Abstract: The first mixing ratio error in a transient state is sampled as a pre-transient error, the last mixing ratio error in the transient state is sampled as a post-transient error, and the peak value of the mixing ratio errors in the transient state is also sampled. The difference between either this pre-transient mixing ratio error or post-transient mixing ratio error depending on whichever is the nearer to a peak value, and the peak value of said mixing ratio errors, is computed. Injection fuel correction amounts in transient running states are learned and the learned values are stored in a memory so as to eliminate this difference. By correcting the injection fuel amounts based on these learned values in transient running states, the effect of steady state errors on the transient learning precision is eliminated and instantaneous lean peaks in the air-fuel ratio are smoothed out.

Abstract: A primary passage and a secondary passage in a parallel relation to each other are disposed between an air cleaner and a branch passage collecting portion of an intake manifold. The primary passage is smaller in diameter than the secondary passage and has at the downstream end thereof a laval nozzle through which air flows at the speed of sound even under low-speed low-load engine operating conditions. The laval nozzle is connected to a carburetion chamber which is elongated straightly in the direction downstream of the laval nozzle. A fuel injection valve is disposed in the primary passage at a place upstream of the laval nozzle. A pressure control valve is adapted so that a vacuum higher than a predetermined value is developed in the carburetion chamber at any time. A primary throttle valve and a secondary throttle valve are interconnected so that the secondary throttle valve is held closed unless the primary throttle valve is fully open.