In a two-stroke engine, cylinder oil is injected by lubricators directly onto the liner at a ring of injection points (quills) part-way up the liner. The purpose of injecting at the correct time relative to the piston is to place the oil where the piston rings will wipe it around the liner circumference and up into the ring contact band. The important parameter is the piston position (crank angle) when the oil is injected:

• The oil should be injected when the piston rings are passing (or just below) the injection level, so that the oil is momentarily held between the liner and the moving ring pack and distributed uniformly. If injection occurs when the piston is well above the injection points (near TDC), the oil is sprayed onto the upper liner which the rings have already passed - it will be wiped into the combustion space and burned without contributing to lubrication of the lower liner/ring area.
• It should not be injected when the piston is below the injection level with the ports open, because then a considerable proportion of the oil drains into the scavenge space or is carried out by the scavenge air, wasting it and soiling the scavenge space.
• The timing relates to speed and direction of motion: because the injection is set for a given crank angle, at higher engine speed the piston passes the point faster, so a fixed crank-angle injection may occur while the piston is at a slightly different physical position; electronic systems time the injection to the piston position instead, keeping it constant. Direction of motion matters because when the engine runs astern, the piston still moves down after TDC, but the "power stroke" direction (up or down) relative to the injection level is unchanged in a two-stroke; the important point is that injection must coincide with the ring passage coming up and down, and the injection relative to the rings (which are on the piston) is independent of direction, but the sequence of the piston positions is reversed, so the timing must also be adjusted when going astern. In practice a fixed set of injection points on the liner is fed when the piston rings are level with them.

• If the oil is injected too early (piston below the injection level, e.g. at BDC), most oil drains down or is lost into the scavenge space, so the upper part of the liner run remains unlubricated, causing higher friction, ring/liner wear, scuffing, and blow-by (loss of sealing). Blow-by of gas past the rings reduces compression and power and leads to burning of oil and more wear.
• If injected too late (piston above the level), the oil is burned in the combustion space, is carried to the exhaust/injector, and the ring band on the downward part is dry, again causing wear and power loss.
• The result is uneven/insufficient oil film, causing metal-to-metal contact (scuffing), higher piston friction (which at high load lowers the brake power and raises fuel consumption), worn rings losing compression, and possibly a scavenge fire - all of which detrimentally affect the developed power.

[Sketch notes: the lubricator pump delivers oil to a pipe which passes through a non-return check valve to the outside of the cylinder jacket, enters through the cylinder wall (liner) at right angles, opening into a small landing/recess on the inside of the bore.]

Each lubrication point consists of a brass/tubular quill or nipple screwed through the jacket/cooling space 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.