Steven N. Herzog
Paul W. Michael
Edited by Kenneth J. Korane
Manufacturers looking to improve the fuel economy of off-road equipment should take a lesson from the automotive industry. Straight-grade engine oils have not been approved for use in passenger cars for decades, in part because they cannot meet fuel-economy requirements. Multigrade oils, which provide a more-consistent viscosity across a wide range of temperatures, are used instead. But straight-grade hydraulic fluids are used in many fluid-power applications.
Where industrial machines operate in temperature-controlled environments, straight-grade fluids are a practical choice. In commercial vehicles and mobile equipment, on the other hand, varying temperatures and energy-efficiency considerations can make multigrade hydraulic fluids a better option.
Nearly a decade of laboratory studies and field trials in off-road equipment have demonstrated that fuel efficiency depends on hydraulic fluid temperature, viscosity, and shear stability, and that shear-stable, multigrade hydraulic fluids can improve fuel economy.
To help engineers take advantage of energy-efficient hydraulic fluids, the National Fluid Power Association’s (NFPA) Fluids Technical Committee has proposed a viscosity classification system analogous to SAE J300 specs for engine oils.
This new classification uses the letter “L” in place of “W” (to avoid possible confusion with engine oils) as the designation for low-temperature grades. For instance, an NFPA 32L-68 hydraulic fluid provides the low-temperature viscosity properties of an ISO 32 hydraulic fluid and the high-temperature viscosity of an ISO 68. Fluids that meet the requirements of the proposed NFPA energy-efficient classification system increase fuel economy and productivity while reducing CO2 emissions.
A critical difference between hydraulic and engine-oil applications is pressure. Oil pressure in an engine-lubrication system is typically 40 to 60 psi. Fluid-power systems routinely operate at pressures two orders of magnitude higher. High pressure creates shear stresses that could destroy the polymers in multigrade engine oils. Consequently, when polymers are used in energy-efficient fluids, they must be shear stable.
Several studies have established that hydraulic-pump efficiency depends on hydraulic-fluid viscosity after shear. And the two elements that make up pump efficiency are volumetric and mechanical efficiency. Mechanical efficiency relates to frictional losses in a hydraulic component and the energy required to generate rotary motion in a pump or motor. Volumetric efficiency relates to flow losses within a hydraulic component and to internal leakage. Both properties are viscosity dependent. The optimal overall efficiency of a pump corresponds to the maximum product of volumetric and mechanical efficiency. See the related sidebar, “Relating viscosity to pump efficiency,” for more details.
Because mobile-hydraulic systems generally operate in the steep portion of the volumetric efficiency curve, increasing hydraulic-fluid viscosity at high temperatures tends to improve volumetric efficiency and reduce energy consumption. During cold-temperature start-up when viscosity is potentially high, a low-viscosity hydraulic fluid would improve mechanical efficiency and lead to better overall efficiency. A shear-stable, high VI (viscosity-index) hydraulic fluid lets users meet both goals.
Low-temperature efficiency. The Vane-pump efficiency graph shows how increasing the VI of an ISO 46 hydraulic fluid from 100 to 160 improves low-temperature efficiency. Depending on the pump displacement, at 0°C hydromechanical-energy losses due to viscous drag are 30 to 60% higher for a 100-VI fluid than a 160-VI shear-stable fluid.
Qa = Qn – k(P/η)
where Qa = actual flow rate, lpm; Qn = nominal flow rate, lpm; k = geometric constant for the pump; P = discharge pressure, bar; and η = kinematic viscosity after shear, mm2/sec. The term k(P/η) represents internal leakage in the pump. Leakage decreases as the fluid viscosity increases, resulting in higher flow rates and improved volumetric efficiency.
In field trials conducted in a medium-size excavator, mini excavator, and skid-steer loader, OEM-recommended oils were used to establish baseline efficiency and productivity levels. An SAE 10W hydraulic fluid (104 VI) was used in the medium-excavator trial and “all-season” ISO 46 hydraulic fluid (142 VI) went into the mini excavator and skid-steer loader trials.
The duty cycle during the field trial differed for each piece of equipment and matched their intended use. Each vehicle relied on hydraulic propulsion and the size of the diesel engine varied from 40 to 125 hp.
The purpose of the field trials was to validate efficiency models developed from pump bench-test data. The Performance improvements table compares predicted efficiency improvement and field test results. This efficiency data is based on the amount of material moved per unit of diesel fuel consumed, and showed improvements of up to 22%.
In addition to better fuel-efficiency, the field trial also demonstrated productivity improvements. As shown in the table, gains ranged from 14 to 16%, as measured by the amount of material moved per hour.
A cost-benefit analysis performed after each field study evaluated the economic value of energy-efficient hydraulic fluids. The nearby table shows one example. A 4,000-hr drain interval was assumed with a value of $9.00/gallon assigned to the 10W fluid and $13.50/gallon to the energy-efficient hydraulic fluid — a 50% premium. The cost of diesel fuel in this analysis was $3.15/gallon. To be conservative, a fuel-economy improvement at full throttle of 18.4% was used instead of the 26% improvement seen at 90% throttle. No value was assigned to the approximately 6% additional work that the excavator could perform when using the 179-VI energy-efficient hydraulic fluid.
For all three tests, savings exceeded $10,000/fluid-drain interval. For the skid-steer loader and the 40-hp excavators, much of the gains resulted from increased productivity.
NFPA’s Technical Committee proposes the following viscosity guidelines for specifying hydraulic fluids that can enhance fuel economy. Based on the field studies referenced here, a shear-stable, multigrade hydraulic fluid is recommended. These high-efficiency fluids have an equivalent viscosity at 100°C that is a minimum of one grade higher than the ISO 40°C viscosity grade. Likewise they have an equivalent viscosity at low temperature one grade lower than the ISO 40°C viscosity grade. For instance, in the medium-size excavator, the ISO 46 energy-efficient fluid had the low-temperature viscosity of an ISO 32 fluid and the high-temperature viscosity of an ISO 68 fluid. This fluid is designated “NFPA 32L-68.” Here’s the basis for this designation.
The first step in selecting an energy-efficient hydraulic fluid is to determine the minimum starting temperature requirements. As in automotive applications, hydraulic systems are least efficient during startup. To define the low-temperature viscosity grade, or L grade, the temperature corresponding to 750 cP (mPa-sec) has been determined for each ISO viscosity grade. The 750-cP limit was selected because it corresponds to the typical requirement for maximum viscosity at pump startup. The L grades and the ranges associated with each were established by extrapolating the 40°C viscosity ranges of each ISO VG, using 100 VI, down to the 750-cP limit. For example, an NFPA 32L has a temperature range of –8 to –14.9°C. All the fluid grades are shown in the accompanying Low-temperature viscosity grades table.
To define the NFPA high-temperature grade, the minimum viscosity for each ISO grade has been extrapolated up to the viscosity at 100°C using a VI of 100. The maximum viscosity for each grade is just below the minimum viscosity for the next higher viscosity grade.
There are two ways to use this data to select an energy-efficient fluid. One may simply add two NFPA grades to the low-temperature viscosity (L grade) and use the High-temperature viscosity grades table to determine the minimum viscosity requirement after shear at 100°C. For instance, if a 32L fluid is required for starting conditions, the minimum viscosity to yield fuel-efficient performance is 8.1 mm2/sec at 100°C after shear. This fluid would be designated NFPA 32L-68. An NFPA 32L-100 would be expected to provide even greater energy efficiency.
Alternatively, a high-temperature viscosity grade may be selected based on specific viscosity requirements for system components. In these applications, determine the minimum fluid viscosity by referring to hydraulic pump and motor specifications or the information provided in NFPA T2.13.13. In either case, a viscosity-temperature chart may be used to determine the minimum viscosity requirement at 100°C.
A widely used laboratory procedure — the 40-min sonic shear test (ASTM D 5621) — determines the shear stability of a fluid and provides a good estimate of in-service viscosity. Studies conducted to evaluate shear-stability tests show that this test correlates with the in-service viscosity of a hydraulic fluid in a 345 bar (5,000 psi) piston pump. Because hydraulic pump efficiency corresponds to in-service viscosity, the sheared-oil viscosity — the viscosity at 100°C after the 40-min sonic shear test — is used to the determine NFPA high-temperature viscosity grades shown in the table.
Relating viscosity to pump efficiency
A recent study performed by Evonik RohMax, however, challenges old perceptions of volumetric and hydromechanical losses versus viscosity. It shows that mechanical efficiency in pumps is much less sensitive to viscosity than previous models indicate, and that overall efficiency closely parallels volumetric efficiency over the temperature operating range.
As the Efficiency versus viscosity graphic shows, over the range of temperatures in this test, volumetric losses far outweigh hydromechanical losses. Thus, the most important factor in overall efficiency is the volumetric efficiency, which varies with viscosity. Thus, overall efficiency of a hydraulic pump depends primarily on fluid viscosity — the higher the viscosity, the more efficient the pump.
This is important because it means the pump volumetric-efficiency gains produced by high-viscosity-index, shear-stable hydraulic fluids are not counteracted by hydromechanical losses.
Update on NFPA standards
The current NFPA standard practice for viscosity selection (T2.13.13) does not recognize the importance of fluid-shear stability or the potential to improve fluid-power efficiency through the use of high-viscosity-index, shear-stable hydraulic fluids. The classification system and modifications the authors describe here are, at this stage, only proposals by the Fluids Technical Committee. They have not yet been vetted by the full NFPA membership.