Improving hydraulic pump energy efficiency requires understanding how temperature, speed, and pressure interact across load conditions.
Understanding how to maximise hydraulic pump energy efficiency is increasingly important for engineers seeking to reduce energy losses across fluid power systems. A new study by Dr Heikki Kauranne from Aalto University sheds light on the real-world performance characteristics of swash-plate type axial piston pumps, and challenges long-held assumptions about how these pumps behave under changing load conditions.
In most design and operating scenarios, engineers have historically relied on manufacturer-supplied efficiency curves to evaluate pump performance. However, these data sets are typically limited to two-dimensional mappings, usually pressure and speed, with no insight into how displacement or fluid temperature affect outcomes. According to Kauranne, this can lead to hydraulic systems operating outside their most efficient range, creating unnecessary energy losses and increasing the burden on auxiliary cooling systems.
To better quantify these variables, the study conducted extensive steady-state measurements on a nine-piston variable displacement axial pump across a range of real-world parameters: pressures from 25 to 250 bar, rotational speeds from 500 to 2000 rpm, multiple displacement settings, and inlet fluid temperatures of 25°C and 60°C. Using a controlled test system aligned with ISO 8426 and ISO 4409 standards, Kauranne calculated total, volumetric, and hydromechanical efficiencies under varied load conditions.
The results highlight a critical takeaway for pump operators and design engineers: total efficiency is not a fixed trait. It is highly dependent on how pressure, speed, displacement, and temperature interact at any given operating point.
At lower pressures, total efficiency rises sharply before tapering off at higher loads. Volumetric efficiency, influenced primarily by pressure-induced internal leakage and fluid compressibility, declines linearly with rising pressure. In contrast, hydromechanical efficiency tends to improve with pressure, as higher loads stabilise lubrication films within the pump. Rotational speed, too, plays a dual role, increasing volumetric efficiency but slightly reducing hydromechanical efficiency at low pressure due to higher friction losses.
One particularly interesting result concerns fluid temperature, a parameter often overlooked in field conditions. While higher temperatures (and therefore lower viscosities) can increase hydromechanical efficiency by reducing friction, they simultaneously reduce volumetric efficiency due to higher internal leakage. The study found that the optimal balance between these competing effects shifts depending on pressure and displacement, underscoring the need for careful thermal management.
Displacement settings also proved critical. Operating at higher displacement levels consistently delivered better overall efficiency, especially at moderate to high pressures. This finding has particular relevance for variable displacement pumps used in load-sensing or energy recovery systems, where displacement is actively modulated to meet changing demand.
For operators of mobile or industrial hydraulic equipment, these findings translate into actionable insight. Running a pump at suboptimal displacement with a high-viscosity fluid at variable speeds can significantly reduce system efficiency, even if other factors appear to be within design limits.
Kauranne notes that pump manufacturers rarely release performance data beyond simplified curves. As a result, engineers designing high-efficiency systems, particularly those pairing electric drives with hydraulic actuation, must either conduct their own tests or make conservative assumptions, both of which add cost and complexity to the design process.
The study further underscores the need for improved modelling tools and performance datasets that reflect real-world variability. While ISO 8426 and ISO 4409 provide valuable frameworks, their limitations in accounting for compressibility, fluid enthalpy, or dead volume mean that next-generation modelling methods, such as those proposed by Schänzle, Pelz, and Barkei, may need to be adopted to capture the full picture.
For now, Kauranne’s work provides a vital resource for pump users and OEMs seeking to enhance their system efficiency. The ability to map how a pump performs across a broad range of operating conditions provides a more complete foundation for design, optimisation, and predictive maintenance.
As hydraulic systems increasingly intersect with digital control, electrification, and energy management goals, knowing where the real performance sweet spots lie could mark the difference between wasted energy and engineered precision.
The full study is published in Energies and can be accessed here.
