Consequences of oil film degradation

11 Jul 2017

In a review of recent studies into two-stroke lubrication, Rathesan Ravendran and Peter Jensen of Hans Jensen Lubricators highlight the importance of cylinder oil distribution.

It is important to address degradation of the cylinder lubrication oil in two-stroke marine diesel engines in order to ensure optimal engine-running conditions. Oil degradation (also known as oil starvation) means the change in physical and chemical properties of the lubricants that results in deterioration of its performance (CIMAC Working Group 8, 2004, Oil Degradation. Guidelines for Diesel Engines Lubrication, 22).

The oil film on the cylinder wall can be considered as an oil reservoir, consisting of several elements needed to ensure sufficient lubrication. When the lubrication oil is in service, the oil film will be exposed to stresses degrading the lubricating properties of the oil. The ability of the oil to treat oil stresses is therefore a key factor to ensure longer life of cylinder liners and piston rings.

Oil stresses are in general classified in four categories, which are all related to engine/operating conditions and the properties of the lubrication oil and the fuel oil (Hammett, J, 2014, Utilising the Latest Findings Engine Oil Stress from Field & Laboratory Engine Testing, 49(3), 6–13). The four categories are:

Humidity stress: Water absorbed in the lubricant compromises its ability to deal with insoluble particles and promotes depletion of the alkalinity reserve.

Acid stress: Sulphuric acid formed by the combustion corrodes the cylinder liner. This must be prohibited (in a certain degree) by the neutralizing agents in the lubricant.

Insoluble stress: The lubrication oil has to clean insoluble particles in order to avoid deposit formation and wear. These particles are; non-combusted fuel components or contaminants (asphalthenes, cat fines), neutralization products (CaSO4, FeSO4) and metal debris from the abrasive wear.

Thermal stress: Temperature and exposure time determine the extent of thermal degradation of lubricant components. It is influenced by engine load, combustion characteristics and cylinder piston cooling.

Large two-stroke engines place greater physical and chemical stresses on the lubricant because of high power output, low oil consumption and long intervals between injections of fresh lubricants.

The rate of oil degradation varies in relation to the oil film thickness. The thinner the oil film the faster degradation. However, distribution of fresh lubricant with non-return valves  is inhomogeneous and lubricant was found to stay in the engine for up to 30 minutes (Doyen et al, 2007, Advanced applied research unravelling the fundamentals of 2-stroke engine cylinder lubrication—an innovative on-line measurement method based on the use of radioactive tracers). Studies have shown that the residence time increases at lower engine loads and also feed rates (Hammett, 2014). This means that the risk of oil degradation is higher when the engine is slow-steaming and when the engine consumes low lubrication oil.

Several studies have shown that the wear of cast iron, caused by reciprocating piston ring under lubrication, is more severe at top dead centre and bottom dead centre than between these reversal points (Felter, C. L. et al, 2010,  Development of a model capable of predicting the performance of piston ring–cylinder liner-like tribological interfaces). Furthermore, sulphuric acid and carbon soot in the lubrication increased the wear drastically (Yahagi, 1987, Corrosive wear of diesel engine cylinder bore. Tribology International, 20(6), 365–373). The level of sulphuric acid in the lubricating oil influences the corrosive wear. The carbon soot on the other hand increases abrasive wear, due to the hard carbon particles rubbed against the surface.

Oil stress measurements are as important as oil thickness measurements in order to prevent significant liner wear or damage of sliding surfaces. However, oil stress may be difficult to measure as several locations must be measured to understand the physical and chemical properties of the lubrication oil film.

Lubrication oil injection at each piston stroke will refresh the oil film at each piston stroke and thereby minimise the stress level of the oil film on the cylinder liner. Degradation of the oil film will therefore be prevented and low wear rates can be obtained. Furthermore, by injecting only the required amount of lubrication reduces furthermore the accumulation of oil additives, such as calcium carbonate particles which will lead to scuffing.

Flexibility in the delivery of the required amount of fresh lubrication oil at each engine stroke has been highly prioritized by Hans Jensen Lubricators. For this purpose, the HJ Lubtronic system operates with automated stepless stroke adjustment and timed lubrication. Combining HJ Lubtronic system and HJ SIP valves is said to ensure good engine condition. The HJ SIP spray injection valves, placed at the liner circumference, direct the lubricating oil spray upwards and into the engines scavenging air. Thus fresh oil is distributed at the top of the cylinder, where the oil film is highly exposed to degrading oil stresses.