Marine engine lubrication

Abstract

A detergent system that includes (i) an overbased C20-28 alkyl-substituted hydroxybenzoate Ca salt of TBN less than 250 and (ii) an overbased C14-18 alkyl-substituted hydroxybenzoate Ca salt of TBN less than 250, the wt % Ca ratio of (ii) to (i) being more than one, is used in a trunk piston marine lubricant to improve its asphaltene dispersancy when lubricating a medium-speed compression-ignited marine engine fuelled by a heavy fuel oil.

What is claimed is: 
     
         1 . A method for improving asphaltene dispersancy in a trunk piston marine engine, the method comprising lubricating the engine with a lubricating oil composition comprising a detergent system that includes (i) an overbased alkyl-substituted hydroxybenzoate calcium salt of TBN less than 250, which alkyl group has 20-28 carbon atoms, and (ii) an overbased alkyl-substituted hydroxybenzoate calcium salt of TBN less than 250, which alkyl group has 14-18 carbon atoms; the wt. % Ca ratio of (ii) to (i) being greater than one. 
     
     
         2 . The method of  claim 1 , where the hydroxybenzoates, (i) and (ii), are each salicylates. 
     
     
         3 . The method of  claim 1 , where the alkyl group(s) of (i) has 20-24 carbon atoms. 
     
     
         4 . The method of  claim 1 , where the composition has a TBN in the range of 20-60 mg KOH/g. 
     
     
         5 . The method of  claim 4 , where the composition has a TBN in the range of 25-55 mg KOH/g. 
     
     
         6 . The method of  claim 1 , where the wt. % Ca ratio is in the range 1 to 50. 
     
     
         7 . The method of  claim 6 , where the wt. % Ca ratio is in the range 1 to 3. 
     
     
         8 . The method of  claim 1 , where the composition comprises an oil of a lubricating viscosity, in a major amount, containing 50 mass % or more of a basestock containing less than 90% saturates and/or greater than 0.03 percent sulphur. 
     
     
         9 . The method of  claim 1 , where the composition comprises an oil of a lubricating viscosity, in a major amount, containing 50 mass % or more of a basestock containing greater than or equal to 90% saturates and less than or equal to 0.03% sulphur.