Cylinder lubrication in a two-stroke engine is entirely separate from the crankcase lubrication system: cylinder oil must form and maintain an oil film between the piston rings and the liner to control wear, seal combustion gases and keep the liner clean. Since the piston skirt and crosshead receive oil from the crankcase, the cylinder oil is supplied fresh by lubricators at each cylinder, and it is burned in the combustion space or passes down to the scavenge space, so it is a total-loss lubricant. Because of MARPOL Annex VI, which limits the sulphur content of fuel (down to 0.50% or 0.10% sulphur in Emission Control Areas and as required by various regulators), the requirement to neutralise the acidic products of sulphur combustion (sulphuric acid) is much reduced. Excessively high cylinder oil feed rates now produce excess alkalinity, deposits, ash and increased oil costs, while feed rates must still be enough to maintain the ring/liner film and prevent corrosion wear. This tension between alkalinising and wear protection is the heart of modern cylinder lubrication practice.
"Two-level lubrication" refers to the ability of the lubrication system to deliver different feed rates of cylinder oil at different engine operating conditions, usually distinguishing between a higher feed rate for normal sea-going service and a reduced feed rate for low-load, slow-steaming or manoeuvring conditions, and increasingly with separate settings to match fuel sulphur content. It is implemented by either;
- Two separate oil injection pumps/systems with different deliveries which are engaged by the control system according to engine speed or load, or
- An electronic pulse lubrication system where the quantity injected per unit time (and even per injection) is programmed as a function of engine speed and load and of the selected fuel-oil sulphur content and the BN (base number) of the oil.
In normal service the feed rate is set to, say, 1.0 to 1.2 g/kWh to allow for the corrosive component of high-sulphur fuel. In slow-steaming or low-load operation the lower setting (e.g. 0.5 to 0.8 g/kWh) is selected because the corrosive load is lower but a minimum film must still be maintained. Some systems physically select a different set of plungers or a different cam to give the two rates; electronic systems simply change the injection programme. The purpose is to avoid both under- and over-lubrication over the full operating range and to minimise total oil consumption and carbon/ash deposit formation.
Under-lubrication: with too little cylinder oil, the oil film between rings and liner breaks down. This causes metal-to-metal contact, high friction, high liner and ring wear, scuffing, seizure of the ring(s), loss of compression and blow-by of combustion gas, a fall in power, and greater risk of a scavenge fire as hot blow-by gases ignite the lubricant deposits collected in the scavenge space. The liner can become polished or badly worn and the running surface can be damaged permanently. Anti-corrosion protection also fails, so acid attack (cold corrosion) increases, especially in low-sulphur/low-load conditions.
Over-lubrication: excessive oil is passed into the cylinder. Parts of the oil are burned, and the ash and carbon deposits build up on the piston crown, ring grooves and gas side, and in the exhaust valves, turbocharger (if not cleaned) and scavenge space. The pour of oil down the liner increases oil consumption, raises costs and produces large quantities of sludge and oily deposits in the scavenge space, which are a serious fire risk. Carbon in the ring grooves causes the rings to become stuck, reducing sealing and leading to blow-by. The excess alkalinity (BN) from the oil can react with fuel-ash and form hard deposits. The overall result is reduced engine reliability and higher running cost.