Discuss the significance of cylinder lubrication in a two-stroke diesel engine considering the impact of Annex VI of MARPOL 73/78. Explain:

Cylinder lubrication in a two-stroke engine is entirely separate from the crankcase lubrication: the cylinder oil is supplied fresh by lubricators at each cylinder and is burned or passes down to the scavenge space (total-loss). Its function is to form and maintain an oil film between the piston rings and the liner to control wear, seal combustion gases, and keep the liner clean. Because of MARPOL Annex VI, which limits the sulphur content of fuel (down to 0.50% or 0.10% sulphur in ECAs), the requirement to neutralise the acidic products of 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.

"Two-level" (two-tier) cylinder lubrication refers to the ability of the system to deliver different feed rates of cylinder oil at different operating conditions, usually a higher feed rate for normal sea-going service and a reduced feed rate for low-load/slow-steaming/manoeuvring conditions, and increasingly with separate settings to match the fuel sulphur content. It is implemented by either two separate oil injection pumps/systems with different deliveries engaged by the control system according to engine speed/load, or by an electronic pulse lubrication system where the quantity injected per unit time is programmed as a function of engine speed, load, and the fuel sulphur/BN. In normal service the feed rate is set to, say, 1.0-1.2 g/kWh to allow for the corrosive component of high-sulphur fuel; in slow-steaming/low-load the lower setting (e.g. 0.5-0.8 g/kWh) is selected because the corrosive load is lower but a minimum film must still be maintained. 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, causing 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 a greater risk of a scavenge fire. The liner can become polished or badly worn. 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, 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) can react with fuel ash and form hard deposits. The overall result is reduced engine reliability and higher running cost.