Piston Ring Function
B-1 Piston & Piston Ring Assembly
We've removed a piston & ring assembly from the engine so we can look at it in detail.
Assembling the rings onto the piston
One of each of the above ring types, 'top', 'second', and 'oil ring' is usually installed on each piston in a typical engine. Such an arrangement makes for what we call a 'three ring set' or a 'three-ring pack'. Using this arrangement, how many rings in total would be installed in a four-cylinder engine? Twelve is the correct answer, i.e., 4 cylinders x 3 rings/cylinder = 12 rings. This can be considered 'the usual' number of rings found in a standard 4-cylinder car engine. A 6-cylinder engine would then have 18 rings, an 8-cylinder engine 24 rings, and so on. Each of the three rings in a set is a 'specialist' and will use a distinct combination of shapes, materials, heat treatment and/or surface coatings in order to perform its assigned function in an optimal way
Looking more closely at the top and second rings, an observant person notices immediately that there are open joints in the periphery of these rings. That is, they are not closed circles. Such a person would also notice something like a spring in the middle of the oil ring. In fact, this 'expander' section does behave like a spring, generating a uniform tension in the oil ring so as to keep it forced tightly against the cylinder wall.