As part of anindustry-wide effort, a coalition of university researchers and industrysuppliers organized through the NSF-sponsored Center for Compact and EfficientFluid Power (CCEFP) is working together to boost the efficiency of fluid powerapplications. The center is supported by seven participating universities and55 industrial partners.

New multi-grade oils, optimized with high-tech polymeradditives and friction modifiers, are delivering promising field results, andnew hydraulic fluids have been introduced by major oil companies. Research is moving ahead on duty cyclestandards for specific vehicles, and the impact that high-efficiency hydraulicfluids (HEHF) could have on overall equipment design, particularlyhigh-pressure and high-temperature applications such as excavators, skid steerloaders and construction equipment.

Field tests on excavators conducted by Evonik RohMax Oil Additives show efficiency gains between 18 and 26 percent depending on the work cycle. That'ssignificant when you consider that if the use of HEHF results in the burning of18 percent less diesel fuel to do the same amount of work, an excavator wouldburn 3,346 fewer gallons of diesel fuel in one drain interval of 4,000 hours.Moreover, reducing emissions by 18 percent would result in 35 metric tons lessCO₂.

"Improving theefficiency of fluid power applications is a promising area of research," saysPaul Michael, a research chemist at the Milwaukee School of Engineering, whichis a member of the CCEFP. Michael says in the past 30 years there have been 15improvements in the engine oil chemistry used in diesel and gasoline engines -largely driven by the desire to improve fuel economy. The total number ofimprovements in hydraulic fluids? Zero.

Today,lower-viscosity oils with friction modifiers are used to provide fuel economyin passenger cars. But in fluid power applications, the vast majority of oilsused now are essentially the same technology used 30 years ago — astraight-grade mineral oil plus a 1- to 2-percent, zinc-based, anti-wearadditive package.

In a passenger car, the oil pressure is approximately 50psi. In a hydraulic application, it can easily be 100 times higher. Higherpressure subjects the fluid to extremely high shear conditions, which breakdown the polymer molecules used in engine oil.

According to SteveHerzog, OEM liaison manager for Evonik RohMax, there is no question thatimprovements in hydraulic pump efficiency can be realized through the use ofshear stable hydraulic fluids. Research data shows that the use of shear stablepolymer additives can result in more than a 10 percent improvement in hydraulicefficiency.

Herzog says efficiencyis achieved by putting a shear stable polymer into the oil and creating a high-viscosityindex fluid that provides improved efficiency at high temperature (due toreduced internal leakage in the pumps and motors) and low temperature (due toreduced fluid drag at start-up). The proposed NFPA recommendation for Energy Efficient HydraulicFluid is a high viscosity index (>160 VI) and good shear stability.

Under high pressure conditions, temperatures are oftenelevated. As a result, the viscosity is reduced and there are higher levels ofinternal leakage through the pump clearances. 

The polymer-enhancedhydraulic fluid has a thickening effect, which causes the oil to thin out at aslower rate as the temperature increases. As a result, there is an increase involumetric efficiency and less internal leakage in the pumps and motors.

Evonik RohMax, a partner in the Engineering Research Center, has developed andproduced shear stable polymers that are especially well-suited for hydraulicapplications. These polymers make the hydraulic fluid resistant to breakingdown under the high-pressure conditions typical in industrial equipment,maintaining a higher level of in-service viscosity, and better volumetricefficiency.

Other companies within the center that are activelyworking on solutions include Afton Chemical (providing friction modifierexpertise) and Shell Oil Co. as the finished oil expert.

Other Side of the Circuit

Research at the CCEFP is alsolooking at the other side of the circuit - the load and hydraulic motor used topropel skid steer loaders, excavators and hydraulic hybrid vehicles. A car isleast efficient in terms of fuel economy when it is starting up, and the sameis true with mobile equipment powered by a hydraulic system. Essentially, it isthis starting efficiency that establishes design parameters such as operatingpressure and size, or displacement of the hydraulic motor.

"The whole point is that if you can lubricate a motor more efficiently, you can get more torque under startingconditions, which means you can use a smaller motor with less displacement,which in turn means you can also have a smaller pump in the system because lessflow will be needed to generate the same speed at the high end," says Michael."This is the area where we are concentrating our research, because it's wherewe can make the biggest impact."

Michael's group hasstudied the effects of various fluids on different types of motors using hislab's hydraulic dynamometer, which is designed specifically for measuringhydraulic motor efficiency under starting and low-speed, high-torqueconditions. Three lubricants with the same chemistry, but different viscositycharacteristics, were tested on an axial piston, radial piston and orbitalmotor. Researchers found that different motors have different appetites forlubricants, depending on the lubrication regime at start-up.

Under test conditions of 50 and 80C, the axial pistonmotor exhibited a 7 percent increase in efficiency at start-up due to theviscosity improver additive. "It is our hypothesis that the starting conditionfor this type of motor depends upon hydrostatic lubrication," says Michael."These motors have shoes sliding on a bearing surface. Oil is pumped underpressure through tiny holes in the bottom of these shoes, lifting them off thebearing surface, so the surfaces aren't actually moving relative to oneanother. The polymer additive here thickens the oil, so you get betterlubrication, which accounts for the higher efficiency."

There was noimprovement in performance at start-up of either the radial or piston motorfrom the viscosity improver additive. Michael says he believes that thecondition at start-up for these motors is what is known as boundarylubrication, in which case chemical additives in the oil react to the metalsurfaces to form a low-friction film that reduces static friction. "It appearsas though the chemistry, rather than the physical properties of the oil, is thedetermining factor here," says Michael.

While research is ongoing in a variety of areas, fieldtrials and a decade of laboratory testing demonstrate that shear stablemulti-grade hydraulic fluids and friction modifiers improve energy efficiencyin hydraulic systems. Fluids that meet the requirements of the NFPA EnergyEfficient Hydraulic Fluid classification system increase fuel economy andproductivity while reducing CO₂ emissions. And in the last year, oil companiesincluding Shell, ExxonMobil and Citgo have introduced new high-efficiencyhydraulic fluids, and OEMs are beginning to specify the fluids in theirproducts.

With the focus of theengineering research center on increasing energy efficiency, some members set a10 percent goal on average energy savings in fluid power applications, achievedthrough the use of new hydraulic fluids. That seems to be well within reach,and now the momentum is for more research on duty cycle standards that documentsavings for specific classes of vehicles and industry adoption.

More Information
Since mobile hydraulic systems generally operate in the steep portion of the volumetric efficiency curve, increasing the viscosity of the hydraulic fluid at high temperatures improves volumetric efficiency and reduces energy consumption. During cold temperature start-up, when the viscosity of the hydraulic fluid is potentially very high, a low-viscosity hydraulic fluid improves mechanical and overall efficiency. Both of these goals can be achieved through the use of a shear stable high VI hydraulic fluid. View chart

Evonik RohMax performed a cost-benefit analysis to determine the economic impact of the energy savings measured during field tests comparing OEM 10W oil and high-efficiency hydraulic fluid (179 VI HF). Though the 179 VI HF requires more work cycles, it consumes less fuel, resulting in a net benefit. A drain is defined as the time interval between fluid changes, or 4,000 hours for the field tests described here. View cost-benefit analysis

The results of field trials on a medium-size excavator confirmed that high efficiency hydraulic fluid consumes less fuel than OEM 10W oil. The difference is even more significant at a lower duty cycle. View results of field trials