System lubricating oil composition for crosshead-type diesel engine

Abstract

The present invention provides a system lubricating oil composition for a crosshead-type diesel engine, which can prevent fuel efficiency deterioration by suppressing the viscosity increase and comprises a mineral base oil and/or a synthetic base oil, (A) (a-1) a star polymer having a vinyl aromatic hydrocarbon structure in the molecule and/or (a-2) an ethylene-α-olefin copolymer or a hydrogenated compound thereof and having a base number of 4 to 20 mgKOH/g and a 100° C. kinematic viscosity of 7.5 to 15.0 mm 2 /s.

1 . A system lubricating oil composition for a crosshead-type diesel engine comprising:
 a mineral base oil and/or a synthetic base oil;   (A) (a-1) a star polymer having a vinyl aromatic hydrocarbon structure in the molecule and/or (a-2) an ethylene-u-olefin copolymer or a hydrogenated compound thereof,   and having a base number of 4 to 20 mgKOH/g and a 100° C. kinematic viscosity of 7.5 to 15.0 mm 2 /s.   
     
     
         2 . The system lubricating oil composition for a crosshead-type diesel engine according to  claim 1  wherein Component (a-1) is a compound having a structure wherein the polymer or copolymer of a diene extends radially from the core of the vinyl aromatic hydrocarbon located in the center of the molecule. 
     
     
         3 . The system lubricating oil composition for a crosshead-type diesel engine according to  claim 1  wherein the vinyl aromatic hydrocarbon is divinylbenzene. 
     
     
         4 . The system lubricating oil composition for a crosshead-type diesel engine according to  claim 1  wherein the polymer or copolymer of a diene is polyisoprene or an isoprene-styrene copolymer. 
     
     
         5 . The system lubricating oil composition for a crosshead-type diesel engine according to  claim 1  wherein Component (A) is contained in an amount of 2 to 15 percent by mass on the basis of the total mass of the composition. 
     
     
         6 . The system lubricating oil composition for a crosshead-type diesel engine according to  claim 1  further comprising at least one type selected from the group consisting of (B) metal detergents, (C) zinc dialkyldithiophosphates, (D) rust inhibitors and (E) ashless dispersants. 
     
     
         7 . The system lubricating oil composition for a crosshead-type diesel engine according to  claim 1  wherein the base oil has a 100° C. kinematic viscosity of 3.5 to 9.3 mm 2 /s.