Karl-Erik Rydberg. Abstract. and a number of energy efficient fluids have been launched on the market.

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1 The 3th Scandinaian International Conference on Fluid Power, SICFP03, June 3-5, 03, Linköing, Sweden Hydraulic Fluid Proerties and their Iact on Energy Efficiency Karl-Erik Rydberg Diision of Fluid and Mechatronic Systes, Deartent of Manageent and Engineering Linköing Uniersity, Linköing, Sweden Abstract To eet the increased requireents on higher efficiency and better functionality of hydraulic systes, new coonents and syste concets hae been deeloed oer the years. Howeer, the ost iortant coonent in a hydraulic syste, which has a ajor iact on syste efficiency and wear are the fluid itself. The last decades, ajor attention on hydraulic fluid deeloent, hae been set uon enironental adation. Today, energy efficiency has been a hot toic and a nuber of energy efficient fluids hae been launched on the arket. Besides energy efficiency, enironental adation of fluids, are extreely iortant in obile alications. In this aer the focus is set on hydraulic fluid roerties and its iact on syste efficiency. Enironental adated fluids will also be discussed. Synthetic saturated esters are analysed and coared with ineral oils. The ain question that will be tried to answer is - How to select the best fluid for a secific alication?. Keywords: Hydraulic fluids, Synthetic esters, Fluid roerties, Viscosity, Energy efficiency. Introduction future roduct with a high deeloent otential. The ressure edia is the ost iortant coonent in the hydraulic syste because it takes care of the energy transfer in the whole syste fro u to cylinder / otor. Additionally the hydraulic fluid has to roide lubrication and cooling. Esecially in obile alications it is of great iortance to iniize the negatie eni- ronental influence fro hydraulics.. Enironental adated fluids In order to ake hydraulic fluids enironentally adated a nuber of synthetic fluids hae been deeloed. One of the ost roising fluids is saturated synthetic esters. This tye of fluid has any interesting roerties to exlore. Synthetic saturated esters hae been on the arket for ore than 0 years, but the ealuation of their roerties is still liited. Howeer, the results so far about the lubrication roerties, iscosity index, and stability etc are ery ositie, see Fig.. There is no doubt that the synthetic esters can be considered as a Fig. : Acid nuber and iscosity stability of saturated ester, Panolin HLP Synth, []. 447

2 Fro Fig. it can be seen that the saturated ester (Panolin HLP Synthetic) has extreely stable acid nuber (TAN) and iscosity coared to unsaturated esters. In coarison with ineral oil the saturated ester also shows iressie shear stability oer the tie.. Energy efficient fluids In order to axiize energy efficiencies in hydraulic systes, echanical and oluetric losses ust be balanced so the su of these losses is iniized. Since the echanical losses in sealing gas are roortional to the fluid iscosity and oluetric losses inerse roor- tional to iscosity, it is clear that an otial iscosity exists. Fluid iscosity is not the only araeter that iroes energy efficiency. Also, friction coefficient roided by the fluid in sealing gas as well as the iscosity index (VI) of the fluid is ery iortant araeters. Looking at the deeloent of engine lubricants for cars it can be noted that today s fluids are low iscosity oils with friction odifiers and VI iroers. It has been roen that this kind of fluids hae a significant iact on fuel consution. The fluid roerties iact on hydraulic syste energy efficiency has been clearly alidated in a NSF-sonsored roject at Center for Coact and Efficient Fluid Power (CCEFP) in US. An excaator has been selected as a riary ehicle for a test bed, [3]. As a eber of this roject, Eonik RohMax has deeloed a new ulti-grade fluid, otiized with high-tecaddities and friction odifiers. Field tests, erfored by olyer Eonik RohMax, on excaators show efficiency gains between 8 and 6%, deending on work cycle, [4]. The iortance of high iscosity index (VI>60) and good shear stability has also been stated..3 Fluid iscosity grade and iscosity index Selection of an energy efficient fluid for a secific hydraulic alication is highly associated with a roer iscosity grade and the iscosity index (VI) of the fluid. The standard iscosity grade, ISO VG gies the fluid iscosity in cst at the teerature 40 o C. In obile alications the ost coonly used iscosity grade is VG 46. Howeer, if the hydraulic syste contains high seed us and otors as well as long lines a lower VG-alue will boost the efficiency. Using fluids with low VG-alue (VG 3 or VG0) in obile achines it is ery iortant to take the iscosity index into account. The lower VG-alue the higher VI-alue is required. In obile alications where the fluid teerature ariations can be ery high the iscosity index (VI) has a high iact on energy efficiency. RohMax in US, a leading anufacturer of hydraulic inestigated the iact of VI efficiency in obile alication, [5]. They hae deeloed a fluid with ery high shear stability and tested the fluid in Eaton-Vickers ane us. The efficiency iroeents ersus fluid teerature at different VI-alues are shown in Fig.. The 0-leel stays for VI00. Fig. : Gain in u efficiency ersuss fluid teerature with VI as araeter, [5]. The aboe figure shows that VI > 60 is needed if the fluid teerature rang is bigger than 50 o C. It is also worth to be entioned that these figures belongs to one secific u. In ractice, the required VI-alue deends uon the tye of u, the u syste ressure range. Viscosity deendent losses in hydraulic ies Hydraulic systes are coonly included long ie. The ressure losses in a straight ie are theoretically calculated as follows: Lainar flow: Turbulent flow: Δ Δla turb fluid addities, has on hydraulic u seed range and the 3 L () d 0,068 L ρ.5 () d where d ie diaeter [], L ie length [], dynaic iscosity [Ns/ ], ρ fluid density [kg/ 3 ], aerage flow elocity [/s]. Calculated ressure dro across a 5 long ie with 448

3 The test results deonstrated in Fig. 5, are all two different iscosities of the fluid (V VG 46), are following the theoretical calculationss according to eq. (3). illustrated in Fig. 3. Fig. 3: Pressure dro in a 5 long ie (φ0 0 ) ersus Fig. 5: Measured ressure dro in a 5 long hose (φ0 ) flow elocity for the iscosity 30 and d 46 cst. ersus flow elocity for 6 different fluids. Coarison with easureents in Fig. 3, 3 shows that 3 the real ressure dro follows the theoretical alues if this All hydraulic systes are conntaining at least one is calculated as, Δ ax{δla, Δturb } Energy efficiency of u s and otors u. The u is a riary sysste unit that has to (3) transfor echanical energy intoo hydraulic energy. Fro the figure it can be obsered that the turbulent Therefore, the u efficiency has a significant iact flow starts to increase the ressure dro at a uch lower on the oerall syste efficiency. Thhe u efficiency is Re-nuber than the critical alue (300). In n Fig 4, Δturb related to the doinant losses, w which are oluetric is bigger than Δla for all Re > 0,56* In ractice, (leakage) and echanical (friction) losses. These losses the transition fro lainar to turbulent stage s is ery are ainly affected by the fluid iscosity. In order to sooth and not as an iediate switch at Ree 300. analyses the iscosity iact on u efficiency a odel can be used.. Pressure dro easureents Based on a sei-eirical efficiiency odel for axial iston achines, [], Fig. 6 shoows siulated u Pressure dro easureents hae been erfored at efficiencies as a function of fluid isccosity. the diision of Fluid and Mechatronic Systes in Linkoing, Sweden. The rincile layout of the used test-stand is illustrated in Fig. 4. Fig. 4: Test-stand for easureents of ressure dro in ies. The test object is 5 long hose with w an inner diaeter of 0. The fluids that hae beeen tested are one ineral oil (VG 46), four different synth hetic saturated esters (three VG 3 fluids and one VG ) and one low iscosity fluid an eco-diesel (VG 3). Meeasured hose ressure dro ersus fluid elocity for these six fluids are shown in Fig. 5. More details about the ressure dro tests and fluid exainations are resented in ref. [6]. Fig. 6: Efficiencies ersus fluid iscosity for an in-line axial iston u at two seeds, 500 and 3000 r. 449

4 Fig. 6 shows that the iscosity, which axiize the u oerall efficiency is related to oeration conditions, ressure and seed. Fro the efficiency odel the dynaic iscosity () that gies ax efficiency can be exressed as, ot n where C and k are achine constants, u ressure and n u seed. Equation (4) is alicable on both u and otors, but it has to be obsered that the achine constants can aries in a wide range, deending on the tye of achines. 3. Hydrostatic transission efficiency Hydrostatic transissions are widely used in drie train for working achines. In a wheel loader or forwarder (forest achine) the drie transission stays for ore than 50% of the energy consution. Therefore, it is highly iortant to ake an analysess of how the fluid roerties will affect the transission oerall efficiency. The transission inestigated in this study is scheatically deicted in Fig. 7. Fig. 7: Scheatic drawing of the test transission. C k (4) h b b 0 H δ L L n n b b 3 b4 b γ 5 b6 Δ n Δ Δ Δ n 0 The efficiencies of the ariable otor are, a 0 a Δ a βe Δ π n b b H δ 0 L L n n h b b 3 b4 b Δ γ 5 b6 n Δ Δ Δ n 0 In equation (5) - (8), a and b are achine deendent loss coefficients, fluid dynaic iscosity, Δ transission ressure difference, n shaft seed and dislaceent setting. The oerall transission efficiency, with exclusion of the boost u, is calculated as, T n t, o T n h h Including the boost u ower, the oerall efficiency is exressed as, T n t, b, o T T n where T shaft torque. ( b ) π (6) 3. Theoretical analysis of transission efficiency In the analysed transission (see Fig 7), the ain u and otor hae ax dislaceent of D 0 c 3 /re and D 50 c 3 /re, resectiely. The boost u dislaceent is D b 0,. D. Siulated efficiency characteristic of the sequence controlled transission, without boost u, is resented in Fig. 8. a 3 Δ n (7) π (8) (9) (0) The transission concet is traditional, with ariable dislaceent axial iston units as u (In-line) and otor (Bent-axis). The ain u is suorted with a fixed dislaceent boost u. For theoretical exaination of transission efficiency the u and otor loss odels, deeloed for axial iston units, reference [], is used. The ariable u, oluetric and hydro-echanical efficiencies are exressed as. a a Δ a a Δ a 0 3 βe π n 4 Δ n (5) Fig. 8: Siulated transission oerall efficiency ersus otor seed, without boost u, ν 46 cst. The transission efficiency characteristic in Fig. 8 is calculated for oeration conditions of constant u 450

5 seed, n 500 r, constant ressure diffference, Δ Energy efficient hydraulic fluidds, is the coonly 0 MPa, constant low ressure (boost ressure), 3 used nae for fluids containinng secific friction MPa and a fluid iscosity of 46 cst. Maax efficiency odifiers. For obile alications tthese fluids also hae (84%) is reached, as exected, when the u and otor high iscosity index (VI > 50) andd high shear stability, hae ax dislaceent stroking (.0 0). When the [7]. otor seed is increased fro 000 r to r the Focusing on the friction odifiers iact on efficiency goes down, ainly because of iscosity and transission efficiency, this cann be ealuated by seed deendent friction and ressure lo osses. At the reduction of the friction coefficcient alues in the otor seed, 4000 r the transission efficiency is efficiency odel (eq. 5 8). In Fig.., the transission reduced to 69%. efficiency characteristics (boost u included) are If the boost u is included in th he efficiency shown for Δ 40 MPa, but two friiction cases - original calculations (Eq. 0), the efficiency grah becae b as in friction coefficients and with reducced friction, 5% of Fig. 9. At constant u seed the boost u u ower is original alues. constant, around 3 % of the ain u ower, when.0 and Δ 0 MPa. Therefore, the boo ost u will reduce the transission oerall efficiency aroxiately a 3% (when.0), coared to the case in Fig. 8. Fig. 9: Siulated transission oerall efficiencyy ersus otor seed, with boost u, ν 46 cst. According to equation (4), the fluid iscosity that axiizes the efficiency of a hydraulic ach hine, deends uon ressure and seed. By using a low iscosity fluid Fig. : (VG ) in the transission, it can be exected that its Siulated transission oeerall efficiency ersus otor seed, with boost u, without and with efficiency will be ostly gained at high otor seed. reduced fricttion. This is confired by the characteristics show wn in Fig. 0. The figure aboe shows that thiis draatic reduction of friction only will increase the eefficiency about %. This exale deonstrates that, too reach an efficiency gain around 0%, according to referrence [4] and [7], it is not enough to use fluids with frictionn odifiers, VG- and VI-alues as well as shear stability ust be adjusted for the alication. It has to be noted that the conduucted work resented aboe Fig. 0: is just a theoretical aanalyses based on Siulated transission oerall efficciency ersus sei-eirical efficiency odels alidated for axial otor seed, without boost u, ν cst. iston units, []. 45

6 3.4 Exeriental alidation of efficiencies In a transission test-stand at FluMeS lab in Linköing, soe efficiency easureentss hae been conducted. The test object is siilar to the transission shown in Fig. 7. The tests hae been erfored at ax dislaceent setting of u and otor (,0). Test results are resented in Fig. -4. ineral oil, at three different fluid teeratures and thereby three different iscosities, 6, 46 and 00 cst. Fro the lowest to the highest iscosity the change in efficiency is about 5 %. It can be noted that this iscosity iact on efficiency corresonds quite well to siulation results resented in this aer (see Fig. 9 and 0). In Fig. 3 two saturated esters (VG and VG 3) are coared with a ineral oil (VG 46) at low u seed (000 r). The figure shows that otial fluid iscosity is load deendent. Increased transission ressure requires higher iscosity to axiize the efficiency. Also the lubrication roerty of saturated esters is better than for ineral oil. Fig. 4 deonstratess that the ester cerates lower friction losses than ineral oil. 4 How to select the best fluid? Fig. : Measure transission oerall efficiency ersus Load Torque, (boost u incl.), ineral oil VG 46. Fig. 3: Measure transission oerall efficiency ersus Load Torque, (boost u incl.), for three different fluids. Fig. 4: Measured transission ressure difference ersus Load Torque, (boost u incl.). Fig. illustrates the transission efficiency for The hydraulic fluid is one on the ost iortant design factors in a hydraulic syste and should be considered as a achine eleent, ossible to deelo as well as the other hydraulic coonents in a syste. Fluid selection ust be based on a detailed syste secification - containing tye of coonents, syste ressure range, u and otor seed range and oeration teerature range. Also, exected oeration tie and reliability ust be includedd in the secification. Otial VG-alue for the secified syste can be found by using theoretical odels, which are alidated by ractical tests. The estiated teerature range for axiu ower oeration gies the requireent on iscosity index (VI). Esecially in high ressure systes the fluid lubricity is of high iortance. It is a well known fact that synthetic esters, gies lower friction and wear than ineral oil. Ester fluids also hae higher heat caacity than ineral oils, which results in lower teerature ariations. Field tests conducted by the Forestry Research Institute of Sweden, confirs that the steady state fluid teerature in a forest achine, will be reduced 5-0 o C with use of synthetic esters instead of ineral oils. Seeral decades of docuented exerience shows that the lifetie of a hydraulic syste (including the fluid) deends ainly on the cleanliness of the fluid, stability and working teerature. A clean fluid (low aount of articles, low water and air content) with good stability regarding iscosity and acid nuber that can work in a 45

7 suitable teerature gies a ery long syste lifetie. It is also a ery well known fact that there is a clear connection between the cleanliness, stability and teerature of the fluid. The reason is that articles and water, in cobination with high teerature gie an accelerating oxidation of the fluid and a faster brake down of the addities. 5 Conclusions The theoretical and ractical results resented in this aer, deonstrates that the efficiency of a syste is strongly deendent on the iscosity of the hydraulic fluid. The otial iscosity is howeer, deendent on the ressure leel and seed. In an alication the iscosity should be selected so that the efficiency is axiized within the ressure and seed where the biggest aount of energy is transferred so that one can reduce the energy losses. The iscosity index is highly iortant in obile systes. It is also stated that synthetic ester fluids gies lower friction losses than standard ineral oil. The shear stability of saturated esters is extreely good coared to ineral oil and unsaturated esters. Howeer, these facts just reresent a art of the inforation needed to find the otial fluid for a secific alication. Besides theoretical facts a solid exerience is needed for a satisfactory fluid selection. Finally, this aer deonstrate, that there is no sile answer of the question How to select the best fluid?. To find a releant answer, a good choice is to start with a detailed syste secification and try to conert the secification into fluid requireents. [3] High Efficiency Excaator. Info_CCEFP_Excaator. Center for Coact and Efficient Fluid Power, US, March 6, 0. [4] A Presher: New Hydraulic Fluids Aied at Energy Saings. Design News, October 009. [5] C K Dustan, Rohax USA. Perforance Benefits of High Viscosity Index Hydraulic Fluids. Lubricants-Russia 006, Moscow, Noeber 4, 006. [6] S Jaalagi, S Singireddy: Hydraulic fluid roerties and its influence on syste erforance. Master s Thesis, LIU-IEI-TEK-A--/084 SE, Linkoing Uni., 0. [7] S Herzog: Iroing Fuel Econoy and Productiity of Mobile Equient through Hydraulic Fluid Selection. Eonik Rohax USA, 0. [8] TechBulletin: HYDREX TM Energy Efficiencies in Mobile Equient. Petro-Canada, 00. Acknowledgeent This work has been suorted by the Swedish Fluid Power Association (IFS). References [] Rydberg K-E.: On erforance otiization and digital control of hydrostatic dries for ehicle alications. Ph.D. thesis no 99, Linköing Uniersity, Sweden, 983. [] K Eriksson: Enironental adated ressure edia ersus ineral oils, (in Swedish). Presentation at National Hydraulic Days, Linköing, May 30-3,