

Claim 1

An air-fuel ratio control system for an internal combustion engine having at least one intake valve, and valve timing changing means for changing valve timing of said at least one intake valve at least between low-speed valve timing suitable for operation of said engine in a low engine rotational speed region, and high-speed valve timing suitable for operation of said engine in a high engine rotational speed region, the system comprising: operating condition detecting means for detecting operating conditions of said engine; air fuel ratio calculating means for calculating a desired air-fuel ratio of a mixture supplied to said engine in response to the detected operating conditions of said engine; changeover detecting means for detecting whether or not said engine has entered a predetermined changeover region in which said valve timing is to be changed; and air-fuel ratio changing means for changing said desired air-fuel ratio when it is detected that said engine has entered said predetermined changeover region; wherein when changeover is to be effected from said low-speed valve timing to said high-speed valve timing, said air-fuel ratio changing means changes said desired air-fuel ratio to be applied when said low-speed valve timing is selected, to one to be applied when said high-speed valve timing is selected, and said valve timing changing means changes said valve timing to said high-speed valve timing after said desired air-fuel ratio has been changed to said one to be applied when said high-speed valve timing is selected.