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Quality Hydraulic Fluid Improves Fuel Economy of Construction Equipment

In the world of power transmission systems, hydraulic fluid power systems have remained popular owing to their high-power density and cost advantages compared to electro-mechanical systems, with hydraulic equipment being the undisputed leader in terms of power density. Today, hydraulics forms an integral and indispensable part of most construction equipment. The almost incompressible nature of oil along with its ability to withstand pressure aids in transmission of power and force (with multiplication if required) from one point to another in hydraulic systems. This power and force can then be utilised to do useful work, be it lifting heavy loads, moving it from one place to another, drilling precise holes, digging and excavating the earth etc.

Hydraulic power is generated through a combination of oil flow and pressure. Oil flow and pressure is created from a hydraulic pump and transmitted through hoses or tubes, via control valves, to the hydraulic motor or cylinder that will do the work. One of the key advantages of hydraulic systems is to be able to transmit large amounts of power from a remote power source (electric motor or internal combustion engine) to a compact actuator.

Hydraulic fluid is a multi-functional tool that helps keep a machine running smoothly. Not only is it responsible for power transmission and transferring of heat, but its functions also include: lubricating critical components, maintain pressure, providing seal and preventing corrosion.

The first thing to know about hydraulic oil is there is no universal spec for it. Each manufacturer has a particular type of hydraulic oil with its own viscosity grade suitable for their machines. There are generic hydraulic oils on the market, but these are often not backwards compatible, meaning they can't be outfitted to older machines, and could cost owners money in the long run.

It's critical that users follow the OEM's recommendations when specifying hydraulic fluid. Owners need to consider the pump and equipment specification, the load used on the fluid, pump type, and the operating conditions and applications.

Hydraulic base fluid options include petroleum/mineral oil, synthetic hydrocarbons, bio-based oils and water/ oil emulsions. Today's typical hydraulic fluid is mineral based with 0.3%-0.8% zinc-based antiwear additive (ZDDP).

Historically the hydraulic fluid is one of the last areas to focus on when developing the equipment and has been viewed as a commodity. But results show that fluids can be developed to improve overall equipment performance—and with that improve the overall efficiency of the system and increase productivity. These fluids have been a focus area over the past few years and will continue since the off-highway segment is now in the spotlight for reducing emissions and improving overall fuel economy.

There are two main considerations for hydraulic fluid that have a bearing on energy efficiency—the viscosity grade and the hydraulic fluid type. As mentioned earlier, these specifications are typically determined by type of hydraulic pump, operating temperature and pressure ranges. Other considerations are base oil, lubricant quality and additives.

Hydraulic fluid performance can have a significant impact on the fuel efficiency of construction equipment, particularly mobile equipment. Much of the fuel consumed in mobile equipment that relies on hydraulic power to perform the work is used for operating the hydraulic pump, which is the heart of the hydraulic system. Power generated by the engine is eventually transferred throughout the hydraulic system. The less efficiently this power is transmitted via the hydraulic fluid the more the engine is required to work, thus more fuel is consumed.

Good hydraulic oil improves pump efficiency. Improved pump efficiency improves fuel consumption. As hydraulic fluids lose the effectiveness to transmit power due to a lower VI or contamination the operator is forced to use more fuel to make the hydraulic pump work harder in order to compensate for the loss of workload that the system is handling. A correct operating viscosity provides the proper lubricating film thickness for hydraulic components, there's a narrower viscosity range where power losses are minimized and power transfer is maximized. Contaminants such as particulate and water will contribute to inefficient power transmission in these systems as well.

Fuel efficiency depends on proper additive formulation, hydraulic fluid operating temperature, viscosity, viscosity index (VI), and shear stability. Shear-stable multigrade hydraulic fluids can improve fuel economy over monogrades. When polymers are used in energy-efficient fluids, they must be shear stable. Hydraulic oil containing friction modifiers in the additive formulation also contribute to the overall fuel efficiency. Another formulation factor for the efficient power transmission is a high resistance to reduction in volume under pressure due to little or no compressibility (high bulk modulus).

Temperature control, contamination control, and aeration are system design factors that will influence how power is transferred throughout the hydraulic system. Systems are least efficient during startup. Determining the minimum starting temperature requirements aids in selecting an energy-efficient hydraulic fluid based on viscosity and the viscosity index. Aeration can also be a factor of system design which increases the fluids compressibility and waste power.

Viscosity variations associated with low starting temperature and high operating temperatures contribute to system efficiency and reliability losses in a variety of ways. Selection of the proper viscosity grade of hydraulic fluid is an important and cost-effective technique that allows equipment to start smoothly at low temperatures, and also deliver adequate oil flow rates needed for efficient operation at high temperatures.

Viscosity is an important criterion in the selection of a hydraulic fluid. At low temperature, excessive viscosity may result in poor mechanical efficiency, difficulty in starting, and wear. As oil temperature increases, viscosity decreases, resulting in lower volumetric efficiency, overheating and wear. Pump and motor manufacturers often provide hydraulic fluid recommendations in their documentation covering:
the maximum startup viscosity under load
the range of optimum operating viscosity
the maximum and minimum operating viscosity

Selection of the optimum fluid viscosity grade will provide the most efficient pump performance at standard operating temperatures, therefore minimizing lost time and energy and fuel costs for the operator.

Recent work by the authors has led to the development of a new performance standard for hydraulic fluids, described as Maximum Efficiency Hydraulic Fluid (MEHF). MEHF fluids are formulated to provide a combination of high viscosity index and good shear stability, which enables all types of hydraulic pumps to deliver increased power at a lower level of energy consumption.