What is cylinder lubrication in a two-stroke marine engine?

Cylinder lubrication is the separate oil system that feeds a metered quantity of special oil directly onto the cylinder liners of large two-stroke crosshead engines. These engines keep the combustion space and the crankcase apart with a diaphragm and stuffing box, so two different oils can be used: clean system oil circulates through the bearings and crosshead gear below, while cylinder oil is injected at the liner surface, does its work once, and is burned or drains away as scavenge drain oil.

The cylinder oil has three jobs: it maintains an oil film between the piston rings and liner to control friction and wear, it helps the rings seal combustion pressure, and it neutralises the sulphuric acid formed when sulphur in the fuel burns and condenses on the relatively cool liner. Neutralisation is why the oil is alkaline, with its reserve measured as base number, BN, and why the oil must match the fuel. High-sulphur fuel burned with a scrubber calls for high BN oil, around 100 or more, while 0.5 percent sulphur fuels typically run on around BN 40, and very low sulphur distillates lower still. Mismatch hurts in both directions: too little alkalinity leaves acid to corrode the liner, known as cold corrosion, while a strongly over-based oil on low-sulphur fuel leaves unspent calcium additive as hard deposits that polish the liner mirror-smooth, destroy the oil film and invite scuffing.

Feed rate is the other lever. Modern electronically controlled lubricators inject timed doses as the rings pass, with feed commonly of the order of 0.6 to 1.2 grams per kilowatt-hour and often controlled in proportion to fuel sulphur. Under-lubrication shows as rising wear and eventually scuffing, a hot seized-surface failure that can wreck a liner in hours; over-lubrication wastes a genuinely expensive oil and builds deposits on the piston topland and in the scavenge space, where they become a scavenge fire risk.

Condition monitoring closes the loop. Regular scavenge port inspections show ring condition and deposits directly; scrape-down or drain oil analysis trends the iron content and residual BN of the oil coming off each liner, revealing corrosive or abrasive wear cylinder by cylinder; and liner gauging at overhauls confirms the wear rate. Adjust feed rate on evidence, not habit, follow the engine maker's guidance when changing fuel or oil grade, and escalate any sudden iron spike or scuffing indication immediately, because liner damage progresses fast and the repair is measured in days off hire.