Many types of valves are on tractors including valves in the hydraulic pump, carburetor, and oil pump but when used alone the term "Valve" refers to the Intake and Exhaust Valves. These open and close based on the action and timing of the camshaft to allow air and fuel to be drawn in and burnt exhaust gases to be push out.

A hydraulic device that controls fluid flow direction, pressure, or flow rate.

(e.g. 24 valves) Signifies the total intake and exhaust valves in an engine. For example, a 24-valve V-6 would have 4 valves per cylinder: 24/6=4. A 16-valve V-8 would have 2 valves per cylinder: 16/8=2. More valves allow the engine to breathe better at high RPM and produce more power.

Metal devices that open and close to allow fuel and air to enter the combustion chamber and exhaust gases to leave it. Operated from the camshaft by means of valve lifters, push rods, rocker arms, and overhead camshaft lobes, the valves can be adjusted with feeler gauges so that they open and close at the proper times. (These adjustments can't be made to valves that are operated by valve lifters).

In a steam engine, steam has to be admitted to and exhausted from, the cylinders at the right moment in the cycle. This is achieved by a valve arrangement inside the steam chest, which is located next to each cylinder.