Energy Efficient Fluids

Project / Leader

1B.2 - Paul Michael, Fluid Power Institute

Title

Energy Efficient Fluids

Statement of Project Goals

The research goal of this project is to systematically investigate the tribology of hydraulic motor lubrication under low-speed high-torque (LSHT) conditions by comparing the boundary friction, mixed-film lubrication, and pressure-viscosity properties of fluids with motor efficiency measurements in a full-scale dynamometer.

Project's Role in Support of the Strategic Plan

This project will increase the efficiency and energy density of hydraulic fluid power systems by identifying the fundamental fluid properties that affect starting torque and low-speed efficiency in hydraulic motors.  This is of strategic importance within the CCEFP because low-speed motor efficiency often determines the minimum displacement (size) and operating pressure of mobile hydraulic equipment.  Thus, solving the problem of low-speed efficiency is essential to improving the compactness of mobile fluid power systems.  The findings of this investigation will be incorporated into formulations of energy efficient fluids for Test Bed 1 (excavator) and Test Bed 2 (HHPV).

Description and explanation of research approach

Description of problem: This research project targets the problem of hydraulic motor starting and LSHT efficiency by investigating the fluid properties that influence friction.  Since starting efficiency often determines the minimum displacement (size) and operating pressure of mobile hydraulic equipment, it is necessary to improve this characteristic of motors in order to improve the efficiency and compactness of fluid power systems.  The current models that describe leakage and hydrostatic load bearing capabilities in hydraulic motors are based on assumptions of Newtonian viscosity behavior, EHL contact conditions, and laminar flow.  However, the hydraulic motors used in off-highway equipment frequently start, stop, and reverse rotation as they transport material during low-speed high-torque (LSHT) “digging” cycles. Under low-speed conditions, the efficiency of hydraulic motors is at a minimum because the entrainment velocity is insufficient to generate a full hydrodynamic separation of surfaces.  As evidenced by scuffing on the piston skirt shown in Figure 1, boundary lubrication clearly plays a role in the lubrication of a radial piston motors.  This research project provides new insights into role of boundary friction, mixed-film lubrication, and elastohydrodynamic lubrication in geroler, radial, and axial piston motors.

Research Approach: Four ISO VG 46 hydraulic fluids were evaluated in this study.  These fluids incorporated the same ashless antiwear additive chemistry but differed in base stock and viscosity index.  Boundary lubrication regime friction coefficients were measured using a PCS Instruments High Frequency Reciprocating Rig (HFRR). Mixed-lubrication regime traction coefficients were measured using a PCS Instruments Mini-Traction Machine (MTM). Low-shear viscosities (µ) were measured using a high-pressure falling body viscometer. The pressure-viscosity coefficient, α*, was determined at 50 and 80C and from measurements at pressure, p, of 0.1, 25, 50 100 150 250 and 350 MPa. Several forms of the pressure-viscosity coefficient are in common use.  In this study, the reciprocal asymptotic isoviscous pressure coefficient, α*, as defined in Equation 1 was used.

Equation 1

Geroler, axial, and radial piston motors were used in the efficiency studies.  Motors were subjected to manufacturer specified break-in conditions prior to testing.   Specifications for the motors are provided in Table 1.

Table 1: Description of Test Motors

Motor efficiency measurements were performed under constant pressure conditions in accordance with the ISO 4392-1 standard test method. In this standard test method, fluids are compared by measuring the torque output of the hydraulic motor as it rotates at 1 RPM with a constant supply pressure. The LSHT dynamometer incorporates a pressure-compensated axial piston pump, a digital torque transducer, two variable frequency drives, and an 18-channel data acquisition system.  The VFD controllers were programmed for semi-automated testing.