In a two-stroke engine the cylinder oil is injected by lubricators onto the liner at a ring of injection points. The timing of injection relative to the piston position is critical. The oil should be injected when the piston rings are passing the injection level so the rings wipe the oil around the liner and up into the ring contact band. If injected when the piston is above the injection level (near TDC), the oil is sprayed onto the upper liner already passed by the rings and is burned in the combustion space; if injected when the piston is below the level (near BDC, ports open), the oil drains into the scavenge space or is carried out by the scavenge air. So the injection must coincide with the ring passage. Speed: at higher engine speed the piston passes the point faster, so a fixed crank-angle injection may occur at a slightly different piston position; electronic systems time injection to piston position to keep it constant. Direction of motion: when running astern the piston still moves down after TDC, but the sequence of piston positions is reversed; the injection must still coincide with the ring passage, so the timing is adjusted for the astern direction. The injection is set so the oil is delivered just as the rings pass the injection level, giving maximum oil retention on the liner.

• If injected too early (piston below the level), most oil drains into the scavenge space or is lost, so the upper liner/ring band is dry, causing high friction, ring/liner wear, scuffing, and blow-by of gas past the rings. Blow-by reduces compression and power, and the burning oil causes deposits.
• If injected too late (piston above the level), the oil is burned in the combustion space, the ring band on the downward stroke is dry, again causing wear and power loss.
• The result is an inadequate oil film, metal-to-metal contact (scuffing), higher piston friction (lowering brake power and raising fuel consumption), worn rings losing compression, and possibly a scavenge fire - all detrimental to the developed power.

[Sketch notes: the lubricator pump delivers oil through a pipe to a non-return valve and then to a quill/nipple screwed through the cylinder jacket (cooling water space) into the liner. A small bore leads radially through the liner wall to open flush on the bore surface (or into a small groove).]

Each lubrication point consists of a brass/tubular quill or nipple screwed through the jacket and into the liner, sealing the cooling water from the liner. A small bore leads radially through the liner wall to emerge flush (or in a small groove) on the bore surface. A non-return valve prevents gas pressure blowing back into the lubricator when the cylinder is at high pressure. The oil is pressure-fed individually (as a pulse) to each point in turn, or to all points of a cylinder together, from the lubricator driven by the engine or electronic system, timed as above.