Marine medium-speed diesel engines operate in a unique environment, often exposing their lubrication lines to the risk of oil-water mixing. For one thing, even minor leaks in the engine cooling system can allow seawater or freshwater to seep into the lubrication lines. Furthermore, the combustion of fuel generates water vapor, which condenses into liquid water upon cooling and mixes with the engine oil. Furthermore, ships operate in high humidity environments, allowing moisture from the air to enter the oil sump through the breather valve. This oil-water mixing can easily lead to oil emulsification. Demulsibility, a core performance indicator of marine medium-speed diesel engine oil, directly determines the oil's ability to quickly separate oil from water, preventing emulsification from lingering and impacting lubrication line function. This is essential for ensuring the proper operation of the lubrication system.
The primary impact of oil emulsification is the disruption of oil film stability in key areas of the lubrication line. Lubricating oil relies on forming a continuous, uniform film on metal surfaces to isolate moving parts and reduce friction. When engine oil remains emulsified due to insufficient demulsibility, the strength and adhesion of the oil film are significantly reduced. This is especially true in high-speed, high-load areas such as crankshaft bearings and camshaft bushings, where the oil film easily breaks, leading to direct contact between metal parts. This dry or semi-dry friction exacerbates component wear, and the resulting metal debris circulates with the emulsified oil in the oil circuit, further contaminating the oil and creating a vicious cycle of wear and contamination.
Insufficient demulsibility can also cause the emulsified oil to form sludge in the lubrication circuit, leading to blockage. When emulsified oil mixes with impurities such as water, metal debris, and fuel dilution products, it easily forms a viscous, sludge-like substance. This sludge initially clogs the oil filter in the lubrication circuit, increasing the pressure differential across the filter. If not cleaned promptly, the sludge can also deposit on the inner walls of small oil passages (such as those in rocker arm shafts and piston cooling nozzles), reducing the cross-sectional area of the passages. Oil line blockage reduces lubricant supply, especially to components located far from the oil pan. This insufficient supply can lead to lubrication failure, and in severe cases, localized overheating or component seizure.
Furthermore, the water content of emulsified oil can exacerbate corrosion of metal components in lubricant lines. During use, the combustion of marine medium-speed diesel engine oil produces acidic substances (such as sulfuric acid and carbonic acid). These acids dissolve in the water in the emulsified oil, forming a corrosive liquid. As this corrosive liquid circulates in the lubricant lines, it chemically corrodes metal surfaces such as steel pipes, joints, and the inner wall of the oil pan, causing pitting and rust. The resulting rust residue not only contaminates the oil but can also compromise oil line seals, creating new leaks and further deteriorating the lubricant line's operating environment.
Emulsification of engine oil also significantly affects the oil pressure stability of lubricant lines. While the viscosity of normal engine oil fluctuates regularly with temperature, the viscosity of emulsified oil can vary erratically depending on its water content, potentially thinning the oil or forming uneven, viscous clumps. The oil pump in the lubrication oil circuit maintains a stable oil pressure by delivering oil. When the viscosity of the emulsified oil is abnormal, the pump's delivery efficiency decreases, causing fluctuations in the oil pressure in the main oil gallery. This unstable oil pressure affects the distribution of lubricating oil to the branch oil circuits. For example, insufficient oil pressure may prevent the valve train, piston pins, and other components from receiving adequate lubrication, thus shortening component life.
Inadequate demulsibility can also indirectly affect the auxiliary cooling function of the lubrication oil circuit, accelerating oil deterioration. During circulation, lubricating oil removes some of the heat generated by engine components, providing auxiliary cooling. The thermal conductivity of emulsified oil differs from that of pure oil. Incomplete oil-water separation can lead to the formation of water clusters or water-in-oil structures, resulting in uneven heat transfer and potentially overheating of local components in the oil circuit. Furthermore, the presence of water accelerates the oxidation and aging of the oil, generating more oxides and colloids. These deposits in the oil circuit further reduce the oil's lubricating properties, shortening oil change intervals and increasing maintenance costs and downtime for the vessel.
The demulsibility of marine medium-speed diesel engine oil impacts the lubrication system across multiple dimensions, including lubrication, cleanliness, corrosion protection, and cooling, directly impacting the engine's operational stability and lifespan. If demulsibility fails to meet standards, the lubrication system may malfunction due to oil film breakdown, blockage, corrosion, and other issues, increasing the risk of engine overhaul. Therefore, during vessel operation and maintenance, it is essential to strictly select medium-speed diesel engine oil that meets marine standards (such as ISO 12922), regularly test the oil's moisture content and demulsibility, and promptly address any oil-water mixing issues to ensure the lubrication system is always in good working condition and guarantee reliable engine operation.
