Not possible to cut a 24" log with a 18" bar. I have proved this several times to my wife and have also shown her that in order to safely cut certain sizes of trees you must have the correct saw and bar. It just isn't safe to cut a 16" tree with a 14" bar, it isn't safe to cut a 20" tree with a 18" bar, you must use a 20" bar, for a 24" tree you must use at least a 28" bar and so on. I have a few 48" plus trees coming up, I'm looking for a 72" bar. It also applies to gas tree trimmers too, you must have several sizes of trimmers and bars to safely cut some trees.
PS, anyone who tells my wife different will get a quick punch in the face.