I was reading about oil control rings in the piston ring topic. Most modern engines use a 3 piece design for oil control rings where you have top rail, bottom rail and spacer-expander (sandwiched between the former two).
My question is what is a "spacer ring"? Is it different from the term "expander ring" or is it just different name for same thing (This is because various sources use the terms interchangeably while some distinguish it. Source1, Source2).
Also what is the purpose of spacer & expander ring (assuming they refer to different things) ?
From What I have read, I understood (seems pretty much wrong) it as follows. Spacer/expander (same thing) ring basically acts as a spring. When the spacer is sandwiched between the two rails and inserted into the ring groove, both of the rails press against the spacer. Since spacer has corrugations it can slightly expand (increase in radius maybe locally) and thus form a tight seal with the cylinder. Thus from what I understood it is the expander that scrapes of oil mainly in oil control ring. Also having corrugations allows oil to easily flow through the oil drain holes as well.
This is a website which seems to solve my doubts on how spacer works