Carbon residue control in marine medium-speed diesel engine oil is a key factor affecting engine deposit formation. Its mechanism of action spans the entire process of fuel combustion, lubricant degradation, and metal component interaction. Carbon residue is essentially the carbonaceous material remaining after high-temperature cracking of oil products. Its content directly determines the rate and type of deposit formation. When the carbon residue value of marine medium-speed diesel engine oil exceeds the standard, the oil contains an increase in unstable hydrocarbons and colloidal substances. These substances easily polymerize under the high temperatures in the combustion chamber, forming a highly adherent carbon layer. This carbon deposit not only covers the piston ring grooves and cylinder walls but also changes the surface roughness of the combustion chamber, leading to localized overheating and reduced heat transfer efficiency.
Contamination of the lubricant system is another important aspect of carbon residue control. Marine medium-speed diesel engine oil is continuously exposed to high-temperature combustion gases during operation. The base oil and additives in the oil are oxidized to form intermediate products such as hydroxy acids and gums. When the carbon residue value is high, these oxidation products accelerate the conversion to asphaltenes, forming a viscous sludge. This type of sludge has strong adsorption properties and can adhere to filters, oil separators, and the inner walls of fuel pipes, increasing system flow resistance. More seriously, the metal particles in the sludge can exacerbate wear on injector components, leading to poor fuel atomization and further deteriorating combustion conditions.
Corrosion and wear of metal components and carbon residue control have a synergistic effect. Acidic substances, such as sulfuric acid and sulfurous acid, produced by the combustion of high-carbon residue oils can penetrate the lubrication system through the cylinder liner cooling water cavity or the piston ring gap. If the lubricant's carbon residue is too high, its detergency and dispersibility will be significantly reduced, making it unable to effectively neutralize the acidic substances. The combined effects of acid corrosion and mechanical wear can cause pitting on the cylinder liner surface. These microscopic defects become the core attachment points for carbon deposits, creating a vicious cycle of corrosion and carbon deposition.
The formation of combustion chamber deposits is also closely related to changes in lubricant viscosity. When the carbon residue of marine medium-speed diesel engine oil exceeds the standard, the oil's oxidation stability at high temperatures decreases and its viscosity increases sharply. High-viscosity oil cannot return to the oil sump promptly, causing the crankcase oil level to rise abnormally. Excess lubricating oil is squeezed into the combustion chamber as the piston descends, forming a liquid oil film. This oil film decomposes at high temperatures to form carbonaceous particles, which, together with particulate matter from fuel combustion, form a composite deposit with greater hardness and adhesion than carbon deposits from a single source.
The formation mechanism of exhaust system deposits is also related to carbon residue control. Unburned hydrocarbons produced by the combustion of high-carbon residue oil condense in the low-temperature zone of the turbocharger turbine end, reacting with sulfur compounds in the exhaust gas to form sulfate particles. These particles flow with the exhaust gas to the exhaust pipe elbow, where they are deposited due to reduced flow velocity. Long-term accumulation of deposits reduces the cross-sectional area of the exhaust passage, leading to increased backpressure, which in turn reduces engine power and increases fuel consumption.
The consumption rate of lubricant additives is also affected by carbon residue. Detergents and dispersants, commonly used in marine medium-speed diesel engine oils, work by adsorbing particulate matter through polar groups, keeping metal surfaces clean. When carbon residue is too high, additive molecules become encapsulated by excessive carbonaceous particles, forming large aggregates and precipitating. This precipitation not only consumes the effective additive components but also forms a thick layer of sludge at the bottom of the oil sump, reducing the effective capacity of the lubricant.
From a systemic perspective, controlling carbon residue in marine medium-speed diesel engine oil is essentially a process of balancing thermal stability and cleanliness. Low-carbon residue oils can reduce deposit formation in the combustion chamber and lubrication system, extending maintenance intervals for key components such as injectors and turbochargers. However, excessive pursuit of low carbon residue can lead to a decrease in oil anti-wear performance, necessitating optimized base oil selection and additive formulations to achieve comprehensive performance improvements. In practical management, dynamic carbon residue control standards should be established, taking into account factors such as fuel sulfur content and engine load characteristics, to ensure stable operation of the lubrication system under complex operating conditions.
