DE68927221T2 - Lubrication process and composition therefor - Google Patents

Description

This invention relates to the lubrication of crosshead diesel engines, e.g. marine low speed crosshead diesel engines.

The lubrication of crosshead diesel engines, and in particular the cylinder liners and piston rings of such machines, presents problems because the temperatures and pressures encountered during their operation can lead to the breakdown of the lubricant film on the inner walls of the cylinder liners. When this occurs, it leads to excessive cylinder liner and/or piston ring wear.

Various additives have been proposed for use in engine lubricant compositions to reduce engine wear. For example, anti-wear additives have been proposed, but although they are effective at low temperatures, they tend to become ineffective at the high temperatures encountered in the cylinder liners of crosshead diesel engines.

Extreme pressure additives have also been proposed which act when the lubricant film breaks down between the metal surfaces when sliding under high pressures. They have been proposed in particular for use in gear oils. The disadvantage of using such additives in lubricant compositions for crosshead diesel engines is that under the temperature conditions at the top of the liners such machines, cylinder liner and piston ring wear can still occur, and indeed may be increased, even if lubricant film breakdown does not occur.

According to the present invention there is provided a lubricant composition for lubricating a crosshead diesel engine, the composition comprising a major amount of a base oil and minor amounts of an antiwear additive and an extreme pressure additive, the antiwear additive being a compound of the formula X=P(OR)₃ (wherein X represents an oxygen or sulfur atom and each R, which may be the same or different, represents a phenyl, tolyl or xylyl group or a halogenated derivative of such a group) and the extreme pressure additive is a sulfurized olefin containing from 25 to 55 weight percent sulfur, the weight ratio of antiwear additive to extreme pressure additive being from 0.2:1 to 10:1 and the total base number being at least 40 (measured by ASTM D2896 in mg KOH/g of composition).

The invention further provides a method of lubricating a crosshead diesel engine, the method comprising lubricating the engine with a composition according to the invention.

Compositions according to the present invention enable good lubrication to be achieved between metal surfaces at elevated temperatures and high contact pressures when compared to lubricant compositions consisting of a base oil containing either the anti-wear additive or the extreme pressure additive alone.

Particularly good and unexpected lubricating properties in Elevated temperatures and pressures can be achieved in crosshead diesel engines using compositions according to the present invention. Examples of specific antiwear additives which can be advantageously used according to the present invention are the phosphothionic acid esters described in British Patent Specification 1018307 and the analogous phosphoric acid esters having an oxygen atom in place of the sulphur atom of these esters. Specific antiwear additives which can be used include tritolyl phosphate, trixylyl phosphate or triphenyl phosphorothioate.

The sulfurized olefins used as extreme pressure additives according to the invention contain from 25 to 55 weight percent sulfur. Particularly preferred extreme pressure additives contain from 35 to 45 weight percent sulfur. The olefinic portion of the sulfurized olefins can be derived from one or more olefins and the olefins can be straight chain or branched chain or cyclic. Furthermore, the unsaturation can be terminal or internal. Preferably, the olefins contain up to 8 carbon atoms. Particularly preferred extreme pressure additives are obtained from olefins containing from 3 to 6 carbon atoms. Particularly preferred sulfurized olefins which can be used as extreme pressure additives in lubricant compositions of the invention are described in British Patent Specification 1308894 and US Patent Specification 4204969.

The amount of extreme pressure additive should be sufficient to provide an extreme pressure effect. This is typically provided by the presence of up to 5.0 percent of the additive based on the total weight of the composition.

In general, it will be necessary to have 0.05 percent of an extreme pressure additive and compositions of Invention preferably contain from 0.05 to 0.2, more preferably from 0.1 to 1.5 and advantageously from 0.3 to 1.0 weight percent of an extreme pressure additive, based on the total weight of the composition.

The amount of antiwear additive present is based on the amount of extreme pressure additive present. The weight ratio of antiwear additive to extreme pressure additive should be from 0.2:1 to 10:1, preferably from 0.2:1 to 5:1, and advantageously about 0.5:1. The relative amounts of these additives are preferably adjusted to provide the compositions with good antiwear properties under high pressures but without excessive corrosion at elevated temperatures. These amounts of antiwear and extreme pressure additives present in an order of magnitude to achieve this will often be less than expected to provide the compositions with satisfactorily low wear at high temperatures based on the known properties of antiwear and extreme pressure additives used.

The base oil, which may consist of a mixture of two or more oils, is preferably a hydrocarbon oil and may contain one or more paraffinic or synthetic oils. In general, it is preferred to use an oil mixture.

The viscosity index of the base oil should generally be at least 50 and preferably at least 70, although it will usually not be higher than 180 and preferably not higher than 130. The base oil preferably has a viscosity index of 95 to 105.

Compositions used according to the present invention will typically contain other additives, e.g. overbased additives, e.g. naphthenates, sulfonates, Phenates, salicylates or carboxylates. The total base number (TBN) of the compositions should be at least 40 and preferably from 70 to 100 (measured by ASTM D2896 in mg KOH/g composition). The TBN of the compositions is preferably not greater than 150.

The viscosity of the compositions is preferably at least 10 and more preferably at least 15 cSt at 100°C. However, the viscosity of the base oil will usually be not more than 35 and preferably not more than 30 cSt at 100°C. Advantageously, the viscosity of the oil will be from 17 to 25 cst at 100°C.

The lubricant compositions can be prepared by known processes. Thus, it is generally practiced to first heat the base oil, usually to a temperature of about 60ºC, then add any overbased additives with stirring and finally add the extreme pressure and anti-wear additives, again with stirring, preferably after allowing the mixture to cool.

It will be appreciated that the compositions may contain various additives used in the lubrication art, such as dispersants, antioxidants and corrosion inhibitors.

The following examples are given for illustrative purposes only. All parts are percentages by weight of the finished lubricant composition unless otherwise indicated.

example 1

58.96 parts 500 SN mineral oil (viscosity index 95) and 13 parts Brightstock mineral oil (viscosity index 95) were mixed and heated to about 60°C. 12.65 parts overbased 250 TBN calcium phenate, 12.65 parts overbased 300 TBN calcium sulfonate and 2.0 parts polyalkylene succinimide were added to the mixture with vigorous stirring. The resulting homogeneous mixture was cooled to about 50°C and 0.50 parts sulfurized olefin (containing about 38% sulfur) and 0.24 parts tritolyl phosphate (TTP) were added with stirring.

The lubricant composition produced (TBN 70.6, Viscosity Index 97 and viscosity 17.2 cst at 100ºC) was then tested using a standard IP 239/85 test "Extreme Pressure Properties: Friction and Wear Tests for Lubricants: Four-Ball Machine" modified to be operable from ambient temperature to 200ºC, the latter temperature being approximately that at the top of the cylinder liner of a low speed crosshead diesel engine. The results obtained are shown in the accompanying table, where weld point and initial seizure values are in kg and scratch values are in mm.

Example 2

In a similar manner as described in Example 1, 59.04 parts of 500 SN mineral oil (viscosity index 95) and 13.0 parts of bright stock mineral oil (viscosity index 95) were mixed and heated to about 60°C. 12.65 parts of overbased 250 TBN calcium phenate, 12.65 parts of overbased 300 TBN calcium sulfonate and 2.0 parts of polyalkylene succinimide were added to the mixture with vigorous stirring. The resulting homogeneous mixture was cooled to about 50°C and 0.45 parts of the sulfurized olefin and 0.21 parts of triphenyl phosphorothionate were added with stirring.

The lubricant composition prepared (TEN 70.3, Viscosity Index 98 and viscosity 17.1 cSt at 100ºC) is then tested as in Example 1, the results being shown in the attached table.

Example 3

A composition produced as in Example 1 was prepared using 0.55 parts of tritolyl phosphate and 0.5 parts of the sulfurized olefin, adjusting the amount of mineral oil accordingly.

The composition was tested as in Example 1 and the results are shown in the attached table.

Example 4

A similar composition to Example 1 was prepared using 0.25 parts trixylyl phosphate in place of the tritolyl phosphate, adjusting the amount of mineral oil accordingly. The composition was tested as in Example 1 and the results are shown in the attached table.

Examples 5 and 6

Two lubricant compositions were prepared using the procedure of Example 1, but with the amounts and types of extreme pressure and anti-wear additives shown in the accompanying table, with the amount of mineral oil adjusted accordingly. The results of the test conducted as in Example 1 are also given in the table.

annotation

In the table, the information corresponds to the following:

Sulphide A-sulfurized olefin with about 38% sulfur

Sulphide B-organic sulphide with about 42 % sulphur

TTP-Tritolyl Phosphate

TPPT-Triphenylphosphorothionate

TXP-trixylyl phosphate. Table

Claims (11)

1. A lubricant composition for lubricating a crosshead diesel engine, the composition comprising a major amount of a base oil and minor amounts of an antiwear additive and an extreme pressure additive, wherein the antiwear additive comprises a compound of the formula X=P(OR)₃ (wherein X represents an oxygen or sulfur atom and each R, which may be the same or different, represents a phenyl, tolyl or xylyl group or a halogenated derivative of such a group) and the extreme pressure additive is a sulfurized olefin containing from 25 to 55 weight percent sulfur, the weight ratio of antiwear additive to extreme pressure additive being from 0.2:1 to 10:1 and the total base number being at least 40 (measured by ASTM D2896 in mg KOH/g of composition). 2. A composition according to claim 1, wherein each R represents a 4-tolyl or 3,5-xylyl group or a halogenated derivative of such a group. 3. A composition according to claim 1 or claim 2, wherein each R has the same meaning. 4. A composition according to any preceding claim, wherein the antiwear additive is tritolyl phosphate, trixylyl phosphate or triphenyl phosphorothioate. 5. A composition according to any preceding claim, containing up to 5.0 percent of a sulfurized olefin. 6. The composition of claim 5 which contains from 0.05 to 2. percent of a sulfurized olefin. 7. Composition according to one of the preceding claims, having a viscosity of 10 to 35 cst at 100ºC. 8. A composition according to any one of the preceding claims, wherein the viscosity index of the base oil is from 50 to 130. 9. A composition according to any preceding claim, which contains an overbased additive. 10. A composition according to any preceding claim having a total base number (measured by ASTM D2896 in mg KOH/g composition) of 40 to 150. 11. A method of lubricating a crosshead diesel engine, the method comprising lubricating the engine with a composition according to any one of the preceding claims.