In an engine having apparatus for a post-exhaust process, surging is
prevented when the air excess coefficient is changed to a lean condition
from a rich condition in order to carry out the post-exhaust process. The
present engine control apparatus comprises a turbo-supercharger driven by
exhaust energy of an engine in a vehicle, an EGR path interconnecting an
exhaust path and an inlet path of the engine, at least one of a NO.sub.x
trap catalyzer and a diesel particulate filter (DPF) mounted in the
exhaust path for purifying exhaust discharged from the engine, an EGR
valve for adjusting the flow rate of air passing through the EGR path, an
air excess coefficient control unit for controlling the air excess
coefficient, depending on driving conditions, and wherein the EGR valve
is operable to close when the air coefficient control unit controls the
air excess coefficient so as to be in a lean condition.