The cam lobes force the valves open by pressing on the valve, or on some intermediate mechanism, as they rotate.
Camshafts can be made out of several types of material.
Another option was to use a triple eccentric with connecting rods; these were used on certain W. Bentley-designed engines and also on the Leyland Eight.
A secondary effect of increased duration can be increased overlap, which is the number of crankshaft degrees during which both intake and exhaust valves are off their seats.
It is overlap which most affects idle quality, inasmuch as the "blow-through" of the intake charge immediately back out thru the exhaust valve which occurs during overlap reduces engine efficiency, and is greatest during low RPM operation.
In internal combustion engines with pistons, the camshaft is used to operate poppet valves.
It consists of a cylindrical rod running the length of the cylinder bank with a number of oblong lobes protruding from it, one for each valve.
With some early fuel injection systems, cams on the camshaft would operate the fuel injectors.