Engine oil composition

Abstract

Engine oil compositions comprise (A) a lubricating base oil having a kinematic viscosity at 100° C. of 2 to 6 mm 2 /S, a viscosity index of 120 or more, and a total aromatics content of 15 percent by mass or less and (B) a polymethacrylate-based viscosity index improver in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm 2 /s. Disclosed are also engine oil compositions which comprise (A) a lubricating base oil having a kinematic viscosity at 100° C. of 2 to 6 mm 2 /S, a viscosity index of 120 or more, and a total aromatics content of 15 percent by mass or less; (B) a polymethacrylate-based viscosity index improver in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm 2 /s; and (C) molybdenumdithiocarbamate.

What is claimed is:  
     
         1 . An engine oil composition which comprises (A) a lubricating base oil having a kinematic viscosity at 100° C. of 2 to 6 mm 2 /S, a viscosity index of 120 or more, and a total aromatic content of 15 percent by mass or less and (B) a polymethacrylate-based viscosity index improver blended in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm 2 /s.  
     
     
         2 . An engine oil composition which comprises (A) a lubricating base oil having a kinematic viscosity at 100° C. of 2 to 6 mm 2 /S, a viscosity index of 120 or more, and a total aromatics content of 15 percent by mass or less; (B) a polymethacrylate-based viscosity index improver blended in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm 2 /s; and (C) molybdenumdithiocarbamate.  
     
     
         3 . The engine oil composition according to    claim 1    wherein said polymethacrylate-based viscosity index improver has a weight-average molecular weight of 180,000 or more.  
     
     
         4 . The engine oil composition according to    claim 2    wherein said polymethacrylate-based viscosity index improver has a weight-average molecular weight of 180,000 or more.  
     
     
         5 . The engine oil composition according to    claim 1    wherein said total aromatics content is 5 percent by mass or less.  
     
     
         6 . The engine oil composition according to    claim 2    wherein said total aromatics content is 5 percent by mass or less.  
     
     
         7 . The engine oil composition according to    claim 1    wherein said polymethacrylate-based viscosity index improver is a polymer of a compound represented by the formula 
                 

  wherein R 1  is a straight or branched alkyl group having 1 to 18 carbon atoms.  
 
     
     
         8 . The engine oil composition according to    claim 2    wherein said polymethacrylate-based viscosity index improver is a polymer of a compound represented by the formula 
                 

  wherein R 1  is a straight or branched alkyl group having 1 to 18 carbon atoms.  
 
     
     
         9 . The engine oil composition according to    claim 1    wherein said polymethacrylate-based viscosity index improver is a copolymer obtained by copolymerizing one or more monomers selected form the group consisting of compounds represented by formula (1) with one or more nitrogen-containing monomers selected from the group consisting of compounds represented by the formulae 
                 

  wherein R 2  and R 4  are each independently hydrogen or methyl, R 3  is a straight or branched alkylene group having 2 to 18 carbon atoms, e is an integer of 0 or 1, and X1 and X2 are each independently an amine residue or heterocyclic ring having 1 or 2 nitrogen and 0 to 2 oxygen.  
 
     
     
         10 . The engine oil composition according to    claim 2    wherein said polymethacrylate-based viscosity index improver is a copolymer obtained by copolymerizing one or more monomers selected form the group consisting of compounds represented by formula (1) with one or more nitrogen-containing monomers selected from the group consisting of compounds represented by the formulae 
                 

  wherein R 2  and R 4  are each independently hydrogen or methyl, R 3  is a straight or branched alkylene group having 2 to 18 carbon atoms, e is an integer of 0 or 1, and X1 and X2 are each independently an amine residue or heterocyclic ring having 1 or 2 nitrogen and 0 to 2 oxygen.  
 
     
     
         11 . The engine oil composition according to    claim 2    wherein said molybdenumdithiocarbamate is contained in an amount of 0.02 to 0.15 percent by mass in terms of molybdenum concentration, based on the total mass of the composition.  
     
     
         12 . The engine oil composition according to    claim 2    wherein said molybdenumdithiocarbamate is represented by the formula 
                 

  wherein R 5 , R 6 , R 7 , and R 8  are the same or different and are each independently an alkyl or alkylaryl having 2 to 18 carbon atoms, and Y 1 , Y 2 , Y 3 , and Y 4  are each independently selected from the group consisting of sulfur and oxygen.  
 
     
     
         13 . The engine oil composition according to    claim 1    which further comprises additives selected from the group consisting of alkaline earth metal detergents, ashless dispersants, wear inhibitors, ashless oxidation inhibitors, friction modifiers other than molybdenumdithiocarbamate, corrosion inhibitors, demulsifying agents, metal deactivators, and antifoamers.  
     
     
         14 . The engine oil composition according to    claim 2    which further comprises additives selected from the group consisting of alkaline earth metal detergents, ashless dispersants, wear inhibitors, ashless oxidation inhibitors, friction modifiers other than molybdenumdithiocarbamate, corrosion inhibitors, demulsifying agents, metal deactivators, and antifoamers.