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.

Abstract: A projection positioned in the center of a concave section of a toroidal combustion chamber is divided into a section with a steep slope extending from the bottom to the head surface and a section with a gentle slope. The head surface is in the form of a flattened conical trapezoid. Because of its shape, the projection makes it possible for the air which is close to the center section to flow close to the surface of a side wall of the concave section. The air which had been at the center section is added to the amount of air supplied for combustion and is mixed with the fuel in the vicinity of the surface of the side wall of the concave section. By setting the angle at which the air flows along the side wall surface (re-entrant angle) in the combustion chamber, the flow of the air close to the side wall surface is made turbulent, which improves the agitation of the fuel.