Abstract: An engine control system comprises a NOx catalyst selectively reducing NOx from exhaust gases, a reducing agent supply providing the reducing agent to an exhaust system and positioned upstream of the NOx catalyst, NOx sensors detecting an amount of NOx in exhaust gases emitted by an internal combustion engine, a fuel injection system injecting fuel to the internal combustion engine in a main injection mode or a pilot-and-main injection mode, and a control unit activating the fuel injection system in the pilot-and-main injection mode when a NOx purifying efficiency determined on the basis of data of emitted NOx is equal to or below a preset purifying efficiency.

Abstract: An engine control system comprises a NOx catalyst selectively reducing NOx from exhaust gases, a reducing agent supply providing the reducing agent to an exhaust system and positioned upstream of the NOx catalyst, NOx sensors detecting an amount of NOx in exhaust gases emitted by an internal combustion engine, a fuel injection system injecting fuel to the internal combustion engine in a main injection mode or a pilot-and-main injection mode, and a control unit activating the fuel injection system in the pilot-and-main injection mode when a NOx purifying efficiency determined on the basis of data of emitted NOx is equal to or below a preset purifying efficiency.

Abstract: A multiple step valve opening control system, particularly for an EGR control valve unit, having a plurality of valve lifts. The system comprises a valve member for opening a fluid passage in multiple steps, a first piston slidably housed in a housing, a second cylinder substantially coaxial with the first piston, a second piston fitted in the second cylinder and coupled to the valve member, and a first piston stroke regulating member which is moved selectively to a first position for regulating an allowable displacement of the first piston to a first predetermined extent (l.sub.1) or to a second position for regulating the allowable displacement of the first piston to the first predetermined first extent (l.sub.1) plus a third predetermined extent (l.sub.3). By changing the position of the first piston displacement regulating member, the valve member is opened by various valve lifts.