Abstract: To induce high compression engine operation hydraulic fluid is fed into a variable volume chamber defined between an outer piston and an inner one which is reciprocatively disposed therein via a supply passage which includes a one-way valve. When low compression engine operation is required the pressure supplied to a valve chamber in which a spool valve is disposed is increased to the point whereat the spool valve moves to a position wherein the supply passage is closed and a drain and transfer passages are opened. The drain passage leads directly to the cylinder bore so as to enable the hydraulic fluid in the variable volume chamber to be vented unrestrictedly. The transfer passage permits a small amount of hydraulic fluid to flow through the variable volume chamber to the drain passage in a manner which cools the same and prevents degradation of the hydraulic fluid retained in the one-way valve and the like.

Abstract: An EGR system has two vacuum modulation valves, one responsive to venturi vacuum and "EGR" pressure (which prevails just upstream of the EGR flow control valve) and the other responsive to venturi vacuum and induction vacuum. The first valve predominately dilutes the vacuum operating the EGR flow control valve during heavy load operation while the second valve predominately dilutes said vacuum during light load and/or high speed operation. With this combination, excessive EGR is avoided during said light load and high speed operations, respectively.