DESC:FIELD OF THE INVENTION:

The present invention provides a novel fuel efficient hydraulic oil additive composition. The additive composition comprises (a) an oxidation inhibitor (b) a sulphur, phosphorus & nitrogen (S-P-N) based anti-wear / extreme pressure containing multi-functional additive system, (c) viscosity index improver (d) a friction modifier, (e) optionally, a pour point depressant (f) optionally, a demulsifier and (g) optionally, a defoamant, (h) prepared in a mixture of refined base stocks, or mixture of hydro-processed iso-dewaxed base stocks or mixture of hydro-processed iso-dewaxed base stocks and alkylated naphthalene, or mixture of synthetic base and alkylated naphthalene or synthetic base and ester, alkylated naphthalene base or mixtures thereof.

BACKGROUND OF THE INVENTION:

Hydraulic systems are widely being used in mining, construction, manufacturing, transportation, forestry, in variety of industrial applications, etc. Hydraulic fluids are the medium by which power is transferred in hydraulic machinery. The energy loss caused in the hydraulic machine is mainly in the engine and hydraulic system. The energy loss in the hydraulic system includes (i) loss due to sliding friction in the hydraulic pump and motor (ii) pressure loss in the piping (iii) loss due to oil leakage inside the hydraulic pump, hydraulic motor and control valves. The hydraulic oil is delivered from the hydraulic pump is sent through the control valve to the actuators such as to the travel motor, work equipment cylinders, etc. to perform work such as digging, travel, and swing, etc.

The hydraulic oil temperature in variety of hydraulic machineries varies from 50 to 70 0C during normal operation however in off-high way hydraulic equipments is about 50 to 80 0C and it may reach to 80 – 100 0C in summer in tropical regions. The internal leakage inside the hydraulic component is less for new oil with comparatively higher viscosity than conventional hydraulic oil. In the operating temperature range, the sliding friction resistance in the hydraulic pump, hydraulic motor and pressure loss inside the piping increases, as a result of which, hydraulic pump efficiency decreases.

On the other hand, new oil has a comparatively lower viscosity than that of conventional hydraulic oil in low temperature range for operation in cold regions. When a hydraulic system is operated in cold region, its workability may be lower than usual because of the low fluidity of the new hydraulic oil in the hydraulic systems. Therefore, it would provide good fluidity at low temperatures, high operability is obtained. This feature would result to increase in the pump efficiency.

Mechanical efficiency (? vol.) = theoretical torque – friction loss / theoretical torque
Volumetric efficiency (? mech.) = theoretical delivery – internal leakage / theoretical delivery

Overall efficiency = mechanical efficiency * volumetric efficiency

? overall = Output power / Input power = P*Qact. / 2*p*N*T = Q act. / Qn X P*Qn / 2*p*N*T = ? vol. X ? mech.

Wherein,
P = Pump outlet pressure
Q act = Actual pump outlet flow
Q n = nominal pump flow
T = shaft torque
N = shaft speed
? vol. a Viscositya * Speedb / Pressurec
? mech. a Pressured / Speede * Viscosityf

The overall efficiency of pump is influenced by pressure drop across the pump and viscosity of the fluid in-service. This means that volumetric efficiency increases with increasing viscosity, the mechanical efficiency decreases with increasing viscosity. This means that hydraulic oil with high viscosity index and lower coefficient of friction can attain higher overall efficiency of the hydraulic system in wide operating temperature range.

In order to enhance the pump efficiency, a higher ISO viscosity grade fluid can conventionally be used at higher operating temperature but care to be taken to sufficient low temperature fluidity during cold start.

All hydraulic oils should prevent components from excessive wear and maintenance costs even under extreme temperatures. The deterioration can result from a variety of factors. Rust is one common problem which may create serious issues in engines, transmissions, and hydraulics systems. This problem often arises from the presence of excess water vapor in the system. Oxidation and foam build up is other problems that build up over time and affect the performance of hydraulics systems. For optimal functioning, a hydraulic fluid must be relatively incompressible and must flow readily. In addition, it should provide adequate lubricity for moving parts, stability under anticipated conditions of use, compatibility with materials used to construct the hydraulic system, and the fluids should have the ability to protect system components against chemical reaction with materials which may enter the system.

The hydraulic oil serves as the power transmission medium in a hydraulic system. The most commonly used fluids are petroleum oils, synthetic lubricants, oil-water emulsion, and water-glycol mixtures. The principal requirements of a hydraulic fluid are proper viscosity high viscosity index, anti-wear protection, good oxidation stability, adequate pour point, good demulsibility, rust inhibition, resistance to foaming, and compatibility with seal materials. Anti-wear oils are frequently used in compact, high-pressure, and high-capacity pumps that require extra lubrication protection. Certain synthetic lubricants and water-containing fluids are used where fire resistance is needed. Synthetic lubricants also are used in extreme-temperature conditions.

The initial oil viscosity is often chosen based on considerations other than energy efficiency such as flow rate, temperature range, wear and external and internal leakages, etc. Energy losses in the lubricants are defined by initial oil viscosity and its temperature, pressure and its shear rate. The conventional hydraulic fluids for plant indoor hydraulic systems do not usually contain viscosity index improvers as these fluids are not generally used at subzero temperatures. The shear thinning effect due to temporary mechanical realignment of a polymeric viscosity index improver (VII) has been observed for multi-grade engine oils to give better fuel economy than single grade oils. Nevertheless, VII can help to utilize the shear thinning effect for friction reduction and energy conservation.

There are a great number of synthetic fluids as well as mineral blends that can be used as base fluids for contemporary industrial oils especially hydraulic oils. The most common alternatives to conventional mineral based oils are synthetic poly-alpha-olefins (PAO), poly-glycols, and esters. The non-conventional mineral based oils so called `very high viscosity index’ (VHVI) petroleum hydrogenated base oil produced either by severe hydro-cracking or by iso-dewaxing technologies. The hydro-cracking process results in base stocks containing more paraffin and less aromatic hydrocarbons while PAO consists of almost only paraffin. This molecular structure has better viscosity temperature and pressure viscosity behavior resulting respectively in high viscosity index and low pressure viscosity coefficient. The high pressure viscosity behavior for VHVI base stocks is substantially lower than solvent refined base stocks. Therefore, hydro-cracked and PAO oils show significant advantage over solvent refined base oils in friction reduction under EHD lubrication regimes.

Compressibility is vitally important for hydraulic fluids; one of the main functions is to transmit power. Precise power transmission in high pressure hydraulic systems requires the fluid viscosity (as well as volume) to be precise and has to be least sensitive to pressure or have the smallest pressure – viscosity coefficient. Low viscosity at high pressure is also beneficial for the reduction of viscosity related churning power losses under load conditions. High viscosity index oils tend to be less compressible. The pressure-viscosity and temperature-viscosity properties of liquid hydrocarbons are strongly dependent on their molecular structure. According to existing theories of liquid structure, the more degree of freedom in the molecular structure of liquid, the higher shall be its compressibility.

The various types of viscosity index improvers are available in the market for preparing industrial fluids including hydraulic fluids which can be olefin copolymer (OCP), styrene butadiene (SBR), styrene isoprene co-polymer (SIP), poly methacrylate (PMA), alkyl polymethacrylate (APMA), maleic anhydride styrene co-polymer, poly isobutylene (PIB), dispersant viscosity modifier (DVM), asteric viscosity modifier (AVM), etc. The most commonly used VIIs are poly methacrylates and olefin copolymers and poly methacrylates are more stable viscosity shear characteristics than olefin copolymer. The shear thinning effect should be more pronounced for high molecular weight polymers and high polymer concentration in oil. The reversible behavior reduces the effective viscosity of the lubricant under high shear operating conditions resulting in reduction in viscous drag and improvement in fuel economy. Simultaneously, the non-Newtonian oil behavior causes the EHD film thickness drop to the value defined by the effective high shear viscosity. Thus, the potential negative effect of oil thinning turns out to be beneficial for energy conservation, providing that effect is reversible.

Friction modifiers create boundary layers on rubbing metal surfaces, physically adsorbing and chemically interacting with the metal surfaces. The structure of the boundary layer is different from surface layer created by regular antiwear or extreme pressure (EP) sulphur, phosphorus or chlorine containing additives. These additives do not decrease often increase friction force and energy consumption.

The friction modifiers react chemically with metal surfaces and are effective for boundary regimes at high temperature. Their friction reduction activity is based on the thermally activated chemical reaction with metal surfaces and their effectiveness is higher at higher temperatures. Long chain polar organic acids or esters work at lower temperatures providing thick antifriction films adsorbed on the metal surfaces. The antifriction efficiency of friction modifier depends on the composition and thickness of the surface layers created on the metal surfaces. The energy losses in the industrial equipment consists of friction losses in the bearings, gear mesh, vane ring, piston liner, frictional losses at couplings, losses due to churning at seals, filters, valves, etc.

In the prior art for producing hydraulic oils, generally, mineral oils or mineral oil with synthetic fluids or complex ester of fatty acids were used. The focus has been on the use of such oil base to enhance the performance.

U.S. Pat. No. 5,360,565 discloses the improved anti-wear, high-pressure hydraulic oil which contains essentially no zinc or phosphorous is described. The hydraulic oil protects against corrosion and oxidation as well as provides anti-wear, anti-weld, and demulsibility properties. This improved hydraulic oil contains (1) petroleum hydrocarbon oil; (2) esters of dibasic and monobasic acids; (3) butylated phenol; (4) phenol; (5) sulfurized fatty oil; (6) fatty acid; and (7) sulfur scavenger. This hydraulic oil has a reduced tendency towards sludge formation and has, therefore, an increased lifetime.

U.S. Pat. No. 6,300,292 discloses the hydraulic oil composition which is excellent in oxidative stability, lubricating properties and biodegradability; comprising vegetable oil as base oil, and one phenol antioxidant, an amine antioxidant and a zinc dithiophosphate antioxidant (edible vegetable oils are used).

U.S. Pat. No. 6,436,883 discloses the functional fluid compositions useful in hydraulic fluid and gear oil formulations. The formulations according to the invention include a predominant amount of at least one polyoxyalkylene glycol derived from the addition polymerization of an alcohol in the presence of an alkylene oxide mixture, which contains a substantial amount of ethylene oxide. Fluids according to the invention exhibit suitable lubricity and stability characteristics and are generally water soluble to a degree sufficient to preclude formation of a sheen on the surface of a body of water into which a fluid according to the invention is brought into contact (polyethylene glycol was used).

U.S. Pat. No. 5,366,658 discloses the use of polymethylalkanes as biodegradable base oils in lubricants and functional fluids. The invention relates to the use of polymethylalkanes having terminal methyl groups and methylene and ethylidene groups in which the total number of C atoms n+2 m+2 is 20 to 100 and the ratio of the methyl and methylene groups to the ethylidene groups is 3 to 20:1 and the ethylidene groups are always separated by at least one methylene group, as biodegradable base oils for lubricants and functional fluids. Suitable polymethylalkanes are obtained by oligomerization of alpha, omega-diolefins, for example in particular according to P 41 19 332.6, or by pyrolysis of ethene/propane copolymers and subsequent hydrogenation in each case. The polymethylalkanes can be combined with conventional additives and other degradable or non-degradable base oils (polymethyl alkanes was used).

U.S. Pat. No. 4,783,274 discloses an anhydrous oily lubricant, which; is based on vegetable oils, which is substituted for mineral lubricant oils, and which, as its main component, contains triglycerides that are esters of saturated and/or unsaturated straight-chained C.sub.10 to C.sub.22 fatty acids and glycerol. The lubricant is characterized in that it contains at least 70 percent by weight of a triglyceride whose iodine number is at least 50 and no more than 125 and whose viscosity index is at least 190. As its basic component, instead of or along with the said triglyceride, the lubricant oil may also contain a polymer prepared by hot-polymerization out of the said triglyceride or out of a corresponding triglyceride. As additives, the lubricant oil may contain solvents, fatty acid derivatives, in particular their metal salts, organic or inorganic, natural or synthetic polymers, and customary additives for lubricants (edible vegetable oils are used).

U.S. Pat. No. 5,538,654 discloses the food grade lubricant composition, which is useful as hydraulic oil, gear oil, and compressor, oil for equipment in the food service industry. This composition comprises (A) a major amount of a genetically modified vegetable oil and (B) a minor amount of a performance additive. In other embodiments the composition contains either (C) a phosphorus compound or (D) a non-genetically modified vegetable oil (edible vegetable oils are used).

U.S. Pat. No. 5,580,482 discloses the lubricant composition stabilized against the deleterious effects of heat and oxygen said composition comprising a triglyceride oil or an oil which is an ester wherein unsaturation is present in either the alcohol moiety or the acid moiety and an effective stabilizing amount of either an N,N-disubstituted aminomethyl-1,2,4-triazole or an N,N-disubstituted aminomethylbenzotriazole and a higher alkyl substituted amide of dodecylene succinic acid (edible vegetable oil with an additive was used).

U.S. Pat. No. 5,888,947 discloses the composition that has three main components: a base oil, an oil source containing hydroxy fatty acids and an oil source containing vegetable or animal waxes. The base oil used in the reference needs to consist of primarily triglycerols (triglycerides) and mono- and diglycerols (glycerides) and free fatty acids. The composition further consists of vegetable oils where the glycerols contain hydroxy fatty acids, preferably making up 5% to 20% of the oil. A third major component is waxes composing 5% to 10% of the oil additives by volume. Additional synthetic mimics or natural products derived from animal or vegetable compounds may be added up to 5% of the compositional volume (glycol fatty esters and fats are used).

SUMMARY OF THE INVENTION:

A novel fuel efficient antiwear hydraulic oil composition has been developed after an exhaustive research work in the laboratory for physico-chemical and tribological performance. The energy efficiency has been demonstrated in the tribological performance tests. The composition of the fuel efficient hydraulic oil includes high performance additive system in combination of premium quality severely hydrotreated / hydroprocessed / iso-dewaxed base oils of Group II, Group III and base oils of Group IV, Group V class or mixtures thereof. It would be within the scope of this invention to use any other suitable base oil, like severely hydrotreated base oils of API Group I, with appropriate modifications. The antiwear hydraulic lubricant should have right mixed lubrication regimes additives along with optimum viscosity to form elasto-hydrodynamic and mixed lubrication films and overall energy efficiency can be achieved through correct operating viscosity and friction modification with appropriate additive systems.

The novel fuel efficient antiwear hydraulic oil composition has been finalized in three phases, in the phase I - laboratory evaluation of candidate blends for physico-chemical properties; in the phase II - tribological studies conducted and finally in the phase III – assessment of energy efficiency on the novel energy efficient antiwear hydraulic oil to establish its performance in a EATON pump test against commercially available conventional antiwear hydraulic oils available in the market.

According to the present invention, a fuel efficient hydraulic oil composition comprising an oxidation inhibitor, a sulphur, phosphorus and nitrogen (S-P-N) based antiwear and extreme pressure containing multi functional additive system, a viscosity index improver, a friction modifier, and a mixture of severely refined base stocks, or hydrotreated / hydro-processed / iso-dewaxed base stocks, or hydrotreated / hydro-processed / iso-dewaxed base stocks and alkylated naphthalene, or mixture of synthetic bases and ester or mixture of synthetic bases and alkylated naphthalene or alkylated naphthalene bases or mixtures thereof.

According to preferred embodiment of the present invention, the sulphur, phosphorus and nitrogen (S-P-N) based antiwear and extreme pressure additive system comprising of amine phosphate, dithiophosphoric acid ester, alkylated diphenyl amine, sterically hindered phenol, (4-nonyl phenoxy) acetic acid, acyl sarkosinate, N, N’-bis (2-ethylhexyl)-(1,2,4-triazole-1-yl)methyl)amine, N-N-bis(2-ethylhexyl)-4-methyl-1-H-benzotriazole -1-methylamine, N,N-bis(2-ethyl-hexyl)-5-methyl-1H-benzotriazole-1-methylamine or package containing N, N-bis(2-ethyl-hexyl)-4-methyl-1H-benzotriazole-1-methylamine, N,N’-bis(2-ethylhexyl) -5-methyl-1H-benzotriazole-1-methylamine, (4-nonylphenolphenoxy) acetic acid, tris (methylphenyl) phosphate, sterically hindered phenol, amine antioxidant, mixture of triphenylthiophosphate and tertiary butylated phenyl derivatives or package containing phosphorothioic acid, O,O,O-triphenyl esters, tertiary butyl derivatives, 1-Naphthaleneamine, N-phenyl-ar-(1,1,3,3-tetramethylbutyl), alcohols C12-16, propionic acid derivative, 3-[[bis(2-methyl(propoxyl)phosphinothionyl]thio]]-2-methyl derivative, N-N-bis(2-ethylhexyl)-4-methyl-1-H-benzotriazole -1-methylamine and N,N-bis(2-ethyl-hexyl)-5-methyl-1H-benzotriazole-1-methylamine.

According to preferred embodiment of the present invention, the sulphur, phosphorus and nitrogen (S-P-N) based antiwear and extreme pressure containing multi functional additive system is present in the range of 0.05 to 1.5 percent by weight of the composition.
According to preferred embodiment of the present invention, the oxidation inhibitor is comprising of mixtures of alkylated phenolic antioxidant having alkyl chain length of C2 to C12 which can preferably be alkyl chain length of C4 to C8.

According to preferred embodiment of the present invention, one of the oxidation inhibitor is present in the range of 0.01 to 1.2 percent by weight of the composition.

According to preferred embodiment of the present invention, the friction modifier is selected from the group comprising of long chain fatty amine / amides, fatty esters, borated compounds and oil soluble molybdenum compounds.
According to preferred embodiment of the present invention, the friction modifier is oil soluble molybdenum compound having molybdenum content of 4% to 12%
According to preferred embodiment of the present invention, the oil soluble molybdenum compounds is present in the range of 0.05 to 0.50 percent by weight of the composition.
According to preferred embodiment of the present invention, the viscosity index improver is selected from group comprising of polyalkyl-methacrylates and copolymers of alkylmethacrylate and alphaolefines preferably having average molecular weight of 1,000 to 1,00,000.
According to preferred embodiment of the present invention, the viscosity index improver is present in the range of 1.0 to 90.0 percent by weight of the composition.
According to preferred embodiment of the present invention, the composition further comprises a pour point depressant, a demulsifier and a defoamer, wherein, the pour point depressant is selected from a group comprising poly methacrylates, polyacrylamides, and olefin copolymer, the demulsifier is selected from a group comprising of condensed polymeric alcohols, esters of fatty acids, fatty alcohols alkoxylated with alkylene oxides, and mixtures thereof, and the defoamer is selected from a group comprising of organic polyacrylate polymer or commercially available ash containing defoamer.

According to preferred embodiment of the present invention, the mixture of severely refined base stocks, or hydrotreated / hydro-processed / iso-dewaxed base stocks, or hydrotreated / hydro-processed / iso-dewaxed base stocks and alkylated naphthalene, or mixture of synthetic bases and ester or mixture of synthetic bases and alkylated naphthalene or alkylated naphthalene bases or mixtures thereof is selected from combination of premium quality base oils of API Group II, Group III and base oils of Group IV, Group V class.

According to another preferred embodiment of the present invention, the composition is used for enhancing fuel efficiency of hydraulic systems.

It is also an object of the invention is to provide a novel hydraulic oil additive composition comprising:
(a) an oxidation inhibitor;
(b) a sulphur, phosphorus & nitrogen (S-P-N) based anti-wear and extreme pressure containing multi functional additive system;
(c) a viscosity index improver; and
(d) a friction modifier, and
(e) optionally a pour point depressant, demulsifier and antifoam.

It is further object of the present invention is to provide a hydraulic oil composition comprising:
(a) an oxidation inhibitor;
(b) a sulphur, phosphorus and nitrogen (S-P-N) based antiwear and extreme pressure containing multi-functional additive system;
(c) viscosity index improver;
(d) a friction modifier;
(e) optionally a pour point depressant; demulsifier, antifoam; and
a mixture of refined base stocks, or mixture of hydro-processed iso-dewaxed base stocks, or mixture of hydro-processed iso-dewaxed base stock and alkylated naphthalene, or mixture of synthetic base and alkylated naphthalene or synthetic base and ester, alkylated naphthalene base or mixtures thereof.

It is further object of the invention is to provide a novel high performance anti-wear hydraulic oil possess superior oxidation stability, antirust & anticorrosive properties, load bearing capability with fuel efficiency.

DETAILED DESCRIPTION OF THE INVENTION:

While the invention is susceptible to various modifications and/or alternative processes and/or compositions, specific embodiment thereof has been shown by way of example in tables and will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular processes and/or compositions disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the invention as defined by the appended claims.
The tables and protocols have been represented where appropriate by conventional representations, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.
The following description is of exemplary embodiments only and is not intended to limit the scope, applicability or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing exemplary embodiments of the invention. Various changes to the described embodiments may be made in the function and arrangement of the elements described without departing from the scope of the invention.
To increase the overall efficiency of the hydraulic systems, the hydraulic oil must have the following characteristics;
- Lower coefficient of friction to reduce the sliding friction loss caused in the hydraulic pump and motor
- Adequate viscosity of the hydraulic oil to reduce
o Pressure loss caused in the piping
o Leakage inside the pump, motor, and control valves
- Ability to operate in wide temperature window during extended operation or in-service from cold to tropical regions.

According to the main embodiment, the present invention provides a novel fuel efficient hydraulic oil additive composition comprising:

(a) an oxidation inhibitor;
(b) a sulphur, phosphorus & nitrogen (S-P-N) based anti-wear and extreme pressure containing multi-functional additive system;
(c) a viscosity index improver; and
(d) a friction modifier, and
(e) optionally a pour point depressant, demulsifier and antifoam

According to the other main embodiment, the present invention provides a hydraulic oil composition comprising:
(a) an oxidation inhibitor;
(b)a sulphur, phosphorus and nitrogen (S-P-N) based antiwear and extreme pressure containing multi functional additive system;
(c) viscosity index improver;
(d) a friction modifier;
(e) optionally a pour point depressant; demulsifier, defoamant; and
a mixture of refined base stocks, or mixture of hydro-processed iso-dewaxed base stocks or mixture of hydro-processed iso-dewaxed base stocks and alkylated naphthalene, or mixture of synthetic base and alkylated naphthalene or synthetic base and ester, alkylated naphthalene base or mixtures thereof.

In a preferred embodiment, the oxidation inhibitor is 0.01 to 1.2 percent by weight of the composition, the amount of sulphur, phosphorus and nitrogen based additive is 0.05 to 1.5 percent by weight of the composition, the viscosity index improver is 1 to 50 percent by weight, and the friction modifier is 0.01 to 1.0 percent by weight of the composition.
In a preferred embodiment, the antioxidant used in this invention comprising of mixtures of alkylated phenolic antioxidant having alkyl chain length of C2 to C12, which can preferably be alkyl chain length of C4 to C8 and mainly of mixed butylated phenol. The antioxidant is in the range of 0.01 to 1.2 percent by weight, more preferably 0.05 to 1.0 percent by weight. The preferred range is from 0.10 to 0.80 percent by weight of the composition.
The antiwear and extreme pressure additives used in this invention comprising of sulphur, phosphorus and nitrogen (S-P-N) containing ashless multi-functional additive system. The SPN system includes package comprising of: amine phosphate, dithiophosphoric acid ester, alkylated diphenyl amine, sterically hindered phenol, (4-nonyl phenoxy) acetic acid, acyl sarkosinate, N, N’-bis (2-ethylhexyl)-((1,2,4-triazole-1-yl)methyl)amine, N-N-bis(2-ethylhexyl)-4-methyl-1-H-benzotriazole -1-methylamine, N,N-bis(2-ethyl-hexyl)-5-methyl-1H-benzotriazole-1-methylamine or package containing N, N-bis(2-ethyl-hexyl)-4-methyl-1H-benzotriazole-1-methylamine, N,N’-bis(2-ethylhexyl)-5-methyl-1H-benzotriazole-1-methylamine, (4-nonylphenolphenoxy)acetic acid, tris (methylphenyl) phosphate, sterically hindered phenol, amine antioxidant, mixture of triphenylthiophosphate and tertiary butylated phenyl derivatives or package containing phosphorothioic acid, O,O,O-triphenyl esters, tertiary butyl derivatives, 1-Naphthaleneamine, N-phenyl-ar-(1,1,3,3—tetramethylbutyl, alcohols C12-16, propionic acid derivative, 3-[[bis(2-methyl(propoxyl)phosphinothionyl]thio]-2-methyl derivative, N-N-bis(2-ethylhexyl)-4-methyl-1-H-benzotriazole -1-methylamine and N,N-bis (2-ethyl-hexyl)- 5-methyl-1H-benzotriazole-1-methylamine. The preferred range in this invention is from 0.05 to 1.5 percent by weight more preferably 0.10 to 1.0 percent by weight in the composition.
In yet another embodiment of the present invention, the viscosity index improver used in the present composition comprising of polyalkyl-methacrylates or copolymers of alkylmethacrylate and alphaolefines. The viscosity index improver has average molecular weight from 1,000 to 3,50,000. The preferred range of average molecular weight of said polymer can be 1,000 to 2,50,000.
Other suitable viscosity index improver such as, but not limited to, those selected from array of ethylene and propylene co-oligomer of average molecular weight of 10,000 to 3,00,000 can be used. Also, other suitable viscosity index improver such as, but not limited to, polymethacrylates can be used with molecular weight range of 10,000 to 2,50,000. The percentage range in the antiwear hydraulic oil composition is from 1 to 90 percent by weight.
In a preferred embodiment, the friction additive is selected from the group comprising of long chain fatty amine / amides, fatty esters, or borated compounds or oil soluble molybdenum compounds containing, etc and the range of the hydraulic oil composition is from 0.05 to 0.50 by weight.
The hydraulic oil composition optionally comprises a sufficient amount of pour point depressant to provide desired pour point depression. The pour point depressant is selected from the group comprising of poly methacrylates, polyacrylamides, olefin copolymer, etc. The preferred molecular weight can be in the range of 2,500 to 3,50,000. The preferred range in the hydraulic oil composition is from 0.01 to 1.0 percent by weight.
The anti-wear hydraulic oil composition optionally contains foam inhibitors, which are selected from array of highly viscous organic polymer and can be of dimethyl polycyclohexane, polyacrylates or array of silicone based defoamer, or mixture thereof, etc. The preferred range in the anti-wear hydraulic oil composition is from 0.001 to 0.10 percent by weight.
The anti-wear hydraulic oil composition optionally contains demulsifier in sufficient amount to provide demulsification property. The demulsifiers selected from the group comprising of condensed polymeric alcohols, esters of fatty acids, fatty alcohols alkoxylated with alkylene oxides, or mixtures thereof. The preferred range in the antiwear hydraulic oil composition is from 0.001 to 0.05 percent by weight.
The composition of novel fuel efficient anti-wear hydraulic oil includes combination of premium quality base oils of API Group II, Group III and base oils of Group IV, Group V class, as defined in the API interchangeability guidelines, or mixtures thereof. These base oils are commercially available in the market.
According to an embodiment, the present invention provides a process for preparing anti-wear hydraulic oil additive composition by mixing the appropriate amount of chosen additives or additive systems in a beaker / container. The additives combinations are further optimized in combination of selected hydrocarbon base oils to achieve desired performance in the laboratory tests.
The hydraulic oil additive composition has been prepared by mixing the appropriate amount of chosen additives or additive systems in a beaker / container. The additives combinations are further optimized in combination of selected hydrocarbon base oils to achieve desired performance in the laboratory tests. The chosen additives are mixed in selected base oils for preparing the candidates at an appropriate temperature such as an average blending temperature of 60 ºC to 65 ºC, so that mixture gets bright, clear and homogeneous.

EXAMPLES:
The examples are listed in Table –1A & Table – 1B and these examples were prepared by mixing the components in percentage by weight. The base oils used in the examples are of API Group I, Group II, Group III, Group IV & Group V types. These base oils are commercially available in the market. The array of commercially available additives and additive systems were selected in various combinations to achieve best performance. The additives includes antiwear / extreme pressure additive system, antioxidant, metal deactivator, friction modifier, pour point depressant, demulsifier, defoament, etc. The candidate blends were prepared and tested for various physico-chemical tests including performance properties such as kinematic viscosity, copper strip corrosion, rust test, demulsibility as per ASTM D 1401 and ASTM D 2711, Cincinnati Milacron thermal stability test method A (CM-A) and tribological tests as per industry accepted antiwear hydraulic oil standard DIN 51524 (Part 3).
The referred formulae are suitable to use as hydraulic oil of different ISO viscosity grades. The viscosity grade can be of ISO VG 10 to ISO VG 1000 as recommended by the hydraulic manufacturer. The composition can be used in various hydraulic applications including off-highway equipments. Various physico-chemical & performance tests were conducted to assess the performance in laboratory and thereafter energy efficiency assessed on promising candidate in an EATON 35VQ25 pump test.
The components used in the examples are as follows:
SN base oils are solvent neutral or solvent refined base oils (API Group I) and are commercially available.
Group II are commercially available API Group II base oils.
Group III are commercially available API Group III base oils.
Group IV are commercially available API Group IV base oils.
Group V are commercially available API Group V base oils.
Additive-1 is commercially available antiwear & extreme pressure additive package and can contain combination of zinc dialkyl dithiophosphate, alkyl phenol, alkaryl amine, calcium alkaryl sulphonate, 4-dodecyl phenol, poly (oxyalkylene) alkyl ether, calcium long chain alkylphenate sulphide
Additive-2 is commercially available antiwear and extreme pressure ashless multi-functional additive package containing amine phosphate, dithiophosphoric acid ester, alkylated diphenyl amine, sterically hindered phenol, (4-nonyl phenoxy) acetic acid, acyl sarkosinate, N,N’-bis (2-ethylhexyl)-((1,2,4-triazole-1-yl)methyl)amine, N-N-bis(2-ethylhexyl)-4-methyl-1-H-benzotriazole -1-methylamine, N,N-bis(2-ethyl-hexyl)-5-methyl-1H-benzotriazole-1-methylamine or package containing N, N-bis(2-ethyl-hexyl)-4-methyl-1H-benzotriazole-1-methylamine, N,N’-bis(2-ethylhexyl)-5-methyl-1H-benzotriazole-1-methylamine, (4-nonylphenolphenoxy)acetic acid, tris (methylphenyl) phosphate, sterically hindered phenol, amine antioxidant, mixture of triphenylthiophosphate and tertiary butylated phenyl derivatives or package containing phosphorothioic acid, O,O,O-triphenyl esters, tertiary butyl derivatives, 1-Naphthaleneamine, N-phenyl-ar-(1,1,3,3—tetramethylbutyl, alcohols C12-16, propionic acid derivative, 3-[[bis(2-methyl(propoxyl)phosphinothionyl]thio]-2-methyl derivative, N-N-bis(2-ethylhexyl)-4-methyl-1-H-benzotriazole-1-methylamine and N,N-bis(2-ethyl-hexyl)-5-methyl-1H-benzotriazole-1-methylamine
Additive-3 is commercially available antioxidant selected from a mixed phenolic, which having alkyl chain length of C2 to C12, which can preferably be alkyl chain length of C4 to C8 and mainly of is mixed butylated phenol.
Additive-4 commercially available hydrocarbon based polymer which is ethylene and propylene co-oligomer or polyalkyl methacrylates or co-polymer of alkylmethacrylate and alphaolefines.
Additive-5 is commercially available friction modifier selected from oil soluble molybdenum compounds having molybdenum content of 4% to 12%, or long chain fatty amines / amides having alkyl chain length C10 to C20, or borated compounds, which is commercially available friction modifier.
Additive-6 is commercially available pour point depressant which can be polymethacrylate or their compounds
Additive-7 is commercially available demulsifier, which can be of alkoxylated fatty alcohols.
Additive-8 is commercially available ashless type defoamer, which can be organic polyacrylate polymer or commercially available ash containing defoamer, which can be silicon based defoamer.
Component/ISO VG Candidate 1 Candidate 2 Candidate
3 Candidate 4 Candidate
5 Candidate
6 Candidate
7 Candidate
8
32 32 32 46 46 46 46 46
API Group I `-' 98.88 `-' `-' 63.03 `-' `-' 15.00
API Group II 98.93 `-' 84.44 98.37 31.00 85.34 35.00 25.00
API Group III `-' `-' `-' `-' `-' `-' `-' 47.73
API Group IV `-' `-' `-' `-' `-' `-' 33.18 `-'
API Group V `-' `-' 10.00 `-' `-' 10.00 30.00 `-'
Additive-1 0.85 0.50 `-' 0.70 `-' `-' 0.90 0.85
Additive-2 `-' `-‘ 0.60 `-' 0.55 0.60 `-' `-'
Additive-3 0.10 0.50 0.20 0.80 0.20 0.20 0.80 `-'
Additive-4 `-' `-' 4.40 `-' 5.00 3.5 `-' 11.20
Additive-5 `-' `-' 0.15 `-' `-' 0.15 `-' `-'
Additive-6 0.10 0.10 0.20 0.10 0.20 0.20 0.10 0.20
Additive-7 `-‘ `-‘ `-‘ 0.01 `-‘ `-‘ `-‘ `-‘
Additive-8 0.02 0.02 0.01 0.02 0.02 0.01 0.02 0.02
Property 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
Appearance C&B C&B C&B C&B C&B C&B C&B C&B
Kin viscosity @ 40 0C, cSt 32.5
32.1 32.6 46.9 47.1 46.5
46.9 46.7
SRV, coefficient of friction (COF) i.e. % reduction in COF in comparison to reference oil) `-‘

`-‘ 7-8 `-‘ 11-12 20-27

6-9 4-5
Remarks `-‘
fails fair `-‘ superior good
fair inferior
Examples 1 to 8: Table – 1A

Example 9 to 15: Table 1B

Component / ISO VG Candidate 9 Candidate 10 Candidate 11 Candidate 12 Candidate 13 Candidate 14 Candidate 15 Candidate 16
46 46 46 68 68 68 46 46
API Group I 15 `-' `-' `-' `-' 56 `-‘
API Group II 71.48 80.84 77.13 98.67 98.48 83.04 28.48 24
API Group III `-' `-' `-' `-' `-' `-' `-' 55
API Group V `-' 10.00 10.00 `-' `-' 10 `-' `-‘
Additive-1 `-' `-' `-' 0.70 0.90 `-' `-' 1.2
Additive-2 0.60 0.60 0.60 `-' `-' 0.6 0.6 `-‘
Additive-3 0.20 0.20 0.20 0.50 0.50 0.2 0.2 `-‘
Additive-4 12.50 8.00 11.70 `-' `-' 5.8 14.5 19.8
Additive-5 `-' 0.15 0.15 `-' `-' 0.15 `-' `-‘
Additive-6 0.20 0.20 0.20 0.10 0.10 0.2 0.2 `-‘
Additive-7 `-' `-' `-' 0.01 `-' `-' `-'
`-‘
Additive-8 0.02 0.01 0.02 0.02 0.02 0.01 0.02
Property 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
Appearance C&B C&B C&B C&B C&B C&B C&B C&B
Kin viscosity @ 40 0C, cSt 46.9 47.1 47.2 68.2 68.5 68.7 46.5 46.8
SRV, coefficient of friction (COF) i.e. % reduction in COF in comparison to reference oil) `-‘ 25-30 30-40 `-‘ 4-5 4-8

10-20

20-30
Remarks `-‘ good excellent `-‘ inferior fair superior good
Note: % reduction in COF in comparison to reference oil; <5=inferior; ~5-10=fair; 10-20=superior; 20-30=good; 30 or above =excellent

Phase – 1: Laboratory Evaluation
The energy efficient anti-wear hydraulic oil (composition of Examples of 3, 6 of Table 1A and Examples of 11 & 14 of Table 1B) is evaluated against DIN 51524 Part 3 specifications.

Physico-chemical properties:
Table – 2 provides the physico-chemical data on energy efficient (EE) anti-wear hydraulic oils vis-a-vis conventional oils as reference oils.

The energy efficient lubricant composition has pour point maximum as (-) 21°C which is indicative of its low temperature properties and its suitability for low temperature application. The flash point as determined by ASTM D 92 is more than 200°C suggesting its suitability for gear systems working at elevated temperatures in actual operation. The composition is having excellent demulsibility characteristics allowing better water separation characteristics. The poor demulsibility of the anti-wear hydraulic oil causes sludge, corrosion / rust formation, clogging of the strainer / filter, reduced lubricant performances and shortening of oils’ life.

Performance of Test oils in Cincinnati Milacron Thermal stability Test – Method A:
For comparison, conventional anti-wear hydraulic oils of various viscosity grades available commercially in the market have been chosen as reference oils for generating base line data. In order to confirm the thermal / oxidation inhibiting properties of the selected novel additive system, Cincinnati Milacron (CM) thermal stability (Method A) as per ASTM D 2070 was conducted on test oils.

Phase – 2: Evaluation in Tribological Tests:
In order to compare the anti-frictional performance of energy efficient anti-wear hydraulic oil with that of reference hydraulic oils, weld load and wear scar dia (WSD) done as per IP 239 & ASTM D 2266 test methods. FZG test was conducted to assess the load bearing behavior in comparison to reference oil (table 3). The newly developed candidate meets the anti-wear hydraulic oil requirements as per industry accepted DIN 51524 Part 3 international standards.

Table 2: Test data of Energy Efficient Oils vs. Reference oil

No. Property Method Candidate 4 Candidate 6 Candidate 11
ISO VG -- 46 46 46
1. Appearance Visual C&B C&B C&B
2. ASTM color D 1500 <2.0 <1.5 <1.5
3. K.Vis @ 40°C, cSt D 445 46.9 46.5 47.2
4. Pour point, COC, °C D 92 (-) 21 (-) 27 (-) 33
5. Flash Point, °C D 97 228 231 232
6. Cu Corrosion test @100 0C for 3 hours D 130 1 1 1
7. Rust test, D 665, B Pass Pass Pass
8. Foaming, tendency/ stability, ml/ml
Seq I,
Seq II,
Seq III D 892

10/Nil
20/Nil
10/Nil

Nil/Nil
Nil/Nil
Nil/Nil

Nil/Nil
Nil/Nil
Nil/Nil
9. Demulsibility @54°C, time in minutes D 1401 40-37-3 (15) 40-37-3 (15) 40-37-3
(15)
10. Cincinnati Milacron (CM) test
- % change in kin. viscosity @ 40 0C CM-A

4.4

3.6

3.8

Table 3: Tribological data of candidate oils vs. reference oil

No.
Property Method Candidate 4 Candidate 6 Candidate 11
ISO VG -- 46 46 46
1. Wear test (mm) ASTM D 4172 0.40 0.40 0.40
2. FZG pass load stage DIN 51354 12 >12 >12

Phase – 3 : Assessment of Energy Efficiency of Antiwear Hydraulic oils :
Energy efficiency of the energy efficient anti-wear hydraulic oil was assessed in the laboratory in SRV test rig. This machine is used to measure the coefficient of friction between oscillating ball on a flat disc in a sliding contact geometry. Reduction in coefficient of friction during the test run was taken as criteria for energy saving potential. The higher is the reduction in the coefficient of friction, better will be the energy efficiency in the oil. In this screening test @ 200N, 50°C, 50 Hz, 1 mm for 1 hour, energy efficient oil provided reduction in coefficient of friction to the extent of 4 to 40 percent approximately over the conventional hydraulic oil as reference oil (table 4).

Table 4 : Frictional study of candidate oils in SRV test:
Candidates % reduction in coefficient of friction Remarks
Candidate 1 (VG 32) Reference oil --
Candidate 3 (VG 32) ~ 7 – 8 fair
Candidate 4 (VG 46) Reference oil --
Candidate 6 (VG 46) ~ 20 – 27 good
Candidate 11 (VG 46) ~ 30 – 40 excellent
Candidate 12 (VG 68) Reference oil --
Candidate 14 (VG 68) ~ 4 – 8 fair

ASTM D 6973 is a standard test method for indicating wear characteristics of petroleum hydraulic fluids in High Pressure Constant Volume Vane Pump (Eaton 35VQ25A-11*20). The pass / fail criteria (Eaton) are weight loss of cam ring and vanes are to be maximum of 90 mg. The details of Eaton pump test is provided in the table 5.

Table 5 : Details of EATON 35VQ25A-11*20 pump test conditions:
Parameter Details of test conditions
cartridge kit part number 4998040-002
Temperature 95 + 3 0C
Pressure 207 + 1.4 bar (3000 + 20 psig)
Speed 2400 + 20 rpm
Test duration 50 hours

In order to assess the volumetric, mechanical and overall pump efficiency 3 x 3 x 7 matrix were used to capture the test data for each of 15 minutes in 35VQ25A-11*20 vane pump, which corroborates output pressure of 70, 140 & 207 bar (3 nos.), shaft speed of 1200, 1800 & 2400 rpm (3 nos.) and test temperature of 30, 40, 50, 60, 70, 80 & 90 0C (7 nos.). The test rig uses 35VQ25A vane pump equipped with standard cartridges. The pump is driven by 93 kW (125 hP) electric motor, flow rate of 81 cm3/revolution fitted with torque meter up-to 500 Nm. The pump circuit includes fluid reservoir, 35VQ25A-11*20 pump, pressure regulator (throttle valve), low pressure filter located after the throttle valve, flow meter, heat exchanger and thermocouples are installed at the following locations about 100 mm before the inlet of the pump, 3 to 5 mm inside the pump suction port, at the pump outlet and immediately after the pressure regulator. The power consumed by the electric motor to drive the pump is recorded. It is about equal to the sum of the nominal hydraulic power and of the power needed to overcome the mechanical losses taking place in the pump.

The examples of candidate in comparison to reference oils assessed for volumetric, mechanical and overall efficiencies are given in the table 6.

Table 6: Evaluation of Energy Efficient Candidate oils in Eaton pump
(Test conditions : 1200 rpm speed, 2000 PSI pressure and 90 0C oil temperature)
Candidate Oil Volumetric Efficiency Mechanical Efficiency Overall Efficiency
Candidate 1 (ISO VG 32) 79.297 91.103 72.242
Candidate 3 (ISO VG 32) 81.486 91.691 74.716
Energy efficiency of candidate oil vis-à-vis reference oil (VG 32) 2.761 0.646 3.425
Candidate 4 (ISO VG 46) 80.427 90.035 72.412
Candidate 6 (ISO VG 46) 82.346 90.887 74.842
Energy efficiency of candidate oil vis-à-vis reference oil (VG 46) 2.386 0.947 3.355
Candidate 4 (ISO VG 46) 80.427 90.035 72.412
Candidate 11 (ISO VG 46) 85.125 90.944 77.416
Energy Efficiency of candidate oil vis-à-vis reference oil (VG 46) 5.842 1.009 6.910
Candidate12(ISO VG 68) 82.070 89.910 73.789
Candidate 14 (ISO VG 68) 85.744 90.745 77.809
Energy efficiency of candidate oil vis-à-vis reference oil (VG 68) 4.477 0.929 5.447

Candidate 3, 6, 11 & 14 provided energy efficiency due to their lower coefficient of friction which helped to reduce sliding friction loss in the hydraulic pump / motors, adequate viscosity to reduce internal leakage in the pump, motor & control valves and candidate 11 additionally provided improved energy efficiency owing to improved mechanical efficiency due to efficient friction modification (table 6). The energy efficient oils could operate in wide temperature window during extended operation or in-service from cold to tropical regions.

The important outcome of the energy efficiency test on novel fuel efficient antiwear hydraulic oil is as follows:
• The novel performance antiwear hydraulic oil possesses superior oxidation stability, antirust & anticorrosive properties, load bearing capability with energy efficiency.
• The energy efficiency of novel energy efficient antiwear hydraulic oil is to the tune of around 3.0 to 6.0 % in comparison to reference oil assessed in Eaton pump rig test.
• The fuel efficient antiwear hydraulic oil composition would provide fuel efficiency, enhancement in the life over reference oil as well as increased productivity and reduction in down time.

The field evaluation of novel energy efficient antiwear hydraulic oil composition in comparison to commercially available conventional oil (reference oil) provided fuel efficiency, enhanced life and improvement in productivity in the actual operations.

Those of ordinary skill in the art will appreciate upon reading this specification, including the examples contained herein, that modifications and alterations to the composition and methodology for making the composition may be made within the scope of the invention and it is intended that the scope of the invention disclosed herein be limited only by the broadest interpretation of the appended claims to which the inventor is legally entitled.
,CLAIMS:Claims:

1. A fuel efficient hydraulic oil composition comprising:
(a) an oxidation inhibitor;
(b) a sulphur, phosphorus and nitrogen (S-P-N) based antiwear and extreme pressure containing multi functional additive system;
(c) a viscosity index improver;
(d) a friction modifier; and
(e) a mixture of severely refined base stocks, or hydrotreated / hydro-processed / iso-dewaxed base stocks and alkylated naphthalene, or mixture of synthetic bases and ester or mixture of synthetic bases and alkylated naphthalene or alkylated naphthalene bases or mixtures thereof.

2. The composition as claimed in claim 1, wherein the sulphur, phosphorus and nitrogen (S-P-N) based antiwear and extreme pressure additive system comprising of amine phosphate, dithiophosphoric acid ester, alkylated diphenyl amine, sterically hindered phenol, (4-nonyl phenoxy) acetic acid, acyl sarkosinate, N, N’-bis (2-ethylhexyl)-(1,2,4-triazole-1-yl)methyl)amine, N-N-bis(2-ethylhexyl)-4-methyl-1-H-benzotriazole -1-methylamine , N,N-bis(2-ethyl-hexyl)-5-methyl-1H-benzotriazole-1-methylamine or package containing N, N-bis(2-ethyl-hexyl)-4-methyl-1H-benzotriazole-1-methylamine, N,N’-bis(2-ethylhexyl) -5-methyl-1H-benzotriazole-1-methylamine, (4-nonylphenolphenoxy) acetic acid, tris (methylphenyl) phosphate, sterically hindered phenol, amine antioxidant, mixture of triphenylthiophosphate and tertiary butylated phenyl derivatives or package containing phosphorothioic acid, O,O,O-triphenyl esters, tertiary butyl derivatives, 1-Naphthaleneamine, N-phenyl-ar-(1,1,3,3-tetramethylbutyl), alcohols C12-16, propionic acid derivative, 3-[[bis(2-methyl(propoxyl)phosphinothionyl]thio]-2-methyl derivative, N-N-bis(2-ethylhexyl)-4-methyl-1-H-benzotriazole -1-methylamine and N,N-bis(2-ethyl-hexyl)-5-methyl-1H-benzotriazole-1-methylamine.

3. The composition as claimed in claim 1, wherein the sulphur, phosphorus and nitrogen (S-P-N) based antiwear and extreme pressure containing multi functional additive system is present in the range of 0.05 to 1.5 percent by weight of the composition.
4. The composition as claimed in claim 1, wherein the oxidation inhibitor is comprising of mixtures of alkylated phenolic antioxidant having alkyl chain length of C2 to C12 which can preferably be alkyl chain length of C4 to C8.

5. The composition as claimed in claim 1, wherein one of the oxidation inhibitor is present in the range of 0.01 to 1.2 percent by weight of the composition.

6. The composition as claimed in claim 1, wherein the friction modifier is selected from the group comprising of long chain fatty amine / amides, fatty esters, borated compounds and oil soluble molybdenum compounds.
7. The composition as claimed in claim 6, wherein the friction modifier is oil soluble molybdenum compound having molybdenum content of 4% to 12%.
8. The composition as claimed in claim 7, wherein the oil soluble molybdenum compound is present in the range of 0.05 to 0.50 percent by weight of the composition.
9. The composition as claimed in claim 1, wherein the viscosity index improver is selected from group comprising of polyalkyl-methacrylates and copolymers of alkylmethacrylate and alphaolefines preferably having average molecular weight of 1,000 to 1,00,000.
10. The composition as claimed in claim 1, wherein the viscosity index improver is present in the range of 1.0 to 90.0 percent by weight of the composition.
11. The composition as claimed in claim 1, wherein the composition further comprises a pour point depressant, a demulsifier and a defoamer, wherein:

the pour point depressant is selected from a group comprising poly methacrylates, polyacrylamides, and olefin copolymer;

the demulsifier is selected from a group comprising of condensed polymeric alcohols, esters of fatty acids, fatty alcohols alkoxylated with alkylene oxides, and mixtures thereof; and

the defoamer is selected from a group comprising of organic polyacrylate polymer or commercially available ash containing defoamer.

12. The composition as claimed in claim 1, wherein the mixture of severely refined base stocks, or hydrotreated / hydro-processed / iso-dewaxed base stocks, or hydrotreated / hydro-processed / iso-dewaxed base stocks and alkylated naphthalene, or mixture of synthetic bases and ester or mixture of synthetic bases and alkylated naphthalene or alkylated naphthalene bases or mixtures thereof is selected from combination of premium quality base oils of API Group II, Group III and base oils of Group IV, Group V class.

13. The composition as claimed in any of the preceding claims 1 to 12, wherein said composition is used for enhancing fuel efficiency of hydraulic systems.