Diesel engine cylinder lubricating oil excess alkalinity

It is well known that the TBN (or amount of alkalinity) of the diesel engine cylinder oil should be dependent on the amount of sulphur present in the fuel oil. A typical value is 70 TBN, which is sufficient to neutralize acidic products of combustion of fuel containing up to about 3.5 per cent sulphur.

However, if the alkalinity is too much in excess of the requirement, e.g. in the case of fuel containing very low sulphur, then the excess alkalinity can cause serious corrosion problems, particularly at high temperatures. For this reason, during running in of new cylinder liner and piston rings, engine makers advise using either a straight mineral oil or an oil of very low TBN for cylinder lubrication.

My question is what exactly happens if the alkalinity is excessive? What are the chemical reactions and mechanisms that cause this type damage.

Any additional related information will be most welcome

 

[nan2001]

Excees alkanity resultscalcium deposit formations. This is a hard substance which you normally find as white/gray deposits when you open the units.

This aids results in bore polishing. and leads to higher lub oil consumtion. I have seen in some cases piston seizures due to excess alkanity. In the above case involving a Yanmar engine. The piston got seized and forunately the engine was stopped before major diaster took place.

Oil TBN should be adjusted by experience by seeing the conditions. If excess lower TBN is advised. If lower TBN inadequate, blending or topping with lower TBN is adviseble provided supplier agrees to compatibility.

 

[AdityaSrinivasan]

Each manufacturer of engine be it B&W or Sulzer have their specific liner running in procedure to aid in the runnig in porocess. This would involve the time for the engine is to be run on low TBN oil and the speed to be maintained so as to gradually shape up the surfase asprites and give the liner surface a smooth but firm structure. As you so rightly mentioned the engine manuals would suggest using of Mineral oil for the first lag of the running in. But mordern advancements have lead to use of some particular CLO's which posses the intricate quality of forming abrassive particles on combustion and aid in running in. I will look up so material text books giving detailed explanation of material reactions to alkaline oils and will get back to you.
As far as excess alkality is concerend it could result in cold corrosion in engines running on low sulphur fuel oils.
Hece trend monitoring is absolutely essential for monitoring the sulphur content and a check on combustion byproducts, indicator cards often reveal the internal condition of the engine, more so than thought.

 

[jmw]

Does change in alkalinity affect the viscosity? if so you could use continuous on line viscosity measurement as one quality check. I believe it is a good indicator of acid number change as well as of contamination with other fluids (fuel, coolant) and of breakdown of the shear resistant component (e.g. PIB). So far as i know, only Solartron (

http://www.solartron.com)

can give you the kinematic viscosity at 40degC irrespective of operating temperature, which is the measurement you need. These are similar to their HFO fuel visometer and returns many of the same parameters e.g. dynamic and kinematic viscosity plus density at operating temp, density at 15degC and kinematic viscosity at 40degC.

 

TBN and alkalinity of cylinder oil is of course relevant factors in achieving a satisfactory cylinder condition, it should be neither too high or too low.

What is even more important, however, is not to use more cylinder oil than necessary and ensure the proper distribution of the oil on the running surface of the liner. By such action you are able to achieve the best results. Then the effect of a too high alkalinity is only marginal because the quantity is so low.

We suggest that you take a look at the results obtained using SIP (Swirl Injection Principle) lubrication - spraying cylinder oil droplets on the upper liner wall, instead of relying on the piston rings to do the distribution.

Using this system you can achieve low liner wear conditions and low consumption at the same time, applying conventional TBN 70 cylinder oils.

Results seen are cylinder oil consumption of 0.43 g/BHPh and a liner wear at the same time of no more than 0.013 mm/1000h. For further see www.hjlubri.dk

Thus, by ensuring to apply a low quantity of cylinder oil but ensuring it is present the right place at the right time you achieve good results without having to worry about alkaline deposits.

If you have questions or comments, please do not hesitate to contact hjl@hjlubri.dk.