Valve Timing

the periodic movement of inlet and outlet members of an internal combustion engine, by which the cylinders are filled with a new charge of fuel (intake, inlet) and exhaust gases are removed (exhaust, outlet). Depending on the type and construction of the engine, valve timing may be categorized as valve, rotary, slide (nonvalve), sleeve, and combined types.

In valve-type engines there are two main arrangements of valves: in the head of the cylinders, as the upper or overhead system, and the so-called lower or side system. In the former the valves are actuated by a rocker arm from a camshaft, which is in turn driven by the crankshaft through a pinion or chain drive.

In diesel marine and locomotive internal combustion engines there are additional cams and reversing devices in the valve timing system. These make it possible to change the direction of rotation of the crankshaft.

Rotary valve timing employs plane rotating gears and disks that have ports cut into them. When the disk rotates, its ports match up with the ports at the bottom and head of the cylinder and the exchange of gases takes place.

The so-called slide-valve system uses slide valves driven by the crankshaft of the engine.

The sleeve valve system is used in two-cycle engines. The cylinder walls have slots that are opened and closed by the piston moving within the cylinder.

The most widespread combined type of valve timing is a modified sleeve-valve system in which exhaust takes place through an outlet valve and intake, through a slotted device.