Basic knowledge of marine diesel engines

1. What causes cylinder liner wear?

Answer: Wear between the cylinder and piston assembly can be divided into normal wear and abnormal wear (such as adhesive wear). Normal wear is caused by the following:

1) Frictional wear. The piston assembly reciprocates within the cylinder under boundary lubrication conditions, often causing metal-to-metal contact wear between the piston rings and cylinder liner.

2) Abrasive wear. Ash and sand brought in by the intake air; ash in the lubricating oil; mechanical impurities; carbon deposits or sediments from combustion products; and worn metal particles fill the friction surfaces as abrasives, causing abrasive wear. 

The scratches appearing on the cylinder wall from this type of wear are uniform, parallel straight lines along the piston's direction of movement.

3) Corrosion wear. After combustion of sulfur-containing fuels, sulfur is converted to sulfur dioxide, hydrogen to water vapor, and trace elements such as vanadium, iron, nickel, and sodium also form oxides. 

Vanadium pentoxide and iron oxide are active catalysts for the re-oxidation of sulfur dioxide to sulfur trioxide. Sulfur trioxide reacts with water vapor below the dew point to form sulfuric acid. This leads to low-temperature corrosion of the cylinder wall by sulfuric acid. 

Sulfur dioxide or sulfur trioxide generated from combustion products in the upper part of the cylinder liner (high-temperature zone) may directly act on the metal surface, causing high-temperature corrosion. 

The corrosion layer on the cylinder wall, weakly bonded to the base metal, peels off during friction, leaving the cylinder wall riddled with loose, fine pores. The products of corrosion wear become abrasives, further accelerating abrasive wear. 

Corrosion wear is particularly severe when burning high-sulfur heavy oil.

These three forms of wear coexist and are interconnected, but under certain conditions, one type of wear tends to be dominant.

4) Fusion Wear. During normal operation of a diesel engine, the oil film between the piston rings, piston skirt, and cylinder liner may be damaged for some reason, resulting in dry friction and overheating of the friction surfaces, creating localized high temperatures. 

Due to the high temperature and pressure at the contact points, the parts fuse together. During the relative movement of the friction surfaces, the weaker parts are worn away and adhere to the other friction surface. The fusion point is heated to welding temperature and then cooled to form a thin, hardened layer. 

During this process, the hardened layer continuously deteriorates and detaches, and the detached metal fragments become abrasive. This type of wear is called adhesive wear or fusion wear. It generally occurs near the second piston ring in the upper part of the cylinder, resulting in uneven and irregular grooves on the cylinder wall. 

This wear may also stop due to the recovery of the oil film, eliminating cylinder scoring and entering a recovery period, or it may continue to operate. If the damaged oil film cannot be recovered, the fusion area expands, leading to severe cylinder scoring. If initial cylinder scoring is detected during operation, the cylinder oil supply to that cylinder should be increased first, and the cylinder should be run in again.

2. What is the general wear pattern of cylinder liners?

A: The wear pattern of cylinder liners is mainly reflected in the wear form, wear rate, and amount as the cylinder liner diameter expands along the axial or radial direction. The maximum wear area of the cylinder liner in the axial direction is near the top dead center of the first piston ring, and the wear gradually decreases downwards along the cylinder, eventually becoming cylindrical. 

This is because the piston speed decreases near top dead center, and the upper part of the cylinder is in a high-temperature zone, making it difficult for an oil film to form and be maintained. When burning heavy fuel oil, there is also severe corrosion and wear. The upper part of the cylinder is also a high-pressure zone during combustion, with the piston rings exerting significant pressure on the cylinder wall, exacerbating wear in this area. 

Furthermore, the presence of carbon deposits forming abrasive particles in this area results in the heaviest abrasive wear. Therefore, the upper part of the cylinder liner experiences the greatest wear. In two-stroke diesel engines with a scavenging flow pattern, in addition to the areas mentioned above, the cylinder liner undergoes significant thermal deformation at the air intake, and the oil film is also damaged by the airflow, leading to severe wear around the air intake.

The radial wear of the cylinder liner varies depending on the type of diesel engine. In cylindrical piston diesel engines, the lateral thrust acts on the cylinder, resulting in severe wear in the lateral direction. However, in high-speed, high-load small diesel engines, the piston deforms severely under the highest combustion pressure, leading to often more severe wear in the longitudinal direction.

In large, low-speed crosshead diesel engines, the wear in both the longitudinal and lateral directions depends on the piston alignment and fuel supply.

3. How to inspect the cylinder piston assembly of a crosshead diesel engine without cylinder deactivation?

A: This inspection is usually performed through the inspection port on the control side of the cylinder block. For turbocharged diesel engines with scavenging and constant pressure, it can also be performed through the exhaust port in the exhaust pipe.

 The engine should be rotated during the inspection. The main inspection items are checking the wear of the piston, piston rings, and cylinder liner surfaces; carbon buildup, adhesion, and breakage of the piston rings; contamination and blockage of the intake ports; and the lubrication of the cylinder walls. 

This inspection can further determine the amount of oil supplied by the lubricator in each cylinder. If a semi-dry, semi-wet lubricating oil film is found on the surface of the first piston ring, and the oil film on the piston ring and cylinder wall surfaces is clean, the oil supply is appropriate; if the surface of the first piston ring is dry and wear marks are found on both the ring and cylinder wall, the oil supply is insufficient. 

Finally, the lubricator is rotated to check the blockage of the cylinder oil filling passages. Based on the technical condition of each cylinder, the timing and order of cylinder inspection can be determined, and the amount of cylinder oil injected into each cylinder can be adjusted appropriately. When deciding to reduce the oil level in a cylinder, do not reduce it too much at once; to ensure safety, it should be reduced gradually.

4. What is roundness? How to check cylinder wear? Under what circumstances is cylinder boring or cylinder liner replacement necessary?

A: Roundness is the variation of the actual circle from the ideal circle on the same cross-section.

Checking cylinder wear generally involves measuring the diameter of various parts of the cylinder and comparing it with the original data. During measurement, a template can be hung on the cylinder liner. Use an inside micrometer to measure the cylinder diameter in the front, back, left, and right directions of the designated measurement points on the template, record the measurements, and conduct comparative analysis. 

Since cylinder boring not only reduces the strength of the cylinder liner but also requires replacing the piston, which is not very economical or managerial, theoretically, cylinder boring is possible, but in practice, replacing the cylinder liner is the preferred method.