The lubrication conditions of a chemical engine cylinder liner are one of the core factors affecting its wear rate, and their effect is felt throughout the entire working process of the friction pair. The essence of lubrication is to reduce direct metal-to-metal contact by forming a lubricating film between the cylinder liner and piston rings, thereby reducing friction and wear. Based on the formation mechanism and thickness of the lubricating film, lubrication conditions can be classified into three types: fluid lubrication, boundary lubrication, and mixed lubrication. The mechanisms by which these different conditions affect the wear rate vary.
Under ideal operating conditions, fluid lubrication completely isolates the cylinder liner and piston rings through a continuous oil film of a certain thickness. In this case, the frictional resistance mainly comes from the shear stress of the lubricating oil, resulting in extremely low wear. However, the actual working environment of a chemical engine cylinder liner is complex. High temperatures, high pressures, and speed variations caused by the reciprocating motion of the piston make it difficult to maintain fluid lubrication for extended periods. Especially in the upper part of the cylinder liner near the combustion chamber, high temperatures reduce the viscosity of the lubricating oil, disrupting the integrity of the oil film and causing the lubrication state to shift towards boundary lubrication or mixed lubrication.
Boundary lubrication is one of the main causes of cylinder liner wear. When the lubricating oil film is insufficient to completely isolate the metal surface, the micro-protrusions of the friction pair will come into direct contact. In this case, additives in the lubricating oil (such as anti-wear agents and extreme pressure agents) will react with the metal surface to form a chemical reaction film, providing temporary protection. However, if the additives are depleted or the temperature is too high, causing the reaction film to fail, the direct metal-to-metal contact will lead to adhesive wear, significantly increasing the wear rate. The upper part of the cylinder liner, due to its highest temperature and thinnest lubricating oil film, often becomes the hardest-hit area for wear under boundary lubrication conditions.
Mixed lubrication is between fluid lubrication and boundary lubrication, characterized by localized oil film and localized direct metal-to-metal contact. This state is more common in the middle of the cylinder liner because the piston speed is highest in this area, resulting in vigorous lubricating oil flow. However, the intrusion of mechanical impurities (such as dust and fuel combustion residue) exacerbates wear. Impurity particles roll or slide between the friction pairs, forming three-body wear, leading to scratches or pits on the cylinder liner surface, further increasing the wear rate.
The performance of the lubricating oil also has a significant impact on the wear rate. Viscosity is one of the core indicators of lubricating oil. Excessive viscosity increases internal friction, leading to starting difficulties and increased energy consumption; insufficient viscosity prevents the formation of a sufficiently thick oil film, exacerbating boundary wear. Furthermore, the detergency, dispersancy, corrosion resistance, and oxidation resistance of lubricating oil directly affect cylinder liner life. For example, sulfur in fuel combustion produces sulfur dioxide, which further oxidizes to sulfur trioxide, combining with water vapor to form sulfuric acid, causing chemical corrosion on the cylinder liner surface. If the lubricating oil lacks sufficient base number (TBN), it cannot neutralize acidic substances, accelerating corrosion and wear.
The choice of lubrication method is equally crucial. Splash lubrication relies on lubricating oil splashed from the connecting rod big end onto the cylinder wall, suitable for low-to-medium speed engines, but may lead to increased wear due to insufficient lubrication supply under high-speed or high-load conditions. Injection lubrication uses specialized equipment to directly inject cylinder oil onto the cylinder wall, allowing precise control of the lubricating oil quantity, especially suitable for high-boost or high-performance engines, but requires a high-precision injection system to avoid over- or under-lubrication. Composite lubrication, combining splash and injection methods, leverages the complementary advantages to enhance lubrication performance and is currently the mainstream lubrication solution for chemical engine cylinder liners.
However, the impact of actual operation and maintenance on lubrication conditions cannot be ignored. During cold starts, lubricating oil viscosity increases and fluidity decreases, leading to a significant increase in the instantaneous wear rate of the cylinder liner. Prolonged low-temperature operation can also cause incomplete combustion and carbon buildup, further exacerbating wear. Furthermore, improper maintenance of the air filter and oil filter can allow mechanical impurities to enter, disrupting lubrication conditions and causing abrasive wear. Therefore, regularly replacing filters and maintaining the engine's normal operating temperature (typically 80-90°C) are crucial measures for optimizing lubrication conditions and reducing wear rates.
