New Hydraulic Fluids Aimed at Energy Savings

As part of an industry-wide effort, a coalition of university researchers and industry suppliers organized through the NSF-sponsored Center for Compact and Efficient Fluid Power (CCEFP) is working together to boost the efficiency of fluid power applications. The center is supported by seven participating universities and 55 industrial partners.

New multi-grade oils, optimized with high-tech polymer additives and friction modifiers, are delivering promising field results, and new hydraulic fluids have been introduced by major oil companies. Research is moving ahead on duty cycle standards for specific vehicles, and the impact that high-efficiency hydraulic fluids (HEHF) could have on overall equipment design, particularly high-pressure and high-temperature applications such as excavators, skid steer loaders 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's significant when you consider that if the use of HEHF results in the burning of 18 percent less diesel fuel to do the same amount of work, an excavator would burn 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 less CO₂.

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

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

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

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

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

Under high pressure conditions, temperatures are often elevated. As a result, the viscosity is reduced and there are higher levels of internal leakage through the pump clearances. 

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

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

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

Other Side of the Circuit

Research at the CCEFP is also looking at the other side of the circuit - the load and hydraulic motor used to propel skid steer loaders, excavators and hydraulic hybrid vehicles. A car is least efficient in terms of fuel economy when it is starting up, and the same is true with mobile equipment powered by a hydraulic system. Essentially, it is this starting efficiency that establishes design parameters such as operating pressure 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 starting conditions, 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 less flow 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 where we can make the biggest impact."

Michael's group has studied the effects of various fluids on different types of motors using his lab's hydraulic dynamometer, which is designed specifically for measuring hydraulic motor efficiency under starting and low-speed, high-torque conditions. Three lubricants with the same chemistry, but different viscosity characteristics, were tested on an axial piston, radial piston and orbital motor. Researchers found that different motors have different appetites for lubricants, depending on the lubrication regime at start-up.

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

There was no improvement in performance at start-up of either the radial or piston motor from the viscosity improver additive. Michael says he believes that the condition at start-up for these motors is what is known as boundary lubrication, in which case chemical additives in the oil react to the metal surfaces to form a low-friction film that reduces static friction. "It appears as though the chemistry, rather than the physical properties of the oil, is the determining factor here," says Michael.

While research is ongoing in a variety of areas, field trials and a decade of laboratory testing demonstrate that shear stable multi-grade hydraulic fluids and friction modifiers improve energy efficiency in hydraulic systems. Fluids that meet the requirements of the NFPA Energy Efficient Hydraulic Fluid classification system increase fuel economy and productivity while reducing CO₂ emissions. And in the last year, oil companies including Shell, ExxonMobil and Citgo have introduced new high-efficiency hydraulic fluids, and OEMs are beginning to specify the fluids in their products.

With the focus of the engineering research center on increasing energy efficiency, some members set a 10 percent goal on average energy savings in fluid power applications, achieved through 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 document savings 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