Lubricant performance in service as influenced by effects of degradation :
Consider what may, even does, happen to a unit mass of lubricant during its passage through a diesel engine - to what is it exposed? It is exposed to numerous stresses, summarised as follows:
・engine derived - kWh/g as described in detail in CEC/CIMAC'98 papers [refs CEC Symposium 1997 EL-07]. In this presentation I use the illustrations of the concept of oil stress as related to oil quality, engine oil volume and oil consumption.
・physical stresses, including temperature, pressure, mechanical shear applied over various degrees of time via the oil age or oil residence time on any component;
・chemical stresses or reactions caused by exposure to combustion gases, whether in the combustion chamber, or through exposure of the lubricant film to blow-by gases or in the engine sump, and through tribo-chemical reactions that occur as two metal surfaces undergo relative motion, but separated by the vital oil film with its active additive components;
・physical contamination, arising from soot, water, fuel, airborne dust, particulates generated as a by-product of correct lubricant functioning, for example the neutralisation of mineral acids, wear particles that may aggravate chemical degradation through catalyic activity.
・oil film thickness, whether as a thin film or as the bulk fluid, will have an impact on the rate of degradation and on mechanisms to be considered.
Lubricant degradation mechanisms
The impact of stresses changes the original properties and performance of the lubricant. This may be observed in terms of volatility losses, oxidation, nitration, acid formation, polymerisation, BN depletion [sulphur acids], reduced molecular weight, additive adsorption onto metal surfaces and onto soot particles, incompatibility with fuel components.
Measurable lubricant responses to stresses
The ability of the lubricant to resist changes induced by stresses forms the basic target for enhanced quality oils.
・changes in oil viscosity
・formation of insolubles
・changes to BN and to TAN. BN depletion may be modelled, using the concepts of oil stress and the conversion of sulphur in the fuel to base depleting acids. This is illustrated using as examples different oil BN values and oil consumption rates.
・chemical changes, eg formation of organic acids, nitrated hydrocarbons...
・reduction or increases in molecular weight distributions
・particulates with measurable particle size distributions.
・loss of functionalities. One such factor is oxidation stability. Using differential scanning calorimetry the oxidative inhibition of an oil in service may be followed. An example is given for a BN50 lubricant. This methodology may be routinely applicable in a research context but would not be part of rapid analysis schemes.
Practical implications and inferences
While a lubricant is stressed, the engine may be affected in many different ways, for example
・deposits
・corrosion
・soot induced wear
・separation problems in filters/centrifuges
Designing more stress-resistant lubricants
Given the progress to date, emphasis for advancing the quality of lubricants for the diesel engine has to be in more stress-resistant fluids. In turn this does demand of additive suppliers and base oil producers corresponding boosts in quality to be matched by the skills of the formulators and for the designers of suitable test procedures.
In this context it is noteworthy that CEC IL-047 is investigating methods available to evaluate important performance aspects of oils in service (field performance parameters). A start has been made by identifying these parameters, - wear, deposits, water tolerance, others (to be determined) - as applicable to 2-and 4-stroke engines.
One such key item identified by MESJ is the oxidation stability measurement as it is indicative of how an oil may or has aged or degraded. Since numerous methods exist, varying in relevance to marine oils, this will surely be the subject of further study and method development.
The control of deposits highlights the relevance of lubricant/heavy fuel oil compatability as significant.
As demonstrated by the CEC IL-047 survey and interpretation of the survey information, much offered by MESJ members, compatibility is important. Methods not only to detect fuel oil contamination but also to determine in the laboratory the potential or actual side effects has yet to be fully explored.
This request was first made known to me at least by one OEM during my first visit to his location. I think some progress has been made since then although the details may remain proprietary for the time being. However, just to whet your appetite and perhaps to stimulate discussion and further studies in this area of simulation by lab. test developments let me show some preliminary results that demonstrate differences in deposit formation that may be related to oil BN and detergency for example. Crucially, this deposit data has been matched by the more critical engine performance parameter, namely piston undercrown deposit formation.
A similar picture could be derived for fuel pump plunger cleanliness ratings. Let me just show one example, that could be nicknamed‘the good, the bad and the ugly' or more appropriately the pump plungers that were free, sluggish or stuck during tests of three different oils in a 4L20 engine in our laboratory.
